Mechanisms of rubidium permeation by rabbit cortical collecting duct during potassium restriction

The renal H+-K+-ATPases: physiology, regulation, and structure

The H(+)-K(+)-ATPases are ion pumps that use the energy of ATP hydrolysis to transport protons (H(+)) in exchange for potassium ions (K(+)). These enzymes consist of a catalytic alpha-subunit and a regulatory beta-subunit. There are two catalytic subunits present in the kidney, the gastric or HKalpha(1) isoform and the colonic or HKalpha(2) isoform. In this review we discuss new information on the physiological function, regulation, and structure of the renal H(+)-K(+)-ATPases. Evaluation of enzymatic functions along the nephron and collecting duct and studies in HKalpha(1) and HKalpha(2) knockout mice suggest that the H(+)-K(+)-ATPases may function to transport ions other than protons and potassium. These reports and recent studies in mice lacking both HKalpha(1) and HKalpha(2) suggest important roles for the renal H(+)-K(+)-ATPases in acid/base balance as well as potassium and sodium homeostasis. Molecular modeling studies based on the crystal structure of a related enzyme have made it possible to evaluate the structures of HKalpha(1) and HKalpha(2) and provide a means to study the specific cation transport properties of H(+)-K(+)-ATPases. Studies to characterize the cation specificity of these enzymes under different physiological conditions are necessary to fully understand the role of the H(+)-K(+) ATPases in renal physiology.

Regulation of potassium (K) handling in the renal collecting duct

This review provides an overview of the molecular mechanisms of K transport in the mammalian connecting tubule (CNT) and cortical collecting duct (CCD), both nephron segments responsible for the regulation of renal K secretion. Aldosterone and dietary K intake are two of the most important factors regulating K secretion in the CNT and CCD. Recently, angiotensin II (AngII) has also been shown to play a role in the regulation of K secretion. In addition, genetic and molecular biological approaches have further identified new mechanisms by which aldosterone and dietary K intake regulate K transport. Thus, the interaction between serum-glucocorticoid-induced kinase 1 (SGK1) and with-no-lysine kinase 4 (WNK4) plays a significant role in mediating the effect of aldosterone on ROMK (Kir1.1), an important apical K channel modulating K secretion. Recent evidence suggests that WNK1, mitogen-activated protein kinases such as P38, ERK, and Src family protein tyrosine kinase are involved in mediating the effect of low K intake on apical K secretory channels.

Inhibition of MAPK stimulates the Ca2+ -dependent big-conductance K channels in cortical collecting duct

The kidney plays a key role in maintaining potassium (K) homeostasis. K excretion is determined by the balance between K secretion and absorption in distal tubule segments such as the connecting tubule and cortical collecting duct. K secretion takes place by K entering principal cells (PC) from blood side through Na+, K+ -ATPase and being secreted into the lumen via both ROMK-like small-conductance K (SK) channels and Ca2+ -activated big-conductance K (BK) channels. K reabsorption occurs by stimulation of apical K/H-ATPase and inhibition of K recycling across the apical membrane in intercalated cells (IC). The role of ROMK channels in K secretion is well documented. However, the importance of BK channels in mediating K secretion is incompletely understood. It has been shown that their activity increases with high tubule flow rate and augmented K intake. However, BK channels have a low open probability and are mainly located in IC, which lack appropriate transporters for effective K secretion. Here we demonstrate that inhibition of ERK and P38 MAPKs stimulates BK channels in both PC and IC in the cortical collecting duct and that changes in K intake modulate their activity. Under control conditions, BK channel activity in PC was low but increased significantly by inhibition of both ERK and P38. Blocking MAPKs also increased channel open probability of BK in IC and thereby it may affect K backflux and net K absorption Thus, modulation of ERK and P38 MAPK activity is involved in controlling net K secretion in the distal nephron.

Regulation of renal K transport by dietary K intake

Extracellular K must be kept within a narrow concentration range for the normal function of neurons, skeletal muscle, and cardiac myocytes. Maintenance of normal plasma K is achieved by a dual mechanism that includes extrarenal factors such as insulin and beta-adrenergic agonists, which stimulate the movement of K from extracellular to intracellular fluid and modulate renal K excretion. Dietary K intake is an important factor for the regulation of K secretion: An increase in K intake stimulates secretion, whereas a decrease inhibits K secretion and enhances absorption. This effect of changes in dietary K intake on tubule K transport is mediated by aldosterone-dependent and -independent mechanisms. Recently, it has been demonstrated that the protein tyrosine kinase (PTK)-dependent signal transduction pathway is an important aldosterone-independent regulatory mechanism that mediates the effect of altered K intake on K secretion. A low-K intake stimulates PTK activity, which leads to increase in phosphorylation of cloned inwardly rectifying renal K (ROMK) channels, whereas a high-K intake has the opposite effect. Stimulation of tyrosine phosphorylation also suppresses K secretion in principal cell by facilitating the internalization of apical K channels in the collecting duct.

