Potassium permeable channels in primary cultures of rabbit cortical collecting tubule

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.

Effect of aldosterone on BK channel expression in mammalian cortical collecting duct

Apical large-conductance Ca(2+)-activated K(+) (BK) channels in the cortical collecting duct (CCD) mediate flow-stimulated K(+) secretion. Dietary K(+) loading for 10-14 days leads to an increase in BK channel mRNA abundance, enhanced flow-stimulated K(+) secretion in microperfused CCDs, and a redistribution of immunodetectable channels from an intracellular pool to the apical membrane (Najjar F, Zhou H, Morimoto T, Bruns JB, Li HS, Liu W, Kleyman TR, Satlin LM. Am J Physiol Renal Physiol 289: F922-F932, 2005). To test whether this adaptation was mediated by a K(+)-induced increase in aldosterone, New Zealand White rabbits were fed a low-Na(+) (LS) or high-Na(+) (HS) diet for 7-10 days to alter circulating levels of aldosterone but not serum K(+) concentration. Single CCDs were isolated for quantitation of BK channel subunit (total, alpha-splice variants, beta-isoforms) mRNA abundance by real-time PCR and measurement of net transepithelial Na(+) (J(Na)) and K(+) (J(K)) transport by microperfusion; kidneys were processed for immunolocalization of BK alpha-subunit by immunofluorescence microscopy. At the time of death, LS rabbits excreted no urinary Na(+) and had higher circulating levels of aldosterone than HS animals. The relative abundance of BK alpha-, beta(2)-, and beta(4)-subunit mRNA and localization of immunodetectable alpha-subunit were similar in CCDs from LS and HS animals. In response to an increase in tubular flow rate from approximately 1 to 5 nl.min(-1).mm(-1), the increase in J(Na) was greater in LS vs. HS rabbits, yet the flow-stimulated increase in J(K) was similar in both groups. These data suggest that aldosterone does not contribute to the regulation of BK channel expression/activity in response to dietary K(+) loading.

BK channels in the kidney: role in K(+) secretion and localization of molecular components

Although it is generally accepted that ROMK is the K(+) secretory channel in the mammalian distal nephron, recent in vitro and in vivo studies have provided evidence that large-conductance Ca(2+)-activated K(+) channels (BK, or maxi K) also secrete K(+) in renal tubules. This review assesses the current evidence relating BK channels with K(+) secretion. We shall consider the component proteins of the BK channel, their localization with respect to segment and cell type, and the electrophysiological forces involved in K(+) secretion. Although the majority of studies have focused on a role for BK channels in flow-mediated K(+) secretion, this review also considers a potential role for BK channels in high-K diet-induced K(+) secretion. The division of workload between ROMK and BK is discussed as a mechanism for ensuring a constant plasma K(+) concentration.

Molecular diversity and regulation of renal potassium channels

K(+) channels are widely distributed in both plant and animal cells where they serve many distinct functions. K(+) channels set the membrane potential, generate electrical signals in excitable cells, and regulate cell volume and cell movement. In renal tubule epithelial cells, K(+) channels are not only involved in basic functions such as the generation of the cell-negative potential and the control of cell volume, but also play a uniquely important role in K(+) secretion. Moreover, K(+) channels participate in the regulation of vascular tone in the glomerular circulation, and they are involved in the mechanisms mediating tubuloglomerular feedback. Significant progress has been made in defining the properties of renal K(+) channels, including their location within tubule cells, their biophysical properties, regulation, and molecular structure. Such progress has been made possible by the application of single-channel analysis and the successful cloning of K(+) channels of renal origin.

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.

