Ca Et Biophysica Acta 1783 (2008) 651–672

View metadata, citation and similar papers at core.ac.uk brought to you by CORE

provided by Elsevier - Publisher Connector

Available online at www.sciencedirect.com

Biochimica et Biophysica Acta 1783 (2008) 651–672 www.elsevier.com/locate/bbamcr

Review Ca2+-permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology ⁎ Gregory J. Barritt a, , Jinglong Chen b, Grigori Y. Rychkov c

a Department of Medical Biochemistry, School of Medicine, Faculty of Health Sciences, Flinders University, G.P.O. Box 2100, Adelaide South Australia 5001; Australia b Cytokine Molecular Biology and Signalling Group, Division of Molecular Biology, The John Curtin School of Medical Research, Australian National University, Canberra ACT 0200, Australia c School of Molecular and Biomedical Science, University of Adelaide, Adelaide South Australia 5005, Australia Received 24 September 2007; received in revised form 16 January 2008; accepted 17 January 2008 Available online 7 February 2008

Abstract

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca2+ in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca2+ signals and the entry of Ca2+ through Ca2+-permeable channels in the plasma membrane are critical to the regulation by Ca2+ of hepatocyte function. Ca2+ entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca2+-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca2+ channels (SOCs) have a high selectivity for Ca2+ and characteristics similar to those of the Ca2+ release activated Ca2+ channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca2+ in a sub-region of the ER enriched in type1 IP3 receptors. Activation requires stromal interaction molecule type 1 (STIM1), and Gi2α, F-actin and PLCγ1 as facilitatory 2+ proteins. P2x purinergic channels are the only ligand-gated Ca -permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca2+ channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca2+- and Na+-permeable channels, comprise many hepatocyte receptor-activated Ca2+-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor- activated Ca2+ permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca2+ permeable channels. © 2008 Elsevier B.V. All rights reserved.

Keywords: Hepatocyte; Liver; Ca2+ channel; Plasma membrane; TRP channel; Hormone

1. Introduction

Abbreviations: ER, endoplasmic reticulum; SERCA, endoplasmic reticulum 2+ 2+ It is now about 30 years since the first clear evidence was (Ca +Mg ) ATP-ase; PM, plasma membrane; IP3R, inositol 1,4,5-trispho- 2+ 2+ obtained that Ca acts as an intracellular messenger in hepa- sphate receptor; SOC, store-operated Ca channel; ISOC, store-operated channel current; CRAC, Ca2+ release activated Ca2+ channel; STIM1, stromal interaction tocytes (reviewed in [1]). Since that time the major components 2+ 2+ molecule type 1; TRP, transient receptor potential; [Ca ]cyt, cytoplasmic free of the hepatocyte Ca signalling pathways have been elu- 2+ 2+ 2+ 2+ Ca concentration; [Ca ]er, free Ca concentration in the ER; [Ca ]mt, free cidated. These include the roles of endoplasmic reticulum (ER) 2+ 2+ 2+ Ca concentration in the mitochondria; Ca ext, extracellular Ca ; PLC, 2+ 2+ and mitochondria as intracellular Ca stores, IP3-induced re- phospholipase C; VOCC, voltage-operated Ca channel; DBHQ, 2,5-di-(tert- 2+ 2+ butyl)1,4-benzohydro-quinone lease of Ca from the ER, plasma membrane Ca entry and 2+ 2+ ⁎ Corresponding author. Tel.: +61 8 8204 4260; fax: +61 8 8374 0139. (Ca +Mg )ATP-ases in the ER and plasma membrane (re- E-mail address: [email protected] (G.J. Barritt). viewed in [2,3]). The nature of waves or oscillations of

2+ 2+ increased cytoplasmic free Ca concentration ([Ca ]cyt) and Hepatocytes, which are often considered as specialised epi- many of the target enzymes which are regulated by changes in thelial cells, are organised in single-cell plates (Fig. 1B). The 2+ [Ca ]cyt have been determined (reviewed in [2,3]). Changes in architecture of the liver is determined by the extracellular matrix [Ca2+] in hepatocytes constitute an essential intracellular sig- or “scaffold”. This is composed of fibrillar collagen in the portal nalling network which functions in unison with numerous other and central veins, and a basement membrane composed of signalling pathways, including the extensive protein kinase collagen, laminin and entactinnidogen [15–17]. The hepatocyte network, in regulating hepatocyte and liver function in normal plates are perfused by blood from the gut (hepatic portal vein) and pathological conditions (reviewed in [3–7]. and heart (hepatic artery) which drains into the central vein Recent studies of hepatocyte Ca2+ signalling have chiefly (Fig. 1B). The space through which blood perfuses, the sinu- been directed towards identifying the different pathways of Ca2+ soids, are lined with sinusoidal endothelial cells (Fig. 1A). Bile entry across the hepatocyte plasma membrane; gaining an un- canaliculi, which are surrounded by the adjacent membranes of derstanding of the molecular constituents of each entry pathway; two hepatocytes (Figs. 1B and 2A), collect bile fluid from each understanding the activation of Ca2+ entry by hormones, growth hepatocyte and move this to the bile duct which is lined with factors and other signalling molecules; and elucidating the im- biliary epithelial cells (cholangiocytes) (Fig. 1A, B). mediate and down-steam physiological functions of each Ca2+ Consistent with the complex function and architecture of the entry pathway (reviewed in [8,9]). These investigations have liver, hepatocytes are highly differentiated cells which exhibit recently been aided by the discoveries of TRP (transient receptor spatial polarisation and a characteristic intracellular organiza- potential channels) in animal cells and elucidation of their pro- tion [13,18–21]. Some features of the spatial polarity of hepa- perties and likely functions [10]. The recent discovery of the tocytes are shown in the electron micrographs in Fig. 2A and Orai1 (CRACM1) and stromal interaction molecule type 1 B and schematically in Fig. 2C. Three distinct structural and (STIM1) as the channel pore and Ca2+ sensor, respectively, of functional regions of the hepatocyte plasma membrane can be Ca2+ release activated Ca2+ (CRAC) channels and some other defined: the basal or sinusoidal membrane, which faces the store-operated Ca2+ channels (SOCs) (reviewed in [11,12]) have blood in the sinusoids, the lateral membrane, which lines the also contributed greatly to understanding the process of Ca2+ intercellular space, and the canalicular membrane, which faces entry to animal cells. While it is likely that all these proteins do the canalicular lumen. Tight junctions define the canalicular play important roles in hepatocyte Ca2+ homeostasis, much is membrane region. Gap junctions permit the movement of mol- still to be learned about the molecular nature and mechanisms ecules between adjacent hepatocytes [13,18–21]. Much of the of activation of Ca2+ entry pathways in the hepatocyte plasma hepatocyte cytoplasmic space is occupied by the ER, mitochon- membrane. dria, other intracellular organelles (Fig. 2A). In the fed state The aim of this review is to summarise current knowledge of glycogen granules are clearly evident. The trafficking of pro- the nature, molecular structures and mechanisms of activation teins and vesicles through the hepatocyte cytoplasmic space is of Ca2+-permeable channels in the plasma membranes of hepa- critical for the maintenance of hepatocyte spatial polarity and tocytes, and to present knowledge of their roles in intracellular for many hepatocyte functions, including the secretion of bile Ca2+ homeostasis and in hepatocyte biology. Before examining acids and protein [18,20]. these topics, it is useful, to briefly describe the morphology and function of hepatocytes, the main elements responsible for intra- 3. Ca2+ homeostasis and the role of Ca2+ as an intracellular cellular Ca2+ homeostasis, and the mechanisms involved in the messenger in hepatocytes regulation of hepatocyte membrane potential and cell volume. 3.1. Biochemical processes regulated by intracellular Ca2+ 2. Liver architecture and the morphology and functions of 2+ hepatocytes Changes in hepatocyte [Ca ]cyt regulate glucose, fatty acid, amino acid and xenobiotic metabolism, bile acid secretion, The liver plays a central role in metabolism, detoxification, protein synthesis and secretion, the movement of lysosomes and and in regulating the overall homeostasis of the body. Functions other vesicles, the cell cycle and cell proliferation, and apoptosis of the liver include the metabolism of carbohydrates, fats, pro- and necrosis [7,13,14,22–24]. Changes in the concentration of teins, and xenobiotic compounds, the synthesis and transcellular Ca2+ in intracellular organelles also play important regulatory movement of bile acids and bile fluid, and the synthesis and roles. Thus the concentration of Ca2+ in the mitochondrial 2+ secretion of proteins [13,14]. Within each liver lobe, the tissue matrix ([Ca ]mt) regulates the citric acid cycle and ATP syn- (liver parenchyma) is arranged into hexagonal-shaped lobules thesis [25] and apoptosis [7], the concentration of Ca2+ in the 2+ each comprised of a central vein surrounded by six portal triads ER ([Ca ]er) regulates protein synthesis and the metabolism of [15,16])(Fig. 1A). Each portal triad is comprised of a hepatic xenobiotic compounds [26], and the concentration of Ca2+ in vein, hepatic artery and bile duct (Fig. 1A). The predominant cell the nucleus regulates cell proliferation [5]. type in the liver is the hepatocyte (parenchymal cell) which An important hepatocyte-specific function regulated by intra- comprises about 70% of all cells in the liver (equivalent to 90% of cellular Ca2+ is the uptake of bile acids from the blood, excretion the liver volume) [13,15,16]. Endothelial cells, biliary epithelial of bile fluid into the bile canaliculus and the movement of bile cells (cholangiocytes), hepatic stellate cells, Kupffer cells (mac- along the canaliculus (Fig. 2C) [13]. Hormones and bile acids rophages) and oval cells also play important roles in the liver. enhance the movement of bile fluid along the bile canaliculus by G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 653

Fig. 1. Features of liver anatomy and the organisation of hepatocytes in liver lobules. (A) A drawing of the major lobes of rat liver (a), the relationship between the central vein and portal triads (b), and the arrangement of the hepatic vein, hepatic artery, bile duct, hepatocyte plate and sinusoidal space (c). (B) A schematic drawing of the hepatocyte plates showing the direction of blood flow from the hepatic portal vein and hepatic artery to the central vein, and the direction of bile flow through the bile canaliculus. (A) (b and c) re-drawn, with permission, from [15], and (B) re-drawn with permission from [206].

2+ 2+ increasing [Ca ]cyt, activating Ca -dependent myosin light reasonably evenly throughout all regions of the ER [4,28] with chain kinase, inducing the polymerisation of F-actin and con- some found in ER closely associated with the plasma membrane traction of the bile canaliculus [27–29] (Fig. 2D). [38–40]. The main extracellular signals which employ Ca2+ as an Ryanodine receptors also mediate Ca2+ release from the intracellular messenger in hepatocytes are hormones, including hepatocyte ER, although not all results are consistent with this epinephrine, norepinephrine, vasopressin, angiotensin II, glu- conclusion [37,41–43]. The results of recent experiments, which cagon, insulin, local hormones, including ATP, ADP, nitric employed RT-PCR to identify ryanodine receptor mRNA, have oxide (NO), prostaglandins, serotonin, cytokines, and extra- shown that hepatocytes possess a truncated form of the type 1 2+ 2+ cellular Ca [30–33]. Changes in [Ca ]cyt are also initiated by ryanodine receptor, and have provided evidence that this re- 2+ cell injury often mediated by the formation of reactive oxygen ceptor plays a role in amplifying IP3-induced Ca release from species [1,22,34]. the ER [44].

3.2. Molecular components of hepatocyte Ca2+ homeostasis 3.3. Hepatocyte Ca2+ signals are often encoded by the frequency of Ca2+ waves (oscillations) 2+ The proteins chiefly responsible for the regulation of [Ca ]cyt, 2+ 2+ [Ca ]er and [Ca ]mt in hepatocytes are summarised in Fig. 3. Physiological concentrations of epinephrine, vasopressin, These are the main components of the hepatocyte Ca2+ signalling ATP, prostaglandins and some other hormones induce repetitive 2+ “tool kit” [35]. Rat hepatocytes express type 1 (20%) and type 2 increases in [Ca ]cyt in isolated hepatocytes [45–49]. Hor- 2+ (80%) inositol 1,4,5 trisphosphate receptors (IP3Rs) with pos- mone-induced increases in [Ca ]cyt in single hepatocytes are 2+ sibly a small amount (b1%) of type 3 IP3Rs [28,36,37].In observed as waves of increased [Ca ]cyt, with each wave hepatocytes, type 2 IP3Rs are expressed chiefly in the peri- beginning at a fixed point in the cell [45–47]. In hepatocyte 2+ canalicular region and are responsible for the initiation of waves couplets waves of [Ca ]cyt originate at the bile canalicular 2+ of increased [Ca ]cyt originating from this region [4,28].As region [28]. Increasing the hormone concentration increases the discussed in more detail below, type 1 IP3R are distributed frequency of the oscillations [45,48,50,51]. The strength of the 654 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

Fig. 2. Some features of the morphology and spatial polarity hepatocytes. (A) Transmission electron micrograph of rat hepatocytes in situ, showing organisation of the cytoplasmic space and organelles around the bile canaliculus. The scale bar represents 2 μm (M. Teo, M. van Baal, M. Scheisser, A. Wittert, R. Padbury and G. Barritt, unpublished results). The abbreviations are: LI, lipid inclusions; lateral PM, lateral plasma membrane; JC, junctional complex; BC, bile canaliculus; P, peroxysome; M, mitochondria; and S, sinusoid. (B) Transmission electron micrograph of an isolated rat hepatocyte couplet showing a bile canaliculus located between the two cells. From [207] with permission. (C) A scheme showing the pathways of bile acid movement and vesicle trafficking in hepatocytes within the hepatocyte plate. (D) A 2+ scheme showing the role of [Ca ]cyt in regulating F-actin movement and contraction of the bile canaliculus. hormone-initiated Ca2+ signal is thought to be determined by Ca2+ channels (SOCs) [55] and possibly by some other types 2+ 2+ the frequency of [Ca ]cyt oscillations [25,52]. Hormone- of Ca entry channels. 2+ induced frequency-modulated waves of increased [Ca ]cyt are also observed in hepatocytes in situ in perfused livers [46, 4. Cytoplasmic Ca2+ and the regulation of hepatocyte 47,50]. In liver lobules, the strength of hormonal signals which membrane potential and cell volume 2+ employ oscillations in [Ca ]cyt is thought to be conveyed to hepatocytes located in different regions of the lobule by the rate Membrane potential is the main driving force for Ca2+ entry 2+ 2+ of propagation of the wave of increased [Ca ]cyt (reviewed through Ca -permeable channels in the plasma membrane 2+ in [3]). [10,56,57]. Furthermore, in hepatocytes changes in [Ca ]cyt are When isolated hepatocytes are incubated in the absence of involved in the regulation of both membrane potential and cell 2+ 2+ extracellular Ca (Ca ext) hormone-induced oscillations in volume [58–60]. Therefore, in a discussion of the nature and 2+ 2+ [Ca ]cyt decline to zero after 5–10 minutes [49]. This is likely function of plasma membrane Ca channels in hepatocytes, it due to the transport of a small amount of Ca2+ out of the cell is relevant to consider the main factors which maintain and during each oscillation (cf [53]), as shown also for pancreatic regulate membrane potential and cell volume. acinar cells [54], resulting in a slow depletion of Ca2+ in the ER In response to increases in the concentrations of metabolites 2+ to a level where there is insufficient to allow [Ca ]cyt oscilla- and bile acids in the portal blood following the digestion and 2+ tions. During hormone-induced [Ca ]cyt oscillations an in- absorption of food, hepatocytes take up considerable quantities of crease in Ca2+ entry is required in order to maintain sufficient glucose, amino acids, bile acids and other organic molecules, as Ca2+ in the ER. This is likely to be mediated by store-operated well as Na+ and other inorganic ions from the blood. This is G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 655

