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Respective Roles of H-Atpase and H-K-Atpase in Ion Transport in the Kidney1

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Respective Roles of H-Atpase and H-K-Atpase in Ion Transport in the Kidney1 Respective Roles of H-ATPase and H-K-ATPase in Ion Transport in the Kidney1 Lal C. Garg2 monia and titrable acid. Both of these functions are L.C. Garg, Department of Pharmacology and Thera- achieved by the acidification of the tubule fluid. Most peutics and Division of Nephrology, Hypertension and of the filtered bicarbonate is reabsorbed in the prox- Transplantation, University of Florida College of Medi- imal tubule. The acidification of the luminal fluid in cine, Gainesville, FL the distal tubule plays an important role in the trans- port of total ammonia and in the titration of phos- 1991; (J. Am. Soc. Nephrol. 2:949-960) phate and other buffers. During the last several years, it has been established that acidification of the tubule fluid is mediated, at least in part, by an ABSTRACT electrogenic proton pump (H-ATPase) that is located in the luminal membrane of renal tubule cells in both Two types of proton-translocating ATPases, H-ATPase proximal and distal nephron segments, including the and H-K-ATPase, are found in the renal tubular cells. collecting duct. Recent evidence suggests that an H-ATPase is present in both endocytic vesicles and electroneutral H-K-ATPase present in the collecting apical membranes In almost all nephron segments. duct may play a role in the acidification of tubule On the other hand, H-K-ATPase is present only in the fluid. In this article, we will review the biochemical, connecting tubule and collecting duct. There is evi- immunohistological, and physiological evidence for dence to suggest that H-ATPase may be involved the respective roles of H-ATPase and H-K-ATPase in In H secretion in almost all nephron segments. ion transport in various nephron segments. we will H-K-ATPase is involved not only in H secretion but also also consider here the effects of certain physiological in K absorption in the collecting duct segments. Al- and pharmacological factors on the two enzymes. dosterone administration and metabolic acidosis The synthesis and transport of ammonia, which also play an important role in maintaining the acid-base stimulate the activity of H-ATPase in all collecting balance, will not be discussed here. Several excellent duct segments, whereas hypokalemia has only a and recent reviews are available on this topic (1,2). lImited effect on H-ATPase activity. On the other hand, hypokalemia, as well as metabolic acidosis, stimulates H-K-ATPase activity in the collecting duct GENERAL PROPERTIES OF DIFFERENT TYPES OF segments, whereas aldosterone administration H-ATPases alone plays a minor role in the regulation of this There are three types of H-ATPases. The FO-Fi enzyme. The physiological role and regulation of H- type of H-ATPase is present in the mitochondria ATPase in the proximal tubule has not been estab- where it acts as ATP synthase. This type of H-ATPase lished. is also present in bacterial membranes and in chlo- Key Words: H-A TPase, H-K-A TPase, nephron segments, acid roplasts (3). It is composed of 8 to 14 polypeptides. secretion, potassium transport, aldosterone The mammalian enzyme is inhibited by oligomycin, azide, and low concentrations of dicyclohexylcarbo- diimide (DCCD), but not by ouabain (inhibitor of Na- o maintain acid-base balance, the human kid- T K-ATPase) or vanadate (inhibitor of El -E2 type ney not only reabsorbs the filtered bicarbonate ATPases, including Na-K-ATPase). but also generates bicarbonate by excretion of am- The second type of H-ATPase is present in the membranes of intracellular organelles, such as en- Received June 4, 1991. Accepted July 22, 1991. dosomes, lysosomes, and clathrin-coated vesicles (4), 2Correspondence to Dr. L.C. Garg, Department of Pharmacology and Thera- peutics, University of Florida College of Medicine, Health Science Center, P.O. and it acidifies the interior of these organelles. Be- Box 100267, GainesvIlle, FL 32610-0267. cause this H-ATPase is present in the intracellular 1046-6673/0205-0949$03.O0/0 vacuoles, it has been called vacuolar (or V) H-ATPase Journal of the American Society of Nephrology Copyright C 1991 by the American Society of Nephrology (5). The vacuolar H-ATPase is also present in the Journal of the American Society of Nephrology 949 H-ATPase and H-K-ATPase plasma membranes of subpopulations of cells in the differences in the enzyme activity of V type mammalian collecting duct and turtle urinary blad- H-ATPases in different organelles may also be be- der where it is responsible for urinary acidification cause of secondary factors present in the organelles. (6,7). In addition, this type of H-ATPase is located in These include the presence or absence of a Cl chan- synaptic vesicles and chromaffin granules (8). The V nd and the lipid composition of the organelle mem- type H-ATPases are composed of