The cytoplasmic tail of large conductance, voltage- and Ca2+-activated K+ (MaxiK) channel is necessary for its cell surface expression

The large conductance, voltage- and Ca(2+)-activated K(+) channel (MaxiK) is expressed in several renal segments and functions in cell volume regulation and flow-mediated K(+) secretion. Previously, we cloned two MaxiK channel isoforms, named rbslo1 and rbslo2, from rabbit renal cells. rbslo1 has a 58-amino acid insertion after the S8 hydrophobic domain, whereas rbslo2 is truncated and cannot be activated. Here we use the sequence differences between the two variants to examine their plasma membrane processing. Plasma membrane localization of rbslo1 and 2 expressed in HEK293 cells was assayed by electrophysiology, immunocytochemistry, and biochemistry studies. Consistent with its functional silence, rbslo2 localized primarily within the cytoplasm, presumably in the endoplasmic reticulum and Golgi region. Coexpression with MaxiK beta subunits did not alter the cellular localization of either rbslo1 or rbslo2. When rbslo1 and 2 are cotransfected in non-polarized cells, they colocalized primarily within the cell with only rbslo1 detected at the plasma membrane. When transfected into polarized, medullary-thick ascending limb (mTAL) cells, rbslo1 is expressed at the apical membrane whereas the majority of rbslo2 localized throughout the cytoplasm. Given the high degree of similarity between the two isoforms, we conclude that the cytoplasmic tail of rbslo1 is important for the cell surface expression of MaxiK channels.

Invited review: biophysical properties of sodium channels in lung alveolar epithelial cells

Amiloride-sensitive sodium channels in the lung play an important role in lung fluid balance. Particularly in the alveoli, sodium transport is closely regulated to maintain an appropriate fluid layer on the surface of the alveoli. Alveolar type II cells appear to play an important role in this sodium transport, with the role of alveolar type I cells being less clear. In alveolar type II cells, there are a variety of different amiloride-sensitive, sodium-permeable channels. This significant diversity appears to play a role in both normal lung physiology and in pathological states. In many epithelial tissues, amiloride-sensitive epithelial sodium channels (ENaC) are formed from three subunit proteins, designated alpha-, beta-, and gamma-ENaC. At least part of the diversity of sodium-permeable channels in lung arises from the assembling of different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. This leads to epithelial tissue in the lung, which has enormous flexibility to alter the magnitude and regulation of salt and water transport. In this review, we discuss the biophysical properties and occurrence of these various channels and some of the mechanisms for their regulation.

K depletion increases protein tyrosine kinase-mediated phosphorylation of ROMK

We purified His-tagged ROMK1 and carried out in vitro phosphorylation assays with (32)P-radiolabeled ATP to determine whether ROMK1 protein is a substrate for PTK. Addition of active c-Src and [(32)P]ATP to the purified ROMK1 protein resulted in the phosphorylation of the ROMK1 protein. However, c-Src did not phosphorylate R1Y337A in which tyrosine residue 337 was mutated to alanine. Furthermore, phosphopeptide mapping identified two phosphopeptides from the trypsin-digested ROMK1 protein. In contrast, no phosphorylated peptide has been found in the trypsin-digested R1Y337A protein. This suggested that two phosphorylated peptides might contain the same tyrosine residue. Also, addition of c-Src and [(32)P]ATP phosphorylated the synthesized peptide corresponding to amino acid sequence 333-362 of the COOH terminus of ROMK1. We then examined the effect of dietary K intake on the tyrosine-phosphorylated ROMK level. Although the ROMK channels pulled down by immunoprecipitation with ROMK antibody were the same from rats on a K-deficient diet or on a high-K diet, more ROMK channels were phosphorylated by PTK in rats on a K-deficient diet than those on a high-K diet. We conclude that ROMK1 can be phosphorylated by PTK and that tyrosine residue 337 is the key site for the phosphorylation. Also, the tyrosine phosphorylation of ROMK is modulated by dietary K intake. This strongly suggests that PTK is an important member of the aldosterone-independent signal transduction pathway for regulating renal K secretion.