cells, membrane potential in hepatocytes is set to a negative value by the K+ conductance [68]. The K+ equilibrium potential is set by the activity of the (Na+ +K+)-ATP-ase which maintains the intracellular Na+ and K+ concentrations at low and high values, respectively, relative to those in the blood (Fig. 4). However, in hepatocytes there is a significant deviation of the observed membrane potential from the predicted K+ equilibrium potential. This suggests that Cl− and Na+ conductances also contribute to determining the observed membrane potential [59,61] (Fig. 4). Ca2+ plays an important role in regulating hepatocyte membrane potential through a variety of Ca2+-dependent pathways that control the activity of different types of plasma membrane ion channels and transporters [58,59,69]. In general, the molecular identities of the channels that maintain the hepatocyte membrane potential are largely unknown, and the regulation of hepatocyte membrane conductance by Ca2+ is not well understood. Several types of Ca2+-regulated channels have been reported in hepatocytes in a variety of species. These are Ca2+- activated small conductance K+ channels (SK), Ca2+-dependent non-selective cation channels which, under physiological conditions, mainly provide a pathway for Na+ entry, and Ca2+- dependent Cl− channels [58,59,62,69]. Some of these channels are expressed at different levels in different species. For Fig. 3. The major elements which regulate the distribution and movement of intracellular Ca2+ in hepatocytes. The plasma membrane Ca2+ entry pathways are Ca2+-selective store-operated Ca2+ channels (SOCs), receptor-activated Ca2+- permeable channels (listed in Tables 2 and 3) and ligand-gated Ca2+-permeable channels. Ca2+ outflow across the plasma membrane is mediated by the plasma membrane (Ca2+ +Mg2+)ATP-ases, PMCA1 and PMCA2w [208], with a possible contribution from PMCA4b [209], and by the Na+-Ca2+ exchanger [2,210,211].Ca2+ uptake by the ER is mediated by the ER (Ca2+ +Mg2+)ATP-ase 2+ (SERCAs) and Ca outflow from the ER by types 1 and 2 IP3R, [28,36] and ryanodine receptors [44].Ca2+ uptake by mitochondria is mediated by an electrogenic Ca2+ uniporter and Ca2+ outflow by Na+/Ca2+ and H+/Ca2+ anti- 2+ 2+ porters (reviewed in [2,3,8]. Golgi also possess IP3R and (Ca +Mg )ATP-ases [212]. Numerous Ca2+ binding proteins are present in the cytoplasmic space and in organelles. associated with the inflow of water to hepatocytes. Bile acids, Na+, and other components of bile fluid are transported across hepatocytes and excreted into the bile canaliculus. These processes create considerable osmotic forces, which, if not countered, would greatly alter the hepatocyte volume. The maintenance of a constant cell volume under these conditions requires mechanisms which tightly control the movement of ions across the sinusoidal, basolateral and canalicular domains of the hepatocyte plasma membrane. The process is termed “regulated volume decrease” [59]. When hepatocytes are subjected to osmotically-induced cell shrinkage, the recovery process is termed “regulatory volume increase”. The major pathways involved in regulating volume are K+ and Cl− channels, K+,Na+,andCl− co-transporters and the (Na+ +K+)ATP-ase [59] (Fig. 4). Measurements of the resting membrane potential of hepa- Fig. 4. The major plasma membrane channels, transporters and co-transporters tocytes in situ in the perfused liver using sharp electrodes have involved in the maintenance of the membrane potential, the electrochemical − − – gradient across the plasma membrane, and cell volume. The activity of several given values between 30 and 40 mV [60 65]. In isolated 2+ 2+ − − channels is regulated by [Ca ]cyt, and [Ca ]cyt plays an important role in hepatocytes, reported values range between 20 mVand 50 to regulating cell volume. Since the plasma membrane potential contributes to the −70 mV [9,60,66,67], reflecting, perhaps, differences in expe- driving force for Ca2+ entry through SOCs, changes in membrane potential will rimental conditions. As in the case of the majority of animal affect the amount of Ca2+ which enters the cell through open SOCs. 656 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 example, Ca2+-activated K+ channels are expressed in rabbit volume in hepatocytes. Often, in experiments designed to in- and guinea pig hepatocytes but not in rat hepatocytes [60]. vestigate Ca2+ entry, the membrane potential is not controlled Depending on which channels are activated and the relative and might change substantially during the experiment. This, in sizes of the corresponding conductances, different effects of turn, may lead to changes in the observed rate of Ca2+ entry. For 2+ 2+ changes in [Ca ]cyt on membrane potential can be expected. voltage-independent Ca channels, membrane hyperpolarisa- Under physiological conditions, changes in hepatocyte tion causes an increase in Ca2+ entry due to an increased driving 2+ 2+ [Ca ]cyt, membrane potential, and cell volume occur in re- force. In contrast, for voltage-dependent Ca channels, mem- sponse to the uptake of metabolites and bile acids and to the brane hyperpolarisation may cause a decrease in Ca2+ entry, binding of hormones and other extracellular signals to plasma despite a larger driving force. The decrease is due to a reduction membrane G-protein- or tyrosine kinase- coupled receptors in open probability of the channels. For the same reasons, [59,69–73]. Often, Ca2+-mobilising hormones such as ATP, membrane depolarisation, induced, for example, by Na+ entry epinephrine and vasopressin, cause characteristically different, through activated TRP channels [77], may also have drastic or even opposite, effects on hepatocyte volume (reviewed in effects on Ca2+ entry [57]. Thus, in studies of Ca2+ entry to liver [59]). This indicates the presence of other signalling pathways cells using techniques other than patch clamp recording, it is 2+ which add to, or modulate, the effects of changes in [Ca ]cyt. important to evaluate and take account of possible changes in Glucagon, a pancreatic hormone that regulates glucose re- membrane potential. lease in liver, hyperpolarizes hepatocytes and induces cell shrinkage [61,74]. The hyperpolarization was found to be inhi- 5. Overview of hepatocyte plasma membrane bited by blockers of K+ channels and by ouabain, suggesting an Ca2+-permeable channels involvement of K+ channels and the (Na+ +K+)-ATP-ase in glucagon action [59]. Glucagon-induced hyperpolarisation was Ligand-gated, store-operated (SOC), receptor-activated, and also found to be blocked by the phospholipase C (PLC) stretch-activated Ca2+-permeable channels are expressed in inhibitor, U73122, and by Gd3+, an inhibitor of some plasma hepatocytes and in liver cell lines. No voltage-operated Ca2+ membrane Ca2+-permeable channels, suggesting requirements channels (VOCCs) have been detected [59,66,78,79]. Ligand- 2+ 2+ for PLC and an increase in [Ca ]cyt [75]. Activation by glucagon gated Ca -permeable channels are defined as plasma mem- of Ca2+ entry through SOCs [9], in addition to the glucagon- brane channels activated by the binding of an extracellular induced increase in cAMP, is the most likely cause of the ob- signal molecule (hormone, neurotransmitter, or growth factor) served glucagon-induced activation of Ca2+-dependent K+ to the extracellular domain of the channel protein or channel conductance and subsequent hyperpolarization. subunit complex. SOCs are defined as plasma membrane Ca2+ The activation by glucagon of the K+ conductance alone, channels activated by a decrease in the concentration of Ca2+ however, is unlikely to induce noticeable changes in cell volume in the ER. Under physiological conditions this occurs when in the absence of the activation of a significant movement of ions G-protein- or tyrosine kinase-coupled receptors are activated of another kind. Since hepatocytes have low resting Na+,K+ and by the binding of a hormone or other agonist leading to the − + Cl conductances [59,61] a small K efflux will bring membrane activation of PLCβ or PLCγ, the formation of IP3,andIP3- potential close to the K+ equilibrium potential and stop any and Ca2+-induced release of Ca2+ from the ER. SOCs can be further K+ efflux. However, in addition to activating Ca2+ activated experimentally using inhibitors of the endoplasmic channels, glucagon also activates a Cl− conductance similar to reticulum (Ca2+ +Mg2+) ATP-ase (SERCA) such as thapsi- that activated by hepatocyte swelling [9]. This requires the cyclic gargin or 2,5-di-(tert-butyl)-1,4-benzohydro-quinone (DBHQ) AMP binding protein, Epac, as well as Ca2+ acting as a co-factor which induce the release of Ca2+ from the ER. [9]. Activation of the Cl− efflux by glucagon simultaneously Receptor-activated Ca2+-permeable channels, as defined here, with activation of the K+ efflux through SK channels is the likely are a heterogeneous group of channels activated by the binding of cause of the glucagon-induced hepatocyte shrinkage. a hormone or other agonist to a G-protein or tyrosine kinase- Interestingly, another hormone epinephrine and its analogue coupled receptor. The receptor protein is separate from the phenylephrine, which activates Ca2+ entry and increases K+ and channel protein and in most cases activation involves, or is likely Cl− conductances, causes hepatocyte swelling [59]. Cell swell- to involve, the generation of an intracellular messenger which ing is usually associated with Na+ entry but, if the cell is binds to a site on the cytoplasmic domain of the channel protein, depolarised, it may also possibly be associated with Cl− entry. leading to activation of the channel. Activation may also involve The activation of non-selective cation channels by Ca2+ mo- protein–protein interaction. The activation of receptor-activated bilising hormones, which would provide an entry pathway for Ca2+-permeable channels would not be expected to depend on a + 2+ 2+ Na , has been shown in hepatocytes and liver cell lines [76]. decrease in [Ca ]er. Stretch-activated (mechano-sensitive) Ca - 2+ Whether or not [Ca ]cyt regulates these non-selective cation permeable channels are defined here as those channels in which channels under physiological conditions is not clear. Other there is a direct transduction of mechanical force into channel causes of hepatocyte swelling, which may, or may not, be opening. 2+ 2+ related to changes in [Ca ]cyt, could include the activation of Liver cell plasma membrane Ca -permeable channels have one or more transporters for amino acids and/or bile acids. been studied in the perfused liver, isolated hepatocytes, and in The above discussion emphasises the interrelationships be- liver cell lines (Table 1). Most liver cell lines are derived from 2+ tween hormone action, [Ca ]cyt, membrane potential and cell hepatocyte precursors and represent hepatocytes with different G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 657

Table 1 Hepatocyte preparations and liver cell lines available for the study plasma membrane Ca2+-permeable channels Hepatocyte preparation or liver cell line References Comments (examples only) Perfused liver [96] Permits the study Ca2+ entry processes in hepatocytes in situ in a fully spatially polarised state, and the measurement of intercellular Ca2+ signals, but technically more difficult than experiments with isolated hepatocytes. Freshly isolated hepatocytes in suspension [1,185] Hepatocytes are fully differentiated but have lost some spatial polarity upon removal from the liver Isolated hepatocytes maintained in primary [82,99,166] Provide a reasonable representation of hepatocytes in situ, but undergo some culture for 2–72 h modification of protein expression and de-differentiation Freshly isolated hepatocyte couplets, [29] Retain greater spatial polarity. Permit studies of intercellular Ca2+ signals and triplets and clusters roles of gap junctions. H4-IIE rat liver cells (derived from [97,98] Offer technical advantages for measurement of Ca2+ entry and cell transfection, Reuber hepatoma) but have the disadvantage that they are partly de-differentiated or altered hepatocytes. HTC rat liver cells (derived from rat hepatoma) [150] HepG2 human liver cells (derived from a [152] liver carcinoma) Clone 9 cells [34] WIF B cells (derived by the fusion of a rat [80] [81] WIF B cells exhibit many features of hepatocytes including retention of spatial polarity,

hepatoma cell line with a human fibroblast canalicular structures and secretion of bile acids. They express all three subtypes of IP3R cell line (cf hepatocytes which expresses types 1 and 2 IP3R) but these appear not to be homogenously distributed. May present difficulties with respect to loading cytoplasmic space with fura-2. degrees of de-differentiation. While liver cell lines offer a hormone) will initiate the activation of Ca2+ entry to hepa- number of technical advantages for studies of the molecular tocytes and liver cell lines [83–94]. Ca2+ entry was assessed by 2+ 2+ nature of Ca permeable channels, they do not exhibit all the measuring increases in [Ca ]cyt using an intracellular fluor- properties of primary hepatocytes, including all the features of escent Ca2+ sensor such as fura-2, 45Ca2+, or, in the case of the spatial polarity of hepatocytes in situ, or the spatial polarity perfused liver, a Ca2+ electrode to measure extracellular [Ca2+]. of freshly-isolated hepatocyte couplets [19]. Moreover, for The results of patch clamp recording also provided evidence for 2+ 2+ many signalling proteins, including hormone receptors, Ca SERCA inhibitor- and IP3-initiated Ca entry [95]. Since channels, and IP3Rs, the relative amount of a given protein SOCs have often been functionally defined as channels which expressed in a liver cell line may differ considerably from that are activated by treatment of cells with SERCA inhibitor or IP3 expressed in isolated hepatocytes [19,80,81]. To fully under- [57] Ca2+ entry in response to these agents has been attributed to stand the structure, mechanisms of activation, and physiological SOCs. 2+ functions of hepatocyte plasma membrane Ca -permeable From the results of some earlier studies with IP3 and SERCA channels, it is desirable to ultimately study the properties of inhibitors it was suggested that more than one type of SOC may these channels in cells which most closely reflect the intra- be expressed in hepatocytes and liver cell lines [86,88,91,96]. cellular organisation of hepatocytes in the intact liver. However, in the majority of these studies the nature of the Ca2+ entry pathway involved was not clearly defined. In recent patch 2+ 2+ 6. P2X ATP-activated ligand-gated Ca -permeable clamp experiments only one type of SOC, a highly Ca - channels selective channel similar to CRAC channels, could be detected [97–99]. It is possible that in some studies IP3 and SERCA 2+ A liver cell P2X purinergic Ca -permeable channel activated inhibitors may have initiated the activation of non-SOCs. Thus by ATP has been identified and partially characterised using IP3 may activate another type (non-store-operated) of plasma patch clamp recording and the intracellular fluorescent Ca2+ membrane Ca2+-permeable channel in hepatocytes [85]. Thap- sensor fluo-3 in freshly-isolated guinea pig hepatocytes [82]. sigargin can, under appropriate circumstances, initiate the ac- These channels may contribute to the mechanisms by which tivation of Ca2+ entry through Ca2+-permeable channels ATP, acting as a local hormone, alters hepatocyte functions activated by Ca2+ or by metabolites of arachidonic acid (formed 2+ following ischaemia reperfusion injury or in response to other by Ca -activated phospholipase A2). Moreover, TRPV1 chan- toxic insults to the liver [7]. nels can be activated by the thapsigargin-induced formation of anandamide, a potential endogenous activator of TRPV1 [100]. 7. Store-operated Ca2+ channels It is also possible that SERCA inhibitors can activate Na+ entry through TRP channels or other non-selective cation channels 7.1. Store-operated Ca2+ channels in hepatocytes and liver cells which, in turn, leads to Ca2+ entry through the Na+-Ca2+ exchanger working in reverse mode [101]. Thus in some studies of Many studies have shown that a SERCA inhibitor or IP3 hepatocytes and liver cells which have employed a SERCA 2+ (introduced by micro injection or generated by addition of a inhibitor or IP3 to initiate plasma membrane Ca entry, one or 658 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 more non-store-operated pathways of Ca2+ entry may have been hepatocytes. As discussed above, there are considerable activated in addition to SOCs. differences in the proteins expressed in hepatocytes from Studies with the H4-IIE rat liver cells and rat hepatocytes different species, and in different liver cell lines. The majority using patch clamp recording in the whole cell mode have of studies have been conducted with rat hepatocytes and rat liver characterised the Ca2+-permeable channels activated by thapsi- cell lines and it is possible, or likely, that different types of SOCs gargin and IP3 [97–99]. The SOCs exhibit a high selectivity for could be expressed in hepatocytes from other species. Ca2+ compared with monovalent cations and exhibit properties Liver cells also express TRPM7 channels [102,103] (Rych- similar, or identical, to those of the CRAC channels found in kov, G., Litjens, T., Chen, J. L. and Barritt, G., unpublished lymphocytes and mast cells [97–99]. This conclusion was based results) and it has been pointed out that the current through on a comparison of time courses of activation, current ampli- TRPM7 channels can be difficult to distinguish from that through 2+ 2+ tudes, dependence on [Ca ]ext, conductance for Ba compared CRAC channels [104]. However, in patch clamp experiments with Ca2+, and inhibition by La3+,Gd3+ and 2-aminoethyl divalent cation entry through Ca2+-selective SOCs in liver cells diphenylborate (2-APB). Ca2+ entry, measured by whole cell has been measured in the presence of intracellular Mg2+. Since patch clamp recording, through SOCs in rat hepatocytes can be intracellular Mg2+ inhibit TRPM-7 [104], it is unlikely that Ca2+ activated by physiological concentrations of vasopressin and entry through TRPM-7 channels would have interfered with ATP [99] (Fig. 5). Taken together, these results provide evidence characterisation of the SOC in hepatocytes and liver cells [99]. that (i) there is only one type of SOC in rat hepatocytes, (ii) the rat hepatocyte SOCs are highly Ca2+-selective channels essentially 7.2. Properties of liver cell store-operated Ca2+ channels identical to CRAC channels in mast cells and lymphocytes, and (iii) the channel detected by electrophysiological techniques can The permeability sequence for the movement of cations be activated by physiological concentrations of hormones in rat through liver cell SOCs is Ca2+ NBa2+ NSr2+ NNa+ NCs+ [97].

