Downloaded from http://cshperspectives.cshlp.org/ on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Calcium Signaling in Cardiac Myocytes Claire J. Fearnley1,3, H. Llewelyn Roderick1,2,3, and Martin D. Bootman1,3 1Laboratory of Signalling and Cell Fate, The Babraham Institute, Babraham, Cambridge CB22 3AT, United Kingdom 2Department of Pharmacology, University of Cambridge, Cambridge CB2 1PD, United Kingdom Correspondence: [email protected]; [email protected] Calcium (Ca2þ) is a critical regulator of cardiac myocyte function. Principally,Ca2þ is the link between the electrical signals that pervade the heart and contraction of the myocytes to propel blood. In addition, Ca2þ controls numerous other myocyte activities, including gene transcription. Cardiac Ca2þ signaling essentially relies on a few critical molecular players—ryanodine receptors, voltage-operated Ca2þ channels, and Ca2þ pumps/transport- ers. These moieties are responsible for generating Ca2þ signals upon cellular depolarization, recovery of Ca2þ signals following cellular contraction, and setting basal conditions. Whereas these are the central players underlying cardiac Ca2þ fluxes, networks of signaling mechanisms and accessory proteins impart complex regulation on cardiac Ca2þ signals. Subtle changes in components of the cardiac Ca2þ signaling machinery, albeit through mutation, disease, or chronic alteration of hemodynamic demand, can have profound con- sequences for the function and phenotype of myocytes. Here, we discuss mechanisms under- lying Ca2þ signaling in ventricular and atrial myocytes. In particular, we describe the roles and regulation of key participants involved in Ca2þ signal generation and reversal. OVERVIEW OF THE CARDIAC CYCLE contraction of these chambers, forcing blood into the ventricles. On reaching the atrioven- he mammalian heart is a complex organ tricular (AV) node, the depolarization pauses Tconsisting of four chambers—the left and for a short time period (0.1 s in humans) right atria and the left and right ventricles. to ensure completion of atrial systole. Impor- Through a highly coordinated series of events, tantly, the AVnode acts as an electrical insulator the muscular heart pumps blood through the between the atria and ventricles. The AV node pulmonary and systemic vasculature (Fukuta prevents the transfer of aberrant contraction and Little 2008). During diastole, all four cham- patterns to the ventricles, such as the spontane- bers are relaxed. Systole is initiated by prop- ous electrical activity occurring during atrial agation of a depolarizing action potential fibrillation. The lower portion of the AV node from the sino-atrial node located in the apex is designated the bundle of His, which then of the right atrium, through the right and then splits into the left and right branches, allow- the left atrium. This depolarization induces ing activation of the left and right ventricles, 3All three authors contributed equally to this article. Editors: Martin Bootman, Michael J. Berridge, James W. Putney, and H. Llewelyn Roderick Additional Perspectives on Calcium Signaling available at www.cshperspectives.org Copyright # 2011 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a004242 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004242 1 Downloaded from http://cshperspectives.cshlp.org/ on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press C.J. Fearnley et al. respectively. These branches give rise to thin et al. 2001; Berridge 2003). When Vm reaches filaments called Purkinje fibers, composed of a critical threshold (240 to 250 mV), plasma noncontractile cells that distribute the action membrane L-type Ca2þ channels are opened 2þ potential to ventricular myocytes and enable (ICa,L), allowing a large influx of Ca into the the heart to contract in a coordinated fash- cytosol and increasing the membrane potential ion. Transduction of the depolarization signal to þ10 mV. It is this depolarization signal through the His-Purkinje system causes ventric- that is transmitted from the SA node through ular systole. The contraction wave, traveling up the cardiac conduction system, culminating in from the ventricular base, expels blood into cardiac myocyte contraction. Within the SA the pulmonary artery then on to the lungs, or node cells, ICa,L activates an outward potassium through the aorta into the arterial system. Ret- current (IK) which hyperpolarizes the mem- rograde flow of blood is prevented by valves brane and curtails the action potential. The between the atria and ventricles. hyperpolarization leads to activation of If, As indicated above, the SA node situated T-type Ca2þ channels and Ca2þ sparks, to begin at the apex of the right atrium is responsible