Hyperkalemia, renal failure, and converting-enzyme inhibition: an overrated connection

Hyperkalemia is widely viewed as a common complication of ACE inhibition in azotemic patients. These renal failure patients are the patients who benefit most from ACE inhibition. Because we could not confirm this notion after a retrospective evaluation of 236 azotemic patients, we studied 2 models of renal mass reduction. In the first, we did a 5/6 nephrectomy (Nx) on rats and studied them 2 weeks after surgery (before chronic renal changes had developed). A second group was studied 16 weeks after Nx, once chronic renal failure was established. Rats in both models were treated with quinapril in drinking water. After baseline evaluation, we challenged them either by a high-K(+) diet or by blocking aldosterone receptors. We found that although quinapril blocked the K(+)-induced increase in aldosterone, serum K(+) levels and K(+) balance were maintained before and during high K(+) intake or during simultaneous spironolactone administration. We conclude that in hemodynamically stable rats with reduced renal mass and renal dysfunction, the administration of an ACE inhibitor does not cause severe hyperkalemia.

Effect of dietary K intake on apical small-conductance K channel in CCD: role of protein tyrosine kinase

We have used Western blot to examine the expression of cSrc protein tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP)-1D in the renal cortex, and the patch-clamp technique to determine the role of PTK in mediating the effect of dietary K intake on the small-conductance K (SK) channel in the cortical collecting duct (CCD). When rats were on a K-deficient (KD) diet for 1, 3, 5, and 7 days, the expression of cSrc increased by 40, 90, 140, and 135%, respectively. In contrast, the expression of cSrc in the renal cortex from rats on a high-K (HK) diet for 1, 2, and 3 days decreased by 40, 60, and 75%, respectively. However, the protein level of PTP-1D was not significantly changed by dietary K intake. The addition of 1 microM herbimycin A increased NP(o), a product of channel number (N) and open probability (P(o)) in the CCD from rats on a normal diet or on a KD diet. The increase in NP(o) was 0.30 (normal), 0.45 (1-day KD), 0.65 (3-day KD), 1.55 (5-day KD), and 1.85 (7-day KD), respectively. Treatment of the CCD with herbimycin A from rats on a KD diet increased NP(o) per patch from the control value (0.7) to 1.4 (1-day KD), 1.6 (3-day KD), 2.6 (5-day KD), and 3.5 (7-day KD), respectively. In contrast, HK intake for as short as 1 day abolished the effect of herbimycin A. Furthermore, the expression of ROMK channels in the renal cortex was the same between rats on a KD diet or on a HK diet. Moreover, treatment with herbimycin A did not further increase NP(o) in the CCDs from rats on a HK diet. We conclude that dietary K intake plays a key role in regulating the activity of the SK channels and that PTK is involved in mediating the effect of the K intake on channel activity in the CCD.

Apical membrane of native OMCD(i) cells has nonselective cation channels

The purpose of this study was to examine cation channel activity in the apical membrane of the outer medullary collecting duct of the inner stripe (OMCD(i)) using the patch-clamp technique. In freshly isolated and lumen-opened rabbit OMCD(i), we have observed a single channel conductance of 23.3 +/- 0.6 pS (n = 17) in cell-attached (c/a) patches with high KCl in the bath and in the pipette at room temperature. Channel open probability varied among patches from 0.06 +/- 0.01 at -60 mV (n = 5) to 0.31 +/- 0.04 at 60 mV (n = 6) and consistently increased upon membrane depolarization. In inside-out (i/o) patches with symmetrical KCl solutions, the channel conductance (22.8 +/- 0.8 pS; n = 10) was similar as in the c/a configuration. Substitution of the majority of Cl- with gluconate from KCl solution in the pipette and bath did not significantly alter reversal potential (E(rev)) or the channel conductance (19.7 +/- 1.1 pS in asymmetrical potassium gluconate, n = 4; 21.4 +/- 0.5 pS in symmetrical potassium gluconate, n = 3). Experiments with 10-fold lower KCl concentration in bath solution in i/o patches shifted E(rev) to near the E(rev) of K+. The estimated permeability of K+ vs. Cl- was over 10, and the conductance was 13.4 +/- 0.1 pS (n = 3). The channel did not discriminate between K+ and Na+, as evidenced by a lack of a shift in the E(rev) with different K+ and Na+ concentration solutions in i/o patches (n = 3). The current studies demonstrate the presence of cation channels in the apical membrane of native OMCD(i) cells that could participate in K+ secretion or Na+ absorption.