Fig. 5. Activation of the SOC current, ISOC, by vasopressin in freshly-isolated rat hepatocytes attached to glass coverslips. (A) Development of ISOC initiated by 20 nM vasopressin. Open symbols are the amplitudes of the inward current of the individual cells at −118 mV. Solid lines represent fits of the Boltzmann curve to the experimental data. (B) Averaged results of the data presented in the panel A. (C) Averaged leak-subtracted I–V plot of the current activated by the application of 20 nM vasopressin to the external solution (n=5). (D) Leak-subtracted current traces obtained in response to 200 ms steps to −118 mV in the presence of vasopressin. Data was averaged from seven cells. Taken from (Rychkov et al 2005), with permission. G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 659

2+ The permeability of liver cell SOCs for Mn has not been hand domains reduced the fast component of liver cell ISOC directly tested in patch clamp experiments, although it is known inactivation. However, no effect of the calmodulin antagonists that the permeability of Mn2+ through CRAC channels is low Mas-7 and calmidazolium was detected. It was concluded that [105]. The results of earlier work which employed Mn2+ and calmodulin is responsible for at least part of the Ca2+-dependent 2+ fura-2 to measure Mn entry, suggest that liver cell SOCs can inactivation of liver cell ISOC. Moreover, the possibility was raised admit Mn2+ [90,93] although it is possible that the Mn2+ entry that calmodulin is tethered to the SOC protein itself and hence is observed may have been through non-store-operated pathways. protected from the actions of calmodulin inhibitors (cf the mecha- Liver cell SOCs are partially blocked by Co2+ and Cd2+ and nism by which calmodulin regulates L-type VOCCs [114]). completely blocked by Zn2+,Gd3+ and La3+. The most potent blocking agents are Gd3+ and La3+ which give complete block 7.3. Likely roles of STIM and Orai (CRACM) proteins in liver 2+ 2+ at about 2 μM in the presence of 10 mM Ca ext [97,99]. Results cell store-operated Ca channels of earlier studies of the inhibition of Ca2+ entry (measured using fura-2) in hepatocytes by different trivalent cations led to the The results of recent experiments employing a wide variety conclusion that there is some similarity between the pore of the of approaches and techniques have led to the conclusion that a hepatocyte SOCs and the pore of the highly Ca2+-selective member of the Orai (CRACM) family of proteins constitutes the VOCCs [106]. pore of SOCs in mast cells, lymphocytes and in some other cell Ca2+ entry through liver cell SOCs is inhibited by 2-aminoethyl types (reviewed in [11,12,115]). Other experiments have shown 2+ diphenylborate (2-APB) (S0.5 approx. 10 μM) [107],SK&F96365 that STIM1 located in the ER constitutes the Ca sensor which 2+ [78,93], arachidonic acid [108], the PLC inhibitor U73122 detects the decrease in [Ca ]er and conveys this information to [109,110], and isotetrandrine and tetrandrine [108]. In addition to Orai leading to activation of the channel and Ca2+ entry. It is providing some pharmacological characterisation of liver cell presently thought that this involves the movement of some SOCs, these results indicate that caution should be exercised in STIM to ER-plasma membrane junctions leading to an inter- employing U73122, an inhibitor of PLC [111] and isotetrandrine action between STIM, located in the ER, and Orai, located in and tetrandrine (inhibitors of phospholipase A2 [112]) in studies the plasma membrane (reviewed in [11,12,115,116]). It is not of the regulation of Ca2+ entry. In H4-IIE rat liver cells, VOCC yet completely clear whether there is a direct interaction be- antagonists, including verapamil, nifedipine, nicardipine and the tween the STIM protein and the Orai protein, or an interaction dihydropyridine analogues AN406 and AN1043, inhibit thapsi- involving additional proteins. Further, proteins other than Orai gargin-stimulated Ca2+ entry measured using a fluorescent intra- and STIM are likely to have roles in the activation mechanism. cellular Ca2+ sensor [78] (cf [113]. However, the concentrations Other experiments suggest that the localisation of Orai and of the VOCC antagonists used were considerably higher than STIM and the Ca2+ entry channel may create domains of in- 2+ those which inhibit VOCCs in excitable cells suggesting that creased [Ca ]cyt at specific locations under the plasma mem- their action on liver cell SOCs is indirect and/or non-specific. brane [115]. There is some evidence to indicate that calmodulin is in- The results of recent experiments with liver cells indicate that volved in the regulation of liver cell SOCs. In rat hepatocytes, it is likely that STIM is required in the mechanism of SOC thapsigargin-initiated Ca2+ entry was found to be inhibited by activation. Thus, it has been shown using H4-IIE rat liver cells the calmodulin antagonists calmidazolium and CGS 9343B that the siRNA-mediated knockdown of STIM1 caused a when the antagonists were added before (but not after) thap- substantial reduction in the amplitude of ISOC initiated by IP3 or sigargin, leading to the conclusion that calmodulin is required thapsigargin [110]. Treatment of H4-IIE cells with thapsigargin for Ca2+ entry [83]. In H4-IIE rat liver cells, thapsigargin- leads to a re-distribution of STIM1 to puncta, as assessed using initiated Ca2+ entry and release from intracellular stores were cells transfected with GFP-STIM1 and by imaging endogenous observed to be inhibited by the calmodulin antagonists W7, STIM1 by immunofluorescence (Castro, J., Jones, L., Litjens, W13 and calmidazolium [78]. No inhibition was observed with T., Barritt, G. and Rychkov, G., unpublished results). The pu- another calmodulin antagonist, KN62. Substantial inhibition of tative roles of STIM1 and Orai1 in the activation of liver cell Ca2+ entry by calmidazolium was only observed when this was SOCs are shown schematically in Fig. 6A. added before thapsigargin [78]. It was concluded that calmo- The proposed mechanism of activation of liver cell SOCs dulin is not involved in the activation of Ca2+ entry. While the involving the interaction of STIM1 with Orai1 at ER-plasma results of some of these experiments suggest that calmodulin membrane junctions requires that such junctions are normally may be required for liver cell SOC activation, in both studies present in hepatocytes or are formed upon depletion of Ca2+ in Ca2+ entry was measured using a fluorescent intracellular Ca2+ the ER. Evidence for a close association of some ER with the sensor and the nature of the Ca2+ entry channel was not defined plasma membrane in hepatocytes comes from previous sub- by patch clamp recording. cellular fractionation experiments which generated highly puri- The results of patch clamp studies with H4-IIE rat liver cells fied plasma membrane fractions and provided evidence that have provided evidence that the fast inactivation of the SOC specialised sub-regions of the ER are located close to the plasma 2+ 2+ Ca current, ISOC, is a calmodulin- and Ca -dependent pro- membrane [38,40]. The presence of ER close to the plasma cess, similar to the Ca2+-dependent fast inactivation of CRAC membrane in hepatocytes is not readily apparent from inspec- channels [98]. Over-expression of either a calmodulin inhibitor tion of electron micrographs, although areas of the ER which peptide or a mutant form of calmodulin lacking functional EF come close to the plasma membrane can be seen (Fig. 6B). ER- 660 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

Fig. 6. Proposed molecular and spatial organization of store-operated Ca2+ channels and their activation pathways in liver cells. (A) Schematic representation of some of the proteins and organelles thought to be involved in the activation of SOCs in hepatocytes and liver cell lines. It is proposed that activation of SOCs requires sub- regions of the ER which are in close proximity to the plasma membrane and form ER-plasma membrane junctions. These ER-sub-regions are enriched in type1IP3R. It is proposed that while each ER sub-region communicates with the bulk of the ER, the movement of Ca2+ between the sub-regions and the bulk of the ER is slow. The 2+ steps in the activation of SOCs are proposed to be: the initiating decrease in [Ca ] in the lumen of the ER induced by IP3 (physiological) or a SERCA inhibitor (experimental), dissociation of Ca2+ from the luminal domain of the Ca2+ sensor STIM1, a conformational change in STIM1, oligomerisation of STIM1, relocalisation of STIM1 in the ER, interaction of STIM1 in close proximity to ER-plasma membrane junctions with CRACM1/Orai1, a conformational change and increase the probability of opening of the Orai1 channel. Other proteins (as yet unidentified) are likely to be involved. The F-actin cytoskeleton, regulated in part by Gi2α and PLCγ1 are thought to play permissive roles in the activation pathway. Ca2+ which moves through SOCs into the ER-plasma membrane junction may cause a local 2+ increase in [Ca ]cyt at the mouth of the channel, before being transported directly to the lumen of the ER via SERCA pumps and to mitochondria. (B) Transmission electron micrograph showing the ER in a part of an isolated rat hepatocyte near the plasma membrane with regions of the ER in the vicinity of the plasma membrane (Wang, Y.J., Gregory, R.B., and Barritt, G.J., unpublished results). The abbreviations are: PM, plasma membrane; and ER, endoplasmic reticulum.

plasma membrane localisations have been observed by electron 7.4. Roles of an endoplasmic reticulum sub-region and type 1 microscopy in some other cell types (reviewed in [116]). IP3 receptors It has been proposed that a TRP (transient receptor potential) protein, possibly TRPC1, TRPC3, TRPC4, TRPV5 and/or Several experimental approaches have addressed the ques- TRPV6, constitutes the pore of SOCs in some types of animal tion of whether all of the ER or a sub-component of the ER is cells (reviewed in [57,117,118]. Some of these TRP proteins are required for the activation of SOCs in liver cells. In hepatocytes expressed in liver cells (Table 3). Ectopic expression of hTRPC1 in situ, in freshly-isolated hepatocytes, and liver cell lines, the in H4-IIE rat liver cells or knockdown of endogenous TRPC1 ER is found to extend throughout most of the cytoplasmic space proteins using siRNA did not cause large changes in thapsi- [125]. The results of several experimental approaches suggest gargin-stimulated Ca2+ entry (assessed using a fluorescent Ca2+ that a small region of the ER, rather than the whole ER, is all sensor and patch clamp recording), indicating that it is unlikely that is necessary for liver cell SOC activation, and that this sub- that the TRPC1 peptide constitutes SOCs in rat liver cells region of the ER is enriched in type 1 IP3R. When microinjected [119,120]. However, a role for TRPC1 in forming or modulating into freshly-isolated hepatocytes, a monoclonal anti-type 1 IP3R liver cell SOCs is not excluded. For example, several recent antibody, which in other studies was shown to inhibit Ca2+ studies have provided evidence that TRP polypeptides interact release mediated by type 1 IP3R, was found to inhibit hormone- with STIM and/or Orai polypeptides [118,121–124]. As de- and thapsigargin-induced Ca2+ entry with little effect on the scribed above, in patch clamp recording experiments only one release of Ca2+ from intracellular stores [126]. The microinjec- type of SOC can be detected in rat liver cells and this has a high tion of a relatively low concentration of adenophostin A, which 2+ selectivity for Ca comparable to that of CRAC channels has a high affinity for IP3Rs relative to that of IP3, induced near- in lymphocytes and mast cells. The Ca2+-permeable channels maximal activation of Ca2+ entry with little detectable release of formed by TRPC1 polypeptides and by most other TRP Ca2+ from intracellular stores [126]. The results of experiments 2+ polypeptides have a relatively low selectivity for Ca compared in which IP3 analogues selective for either type 1 or type 2 IP3R + with Na [10,56]. This comparison provides further evidence were microinjected to rat hepatocytes indicate that type 1 IP3R that it is unlikely that any of the known TRP polypeptides are preferentially involved in SOC activation [127]. constitutes the Ca2+-selective SOCs found in rat hepatocytes and As mentioned above, the results of immunofluorescence liver cells, although TRP polypeptides may contribute to SOCs experiments conducted with spatially polarised rat hepatocytes in in hepatocytes from some species. situ, and with isolated hepatocyte couplets or triplets, indicate that G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 661