the next conduction cycle. for initiation of the cardiac action potential. At rest, the membrane potential starts around EXCITATION-CONTRACTION COUPLING –70mV (V ) and slowly depolarizes until an m (EC-COUPLING) action potential is triggered. Ca2þ signals may play a key role in action potential generation, EC-coupling is the process pairing myocyte although there is considerable debate regard- depolarization with mechanical contraction. ing the major mechanisms controlling the rate Ca2þ is the critical intermediary (Bers 2008). of SA node depolarization (see Lakatta and Indeed, since Ringer’s experiments more than DiFrancesco 2009). One primary component a century ago, Ca2þ has been known to be an of SA node depolarization is known as If (f essential mediator of this process (Ringer stands for funny) (Brown et al. 1979), an ion 1883). As the action potential sweeps over the current mediated by hyperpolarizing-activated heart, the plasma membrane (sarcolemma) of cyclic nucleotide-gated (HCN) channels (Di- each myocyte becomes depolarized (290 mV Francesco 1993). Because this current is trig- to þ20 mV) thereby causing concerted open- gered by hyperpolarization, it is activated at ing of L-type VOCCs (“long-lasting current;” 2þ the start of diastole and slowly declines Cav1.2). Ca flows via the VOCCs into a throughout the pacemaker period. HCN chan- restricted space between the sarcolemma and nels are relatively nonselective, and they there- the underlying sarcoplasmic reticulum (SR) fore generate an inward current depolarizing known as the “junctional zone” or “dyadic 2þ Vm toward the threshold for firing an action cleft.” The accumulation of Ca ions during potential. Intracellular Ca2þ cycling has also an action potential increases the Ca2þ con- been proposed to act as a primary regulator of centration within this microdomain from SA node depolarization. Imaging SA node cells 100 nM to 10 mM. This elementary Ca2þ reveals spontaneous elementary Ca2þ signals influx signal, derived from the activation of known as Ca2þ sparks arising from the SR and VOCCs is known as a “Ca2þ sparklet” (Fig. 1) preceding action potential generation (Huser (Wang et al. 2001). The distribution of Ca2þ et al. 2000). Ca2þ sparks reflect the concerted sparklet magnitudes suggests that one or sev- opening of a cluster of RyRs. The Ca2þ sparks eral VOCCs can give rise to such signals within activate sodium/calcium exchange (NCX), myocytes (Cheng and Wang 2002). which promotes membrane depolarization Ca2þ sparklets themselves are not adequate because three Naþ ions enter for each Ca2þ to cause substantial contraction. However, ion that leaves. T-type Ca2þ channels (“tran- they are sufficient to induce opening of RyRs sient current;” Cav3) may also provide a source (type 2 RyRs) on the closely apposed SR, via a of Ca2þ for triggering Ca2þ sparks (Bogdanov process known as “Ca2þ-induced Ca2þ release” 2 Cite this article as Cold Spring Harb Perspect Biol 2011;3:a004242 Downloaded from http://cshperspectives.cshlp.org/ on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Calcium Signaling in Cardiac Myocytes Ai L-type VOCC ii RyR distribution distribution Bi ii L-type VOCC iii RyR Diastole activation activation Action potential arrives- sarcolemmal depolarization Ca2+ sparklet Ca2+ spark T-tubule SR Dyadic cleft Ci ii Control myocyte Detubulated 12 12 myocyte Edge of myocyte Center of myocyte 8 8 4 4 emission (f/f0) emission (f/f0) Fluo4 fluorescence 0 Fluo4 fluorescence 0 0123 0123 Time (s) Time (s) Figure 1. Excitation contraction coupling in ventricular myocytes. Panel A illustrates the distribution of L-type VOCCs (Ai) and type 2 RyRs (Aii) in a section of a ventricular myocyte. The distributions of these proteins are essentially overlapping at the level of the light microscope. Panel B is a cartoon sequence of events leading to the generation of a Ca2þ signal within a ventricular myocyte. A small section of a ventricular myocyte is depicted with two T-tubule projections (T-tubule spacing 1.8 mm). During the diastolic phase (Bi), the L-type VOCCs (red channels on the T-tubule membranes) and RyRs (blue channels on SR membrane) are silent. Arrival of the action potential causes depolarization of the sarcolemma and activation of the L-type VOCCs thereby generating “Ca2þ sparklets” (Bii). The Ca2þ sparklets trigger activation of the RyRs thereby producing “Ca2þ sparks” (Biii). Panel Ci depicts the consistent, global Ca2þ responses observed in an electrically paced ventricular myocyte. The black and gray traces indicate the Ca2þ concentration (measured with fluo4) at the center and edge of the myo- cyte. The profile of the Ca2þ signal was essentially the same in both locations. Panel Cii illustrates