Flow-dependent K+ secretion in the cortical collecting duct is mediated by a maxi-K channel

K+ secretion by the cortical collecting duct (CCD) is stimulated at high flow rates. Patch-clamp analysis has identified a small-conductance secretory K+ (SK) and a high-conductance Ca(2+)-activated K+ (maxi-K) channel in the apical membrane of the CCD. The SK channel, encoded by ROMK, is believed to mediate baseline K+ secretion. The role of the stretch- and Ca2+-activated maxi-K channel is still uncertain. The purpose of this study was to identify the K+ channel mediating flow-dependent K+ secretion in the CCD. Segments isolated from New Zealand White rabbits were microperfused in the absence and presence of luminal tetraethylammonium (TEA) or charybdotoxin, both inhibitors of maxi-K but not SK channels, or apamin, an inhibitor of small-conductance maxi-K+ channels. Net K+ secretion and Na+ absorption were measured at varying flow rates. In the absence of TEA, net K+ secretion increased from 8.3 +/- 1.0 to 23.4 +/- 4.7 pmol. min(-1). mm(-1) (P < 0.03) as the tubular flow rate was increased from 0.5 to 6 nl. min(-1). mm(-1). Flow stimulation of net K+ secretion was blocked by luminal TEA (8.2 +/- 1.2 vs. 9.9 +/- 2.7 pmol. min(-1). mm(-1) at 0.6 and 6 nl. min(-1). mm(-1) flow rates, respectively) or charybdotoxin (6.8 +/- 1.6 vs. 8.3 +/- 1.6 pmol. min(-1). mm(-1) at 1 and 4 nl. min(-1). mm(-1) flow rates, respectively) but not by apamin. These results suggest that flow-dependent K+ secretion is mediated by a maxi-K channel, whereas baseline K+ secretion occurs through a TEA- and charybdotoxin-insensitive SK (ROMK) channel.

Protein-tyrosine phosphatase reduces the number of apical small conductance K+ channels in the rat cortical collecting duct

Previous studies have demonstrated that an increase in the activity of protein-tyrosine kinase (PTK) is involved in the down-regulation of the activity of apical small conductance K(+) (SK) channels in the cortical collecting duct (CCD) from rats on a K(+)-deficient diet (). We used the patch clamp technique to investigate the role of protein-tyrosine phosphatase (PTP) in the regulation of the activity of SK channels in the CCD from rats on a high K(+) diet. Western blot analysis indicated that PTP-1D is expressed in the renal cortex. Application of 1 microm phenylarsine oxide (PAO) or 1 mm benzylphosphonic acid, agents that inhibit PTP, reversibly reduced channel activity by 95%. Pretreatment of CCDs with PAO for 30 min decreased the mean NP(o) reversibly from control value 3.20 to 0.40. Addition of 1 microm herbimycin A, an inhibitor of PTK, had no significant effect on channel activity in the CCDs from rats on a high K(+) diet. However, herbimycin A abolished the inhibitory effect of PAO, indicating that the effect of PAO is the result of interaction between PTK and PTP. Addition of brefeldin A, an agent that blocks protein trafficking from Golgi complex to the membrane, had no effect on channel activity. Moreover, application of colchicine, a microtubule inhibitor, or paclitaxel, a microtubule stabilizer, had no effect on channel activity. In contrast, PAO still reduced channel activity in the presence of brefeldin A, colchicine, or paclitaxel. Furthermore, the effect of PAO on channel activity was absent when the tubules were bathed in 16% sucrose-containing bath solution or treated with concanavalin A. We conclude that PTP is involved in the regulation of the activity of SK channels and that inhibition of PTP may facilitate the internalization of the SK channels.