type 2 IP3R are predominantly located in the ER near the bile F-actin [137]. Disruption of F-actin with cytochalasin D, within canaliculus while type 1 IP3R are distributed throughout most a narrow effective concentration range, inhibited thapsigargin- 2+ regions of the ER with some type 1 IP3R concentrated in ER close and IP3-induced Ca entry [138]. Taken together, these results to the plasma membrane in the sinusoidal and canalicular do- indicate that the normal functions of Gi2α and F-actin are mains [4,28,127]. The results of subcellular fractionation studies required for the activation of hepatocyte SOCs. indicate that type 1 IP3R are found in regions of the ER very close Since the interventions described above inhibited the acti- to the plasma membrane, and are held in this location by F-actin vation of SOCs when this was initiated by thapsigargin, as well [38–40]. Taken together, the results obtained using these different as by IP3 and hormones which generate IP3, it was concluded experimental approaches indicate that a small sub-region of the that the requirements for Gi2α and F-actin are downstream of the 2+ ER enriched in type 1 IP3Rs is required for SOC activation. step in which Ca is released from the ER. Thus, it was pro- In an attempt to investigate the role of a putative sub- posed that Gi2α is not involved in the formation of IP3 catalysed component of the ER in the activation of SOCs, H4-IIE cells by PLCβ, coupled to the vasopressin receptor, and that its role in expressing aequorin targeted to the cytoplasmic space or ER the activation of SOCs represents a “receptor-independent” 2+ were used to measure changes in [Ca ] in these compartments function of Gi2α (cf the role of the Gi3 in vesicle trafficking) and under conditions of Ca2+ entry through SOCs. SERCA inhi- other receptor-independent functions of G-proteins (reviewed in bitors were used to probe the role of SERCAs in the Ca2+ entry [139]). The results of experiments conducted with H4-IIE rat pathway [128]. The results led to the conclusions that the liver cells treated with pertussis toxin suggest that there is no 2+ activation of SOCs and the maintenance of Ca entry through requirement for Gi2 in the activation of SOCs in this liver cell 2+ SOCs is achieved by a small sub-region of the ER Ca store line [78]. In rat hepatocytes, the role of Gi2 may be to maintain near the plasma membrane and that SERCAs with a low hepatocyte spatial polarity since it has been shown that trimeric sensitivity to inhibition by thapsigargin are required to draw Ca2 G-proteins are involved in determining cell polarity [140]. The + into this small ER Ca2+ store and to maintain the flow of Ca2+ results of other experiments suggest that the normal function of a through SOCs. It was also suggested that diffusion of Ca2+ monomeric G-protein, possible ARF-1, is also required for the through the lumen of the ER between the bulk of the ER and the activation of SOCs in hepatocytes [141]. sub-region near the plasma membrane involved in SOC acti- PLCγ1, Gi2α, a monomeric G-protein and F-actin may play vation is relatively slow [128]. “permissive” roles in SOC activation in spatially polarised hepatocytes (Fig. 6A). The permissive roles of these proteins 7.5. Roles of Gi2, F-actin and PLCγ1 in facilitating may include maintenance of the integrity of the ER and the store-operated Ca2+ channel activation putative ER-plasma membrane junctions. One reservation about this conclusion is that, in the experiments designed to test the 2+ While it is likely that STIM1 and Orai1 proteins are com- roles of Gi2 and F-actin, the Ca entry pathway involved was ponents of the mechanism of activation of liver cell SOCs, not characterised using electrophysiological techniques. several other proteins appear to be required. Knockdown of PLCγ1 in H4-IIE rat liver cells using siRNA was found to be 7.6. Likely physiological functions of store-operated Ca2+ associated with a substantial decrease in the amplitude of ISOC channels in liver cells initiated by either IP3 or thapsigargin. No interaction between PLCγ1 and STIM1 was detected in immunoprecipitation expe- In the original concept of capacitative Ca2+ entry developed riments [110]. It was concluded that PLC-γ1 is required to by Putney, it was hypothesised that the physiological function couple ER Ca2+ release to the activation of SOCs independently of SOCs is to supply extracellular Ca2+ to refill the ER after Ca2+ of any PLCγ1-mediated generation of IP3 and independently of is released from this organelle following the actions of PLC- a direct interaction between PLCγ1 and STIM1. These ideas are coupled hormones [142]. This remains a reasonable teleological similar to those reached in experiments conducted with some hypothesis for the major function of SOCs, although direct other cell types where it was concluded that a PLCγ protein is evidence for a role for this pathway under physiological con- 2+ 2+ involved in the activation of receptor-activated Ca -permeable ditions (normal [Ca ext]) in hepatocytes and in other non- channels and/or SOCs independently of PLCγ-mediated gene- excitable cells is limited. 2+ ration of IP3 [129–131]. In the absence of Ca ext (i.e. in a non-physiological situation), 2+ 2+ ADP-ribosylation of Gi2α by treatment of livers with per- hormones induce the release of Ca from the ER and this Ca , tussis toxin or the inhibition of Gi2α function using an inhi- in turn, is transported out of the cell by the plasma membrane 2+ 2+ 2+ bitory antibody or an inhibitory peptide were each found to (Ca +Mg )ATP-ase. Re-addition of Ca ext leads to refilling 2+ inhibit thapsigargin- and IP3-induced Ca entry (measured of the ER via SOCs [57,143]. However, it is less clear whether using fura-2) to freshly-isolated rat hepatocytes [132–136] SOCs play a role in liver cells exposed to hormones under (Fig. 7C). ADP-ribosylation of Gi2α was associated with in- physiological conditions. The results of experiments which em- hibition of the formation of the band of cortical F-actin around ployed 2-aminoethyl diphenylborate, Gd3+ and SK&F96365 to the canaliculus in isolated hepatocyte doublets when spatial inhibit SOCs have provided some evidence that SOCs are re- polarity was regained (Fig. 7A), and with some disruption of quired for the maintenance of hormone-induced Ca2+ oscillations the ER (Fig. 7B) [137]. Moreover, studies with hepatocytes, in hepatocytes [55]. However, interpretation of these results and some other cell types have shown that Gi2α interacts with assumes specificity for the SOC inhibitors employed, and these 662 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

2+ Fig. 7. Disruption of Gi2α function alters F-actin distribution, disrupts the ER, and inhibits vasopressin-activated Ca entry in rat hepatocytes. (A) Pre-treatment of livers with pertussis toxin inhibits re-organisation of F-actin in isolated rat hepatocytes which have been plated for 4 h (a, b, pertussis toxin, c, d, control). In freshly- isolated hepatocytes F-actin (stained using Texas Red-X phalloidin) is principally localised at the cortex (A, a). In control cells plated for 4 h, spatial polarity begins to be regained and this process is associated with the loss of cortical F-actin and an increase in F-actin around the bile canaliculus (arrow) (A, c). In hepatocytes isolated from livers treated with pertussis toxin, the re-organisation of F-actin is inhibited (Ab, Ad). (B) Pre-treatment of livers with pertussis toxin induces fragmentation of the smooth ER in isolated hepatocytes (from [137], with permission). (C) Pre-treatment of livers with pertussis toxin inhibits vasopressin-stimulated Ca2+ entry. Hepatocytes loaded with fura-2 were incubated in the absence of extracellular Ca2+ and in the presence of vasopressin. Extracellular Ca2+ (1.3 mM) was subsequently added to initiate Ca2+ entry. (Adapted from [132], with permission.) G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 663 are known to inhibit other types of Ca2+ channels [55].The selective cation channels activated by ATP, vasopressin and other results of other experiments suggest that SOCs are more effective agonists [150] have been reasonably well characterised. Ara- than at least one type of receptor-activated Ca2+-permeable chidonic acid-activated plasma membrane Ca2+-permeable chan- channel, the maitotoxin-activated Ca2+ entry pathway, in refilling nels are present in several types of animal cell, and are thought to the hepatocyte ER [143]. The proposed role of SOCs in hepa- be involved in refilling intracellular Ca2+ stores [151]. Arachi- tocytes in delivering Ca2+ to refill the ER and of (Ca2+ +Mg2+) donic acid-activated Ca2+ entry attributed to the TRPV4 channel ATP-ases in the ER in transporting Ca2+ from the cytoplasmic has been described in the HepG2 liver cell line [152]. However, space at the mouth of the SOC to the lumen of the ER is analogous no arachidonic acid-activated Ca2+-permeable channels could be to the roles previously proposed for Ca2+ entry channels and detected by patch clamping in H4-IIE rat liver cells or in rat SERCA pumps in pancreatic acinar cells in re-filing regions of hepatocytes [99]. the ER located next to the acinus with Ca2+ which enters from the The physiological functions of the different types of receptor- basal region and is moved (tunnelled) through the lumen of the activated Ca2+-permeable channels expressed in hepatocytes are ER [144]. not well defined. Many admit Na+ as well as Ca2+, and increases Another function of SOCs in hepatocytes may be to deliver in intracellular Na+ may play important roles in regulating nor- Ca2+ to specific locations in the cytoplasmic space near the mal and pathophysiological hepatocyte functions [59,120,150]. plasma membrane, as suggested for other cell types [115,145]. Some channels may have quite specific localisations in the While there is some evidence for such a role for SOCs in some spatially polarised hepatocyte and deliver Ca2+ and Na+ to these cell types, for example, in the regulation of adenylate cyclase specific regions. Channels with specific activation and inhibition activity [145], no direct evidence for this function of SOCs in kinetics may deliver specific types of Ca2+ and Na+ signals hepatocytes has so far been reported. SOCs may also contribute (“pulses”) to the cytoplasmic space. to transcellular Ca2+ movement. Thus hormone-induced in- 2+ 2+ creases in [Ca ]cyt are accompanied by the extrusion of Ca 9. TRP polypeptides as the molecular counterparts of liver across the canalicular membrane into bile fluid [146]. cell receptor-activated Ca2+-permeable channels The results of experiments conducted using inhibitors of SOCs suggest that Ca2+ entry through SOCs is required for the The TRP super family of non-selective cation channels is maintenance of bile flow in rats [147]. Recent studies with H4- comprised of seven sub-families: the TRPC (canonical), the IIE rat liver cells and isolated rat hepatocytes employing patch TRPV (vanilloid), and the TRPM (melastatin) families; the clamp recording and fura-2 have provided evidence that tauro- TRPML (mucolipin) and TRPP (polycysteine) families; and deoxycholic acid and some other choleretic bile acids activate the TRPA (ankyrin) and TRPN (no mechano receptor SOCs, while lithocholic acid and some other cholestatic bile potential C) channels [10,56]. Each TRP channel seems to acids inhibit SOCs. These actions of bile acids are associated with have an extraordinary range of cellular functions [10,56]. TRP a re-distribution of STIM1 (Aromataris, E., Castro, J., Rychkov, channel expression in liver tissue, hepatocytes, and liver cell G., and Barritt, G.J., unpublished results). Thus SOCs may lines has been detected using RT-PCR, Northern Blot, in situ mediate some physiological, pathological and pharmacological hybridisation, anti-TRP antibodies and functional assays actions of bile acids on hepatocytes. (Table 3). Some TRP mRNA detected in liver tissue is likely Pharmacological approaches also suggest that Ca2+ entry expressed in non-hepatocyte cell types, such as Kupffer cells through SOCs is required for cell proliferation in human hepa- and bile duct epithelial cells. While these results provide toma cells. Thus 2-aminoethyldiphenylborate and carboxyami- evidence for expression of TRPC1, C3 and M7 in rat hepa- dotriazole inhibited thapsigargin-initiated Ca2+ entry in HepG2 tocytes, and there is some knowledge of possible functions of and Huh-7 cells, and this was associated with an inhibition of TRPC1 in hepatocytes, and of TRPV4 in HepG2 liver cells, cell proliferation [24]. surprisingly little is known about which other TRP proteins are expressed in primary hepatocytes and what their cellular 8. Receptor-activated and stretch-activated Ca2+-permeable functions are. channels Results of immunofluorescence experiments indicate that in H4-IIE rat liver cells, endogenous TRPC1 is principally located Several non-SOC plasma membrane Ca2+-permeable chan- in intracellular organelles with some expressed at the plasma nels, which can broadly be defined as receptor-activated chan- membrane [119,120]. Ectopic expression of human TRPC1 in nels, have been identified in liver cells and hepatocytes. These are H4-IIE cells led to a significant enhancement of maitotoxin- listed in Table 2, although it should be noted that only a limited stimulated Ca2+ and Na+ entry compared with the small en- number of species and liver cell lines have so far been studied in hancement of thapsigargin-stimulated Ca2+ entry mentioned any detail. Receptor-activated Ca2+-permeable channels found in above [119]. Suppression of endogenous TRPC1 expression liver cells include channels activated by intracellular messengers with trpc1 siRNA led to a small decrease in thapsigargin-sti- and/or possibly by protein–protein interaction. These different mulated Ca2+ entry and to a larger decrease in maitotoxin- and channels were discovered by measuring Ca2+ entry in response to ATP-stimulated Ca2+ entry. The suppression of TRPC1 expres- hormones, specific intracellular messengers (e.g. IP3), toxins sion also led to an enhancement of swelling in hypotonic solu- (e.g. maitotoxin) or free radicals. The stretch-activated channels tion and to enhanced regulated volume decrease [120]. It was (also activated by ATP) [148,149] and Ca2+-activated non- concluded that TRPC1 can be activated by maitotoxin. One 664 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

Table 2 Receptor-activated Ca2+-permeable non-selective Ca2+ channels detected in the perfused liver, freshly-isolated hepatocytes, hepatocytes in primary culture, and liver cell lines Type of Ca2+-permeable Liver cell preparation Agents used to initiate Technique employed to measure References channel Ca2+ inflow Ca2+ inflow Stretch- and ATP-activated Isolated hepatocytes (rat) Stretch Patch clamp recording [149] (chiefly admits Na+) Liver cell line (rat hepatoma) Stretch and ATP Patch clamp recording [148] (16 pS channel) Cyclic AMP-activated Isolated hepatocytes (rat) Glucagon, cyclic AMP Intracellular fluorescent Ca2+ sensor [88,94,186] analogues, parathyroid hormone Isolated hepatocytes (rat) Cyclic AMP Northern blot to detect expression of mRNA [165] and rat liver encoding the CNGCα pore-forming subunit of cyclic nucleotide-gated channel Isolated hepatocytes (axolotl) Cyclic AMP analogues Intracellular fluorescent Ca2+ sensor [166] Maitotoxin-activated Liver cell line (H4-IIE rat Maitotoxin Intracellular fluorescent Ca2+ sensor [107,119,138] hepatoma) and isolated (also Mn2+ as substitute for Ca2+) patch hepatocytes (rat) clamp recording 2+ IP3-activated plasma Isolated hepatocytes IP3 Intracellular fluorescent Ca sensor [85–87] membrane Ca2+ permeable (guinea pig, rat) channel Hormone (receptor)-activated Isolated hepatocytes (rat) Vasopressin Intracellular fluorescent Ca2+ sensor [91] (quenching by Mn2+) Ca2+-activated non-selective Isolated hepatocytes (rat) Vasopressin Intracellular fluorescent Na+ sensor, [76] cation channel patch clamp recording Liver cell line ATP, nucleotide analogues Patch clamp recording [150] (HTC rat hepatoma) Liver cell line Ca2+ Patch clamp recording [150] (HTC rat hepatoma) Liver cell line (HepG2) Ca2+ Patch clamp recording [187] Insulin-activated Isolated hepatocytes (rat) Insulin Intracellular fluorescent Ca2+ sensor [188,189] Liver cell line Patch clamp recording, intracellular [190] (HTC rat hepatoma) fluorescent Ca2+ sensor Arachidonic acid-activated Liver cell line (HepG2) Arachidonic acid Intracellular fluorescent Ca2+ sensor [152] Free radical-activated non- Liver cell line (clone 9 cells) Reactive oxygen species Patch clamp recording, intracellular [34] selective cation channel fluorescent Ca2+ sensor (16-ps) Ca2+–Mg2+ exchanger Hepatocytes and microsomes Isoproterenol [191] (plasma membrane vesicles) (via cyclic AMP)

possible physiological function of TRPC1 is in the regulation of Ca2+ inflow in HepG2 cells was also found to be stimulated hepatocyte volume [119,120]. by 4α-phorbol-12,13-didecanoate and arachidonic acid, known Studies with HepG2 cells have provided evidence which activators of TRPV4, suggesting that functional TRPV4 is suggests the presence of functional TRPV1 channels in this expressed in this liver cell line. As mentioned above, a current human liver cell line [152,153]. Thus, capsaicin and RTX, attributable to TRPM7 has be been detected in liver cells known activators of TRPV1, were found to stimulate Ca2+ (Fig. 8A,B) and this could be suppressed using trpm7 siRNA entry (assessed using a fluorescent Ca2+ sensor) and the action (Litjens, T., Rychkov, G., Roberts, M., Chen, J. and Barritt, G., of capsaicin was blocked by the known capsaicin antagonist, unpublished results) (Fig. 8C). capsazepin. It was also observed that capsaicin induces the release of Ca2+ from intracellular stores, suggesting that 10. Plasma membrane Ca2+-permeable channels activated TRPV1 is located in intracellular membranes and, if appro- by glucagon priately activated, can mediate intracellular Ca2+ release. Incubation of HepG2 cells with hepatocyte growth factor/ Glucagon regulates hepatic carbohydrate, lipid and protein scatter factor for 20 h increased capsaicin-stimulated Ca2+ metabolism, bile flow, and hepatocyte volume and membrane entry in migrating HepG2 cells, leading to the suggestion that potential [59,154–156]. The results of experiments conducted 2+ 2+ 2+ Ca entry through TRPV1 may be involved in the regulation using intracellular fluorescent Ca sensors to measure [Ca ]cyt of liver cell migration [152,153]. While no endogenous indicate that when added alone glucagon or cyclic AMP en- TRPV1 protein could be detected in H4-IIE rat liver cells by hance Ca2+ entry to hepatocytes. The magnitude of the en- immunofluorescence or by measuring Ca2+ entry in response hancement varies considerably from one laboratory to another to TRPV1 agonists, there is evidence for Ca2+ entry through [94,157–159]. The results of other experiments conducted with TRPV1 in rat hepatocytes (Castro, J., Rychkov, G., and Barritt, the perfused liver and isolated hepatocytes indicate that glu- G., unpublished results). cagon can release Ca2+ from intracellular stores in hepatocytes, G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 665