Increased CO(2) stimulates K/Rb reabsorption mediated by H-K-ATPase in CCD of potassium-restricted rabbit

Apical H-K-ATPase in the cortical collecting duct (CCD) plays an important role in urinary acidification and K reabsorption. Our previous studies demonstrated that an H-K-ATPase mediates, in part, Rb reabsorption in rabbit CCD (Zhou X and Wingo CS. Am J Physiol Renal Fluid Electrolyte Physiol 263: F1134-F1141, 1992). The purpose of these experiments was to examine using in vitro microperfused CCD from K-restricted rabbits 1) whether an acute increase in PCO(2) and, presumably, intracellular acidosis stimulate K absorptive flux; and 2) whether this stimulation was dependent on the presence of a functional H-K-ATPase. Rb reabsorption was significantly increased after exposure to 10% CO(2) in CCD, and this effect was persistent for the entire 10% CO(2) period, whereas 10 microM SCH-28080 in the perfusate totally abolished the stimulation of Rb reabsorption by 10% CO(2). After stimulation of Rb reabsorption by 10% CO(2), subsequent addition of 0.1 mM methazolamide, an inhibitor of carbonic anhydrase, failed to affect Rb reabsorption. However, simultaneous exposure to 10% CO(2) and methazolamide prevented the stimulation of Rb reabsorption. Treatment with the intracellular calcium chelator MAPTAM (0.5 microM) inhibited the stimulation of Rb reabsorption by 10% CO(2). Similar inhibition was also observed in the presence of either a calmodulin inhibitor, W-7 (0.5 microM), or colchicine (0.5 mM), an inhibitor of tubulin polymerization. In time control studies, the perfusion time did not significantly affect Rb reabsorption. We conclude the following: 1) stimulation of Rb reabsorption on exposure to 10% CO(2) is dependent on the presence of a functional H-K-ATPase and appears to be regulated in part by the insertion of this enzyme into the apical plasma membrane by exocytosis; 2) insertion of H-K-ATPase requires changes in intracellular pH and needs a basal level of intracellular calcium concentration; and 3) H-K-ATPase insertion occurs by a microtubule-dependent process.

Activation of H(+)-K(+)-ATPase by CO(2) requires a basolateral Ba(2+)-sensitive pathway during K restriction

We studied the activation of H(+)-K(+)-ATPase by CO(2) in the renal cortical collecting duct (CCD) of K-restricted animals. Exposure of microperfused CCD to 10% CO(2) increased net total CO(2) flux (J(t CO(2))) from 4.9 +/- 2.1 to 14.7 +/- 4 pmol. mm(-1). min(-1) (P < 0. 05), and this effect was blocked by luminal application of the H(+)-K(+)-ATPase inhibitor Sch-28080. In the presence of luminal Ba, a K channel blocker, exposure to CO(2) still stimulated J(t CO(2)) from 6.0 +/- 1.0 to 16.8 +/- 2.8 pmol. mm(-1). min(-1) (P < 0.01), but peritubular application of Ba inhibited the stimulation. CO(2) substantially increased (86)Rb efflux (a K tracer marker) from 93.1 +/- 23.8 to 249 +/- 60.2 nm/s (P < 0.05). These observations suggest that during K restriction 1) the enhanced H(+)-K(+)-ATPase-mediated acidification after exposure to CO(2) is dependent on a basolateral Ba-sensitive mechanism, which is different from the response of rabbits fed a normal-K diet, where activation of the H(+)-K(+)-ATPase by exposure to CO(2) is dependent on an apical Ba-sensitive pathway; and 2) K/Rb absorption via the apical H(+)-K(+)-ATPase exits through a basolateral Ba-sensitive pathway. Together, these data are consistent with the hypothesis of cooperation between H(+)-K(+)-ATPase-mediated acidification and K exit pathways in the CCD that regulate K homeostasis.