Protein tyrosine kinase regulates the number of renal secretory K channels

The apical small conductance (SK) channel plays a key role in K secretion in the cortical collecting duct (CCD). A high-K intake stimulates renal K secretion and involves a significant increase in the number of SK channels in the apical membrane of the CCD. We used the patch-clamp technique to examine the role of protein tyrosine kinase (PTK) in regulating the activity of SK channels in the CCD. The application of 100 microM genistein stimulated SK channels in 11 of 12 patches in CCDs from rats on a K-deficient diet, and the mean increase in NP(o), a product of channel number (N) and open probability (P(o)), was 2.5. In contrast, inhibition of PTK had no effect in tubules from animals on a high-K diet in all 10 experiments. Western blot analysis further shows that the level of cSrc, a nonreceptor type of PTK, is 261% higher in the kidneys from rats on a K-deficient diet than those on a high-K diet. However, the effect of cSrc was not the result of direct inhibition of channel itself, because addition of exogenous cSrc had no effect on SK channels in inside-out patches. In cell-attached patches, application of herbimycin A increased channel activity in 14 of 16 patches, and the mean increase in NP(o) was 2.4 in tubules from rats on a K-deficient diet. In contrast, herbimycin A had no effect on channel activity in any of 15 tubules from rats on a high-K diet. Furthermore, herbimycin A pretreatment increased NP(o) per patch from the control value (0.4) to 2.25 in CCDs from rats on a K-deficient diet, whereas herbimycin failed to increase channel activity (NP(o): control, 3.10; herbimycin A, 3.25) in the CCDs from animals on a high-K diet. We conclude that PTK is involved in regulating the number of apical SK channels in the kidney.

Potassium secretion and the regulation of distal nephron K channels

K-selective channels in the luminal membranes of distal nephron segments form a key pathway for the secretion of K ions into the urine. This process is important to the control of K balance, particularly under conditions of normal or high K intake. This brief review will cover three issues: 1) the identification of apical K channels, 2) the role of these channels in the maintenance of K homeostasis, and 3) the role of aldosterone in this regulatory process. The large amount of literature on renal K transport has been elegantly summarized in a recent review in this journal [G. Giebisch. Am. J. Physiol. 274 (Renal Physiol. 43): F817-F833, 1998]. Here I will focus on a few prominent unsolved problems.

Regulation of apical K channels in rat cortical collecting tubule during changes in dietary K intake

Long-term adaptation to a high-K diet is known to increase the density of conducting secretory K (SK) channels in the luminal membrane of the rat cortical collecting tubule (CCT). To examine whether these channels are involved in the short-term, day-to-day regulation of K secretion, we examined the density of K channels in animals fed a high-K diet for 6 or 48 h. CCTs were isolated and split open to provide access to the luminal membrane. Cell-attached patches were formed on principal cells with 140 mM KCl in the patch-clamp pipette. SK channels were recognized from their characteristic single-channel conductance (40-50 pS) and gating patterns. Animals fed a control diet had SK channel densities of 0.40 channels/micrometer(2). When the diet was changed for one containing 10% KCl for 6 h, the channel density increased to 1.51 channels/micrometer(2). Maintaining the animals on a high-K diet for 48 h resulted in a further increase in SK channels to 2.29 channels/micrometer(2). Animals fed a low-K diet for 5 days or longer had SK densities of 0.53 channels/micrometer(2), not significantly different from control values. The presence of conducting Na channels in the luminal membrane will also affect K secretion by the CCT by altering the electrical driving force through the K channels. The density of Na channels, measured with LiCl in the pipette, was 0. 08 for controls and 1.00 and 1.08 channels/micrometer(2) after 6 h and 48 h on a high-K diet. Plasma aldosterone increased from 15 +/- 4 ng/dl (controls ) to 36 +/- 8 and 98 +/- 23 ng/dl after 6 and 48 h of K loading, respectively. The increase in K channel density could not be reproduced by infusion of the animals with aldosterone. We conclude that regulation of the density of conducting Na and K channels may contribute to day-to-day variation in the rate of renal K secretion and to the short-term maintenance of K balance.