Table 3 TRP channels expressed in liver and liver cell lines TRP Technique employed Species Liver tissue, hepatocytes Comments References protein for detection or liver cell line TRPC1 RT-PCR, Western blot, Rat, mouse, AML12, H4-IIE cells, TRPC1 mRNA was detected in all cell [119,120,192–194] immunofluorescence human hepatocytes, liver, types examined, mRNA level in foetal foetal liver liver is higher than that in liver. TRPC1 mRNA endogenous TRPC1 protein was detected in H4-IIE cells with three anti-TRPC1 antibodies TRPC2 RT-PCR Rat, mouse AML12, H4-IIE Lower mRNA level was detected in [119,192] cells hepatocytes than that in brain TRPC3 RT-PCR Rat, mouse, AML12, H4-IIE cells, mRNA level variable among cells types [119,192,194] human hepatocytes, liver, and higher expression level detected in foetal liver brain, mRNA not detected in human liver TRPC5 RT-PCR Human Liver, foetal liver mRNA detected in low expression level [194] TRPC6 RT-PCR Human Liver, foetal liver mRNA detected in low expression level TRPC7 RT-PCR Rat, human H4-IIE cells, hepatocytes, mRNA detected in rat liver cells, not [Chen, J L and Barritt, G J liver and foetal liver detected in human liver tissues unpublished results; 194] TRPV1 RT-PCR, patch clamp Human Hep G2 cells mRNA detected in low expression level. [152,153,195] recording and intracellular Capsaicin-activated Ca2+ inflow and fluorescent Ca2+ sensor TRPV1 current measured. Proposed role TRPV1 in cell migration. TRPV2 Dot-blot analysis RT-PCR Rat, human Liver HepG2 cells mRNA detected [152,196] TRPV3 RT-PCR HepG2 cells mRNA detected [152] TRPV4 RT-PCR, Northern blot Rat, mouse H4-IIE cells, HepG2 cells, mRNA detected, with abundant [152,197; Chen, J L and hepatocyte, liver expression in liver cell lines and tissue Barritt, G J unpublished 4α-phorbol-12,13-didecanoate- and results] arachidonic acid-activated Ca2+ inflow measured Dot-blot analysis Human, mouse Liver Low expression level [198] TRPV6 RT-PCR Zebra fish Liver [199] TRPM4 Northern blot Human Liver Moderate expression level [200] TRPM5 Northern blot, dot-blot Human Liver Strong signal detected by Northern blot [201,202] RT-PCR TRPM7 Northern blot Rat Liver Strong signal detected by Northern blot [103] In situ hybridisation Zebra fish Liver Abundant expression [102] Patch clamp recording, Rat H4-IIE cells TRPM7 mRNA (RT-PCR) and TRPM7 [Rychkov, G., Litjens, T., RT-PCR current detected Chen, J.L., and Barritt, G.J. unpublished results] PKD2 RT-PCR, immunofluorescence Human Liver High expression level [203–205]

although the magnitude of observed Ca2+ release also varies those of the Ca2+-selective hepatocyte SOC, and as discussed between different laboratories (reviewed in [160]). above, a larger outwardly rectifying Cl− current similar to that The primary pathway of glucagon action involves the G-protein activated by cell swelling [9]. Evidence was presented to coupled receptor for glucagon, GS, adenylate cyclase and the indicate that these effects of glucagon involve IP3, cyclic AMP generation of cyclic AMP. Most intracellular effects of cyclic and Epac, but do not involve protein kinase A. AMP are mediated by the activation of protein kinase A, but Another Ca2+-permeable channel in the hepatocyte plasma some are mediated by the binding of cyclic AMP to Epac (ex- membrane which has been suggested as a possible target of change protein directly activated by cyclic AMP) and cyclic glucagon and cyclic AMP is the photoreceptor cyclic nucleo- nucleotide-gated cation channels [156,161]. Glucagon also in- tide-gated non-selective cation channel [165]. While there is creases IP3 concentrations in hepatocytes through coupling of the evidence that mRNA encoding this channel is expressed in liver glucagon receptor to Gq and PLCβ [162–164]. Thus the glucagon [165], no convincing evidence for the presence of functional 2+ receptor can interact with two trimeric G-proteins, Gs and Gq.In cyclic nucleotide-gated Ca permeable channels in mammalian hepatocytes increases in IP3 induced by glucagon are substan- hepatocytes has so far been reported. It is possible, though, that tially lower than those induced by vasopressin and epinephrine, a cyclic nucleotide-gated non-selective cation channel accounts 2+ the receptors for which are predominantly coupled to Gq and for the observed cyclic AMP-activated Ca entry in axolotl PLCβ [162–164]. hepatocytes which exhibit a substantial and convincing cyclic The results of recent studies using patch clamp recording AMP-activated Ca2+ entry [166]. with rat hepatocytes have shown that glucagon activates a small Glucagon acts synergistically with vasopressin, epinephrine, inwardly rectifying Ca2+ current with characteristics similar to phenylephrine, and other G-protein coupled receptors which 666 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

Fig. 8. Currents attributable to TRPM7 in H4-IIE rat liver cells. (A) Current–time plot showing development of an outward current, measured at 100 mV,due to washout of intracellular Mg2+. (B) Current–voltage plots recorded at the time points indicated in panel A, immediately after achieving whole cell configuration (1), and after the TRPM7 current has fully developed (2). (C) Averaged current–voltage plots obtained 200 s after achieving whole cell configuration, for cells in which TRPM7 expression is ablated by trpm7 siRNA, and for control cells transfected with control siRNA. Traces in B and C are recorded in response to 100 ms voltage ramps ranging from −138 to 102 mV. To activate TRPM7 currents, Mg2+ was omitted from the pipette solution and intracellular Ca2+ was buffered to 100 nM by EGTA.

2+ induce IP3 formation and Ca release from intracellular stores, upon re-oxygenation [178–180]. This is due to enhanced plasma to potentiate Ca2+ entry [157,167–171]. The degree of poten- membrane Ca2+ entry as well as release of Ca2+ from intracellular tiation varies somewhat between liver and hepatocyte prepara- stores [179,180]. tions in different laboratories. The synergistic or potentiating Reactive oxygen species generated by Kupffer cells and action of glucagon is mediated by cyclic AMP, since this intra- hepatocytes during the initial stages of reperfusion are thought to cellular messenger can substitute for glucagon [160]. Synergy be one of the main mediators of enhanced Ca2+ entry to involves the release of Ca2+ from the ER, since it has been hepatocytes (reviewed in [181]). NO and reactive nitrogen shown that glucagon or cyclic AMP enhance Ca2+ entry when species, generated at later stages of reperfusion, may also affect this is initiated by SERCA inhibitors such as DBHQ [94]. The Ca2+ entry [181]. While the Ca2+ permeable channels involved Ca2+ permeable channel which is synergistically activated has in enhanced Ca2+ entry to hepatocytes in ischemia reperfusion not been characterised by electrophysiological studies, but is injury have not been clearly identified, the results of experiments most likely the liver cell Ca2+-selective SOC. conducted with a liver cell line have shown that reactive oxygen 2+ The mechanism(s) by which cyclic AMP enhances IP3-or species can activate a 16 pS Ca -permeable non-selective SERCA inhibitor-initiated Ca2+ entry has not been elucidated, but cation channel [34] (Table 2). TRPM7 and several other TRP several possibilities have been suggested. Cyclic AMP acting channels are known to be activated by reactive oxygen species through protein kinase A may enhance Ca2+ uptake by mito- [182,183] and NO [184]. In response to toxic insults which chondria located near the plasma membrane which, in turn, may induce the generation of reactive oxygen species, TRPM7 me- reduce feedback inhibition by Ca2+ of SOCs [167]. Cyclic AMP diates Ca2+ and Na+ entry to neurons which, in turn, leads to may also enhance the formation of IP3 and/or increases the cell death [182]. TRPM7 is expressed in liver cells (Fig. 8 and 2+ sensitivity of IP3 receptors to IP3 and hence, potentiate the release Table 3) and is a possible candidate for mediating Ca entry in of Ca2+ from the ER and the subsequent activation of SOCs [160]. hepatocyte death initiated by reactive oxygen species. It has also been shown that cyclic GMP potentiates the synergistic action of glucagon and vasopressin on Ca2+ entry in 12. Conclusions rat liver, and in guinea pig hepatocytes potentiates the release of 2+ Ca from the ER mediated by IP3 [172,173]. The mechanism The following conclusions can be reached from the above of action of cyclic GMP on Ca2+ entry appears to be complex discussions of liver cell plasma membrane Ca2+-permeable but may include enhancement of Ca2+ release from the ER and a channels. The spatial and temporal organisation of Ca2+ entry subsequent enhanced activation of SOCs. through Ca2+-permeable channels in the hepatocyte plasma membrane are critical in generating specific cytoplasmic Ca2+ 11. Hepatocyte plasma membrane Ca2+-permeable signals. The liver cell Ca2+-selective SOC, with characteristics channels activated in response to ischaemia reperfusion similar to those of the CRAC channels in lymphocytes and mast injury of the liver cells most likely plays a major role in Ca2+ entry to liver cells. This is complemented by some receptor-activated Ca2+-perme- When livers are subjected to ischaemic reperfusion injury, an able channels, at least one type of ligand-gated Ca2+ permeable 2+ 2+ increase in total hepatocyte Ca and in the amount of Ca in channel (the P2X purinergic-activated channel) and stretch- mitochondria is observed immediately following the onset of activated Ca2+-permeable channels. Several receptor-activated reperfusion [174–177]. Studies with isolated hepatocytes sub- Ca2+ channels have been identified and partially (often in- 2+ 2+ + jected to hypoxia or anoxia show a sustained increase in [Ca ]cyt completely) characterised. Some TRP Ca -andNa- G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 667 permeable channels are known to be expressed in liver cells. [13] J.L. Boyer, Schiffs diseases of the liver, Lippincott, Williams & Wilkins, – While TRP polypeptides are unlikely to be the molecular Philadelphia, 2002, pp. 135 165. [14] M.F. Leite, M.H. Nathanson, The Liver Biology and Pathobiology, components of SOCs, they are likely to comprise many of the – 2+ Lippincott, Williams and Wilkiins, Philadelphia, 2001, pp. 537 554. liver cell receptor-activated and stretch-activated Ca -perme- [15] F.F. Mohammed, R. Khokha, Thinking outside the cell: proteases regulate able channels. Further experiments are required to characterise hepatocyte division, Trends Cell Biol. 15 (2005) 555–563. liver cell receptor-activated Ca2+ permeable channels more [16] B. Young, J.W. Heath, Wheater's Functional Histology, Churchill – completely, and to determine the molecular nature, mechanisms Livingstone, Edinburgh, 2000, pp. 274 275. [17] P. Bedossa, V. Paradis, Liver extracellular matrix in health and disease, of activation, and precise physiological function of all – 2+ J. Pathol. 200 (2003) 504 515. hepatocyte plasma membrane Ca permeable channels. It [18] Y. Wakabayashi, H. Kipp, I.M. Arias, Transporters on demand: intra- will be important that experiments directed towards further cellular reservoirs and cycling of bile canalicular ABC transporters, elucidation of the molecular properties and physiological J. Biol. Chem. 281 (2006) 27669–27673. functions of Ca2+ permeable channels be conducted as far as [19] A.L. Hubbard, V.A. Barr, L.J. Scott, The Liver: Biology and pathobiology, Raven Press Ltd, New York, 1994, pp. 189–214. possible with primary hepatocytes since the spatial polarity and 2+ [20] M.M. Zegers, D. Hoekstra, Mechanisms and functional features of pola- differentiation of these cells is critical to the intracellular Ca rized membrane traffic in epithelial and hepatic cells, Biochem. J. 336 signals. (1998) 257–269. [21] M.N. Berry, A.M. Edwards, G. Barritt, Isolated hepatocytes — pre- – Acknowledgements paration, properties and applications, Elsevier, 1991, pp. 83 98. [22] C.J. Dixon, P.J. White, J.F. Hall, S. Kingston, M.R. Boarder, Regulation of human hepatocytes by P2Y receptors: control of glycogen phosphor- The authors gratefully acknowledge the assistance of Karen ylase, Ca2+, and mitogen-activated protein kinases, J. Pharmacol. Exp. Jennings in preparation of the manuscript. Research conducted Ther. 313 (2005) 1305–1313. in the authors' laboratories which has contributed to this review [23] E.M. O'Brien, D.A. Gomes, S. Sehgal, M.H. Nathanson, Hormonal – is supported by grants from the National Health and Medical regulation of nuclear permeability, J. Biol. Chem. 282 (2007) 4210 4217. [24] A. Enfissi, S. Prigent, P. Colosetti, T. Capiod, The blocking of capa- Research Council of Australia, the Australian Research Council, citative calcium entry by 2-aminoethyl diphenylborate (2-APB) and and the Flinders Medical Centre Foundation of South Australia. carboxyamidotriazole (CAI) inhibits proliferation in Hep G2 and Huh-7 human hepatoma cells, Cell Calcium 36 (2004) 459–467. References [25] L.D. Robb-Gaspers, P. Burnett, G.A. Rutter, R.M. Denton, R. Rizzuto, A.P. Thomas, Integrating cytosolic calcium signals into mitochondrial metabolic responses, EMBO J. 17 (1998) 4987–5000. [1] G.J. Barritt, J.C. Parker, J.C. Wadsworth, A kinetic analysis of the effects [26] M.J. Berridge, The endoplasmic reticulum: a multifunctional signaling of adrenaline on calcium distribution in isolated rat liver parenchymal organelle, Cell Calcium 32 (2002) 235–249. cells, J. Physiol. 312 (1981) 29–55. [27] S. Watanabe, M.J. Phillips, Ca2+ causes active contraction of bile cana- [2] B. Delgado-Coello, R. Trejo, J. Mas-Oliva, Is there a specific role for the liculi: direct evidence from microinjection studies, Proc. Natl. Acad. Sci. plasma membrane Ca2+-ATPase in the hepatocyte? Mol. Cell. Biochem. U. S. A., 81, 1984, pp. 6164–6168. 285 (2006) 1–15. [28] K. Hirata, T. Pusl, A.F. O'Neill, J.A. Dranoff, M.H. Nathanson, The type [3] L.D. Gaspers, A.P. Thomas, Calcium signaling in liver, Cell Calcium 38 II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat (2005) 329–342. hepatocytes, Gastroenterology 122 (2002) 1088–1100. [4] E. Hernandez, M.F. Leite, M.T. Guerra, E.A. Kruglov, O. Bruna-Romero, [29] T. Tordjmann, B. Berthon, E. Jacquemin, C. Clair, N. Stelly, G. M.A. Rodrigues, D.A. Gomes, F.J. Giordano, J.A. Dranoff, M.H. Na- Guillon, M. Claret, L. Combettes, Receptor-oriented intercellular cal- thanson, The spatial distribution of inositol 1,4,5-trisphosphate receptor cium waves evoked by vasopressin in rat hepatocytes, EMBO J. 17 isoforms shapes Ca2+ waves, J. Biol. Chem. 282 (2007) 10057–10067. (1998) 4695–4703. [5] M.A. Rodrigues, D.A. Gomes, M.F. Leite, W. Grant, L. Zhang, W. [30] L. Canaff, J.L. Petit, M. Kisiel, P.H. Watson, M. Gascon-Barre, G.N. Lam, Y.C. Cheng, A.M. Bennett, M.H. Nathanson, Nucleoplasmic Hendy, Extracellular calcium-sensing receptor is expressed in rat hepa- calcium is required for cell proliferation, J. Biol. Chem. 282 (2007) tocytes. coupling to intracellular calcium mobilization and stimulation of 17061–17068. bile flow, J. Biol. Chem. 276 (2001) 4070–4079. [6] N.C. Teoh, G.C. Farrell, Hepatic ischemia reperfusion injury: pathogenic [31] M. Lesurtel, R. Graf, B. Aleil, D.J. Walther, Y. Tian, W. Jochum, C. mechanisms and basis for hepatoprotection, J. Gastroenterol. Hepatol. 18 Gachet, M. Bader, P.A. Clavien, Platelet-derived serotonin mediates liver (2003) 891–902. regeneration, Science 312 (2006) 104–107. [7] V.B. Nieuwenhuijs, M.T. De Bruijn, R.T. Padbury, G.J. Barritt, Hepatic [32] S. Patel, L.D. Robb-Gaspers, K.A. Stellato, M. Shon, A.P. Thomas, ischemia-reperfusion injury: roles of Ca2+ and other intracellular media- Coordination of calcium signalling by endothelial-derived nitric oxide in tors of impaired bile flow and hepatocyte damage, Dig. Dis. Sci. 51 the intact liver, Nat. Cell Biol. 1 (1999) 467–471. (2006) 1087–1102. [33] C. Schofl, M. Ponczek, T. Mader, M. Waring, H. Benecke, A. von zur [8] G.J. Barritt, in: R. Pochet, R. Donato, J. Haiech, C.W. Heizmann, V. Muhlen, H. Mix, M. Cornberg, K.H. Boker, M.P. Manns, S. Wagner, Gerke (Eds.), Calcium: the molecular basis of calcium action in biology Regulation of cytosolic free calcium concentration by extracellular and medicine, Kluwer Academic Publishers, 2000, pp. 73–94. nucleotides in human hepatocytes, Am. J. Physiol. 276 (1999) [9] E.C. Aromataris, M.L. Roberts, G.J. Barritt, G.Y. Rychkov, Glucagon G164–G172. activates Ca2+ and Cl− channels in rat hepatocytes, J. Physiol. 573 (2006) [34] L.F. Barros, A. Stutzin, A. Calixto, M. Catalan, J. Castro, C. Hetz, T. 611–625. Hermosilla, Nonselective cation channels as effectors of free radical- [10] I.S. Ramsey, M. Delling, D.E. Clapham, An introduction to TRP chan- induced rat liver cell necrosis, Hepatology 33 (2001) 114–122. nels, Ann. Rev. Physiol. 68 (2006) 619–647. [35] M.J. Berridge, M.D. Bootman, H.L. Roderick, Calcium signalling: dy- [11] R.S. Lewis, The molecular choreography of a store-operated calcium namics, homeostasis and remodelling, Nat. Rev. Mol. Cell Biol. 4 (2003) channel, Nature 446 (2007) 284–287. 517–529. [12] C.W. Taylor, Store-operated Ca2+ entry: a STIMulating stOrai, Trends [36] R.J. Wojcikiewicz, Type I, II, and III inositol 1,4,5-trisphosphate re- Biochem. Sci. 31 (2006) 597–601. ceptors are unequally susceptible to down-regulation and are expressed in 668 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