nine or more differ- brane (8). The acidification by V type H-ATPase in ent polypeptides. They contain more cystelne resi- various organelles may be controlled by the primary dues than mitochondrial H-ATPase and are Inhibited and secondary factors mentioned above. In contrast, by low concentrations of N-ethylmaleimide (NEM). a the acidification of urine by V type H-ATPase is sulfhydryl alkylating agent (9). In addition, V type probably controlled by the rates of enzyme biogenesis H-ATPases are inhibited by higher concentrations of and by enzyme insertion into and retrieval from the DCCD than is the mitochondrial H-ATPase (ATP syn- plasma membrane (7). There is evidence that the thase) (5). They are not inhibited by azide, ouabain, activity of the H-ATPase and its insertion into and or vanadate. retrieval from the plasma membrane in the collecting The functions of V type H-ATPases in various spe- duct are influenced by adrenal hormones and cies and in various organs in mammals have been changes in acid-base balance (discussed below). reviewed recently (3,5,8). In brief, the major function The third type of H-ATPase is characteristic of the of V type H-ATPases is to acidify the interior of mammalian gastric mucosa (13). It belongs to the El- intracellular organelles. This acidification leads to E2 type of ATPases that form phosphorylated inter- activation of several secondary processes. For cx- mediates and are vanadate sensitive. It is composed ample, it has been demonstrated In several tissues of two subunits and is inhibited by omeprazole and that the acidification of endosomes is important for an imidazopyridine compound (SCH 28080). It resem- the dissociation of ligands from receptors after recep- bles Na-K-ATPase but is not inhibited by ouabain. tor-mediated endocytosis and the subsequent recy- This type of H-ATPase is also present in certain cling of the receptors (5). Under normal conditions, fungi. Recently, ouabain-insensitive K-ATPase activ- endocytosed ligands such as low-density lipopro- ity has been demonstrated in the mammalian col- teins, Insulin, and epidermal growth factor dissociate lecting duct (14,15). The renal K-ATPase is pharma- from their receptors in the acidic pH of the endo- cologically similar to gastric H-K-ATPase (15). somes. However, treatment of cells with weak bases, The gastric H-K-ATPase is responsible for trans- such as NH4C1 or chloroquine, that neutralize acidic port of H into the lumen in exchange for K. This compartments in the cells results in accumulation of transport process is accompanied by a back diffusion ligand-receptor complexes in the endosomes and a of K into the lumen via a K channel and transport of depletion of cell surface receptors. Similarly, the Cl from cell to the lumen via an apical Cl channel. maintenance of an acidic pH within lysosomes is These processes lead to net secretion of HC1 into the necessary for optimum activity of lysosomal hydro- lumen (16). The recognition of the role of H-K-ATPase lases (10). in gastric acid secretion led to the development of The proton-motive force generated by V type omeprazole (PRILOSEC; Merck Sharp & Dohme, H-ATPases in synaptic vesicles and chromaffin West Point, PA), which is used in pathological hy- granules is used for the uptake and storage of neu- persecretory conditions such as Zollinger-Ellison rotransmitters (8). The mechanism of neurotrans- syndrome and in gastroesophageal reflux diseases mitter uptake may vary with the transporter of the (13). The physiological role of renal H-K-ATPase in H neurotransmitter. Thus, like Na-K-ATPase, V type and K transport is discussed in a later section. H-ATPases provide energy for several secondary processes. In fact, it has been suggested that the DISTRIBUTION OF H-ATPase ALONG THE NEPHRON nervous system cannot function without V type H- ATPases because of a lack of accumulation of neu- The distribution of H-ATPase in the various neph- rotransmitters in the absence of H-ATPase (8). ron segments has been studied by both biochemical V type H-ATPases isolated from different kidney and immunohistochemical techniques. Because NEM membrane compartments have been shown to differ has been used as an inhibitor of the V type H-ATPase, in their enzymatic and structural properties. For ex- the enzyme is also called NEM-sensitive ATPase. ample, two H-ATPases isolated from brush borders We will use the term NEM-sensitive ATPase and and microsomes of the bovine kidney differ in their H-ATPase interchangeably. Figure 1 illustrates the 56K subunit, substrate specificity. pH optimum, and locations of the major nephron segments in which sensitivity to inhibition by Cu2’ (11). It has been enzyme activities have been measured in both the rat suggested that the activity of V type H-ATPase In vlvo and rabbit. The distribution of NEM-sensitive may be regulated by two cytosolic proteins, a stimu- ATPase activity along the rabbit nephron is given in latory protein and an inhibitory protein (12).
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