Potassium channel activity recorded from the apical membrane of freshly isolated epithelial cells in rat caudal epididymis

K+ channels were recorded in excised, inside-out patches from the apical membrane of the freshly isolated tubule of the caudal portion of the rat epididymis. With asymmetric K+ concentrations in bath and pipette (140 mM K+in/6 mM K+out), the channels had a slope conductance of 54.2 pS at 0 mV. The relative permeability of K+ over Na+ was about 171 to 1. The channels were activated by intracellular Ca2+ and by membrane depolarization. These channels belong to a class defined as "intermediate-conductance Ca2+-activated K+ channel. " External tetraethylammonium ions (TEA+) caused a flickery block of the channel with reduction in single-channel current amplitude measured at a range of holding membrane potentials (-40 to 60 mV). Activity of the K+ channels was inhibited by intracellular ATP (KD =1.188 mM). The channel activity was detected only occasionally in patches from the apical membrane (about 1 in 17 patches containing active channels). The presence of the intermediate-conductance Ca2+-activated K+ channels indicates that they could provide a route for K+ secretion in a Ca2+-dependent process responsible for a high luminal K+ concentration found in the epididymal duct of the rat.

Elevation of basolateral K+ induces K+ secretion by apical maxi K+ channels in Ambystoma collecting tubule

We previously reported that exposure of aquatic-phase Ambystoma tigrinum to a solution containing 50 mM K+ (K+ adaptation) caused a nearly 10-fold increase in the number of detectable maxi K+ channels on the apical membrane of their initial collecting tubules. In apparent contradiction to the notion that maxi K+ channels contribute to K+ secretion, these channels were not routinely active at the resting membrane potential (0 mV voltage clamp). To test the possibility that hyperkalemia yields maxi K+ channels that are secreting K+ (i.e., active at 0 mV), we patch-clamped the apical membranes of initial collecting tubules under conditions of elevated basolateral K+ (15 mM). Seven patches containing maxi K+ channels were studied. Six of the seven patches showed maxi K+ channel activity when voltage was clamped at 0 mV. Open probability and unitary current averaged 0.059 +/- 0.016 and 1.65 +/- 0.50 pA, respectively. This activity, together with the high density of channels observed (1.06 channels/micrometer2), indicates that after K+ adaptation, maxi K+ channels contribute to the ability of the late distal nephron of amphibians to secrete K+.

Renal potassium transport: mechanisms and regulation

The regulation of potassium metabolism involves mechanisms for the appropriate distribution between the intra- and extracellular fluid compartments and for the excretion by the kidney. Clearance and single nephron studies show that renal excretion is determined by regulated potassium secretion and potassium reabsorption, respectively, in principal and intercalated cells of the distal nephron. Measurement of the electrochemical driving forces acting on potassium transport across individual cell membranes and characterization of several ATPases and potassium channels provide insights into the transport and regulation of renal potassium excretion.

Cloning and characterization of maxi K+ channel alpha-subunit in rabbit kidney

We have identified in rabbit renal cells two alternatively spliced transcripts of the alpha-subunit rbslo1 and rbslo2. Rbslo1 has a novel "in-frame" 174-bp insertion immediately after the predicted S8 transmembrane segment, whereas rbslo2 has a 104-bp deletion between S9 and S10, creating a frameshift and a premature termination codon. Amino acid identity between mouse maxi K- channel alpha-subunit (mslo) and rbslol was 99%. Two transcript sizes of 4.2 and 7.5 kb were detected in brain, kidney, stomach, testis, and lung. Rbslo is expressed in glomeruli, thin limbs of Henle's loop, medullary and cortical thick ascending limbs of Henle's loop, and cortical, outer, and inner medullary collecting ducts; however, it was rarely detected in proximal convoluted tubules. Rbslo1 is most abundant in inner medulla. Expressed in Xenopus oocytes, rbslo1 generates depolarization-activated, outwardly rectifying K+ currents. Rbslo1 expressed in Chinese hamster ovary cells could be activated by depolarization and Ca2+. These data suggest that rbslo transcripts are expressed in multiple nephron segments and that the magnitude of mRNA expression varies among different nephron segments.