markedly different proportions in different cell types, J. Biol. Chem. 270 [60] S.K. Moule, J.D. McGivan, Regulation of the plasma membrane potential (1995) 11678–11683. in hepatocytes—mechanism and physiological significance, Biochim. [37] L.B. Lilly, J.L. Gollan, Ryanodine-induced calcium release from hepatic Biophys. Acta 1031 (1990) 383–397. microsomes and permeabilized hepatocytes, Am. J. Physiol. 268 (1995) [61] J. Graf, O.H. Petersen, Cell membrane potential and resistance in liver, G1017–G1024. J. Physiol. 284 (1978) 105–126. [38] J.-P. Lievremont, A.-M. Hill, M. Hilly, J.-P. Mauger, The inositol 1,4, [62] T. Capiod, D.C. Ogden, Properties of membrane ion conductances 5-trisphosphate receptor is localised on specialised sub-regions of the evoked by hormonal stimulation of guinea-pig and rabbit isolated hepa- endoplasmic reticulum in rat liver, Biochem. J. 300 (1994) 419–427. tocytes, Proc. R. Soc. Lond., B Biol. Sci., 236, 1989, pp. 187–201. [39] M.F. Rossier, G.S. Bird, J.W.J. Putney, Subcellular distribution of the [63] A.C. Field, D.H. Jenkinson, The effect of noradrenaline on the ion calcium-storing inositol 1,4,5-trisphosphate-sensitive organelle in rat liver. permeability of isolated mammalian hepatocytes, studied by intracellular Possible linkage to the plasma membrane through the actin microfilaments, recording, J. Physiol. 392 (1987) 493–512. Biochem. J. 274 (1991) 643–650. [64] J.G. Fitz, B.F. Scharschmidt, Regulation of transmembrane electrical [40] J.P. Lievremont, A.M. Hill, D. Tran, J.F. Coquil, N. Stelly, J.P. Mauger, potential gradient in rat hepatocytes in situ, Am. J. Physiol. 252 (1987) Intracellular calcium stores and inositol 1,4,5-trisphosphate receptor in rat G56–G64. liver cells, Biochem. J. 314 (1996) 189–197. [65] T.A. Lutz, S. Wild, S. Boutellier, D. Sutter, M. Volkert, E. Scharrer, [41] V. Shoshan-Barmatz, High affinity ryanodine binding sites in rat liver Hyperpolarization of the cell membrane of mouse hepatocytes by lactate, endoplasmic reticulum, FEBS Lett. 263 (1990) 317–320. pyruvate, and fructose is due to Ca2+-dependent activation of K+ channels [42] V. Shoshan-Barmatz, T.A. Pressley, S. Higham, N. Kraus-Friedmann, and of the Na+/K+-ATPase, Biochim. Biophys. Acta 1372 (1998) 359–369. Characterization of high-affinity ryanodine-binding sites of rat liver en- [66] T. Sawanobori, H. Takanashi, M. Hiraoka, Y. Iida, K. Kamisaka, H. doplasmic reticulum. Differences between liver and skeletal muscle, Maezawa, Electrophysiological properties of isolated rat liver cells, Biochem. J. 276 (1991) 41–46. J. Cell. Physiol. 139 (1989) 580–585. [43] R.B. Bazotte, B. Pereira, S. Higham, V. Shoshan-Barmatz, N. Kraus- [67] R. Cermak, E. Scharrer, Effect of 2,5-anhydro-D-mannitol on membrane Friedmann, Effects of ryanodine on calcium sequestration in the rat liver, potential in rat hepatocyte couplets and hepatocyte monolayer cultures, Biochem. Pharmacol. 42 (1991) 1799–1803. Biochim. Biophys. Acta 1421 (1999) 116–124. [44] N. Pierobon, D.C. Renard-Rooney, L.D. Gaspers, A.P. Thomas, [68] B. Hille, Ionic channels of excitable membranes, Sinauer Associates Inc, Ryanodine receptors in liver, J. Biol. Chem. 281 (2006) 34086–34095. Sunderland, Massachusetts, 1992, pp. 1–20. [45] T.A. Rooney, E.J. Sass, A.P. Thomas, Characterization of cytosolic [69] J.G. Fitz, A.H. Sostman, Nucleotide receptors activate cation, potassium, calcium oscillations induced by phenylephrine and vasopressin in single and chloride currents in a liver cell line, Am. J. Physiol. 266 (1994) fura-2-loaded hepatocytes, J. Biol. Chem. 264 (1989) 17131–17141. G544–G553. [46] T.A. Rooney, E.J. Sass, A.P. Thomas, Agonist-induced cytosolic calcium [70] G. Kilic, R.B. Doctor, J.G. Fitz, Insulin stimulates membrane conduc- oscillations originate from a specific locus in single hepatocytes, J. Biol. tance in a liver cell line: evidence for insertion of ion channels through a Chem. 265 (1990) 10792–10796. phosphoinositide 3-kinase-dependent mechanism, J. Biol. Chem. 276 [47] A.P. Thomas, D.C. Renard, T.A. Rooney, Spatial and temporal organi- (2001) 26762–26768. zation of calcium signalling in hepatocytes, Cell Calcium 12 (1991) [71] G. Kilic, J.G. Fitz, Heterotrimeric G-proteins activate Cl− channels through 111–126. stimulation of a cyclooxygenase-dependent pathway in a model liver cell [48] N.M. Woods, K.S. Cuthbertson, P.H. Cobbold, Agonist-induced oscilla- line, J. Biol. Chem. 277 (2002) 11721–11727. tions in cytoplasmic free calcium concentration in single rat hepatocytes, [72] R.M. Roman, K.O. Bodily, Y. Wang, J.R. Raymond, J.G. Fitz, Activation Cell Calcium 8 (1987) 79–100. of protein kinase Calpha couples cell volume to membrane Cl− per- [49] N.M. Woods, K.S. Cuthbertson, P.H. Cobbold, Repetitive transient rises meability in HTC hepatoma and Mz-ChA-1 cholangiocarcinoma cells, in cytoplasmic free calcium in hormone-stimulated hepatocytes, Nature Hepatology 28 (1998) 1073–1080. 319 (1986) 600–602. [73] Y.Wang, R. Roman, S.D. Lidofsky, J.G. Fitz, Autocrine signaling through [50] L.D. Robb-Gaspers, A.P. Thomas, Coordination of Ca2+ signaling by ATP release represents a novel mechanism for cell volume regulation, intercellular propagation of Ca2+ waves in the intact liver, J. Biol. Chem. Proc. Natl. Acad. Sci. U. S. A., 93, 1996, pp. 12020–12025. 270 (1995) 8102–8107. [74] N. Friedmann, G. Dambach, Antagonistic effect of insulin on glucagon- [51] A.P. Thomas, D.C. Renard-Rooney, G. Hajnoczky, L.D. Robb-Gaspers, evoked hyperpolarization. A correlation between changes in membrane po- C. Lin, T.A. Rooney, Subcellular organization of calcium signalling in tential and gluconeogenesis, Biochim. Biophys. Acta 596 (1980) 180–185. hepatocytes and the intact liver, Ciba Found. Symp. 188 (1995) 18–35 [75] L. Fischer, S. Haag-Diergarten, E. Scharrer, T.A. Lutz, Leukotriene and discussion 35–49. purinergic receptors are involved in the hyperpolarizing effect of glu- [52] M.J. Berridge, A. Galione, Cytosolic calcium oscillators, FASEB J. 2 cagon in liver cells, Biochim. Biophys. Acta 1669 (2005) 26–33. (1988) 3074–3082. [76] S.D. Lidofsky, M.H. Xie, A. Sostman, B.F. Scharschmidt, J.G. Fitz, [53] S.K. Duddy, G.E. Kass, S. Orrenius, Ca2+-mobilizing hormones stimulate Vasopressin increases cytosolic sodium concentration in hepatocytes and Ca2+ efflux from hepatocytes, J. Biol. Chem. 264 (1989) 20863–20866. activates calcium influx through cation-selective channels, J. Biol. Chem. [54] A.V.Tepikin, S.G. Voronina,D.V.Gallacher, O.H. Petersen, Pulsatile Ca2+ 268 (1993) 14632–14636. extrusion from single pancreatic acinar cells during receptor-activated [77] J. Soboloff, M. Spassova, W. Xu, L.P. He, N. Cuesta, D.L. Gill, Role of cytosolic Ca2+ spiking, J. Biol. Chem. 267 (1992) 14073–14076. endogenous TRPC6 channels in Ca2+ signal generation in A7r5 smooth [55] R.B. Gregory, G.J. Barritt, Evidence that Ca2+-release-activated Ca2+ muscle cells, J. Biol. Chem. 280 (2005) 39786–39794. channels in rat hepatocytes are required for the maintenance of hormone- [78] A. Auld, J. Chen, H.M. Brereton, Y.J. Wang, R.B. Gregory, G.J. Barritt, induced Ca2+ oscillations, Biochem. J. 370 (2003) 695–702. Store-operated Ca2+ inflow in Reuber hepatoma cells is inhibited by [56] B. Nilius, G. Owsianik, T. Voets, J.A. Peters, Transient receptor potential voltage-operated Ca2+ channel antagonists and, in contrast to freshly cation channels in disease, Physiol. Rev. 87 (2007) 165–217. isolated hepatocytes, does not require a pertussis toxin-sensitive trimeric [57] A.B. Parekh, J.W. Putney Jr., Store-operated calcium channels, Physiol. GTP-binding protein, Biochim. Biophys. Acta 1497 (2000) 11–26. Rev. 85 (2005) 757–810. [79] H.M. Brereton, M.L. Harland, M. Froscio, T. Petronijevic, G.J. Barritt, [58] E.T. Barfod, A.L. Moore, M.W. Roe, S.D. Lidofsky, Ca2+-activated IK1 Novel variants of voltage-operated calcium channel alpha 1-subunit tran- channels associate with lipid rafts upon cell swelling and mediate volume scripts in a rat liver-derived cell line: deletion in the IVS4 voltage sensing recovery, J. Biol. Chem. 282 (2007) 8984–8993. region, Cell Calcium 22 (1997) 39–52. [59] J. Graf, D. Haussinger, Ion transport in hepatocytes: mechanisms and [80] G. Ihrke, E.B. Neufeld, T. Meads, M.R. Shanks, D. Cassio, M. Laurent, correlations to cell volume, hormone actions and metabolism, J. Hepatol. T.A. Schroer, R.E. Pagano, A.L. Hubbard, WIF-B cells: an in vitro model 24 (Suppl 1) (1996) 53–77. for studies of hepatocyte polarity, J. Cell Biol. 123 (1993) 1761–1775. G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 669

[81] S. Sehgal, M.T. Guerra, E.A. Kruglov, J. Wang, M.H. Nathanson, Protein [103] L.W. Runnels, L. Yue, D.E. Clapham, TRP-PLIK, a bifunctional protein 4.1N does not interact with the inositol 1,4,5-trisphosphate receptor in an with kinase and ion channel activities, Science 291 (2001) 1043–1047. epithelial cell line, Cell Calcium 38 (2005) 469–480. [104] M.C. Hermosura, M.K. Monteilh-Zoller, A.M. Scharenberg, R. Penner,