Potassium depletion increases proton pump (H(+)-ATPase) activity in intercalated cells of cortical collecting duct

Intercalated cells (ICs) from kidney collecting ducts contain proton-transporting ATPases (H(+)-ATPases) whose plasma membrane expression is regulated under a variety of conditions. It has been shown that net proton secretion occurs in the distal nephron from chronically K(+)-depleted rats and that upregulation of tubular H(+)- ATPase is involved in this process. However, regulation of this protein at the level of individual cells has not so far been examined. In the present study, H(+)-ATPase activity was determined in individually identified ICs from control and chronically K(+)-depleted rats (9-14 days on a low-K(+) diet) by monitoring K(+)- and Na(+)-independent H(+) extrusion rates after an acute acid load. Split-open rat cortical collecting tubules were loaded with the intracellular pH (pH(i)) indicator 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and pH(i) was determined by using ratiometric fluorescence imaging. The rate of pH(i) recovery in ICs in response to an acute acid load, a measure of plasma membrane H(+)-ATPase activity, was increased after K(+) depletion to almost three times that of controls. Furthermore, the lag time before the start of pH(i) recovery after the cells were maximally acidified fell from 93.5 +/- 13.7 s in controls to 24.5 +/- 2.1 s in K(+)-depleted rats. In all ICs tested, Na(+)- and K(+)-independent pH(i) recovery was abolished in the presence of bafilomycin (100 nM), an inhibitor of the H(+)-ATPase. Analysis of the cell-to-cell variability in the rate of pH(i) recovery reveals a change in the distribution of membrane-bound proton pumps in the IC population of cortical collecting duct from K(+)-depleted rats. Immunocytochemical analysis of collecting ducts from control and K(+)-depleted rats showed that K(+)-depletion increased the number of ICs with tight apical H(+)ATPase staining and decreased the number of cells with diffuse or basolateral H(+)-ATPase staining. Taken together, these data indicate that chronic K(+) depletion induces a marked increase in plasma membrane H(+)ATPase activity in individual ICs.

Metabolic acidosis stimulates H+ secretion in the rabbit outer medullary collecting duct (inner stripe) of the kidney

The outer medullary collecting duct (OMCD) absorbs HCO3- at high rates, but it is not clear if it responds to metabolic acidosis to increase H+ secretion. We measured net HCO3- transport in isolated perfused OMCDs taken from deep in the inner stripes of kidneys from control and acidotic (NH4Cl-fed for 3 d) rabbits. We used specific inhibitors to characterize the mechanisms of HCO3- transport: 10 microM Sch 28080 or luminal K+ removal to inhibit P-type H+,K+-ATPase activity, and 5-10 nM bafilomycin A1 or 1-10 nM concanamycin A to inhibit H+-ATPase activity. The results were comparable using either of each pair of inhibitors, and allowed us to show in control rabbits that 65% of net HCO3- absorption depended on H+-ATPase (H flux), and 35% depended on H+,K+-ATPase (H,K flux). Tubules from acidotic rabbits showed higher rates of HCO3- absorption (16.8+/-0.3 vs. 12.8+/-0.2 pmol/min per mm, P < 0.01). There was no difference in the H,K flux (5.9+/-0.2 vs. 5.8+/-0.2 pmol/min per mm), whereas there was a 61% higher H flux in segments from acidotic rabbits (11.3+/-0.2 vs. 7.0+/-0.2 pmol/min per mm, P < 0.01). Transport was then measured in other OMCDs before and after incubation for 1 h at pH 6.8, followed by 2 h at pH 7.4 (in vitro metabolic acidosis). Acid incubation in vitro stimulated HCO3- absorption (12.3+/-0.3 to 16.2+/-0.3 pmol/min per mm, P < 0.01), while incubation at pH 7.4 for 3 h did not change basal rate (11.8+/-0.4 to 11.7+/-0.4 pmol/min per mm). After acid incubation the H,K flux did not change, (4.7+/-0.4 to 4.6+/-0.4 pmol/min per mm), however, there was a 60% increase in H flux (6.6+/-0.3 to 10.8+/-0.3 pmol/min per mm, P < 0.01). In OMCDs from acidotic animals, and in OMCDs incubated in acid in vitro, there was a higher basal rate and a further increase in HCO3- absorption (16.7+/-0.4 to 21.3+/-0.3 pmol/min per mm, P < 0.01) because of increased H flux (11.5+/-0.3 to 15.7+/-0.2 pmol/min per mm, P < 0.01) without any change in H,K flux (5.4+/-0.3 to 5.6+/-0.3 pmol/min per mm). These data indicate that HCO3- absorption (H+ secretion) in OMCD is stimulated by metabolic acidosis in vivo and in vitro by an increase in H+-ATPase-sensitive HCO3- absorption. The mechanism of adaptation may involve increased synthesis and exocytosis to the apical membrane of proton pumps. This adaptation helps maintain homeostasis during metabolic acidosis.