Renal K+ channels: structure and function

The activity of potassium (K+) channels is intimately linked to several important transport functions in renal tubules. We review recent progress concerning the properties, site along the nephron, and physiological regulation of native K+ channels, and compare their characteristics with those of recently cloned K+ channels. We do not fully cover work on K+ channels in amphibian tubules, cell cultures, and single tubule cells and do not review K+ channels in mesangial cells.

View of K+ secretion through the apical K channel of cortical collecting duct

The apical small-conductance K+ channel plays an important role in renal K+ secretion, as evidenced by the presence of the extensive modulatory pathways. Figure 3 summarizes the current understanding of the mechanisms that modulate the apical small-conductance K+ channel. Stimulation of adenylate cyclase enhances channel activity and consequently K+ secretion. In contrast, increases in intracellular Ca2+ concentration and activation of Ca(2+)-dependent signal transduction pathways inhibit the K+ channel and thus decrease K+ secretion. The vasopressin-induced stimulation of K+ secretion in CCD results at least in part from cAMP-dependent signal transduction pathways. The Ca(2+)-dependent signal transduction pathway is responsible for modulatory coupling between Na+ pump turnover and apical K+ conductance when the Na+ pump is inhibited.

Hypertonicity activates nonselective cation channels in mouse cortical collecting duct cells

We investigated the effect of cell shrinkage on whole-cell currents of M-1 mouse cortical collecting duct cells. Addition of 100 mM sucrose to an isotonic NaCl bath solution induced cell shrinkage and increased whole-cell currents within 5-10 min by approximately 12-fold. The effect was reversible upon return to isotonic solution and could also be elicited by adding 100 mM urea or 50 mM NaCl. Replacement of bath Na+ by K+, Cs+, Li+, or Rb+ did not significantly affect the stimulated inward current, but replacement by N-methyl-D-glucamine reduced it by 88.1 +/- 1.3% (n = 34); this demonstrates that hypertonicity activates a nonselective alkali cation conductance. The activation was independent of extra- and intracellular Ca2+, but 1 or 10 mM ATP in the pipette suppressed it in a concentration-dependent manner, indicating that intracellular ATP levels may modulate the degree of channel activation. Flufenamic acid (0.1 mM) and gadolinium (0.1 mM) inhibited the stimulated current by 68.7 +/- 5.9% (n = 9) and 32.4 +/- 11.7% (n = 6), respectively, whereas 0.1 mM amiloride had no significant effect. During the early phase of hypertonic stimulation single-channel transitions could be detected in whole-cell current recordings, and a gradual activation of 30 and more individual channels with a single-channel conductance of 26.7 +/- 0.4 pS (n = 29) could be resolved. Thus, we identified the nonselective cation channel underlying the shrinkage-induced whole-cell conductance that may play a role in volume regulation.

K(+)-sparing diuretic actions of trimethoprim: inhibition of Na+ channels in A6 distal nephron cells