[82] T. Capiod, ATP-activated cation currents in single guinea-pig hepato- A. Fleig, Dissociation of the store-operated calcium current ICRAC and the cytes, J. Physiol. 507 (1998) 795–805. Mg-nucleotide-regulated metal ion current MagNuM, J. Physiol. 539 [83] Y. Cao, J.Y. Chatton, Involvement of calmodulin in the activation of store- (2002) 445–458. operated Ca2+ entry in rat hepatocytes, FEBS Lett. 424 (1998) 33–36. [105] A.B. Parekh, R. Penner, Store depletion and calcium influx, Physiol Rev [84] R. Carini, G. Bellomo, L. Paradisi, M.U. Dianzani, E. Albano, 4-Hydro- 77 (1997) 901–930. xynonenal triggers Ca2+ influx in isolated rat hepatocytes, Biochem. [106] K.C. Fernando, G.J. Barritt, Characterisation of the divalent cation chan- Biophys. Res. Commun. 218 (1996) 772–776. nels of the hepatocyte plasma membrane receptor-activated Ca2+ inflow [85] G. Guihard, J. Noel, T. Capiod, Ca2+ depletion and inositol 1,4,5-tri- system using lanthanide ions, Biochim. Biophys. Acta 1268 (1995) sphosphate-evoked activation of Ca2+ entry in single guinea pig hepa- 97–106. tocytes, J. Biol. Chem. 275 (2000) 13411–13414. [107] R.B. Gregory, G. Rychkov, G.J. Barritt, Evidence that 2-aminoethyl [86] F. Striggow, R. Bohnensack, Inositol 1,4,5-trisphosphate activates receptor- diphenylborate is a novel inhibitor of store-operated Ca2+ channels in mediated calcium entry by two different pathways in hepatocytes, Eur. J. liver cells, and acts through a mechanism which does not involve inositol Biochem. 222 (1994) 229–234. trisphosphate receptors, Biochem. J. 354 (2001) 285–290. [87] C.A. Hansen, L.J. Yang, J.R. Williamson, Mechanisms of receptor- [108] G.Y. Rychkov, T. Litjens, M.L. Roberts, G.J. Barritt, Arachidonic acid mediated Ca2+ signaling in rat hepatocytes, J. Biol. Chem. 266 (1991) inhibits the store-operated Ca2+ current in rat liver cells, Biochem. J. 385 18573–18579. (2005) 551–556. [88] A. Ikari, H. Sakai, N. Takeguchi, ATP, thapsigargin and cAMP increase [109] L.A. Berven, G.J. Barritt, Evidence obtained using single hepatocytes for Ca2+ in rat hepatocytes by activating three different Ca2+ influx pathways, inhibition by the phospholipase C inhibitor U73122 of store-operated Ca2+ Japanese J. Physiol. 47 (1997) 235–239. inflow, Biochem. Pharmacol. 49 (1995) 1373–1379. [89] P.R. Junankar, A. Karjalainen, K. Kirk, The role of P2Y1 purinergic [110] T. Litjens, T. Nguyen, J. Castro, E.C. Aromataris, L. Jones, G.J. Barritt, receptors and cytosolic Ca2+ in hypotonically activated osmolyte efflux G.Y. Rychkov, Phospholipase C-gamma1 is required for the activation from a rat hepatoma cell line, J. Biol. Chem. 277 (2002) 40324–40334. of store-operated Ca2+ channels in liver cells, Biochem. J. 405 (2007) [90] G.E. Kass, J. Llopis, S.C. Chow, S.K. Duddy, S. Orrenius, Receptor- 269–276. operated calcium influx in rat hepatocytes. Identification and character- [111] J.E. Bleasdale, N.R. Thakur, R.S. Gremban, G.L. Bundy, F.A. Fitzpatrick, ization using manganese, J. Biol. Chem. 265 (1990) 17486–17492. R.J. Smith, S. Bunting, Selective inhibition of receptor-coupled phos- [91] J. Llopis, G.E. Kass, A. Gahm, S. Orrenius, Evidence for two pathways pholipase C-dependent processes in human platelets and polymorpho- of receptor-mediated Ca2+ entry in hepatocytes, Biochem. J. 284 (1992) nuclear neutrophils, J. Pharmacol. Exp. Ther. 255 (1990) 756–768. 243–247. [112] S. Akiba, E. Kato, T. Sato, T. Fujii, Biscoclaurine alkaloids inhibit [92] Y. Zhang, J. Duszynski, S. Hreniuk, M.M. Waybill, K.F. LaNoue, receptor-mediated phospholipase A2 activation probably through uncou- Regulation of plasma membrane permeability to calcium in primary pling of a GTP-binding protein from the enzyme in rat peritoneal mast cultures of rat hepatocytes, Cell Calcium 12 (1991) 559–575. cells, Biochem. Pharmacol. 44 (1992) 45–50. [93] K.C. Fernando, G.J. Barritt, Characterisation of the inhibition of the [113] J.L. Harper, C.S. Camerini-Otero, A.H. Li, S.A. Kim, K.A. Jacobson, hepatocyte receptor-activated Ca2+ inflow system by gadolinium and J.W. Daly, Dihydropyridines as inhibitors of capacitative calcium SK&F 96365, Biochim. Biophys. Acta 1222 (1994) 383–389. entry in leukemic HL-60 cells, Biochem. Pharmacol. 65 (2003) [94] T.L. Applegate, A. Karjalainen, F.L. Bygrave, Rapid Ca2+ influx induced 329–338. by the action of dibutylhydroquinone and glucagon in the perfused rat [114] A. Lee, T. Scheuer, W.A. Catterall, Ca2+/calmodulin-dependent facilita- liver, Biochem. J. 323 (1997) 463–467. tion and inactivation of P/Q-type Ca2+ channels, J. Neurosci. 20 (2000) [95] J. Duszynski, M. Elensky, J.Y. Cheung, D.L. Tillotson, K.F. LaNoue, 6830–6838. Hormone-regulated Ca2+ channel in rat hepatocytes revealed by whole [115] J. Liou, M. Fivaz, T. Inoue, T. Meyer, Live-cell imaging reveals se- cell patch clamp, Cell Calcium 18 (1995) 19–29. quential oligomerization and local plasma membrane targeting of stromal [96] J.G. Altin, F.L. Bygrave, The influx of Ca2+ induced by the adminis- interaction molecule 1 after Ca2+ store depletion, Proc. Natl. Acad. Sci. tration of glucagon and Ca2+-mobilizing agents to the perfused rat liver U. S. A., 104, 2007, pp. 9301–9306. could involve at least two separate pathways, Biochem. J. 242 (1987) [116] P. Varnai, B. Toth, D.J. Toth, L. Hunyady, T. Balla, Visualization and 43–50. manipulation of plasma membrane-endoplasmic reticulum contact sites [97] G. Rychkov, H.M. Brereton, M.L. Harland, G.J. Barritt, Plasma indicates the presence of additional molecular components within the membrane Ca2+ release-activated Ca2+ channels with a high selectivity STIM1-Orai1 Complex, J. Biol. Chem. 282 (2007) 29678–29690. for Ca2+ identified by patch-clamp recording in rat liver cells, Hepatology [117] G. Rychkov, G.J. Barritt, TRPC1 Ca2+-permeable channels in animal 33 (2001) 938–947. cells, Handbook Exp. Pharmacol. (2007) 23–52. [98] T. Litjens, M.L. Harland, M.L. Roberts, G.J. Barritt, G.Y. Rychkov, Fast [118] J.P. Yuan, W. Zeng, G.N. Huang, P.F. Worley, S. Muallem, STIM1 2+ 2+ Ca -dependent inactivation of the store-operated Ca current (ISOC)in heteromultimerizes TRPC channels to determine their function as store- liver cells: a role for calmodulin, J. Physiol. 558 (2004) 85–97. operated channels, Nat. Cell Biol. 9 (2007) 636–645. [99] G.Y. Rychkov, T. Litjens, M.L. Roberts, G.J. Barritt, ATP and vasopressin [119] H.M. Brereton, J. Chen, G. Rychkov, M.L. Harland, G.J. Barritt, Maito- activate a single type of store-operated Ca2+ channel, identified by patch- toxin activates an endogenous non-selective cation channel and is an clamp recording, in rat hepatocytes, Cell Calcium 37 (2005) 183–191. effective initiator of the activation of the heterologously expressed hTRPC- [100] M. van der Stelt, M. Trevisani, V. Vellani, L. De Petrocellis, A. Schiano 1 (transient receptor potential) non-selective cation channel in H4-IIE liver Moriello, B. Campi, P. McNaughton, P. Geppetti, V. Di Marzo, Anan- cells, Biochim. Biophys. Acta 1540 (2001) 107–126. damide acts as an intracellular messenger amplifying Ca2+ influx via [120] J. Chen, G.J. Barritt, Evidence that TRPC1 (transient receptor potential TRPV1 channels, EMBO J. 24 (2005) 3026–3037. canonical 1) forms a Ca2+-permeable channel linked to the regulation of [101] P. Eder, M. Poteser, C. Romanin, K. Groschner, Na+ entry and modulation cell volume in liver cells obtained using small interfering RNA targeted of Na+/Ca2+ exchange as a key mechanism of TRPC signaling, Pflugers against TRPC1, Biochem. J. 373 (2003) 327–336. Archi. - Eur. J. Physiol. 451 (2005) 99–104. [121] G.N. Huang, W. Zeng, J.Y. Kim, J.P. Yuan, L. Han, S. Muallem, P.F. [102] M.R. Elizondo, B.L. Arduini, J. Paulsen, E.L. MacDonald, J.L. Sabel, P.D. Worley, STIM1 carboxyl-terminus activates native SOC, ICRAC and Henion, R.A. Cornell, D.M. Parichy, Defective skeletogenesis with TRPC1 channels, Nat. Cell Biol. 8 (2006) 1003–1010. kidney stone formation in dwarf zebrafish mutant for trpm7, Curr. Biol. 15 [122] Y. Liao, C. Erxleben, E. Yildirim, J. Abramowitz, D.L. Armstrong, L. (2005) 667–671. Birnbaumer, Orai proteins interact with TRPC channels and confer 670 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

responsiveness to store depletion, Proc. Natl. Acad. Sci. U. S. A., 104, [141] K.C. Fernando, R.B. Gregory, F. Katsis, B.E. Kemp, G.J. Barritt, 2007, pp. 4682–4687. Evidence that a low-molecular-mass GTP-binding protein is required [123] J.J. Lopez, G.M. Salido, J.A. Pariente, J.A. Rosado, Interaction of STIM1 for store-activated Ca2+ inflow in hepatocytes, Biochem. J. 328 (1997) with endogenously expressed human canonical TRP1 upon depletion of 463–471. intracellular Ca2+ stores, J. Biol. Chem. 281 (2006) 28254–28264. [142] J.W. Putney Jr., Capacitative calcium entry revisited, Cell Calcium 11 [124] H.L. Ong, K.T. Cheng, X. Liu, B.C. Bandyopadhyay, B.C. Paria, J. (1990) 611–624. Soboloff, B. Pani, Y. Gwack, S. Srikanth, B.B. Singh, D.L. Gill, I.S. [143] R.B. Gregory, D. Sykiotis, G.J. Barritt, Evidence that store-operated Ca2+ Ambudkar, Dynamic assembly of TRPC1-STIM1-Orai1 ternary complex channels are more effective than intracellular messenger-activated non- is involved in store-operated calcium influx. Evidence for similarities in selective cation channels in refilling rat hepatocyte intracellular Ca2+ store-operated and calcium release-activated calcium channel compo- stores, Cell Calcium 34 (2003) 241–251. nents, J. Biol. Chem. 282 (2007) 9105–9116. [144] M.K. Park, O.H. Petersen, A.V. Tepikin, The endoplasmic reticulum as [125] J. Lippincott-Schwartz, The liver: Biology and pathobiology, Raven Press one continuous Ca2+ pool: visualization of rapid Ca2+ movements and Ltd, New York, 1994, pp. 215–226. equilibration, EMBO J. 19 (2000) 5729–5739. [126] R.B. Gregory, R.A. Wilcox, L.A. Berven, N.C. van Straten, G.A. van der [145] K.E. Smith, C. Gu, K.A. Fagan, B. Hu, D.M. Cooper, Residence of Marel, J.H. van Boom, G.J. Barritt, Evidence for the involvement of a adenylyl cyclase type 8 in caveolae is necessary but not sufficient small subregion of the endoplasmic reticulum in the inositol trisphosphate for regulation by capacitative Ca2+ entry, J. Biol. Chem. 277 (2002) receptor-induced activation of Ca2+ inflow in rat hepatocytes, Biochem. J. 6025–6031. 341 (1999) 401–408. [146] Y. Hamada, A. Karjalainen, F.L. Bygrave, Hormone-induced bile flow [127] R.B. Gregory, R. Hughes, A.M. Riley, B.V. Potter, R.A. Wilcox, G.J. and hepatobiliary calcium fluxes are attenuated in the perfused liver of Barritt, Inositol trisphosphate analogues selective for types I and II rats made cholestatic with ethynylestradiol in vivo and with phalloidin in inositol trisphosphate receptors exert differential effects on vasopressin- vitro, Hepatology 21 (1995) 1455–1464. stimulated Ca2+ inflow and Ca2+ release from intracellular stores in rat [147] R.B. Gregory, R. Hughes, G.J. Barritt, Induction of cholestasis in the hepatocytes, Biochem. J. 381 (2004) 519–526. perfused rat liver by 2-aminoethyl diphenylborate, an inhibitor of the [128] C. Chan, M.L. Harland, S.E. Webb, J. Chen, A.L. Miller, G.J. Barritt, hepatocyte plasma membrane Ca2+ channels, J. Gastroenterol. Hepatol. Evaluation, using targeted aequorins, of the roles of the endoplasmic 19 (2004) 1128–1134. reticulum and its (Ca2++Mg2+)ATP-ases in the activation of store- [148] C.E. Bear, A nonselective cation channel in rat liver cells is activated by operated Ca2+ channels in liver cells, Cell Calcium 35 (2004) 317–331. membrane stretch, Am. J. Physiol. 258 (1990) C421–C428. [129] L.M. Broad, F.J. Braun, J.P. Lievremont, G.S. Bird, T. Kurosaki, J.W. [149] C.E. Bear, C.H. Li, Calcium-permeable channels in rat hepatoma cells Putney Jr., Role of the phospholipase C-inositol 1,4,5-trisphosphate are activated by extracellular nucleotides, Am. J. Physiol. 261 (1991) pathway in calcium release-activated calcium current and capacitative C1018–C1024. calcium entry, J. Biol. Chem. 276 (2001) 15945–15952. [150] S.D. Lidofsky, A. Sostman, J.G. Fitz, Regulation of cation-selective [130] R.L. Patterson, D.B. van Rossum, D.L. Ford, K.J. Hurt, S.S. Bae, P.G. channels in liver cells, J. Membr. Biol. 157 (1997) 231–236. Suh, T. Kurosaki, S.H. Snyder, D.L. Gill, Phospholipase C-gamma is [151] O. Mignen, T.J. Shuttleworth, I(ARC), a novel arachidonate-regulated, required for agonist-induced Ca2+ entry, Cell 111 (2002) 529–541. noncapacitative Ca2+ entry channel, J. Biol. Chem. 275 (2000) [131] C.L. Tu, W. Chang, D.D. Bikle, Phospholipase cgamma1 is required for 9114–9119. activation of store-operated channels in human keratinocytes, J. Invest. [152] J. Vriens, A. Janssens, J. Prenen, B. Nilius, R. Wondergem, TRPV Dermatol. 124 (2005) 187–197. channels and modulation by hepatocyte growth factor/scatter factor in [132] L.A. Berven, B.P. Hughes, G.J. Barritt, A slowly ADP-ribosylated human hepatoblastoma (HepG2) cells, Cell Calcium 36 (2004) 19–28. pertussis-toxin-sensitive GTP-binding regulatory protein is required for [153] J. Waning, J. Vriens, G. Owsianik, L. Stuwe, S. Mally, A. Fabian, C. vasopressin-stimulated Ca2+ inflow in hepatocytes, Biochem. J. 299 Frippiat, B. Nilius, A. Schwab, A novel function of capsaicin-sensitive (1994) 399–407. TRPV1 channels: involvement in cell migration, Cell Calcium 42 (2007) [133] N. Butta, E. Urcelay, C. Gonzalez-Manchon, R. Parrilla, M.S. Ayuso, 17–25. Pertussis toxin inhibition of alpha 1-adrenergic or vasopressin-induced Ca2+ [154] D. Alvaro, P. Della Guardia, A. Bini, A. Gigliozzi, S. Furfaro, T. La fluxes in rat liver. Selective inhibition of the alpha 1-adrenergic receptor- Rosa, C. Piat, L. Capocaccia, Effect of glucagon on intracellular pH coupled metabolic activation, J. Biol. Chem. 268 (1993) 6081–6089. regulation in isolated rat hepatocyte couplets, J. Clin. Invest. 96 (1995) [134] L.A. Berven, G.J. Barritt, A role for a pertussis toxin-sensitive trimeric 665–675. G-protein in store-operated Ca2+ inflow in hepatocytes, FEBS Lett. 346 [155] A. Tanaka, H. Shinohara, Y. Yamaoka, Receptor-mediated biphasic (1994) 235–240. alteration of hepatocellular transport from hepatocyte to bile canaliculi as [135] K.C. Fernando, G.J. Barritt, Evidence from studies with hepatocyte measured by near-infrared spectroscopy: a novel test with glucagon for suspensions that store-operated Ca2+ inflow requires a pertussis toxin- biliary excretion, Eur. Surg. Res. 35 (2003) 464–469. sensitive trimeric G-protein, Biochem. J. 303 (1994) 351–356. [156] G. Jiang, B.B. Zhang, Glucagon and regulation of glucose metabolism, [136] L.A. Berven, M.F. Crouch, F. Katsis, B.E. Kemp, L.M. Harland, G.J. Am. J. Physiol. – Endocrinol. Metabol. 284 (2003) E671–E678. Barritt, Evidence that the pertussis toxin-sensitive trimeric GTP-binding [157] L. Combettes, B. Berthon, A. Binet, M. Claret, Glucagon and vasopressin 2+ 2+ protein Gi2 is required for agonist- and store-activated Ca inflow in interactions on Ca movements in isolated hepatocytes, Biochem. J. 237 hepatocytes, J. Biol. Chem. 270 (1995) 25893–25897. (1986) 675–683. [137] Y.J. Wang, R.B. Gregory, G.J. Barritt, Regulation of F-actin and endo- [158] F.L. Bygrave, A. Gamberucci, R. Fulceri, A. Benedetti, Evidence that 2+ plasmic reticulum organization by the trimeric G-protein Gi2 in rat he- stimulation of plasma-membrane Ca inflow is an early action of glu- patocytes. Implication for the activation of store-operated Ca2+ inflow, cagon and dibutyryl cyclic AMP in rat hepatocytes, Biochem. J. 292 J. Biol. Chem. 275 (2000) 22229–22237. (1993) 19–22. [138] Y.J. Wang, R.B. Gregory, G.J. Barritt, Maintenance of the filamentous [159] J.P. Mauger, J. Poggioli, M. Claret, Synergistic stimulation of the Ca2+ actin cytoskeleton is necessary for the activation of store-operated Ca2+ influx in rat hepatocytes by glucagon and the Ca2+-linked hormones channels, but not other types of plasma-membrane Ca2+ channels, in rat vasopressin and angiotensin II, J. Biol. Chem. 260 (1985) 11635–11642. hepatocytes, Biochem. J. 363 (2002) 117–126. [160] F.L. Bygrave, H.R. Roberts, Regulation of cellular calcium through [139] M.R. Koelle, Heterotrimeric G protein signaling: Getting inside the cell, signaling cross-talk involves an intricate interplay between the actions Cell 126 (2006) 25–27. of receptors, G-proteins, and second messengers, FASEB J. 9 (1995) [140] M. Schaefer, M. Petronczki, D. Dorner, M. Forte, J.A. Knoblich, 1297–1303. Heterotrimeric G proteins direct two modes of asymmetric cell division in [161] G.G. Holz, G. Kang, M. Harbeck, M.W. Roe, O.G. Chepurny, Cell the Drosophila nervous system, Cell 107 (2001) 183–194. physiology of cAMP sensor Epac, J. Physiol. 577 (2006) 5–15. G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672 671

[162] R. Burcelin, E.B. Katz, M.J. Charron, Molecular and cellular aspects of [182] M. Aarts, K. Iihara, W.L. Wei, Z.G. Xiong, M. Arundine, W. Cerwinski, the glucagon receptor: role in diabetes and metabolism, Diabetes Metab. J.F. MacDonald, M. Tymianski, A key role for TRPM7 channels in 22 (1996) 373–396. anoxic neuronal death, Cell 115 (2003) 863–877. [163] C. Morand, C. Yacoub, C. Remesy, C. Demigne, Characterization of [183] B.A. Miller, The role of TRP channels in oxidative stress-induced cell glucagon and catecholamine effects on isolated sheep hepatocytes, Am. J. death, J. Membr. Biol. 209 (2006) 31–41. Physiol. 255 (1988) R539–R546. [184] T. Yoshida, R. Inoue, T. Morii, N. Takahashi, S. Yamamoto, Y. Hara, M. [164] T. Mine, I. Kojima, E. Ogata, Role of calcium fluxes in the action of Tominaga, S. Shimizu, Y. Sato, Y. Mori, Nitric oxide activates TRP glucagon on glucose metabolism in rat hepatocytes, Am. J. Physiol. 265 channels by cysteine S-nitrosylation.[see comment], Nat. Chem. Biol. 2 (1993) G35–G42. (2006) 596–607. [165] L. Feng, I. Subbaraya, N. Yamamoto, W. Baehr, N. Kraus-Friedmann, [185] J.N. Crofts, G.J. Barritt, The measurement of Ca2+ inflow across the liver Expression of photoreceptor cyclic nucleotide-gated cation channel alpha cell plasma membrane by using quin2 and studies of the roles of Na+ and subunit (CNGCalpha) in the liver and skeletal muscle, FEBS Lett. 395 extracellular Ca2+ in the mechanism of Ca2+ inflow, Biochem. J. 264 (1996) 77–81. (1989) 61–70. [166] T. Lenz, J.W. Kleineke, Hormone-induced rise in cytosolic Ca2+ in [186] G.E. Kass, A. Gahm, J. Llopis, Cyclic AMP stimulates Ca2+ entry in rat axolotl hepatocytes: properties of the Ca2+ influx channel, Am. J. Physiol. hepatocytes by interacting with the plasma membrane carriers involved in 273 (1997) C1526–C1532. receptor-mediated Ca2+ influx, Cell. Signal. 6 (1994) 493–501. [167] K.C. Fernando, R.B. Gregory, G.J. Barritt, Protein kinase A regulates the [187] W.H. Chen, T.H. Yeh, M.C. Tsai, D.S. Chen, T.H. Wang, Characterization disposition of Ca2+ which enters the cytoplasmic space through store- of Ca2+- and voltage-dependent nonselective cation channels in human activated Ca2+ channels in rat hepatocytes by diverting inflowing Ca2+ to HepG2 cells, J. Formos. Med. Assoc. 96 (1997) 503–510. mitochondria, Biochem. J. 330 (1998) 1179–1187. [188] K. Benzeroual, S.K. Pandey, A.K. Srivastava, G. van de Werve, P.S. [168] N.G. Morgan, P.F. Blackmore, J.H. Exton, Modulation of the alpha Haddad, Insulin-induced Ca2+ entry in hepatocytes is important for PI 1-adrenergic control of hepatocyte calcium redistribution by increases 3-kinase activation, but not for insulin receptor and IRS-1 tyrosine phos- in cyclic AMP, J. Biol. Chem. 258 (1983) 5110–5116. phorylation, Biochim. Biophys. Acta 1495 (2000) 14–23. [169] N.G. Morgan, R. Charest, P.F. Blackmore, J.H. Exton, Potentiation of [189] K. Benzeroual, G. van de Werve, S. Meloche, L. Mathe, A. Romanelli, P. alpha 1-adrenergic responses in rat liver by a cAMP-dependent mecha- Haddad, Insulin induces Ca2+ influx into isolated rat hepatocyte couplets, nism, Proc. Natl. Acad. Sci. U. S. A., 81, 1984, pp. 4208–4212. Am. J. Physiol. 272 (1997) G1425–G1432. [170] R. Somogyi, M. Zhao, J.W. Stucki, Modulation of cytosolic-[Ca2+] [190] L. Mathe, D. Vallerand, P.S. Haddad, Insulin induces cation fluxes and oscillations in hepatocytes results from cross-talk among second mes- increases intracellular calcium in the HTC rat hepatoma cell line, Can. sengers. The synergism between the alpha 1-adrenergic response, gluca- J. Gastroenterol. 14 (2000) 389–396. gon and cyclic AMP, and their antagonism by insulin and diacylglycerol [191] C. Cefaratti, Mg2+ release coupled to Ca2+ uptake: a novel Ca2+ accu- manifest themselves in the control of the cytosolic-[Ca2+] oscillations, mulation mechanism in rat liver, Mol. Cell. Biochem. 295 (2007) Biochem. J. 286 (1992) 869–877. 241–247. [171] F.L. Bygrave, A. Benedetti, Calcium: its modulation in liver by cross-talk [192] H.M. Brereton, M.L. Harland, A.M. Auld, G.J. Barritt, Evidence that the between the actions of glucagon and calcium-mobilizing agonists, Bio- TRP-1 protein in unlikely to account for store-operated Ca2+ inflow in chem. J. 296 (1993) 1–14. Xenopus laevis oocytes, Mol. Cell Biochem. 214 (2000) 63–74. [172] G. Guihard, L. Combettes, T. Capiod, 3¢:5¢-cyclic guanosine monopho- [193] H.L. Ong, J. Chen, T. Chataway, H. Brereton, L. Zhang, T. Downs, L. sphate (cGMP) potentiates the inositol 1,4,5-trisphosphate-evoked Ca2+ Tsiokas, G. Barritt, Specific detection of the endogenous transient receptor release in guinea-pig hepatocytes, Biochem. J. 318 (1996) 849–855. potential (TRP)-1 protein in liver and airway smooth muscle cells using [173] L. Nguyen, A. Karjalainen, E.A. Milbourne, F.L. Bygrave, Permeable immunoprecipitation and Western-blot analysis, Biochem. J. 364 (2002) analogues of cGMP promote hepatic calcium inflow induced by the 641–648. synergistic action of glucagon and vasopressin but inhibit that induced by [194] A. Riccio, A.D. Medhurst, C. Mattei, R.E. Kelsell, A.R. Calver, A.D. vasopressin alone, Biochem. J. 330 (1998) 877–880. Randall, C.D. Benham, mRNA distribution analysis of human TRPC [174] D.K. Dhar, Y. Takemoto, N. Nagasue, M. Uchida, T. Ono, T. Nakamura, family in CNS and peripheral tissues, Brain Res. Mol. Brain Res. 109 FK506 maintains cellular calcium homeostasis in ischemia-reperfusion (2002) 95–104. injury of the canine liver, J. Surg. Res. 60 (1996) 142–146. [195] C.A. Reilly, J.L. Taylor, D.L. Lanza, B.A. Carr, D.J. Crouch, G.S. Yost, [175] H. Isozaki, K. Fujii, E. Nomura, H. Hara, Calcium concentration in Capsaicinoids cause inflammation and epithelial cell death through hepatocytes during liver ischaemia-reperfusion injury and the effects of activation of vanilloid receptors, Toxicol. Sci. 73 (2003) 170–181. diltiazem and citrate on perfused rat liver, Eur. J. Gastroenterol. Hepatol. [196] J.B. Peng, M.A. Hediger, A family of calcium-permeable channels in the 12 (2000) 291–297. kidney: distinct roles in renal calcium handling, Curr. Opin. Nephrol. [176] K. Kurita, G. Tanabe, T. Aikou, H. Shimazu, Ischemic liver cell damage Hypertens. 11 (2002) 555–561. and calcium accumulation in rats, J. Hepatol. 18 (1993) 196–204. [197] R. Strotmann, C. Harteneck, K. Nunnenmacher, G. Schultz, T.D. Plant, [177] Y. Takemoto, M. Uchida, N. Nagasue, K. Ohiwa, T. Kimoto, D.K. Dhar, OTRPC4, a non-selective cation channel that confers sensitivity to T. Nakamura, Changes in calcium content of the liver during hepatic extracellular osmolarity, Nat. Cell Biol. 2 (2000) 695–702. ischemia-reperfusion in dogs, J. Hepatol. 21 (1994) 743–747. [198] J.B. Peng, X.Z. Chen, U.V. Berger, S. Weremowicz, C.C. Morton, P.M. [178] R. Carini, R. Castino, M.G. De Cesaris, R. Splendore, M. Demoz, E. Vassilev, E.M. Brown, Human calcium transport protein CaT1, Biochem. Albano, C. Isidoro, Preconditioning-induced cytoprotection in hepato- Biophys. Res. Commun. 278 (2000) 326–332. cytes requires Ca2+-dependent exocytosis of lysosomes, J. Cell Sci. 117 [199] T.C. Pan, B.K. Liao, C.J. Huang, L.Y. Lin, P.P. Hwang, Epithelial Ca2+ (2004) 1065–1077. channel expression and Ca2+ uptake in developing zebrafish, Am. J. [179] D. Crenesse, M. Hugues, C. Ferre, J.C. Poiree, J. Benoliel, C. Dolisi, J. Physiol. - Regul. Integr. Comp. Physiol. 289 (2005) R1202–R1211. Gugenheim, Inhibition of calcium influx during hypoxia/reoxygenation [200] P. Launay, A. Fleig, A.L. Perraud, A.M. Scharenberg, R. Penner, J.P. in primary cultured rat hepatocytes, Pharmacology 58 (1999) 160–170. Kinet, TRPM4 is a Ca2+-activated non-selective cation channel mediating [180] A. Gasbarrini, A.B. Borle, H. Farghali, C. Bender, A. Francavilla, D. Van cell membrane depolarization, Cell 109 (2002) 397–407. Thiel, Effect of anoxia on intracellular ATP, Na+i, Ca2+i, Mg2+i, and [201] T. Enklaar, M. Esswein, M. Oswald, K. Hilbert, A. Winterpacht, M. cytotoxicity in rat hepatocytes, J. Biol. Chem. 267 (1992) 6654–6663. Higgins, B. Zabel, MTR1, a novel biallelically expressed gene in the [181] G.K. Glantzounis, H.J. Salacinski, W. Yang, B.R. Davidson, A.M. centre of the mouse distal chromosome 7 imprinting cluster, is a member Seifalian, The contemporary role of antioxidant therapy in attenuating of the TRP gene family, Genomics 67 (2000) 179–197. liver ischemia-reperfusion injury: a review.[see comment], Liver Trans- [202] D. Prawitt, T. Enklaar, G. Klemm, B. Gartner, C. Spangenberg, A. plantation 11 (2005) 1031–1047. Winterpacht, M. Higgins, Identification and characterization of MTR1, a 672 G.J. Barritt et al. / Biochimica et Biophysica Acta 1783 (2008) 651–672

novel gene with homology to melastatin (MLSN1) and the TRP gene Electrophysiological properties of gap junctions between dissociated family located in the BWS-WT2 critical region on chromosome 11p15.5 pairs of rat hepatocytes, J. Cell Biol. 103 (1986) 135–144. and showing allele-specific expression, Hum. Mol. Genet. 9 (2000) [208] A. Howard, N.F. Barley, S. Legon, J.R. Walters, Plasma-membrane 203–216. calcium-pump isoforms in human and rat liver, Biochem. J. 303 (1994) [203] A.C.M. Ong, C.J. Ward, R.J. Butler, S. Biddolph, C. Bowker, R. Torra, Y. 275–279. Pei, Coordinate expression of the autosomal dominant polycystic kidney [209] B. Delgado-Coello, J. Santiago-Garcia, A. Zarain-Herzberg, J. Mas- disease proteins, polycystin-2 and polycystin-1, in normal and cystic Oliva, Plasma membrane Ca2+-ATPase mRNA expression in murine tissue, Am. J. Pathol. 154 (1999) 1721–1729. hepatocarcinoma and regenerating liver cells, Mol. Cell. Biochem. 247 [204] L. Tsiokas, T. Arnould, C.W. Zhu, E. Kim, G. Walz, V.P. Sukhatme, (2003) 177–184. Specific association of the gene product of PKD2 with the TRPC1 [210] A. Gasbarrini, A.B. Borle, D.H. Van Thiel, Ca2+ antagonists do not protect channel, Proc. Natl. Acad. Sci. U. S. A., 96, 1999, pp. 3934–3939. isolated perfused rat hepatocytes from anoxic injury, Biochim. Biophys. [205] B. Veldhuisen, L. Spruit, H.G. Dauwerde, M.H. Breuning, D.J. Peters, Acta 1177 (1993) 1–7. Genes homologous to the autosomal dominant polycystic kidney disease [211] R.K. Studer, A.B. Borle, Na+–Ca2+ antiporter activity of rat hepatocytes. genes (PKD1 and PKD2), Eur. J. Hum. Genet. 7 (1999) 860–872. Effect of adrenalectomy on Ca2+ uptake and release from plasma mem- [206] A.W. Ham, Textbook of Histology, 5th edLippincott, J.B., Philadelphia, brane vesicles, Biochim. Biophys. Acta 1134 (1992) 7–16. 1965. [212] L. Missiaen, L. Dode, J. Vanoevelen, L. Raeymaekers, F. Wuytack, [207] D.C. Spray, R.D. Ginzberg, E.A. Morales, Z. Gatmaitan, I.M. Arias, Calcium in the Golgi apparatus, Cell Calcium 41 (2007) 405–416.