Hyperkalemia complicates trimethoprim-sulfamethoxazole (TMP-SMX) therapy in over 20% of HIV-infected patients. TMP is a heterocyclic weak base, similar to amiloride, a "K(+)-sparing" diuretic and Na+ channel blocker. Apical TMP is known to inhibit amiloride-sensitive short circuit current in A6 cells, a tissue culture model for mammalian cortical collecting tubule principal cells [1]. We used cell-attached patch clamp techniques to investigate the effect of TMP on the 4 pS, highly selective Na+ channel in the apical membrane of A6 cells grown on permeable supports in the presence of 1.5 microM aldosterone. Baseline channel activity at resting membrane potential, measured as NPo (N of channels x open probability), was 1.09 +/- 0.50 (N = 18). NPo (0.92 +/- 0.38; N = 9) was unchanged when 10(-3) M TMP was added to the basolateral bath for 30 minutes. However, apical exposure with pipettes containing 10(-3) or 10(-5) M TMP reduced NPo approximately tenfold (0.12 +/- 0.08; N = 7 and 0.18 +/- 0.14; N = 12, respectively). Kinetic analysis revealed the appearance of a new closed state after apical TMP treatment. Another group of A6 cells were pretreated with 10(-3) M apical TMP for 30 minutes prior to patching with pipettes filled with TMP-free saline. NPo progressively rose from 0.07 +/- 0.09 to 0.87 +/- 0.23 (N = 5) as the residual TMP was diluted within the pipette. Apical or basolateral pretreatment (30 min) with 10(-3) M SMX did not change Na+ channel activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandin E2 activates clusters of apical Cl- channels in principal cells via a cyclic adenosine monophosphate-dependent pathway

We examined cell-attached patches on principal cells of primary cultured, rabbit cortical collecting tubules. Under basal conditions, apical 9-pS Cl(-)-selective channels were observed in 9% of patches (11/126), and number of channels times open probability (NP0) was 0.56 +/- 0.21. The channel had a linear current-voltage relationship, reversal potential (Erev) near resting membrane potential, a P0 (0.30-0.70) that was independent of voltage, and complicated kinetics (i.e., bursting) at hyperpolarized potentials. NP0 and channel frequency were increased after 30 min of basolateral exposure to 0.5 microM PGE2 (18/56), 10 microM forskolin (23/36), or 0.5 mM dibutyryl cyclic adenosine monophosphate (cAMP) (25/41). Increases in NP0 appeared to be mediated primarily through an increase in the number of observed channels per patch (N), not changes in P0. After these cAMP-increasing maneuvers, N was inconsistent with a uniform distribution of channels in the apical membrane (P < 0.001), but rather the channels appeared to be clustered in pairs. Apical 0.5 microM PGE2 (12/91), apical or basolateral 0.5 microM PGF2 alpha (8/110), or 0.25 microM thapsigargin (releaser of intracellular Ca2+ stores) (7/73) did not increase NP0 or channel frequency.

CONCLUSIONS

(a) 9-pS Cl- channels provide a conductive pathway for apical membrane Cl- transport across principal cells. (b) Channel activation by basolateral PGE2 is mediated via a cAMP-, but not a Ca(2+)-dependent mechanism. (c) Apical channels are clustered in pairs. (d) With its low baseline frequency and Erev near resting membrane potential, this channel would not contribute significantly to transcellular Cl- flux under basal conditions. (e) However, cAMP-producing agonists (i.e., PGE2, arginine vasopressin) would increase apical Cl- transport with the direction determined by the apical membrane potential.

Cyclosporin A inhibits apical secretory K+ channels in rabbit cortical collecting tubule principal cells

We used the cell-attached patch clamp configuration to examine the effect of basolateral cyclosporin A (CsA) exposure on low conductance K+ channels found in the principal cell apical membrane of rabbit cortical collecting tubule (CCT) primary cultures. Baseline K+ channel activity, measured as mean NPo (number of channels x open probability), was 2.7 +/- 1.1 (N = 29). NPo fell by 69% (0.84 +/- 0.32; N = 32) in cultures pretreated with 500 ng/ml CsA for 30 minutes prior to patching. Chelation of intracellular [Ca2+]i (10 mM BAPTA/AM; N = 8) or removal of extracellular Ca2+ (N = 9), but not prevention of [Ca2+]i store release (10 microM TMB-8; N = 7), abolished CsA-induced inhibition. This suggested that CsA effects were mediated by an initial rise in [Ca2+]i via Ca2+ influx. Either 25 nM AVP (N = 10) or 0.25 microM thapsigargin (N = 8) (causing IP3-dependent and -independent release of [Ca2+]i stores, respectively) augmented, while 25 pM (N = 6) or 250 pM AVP (N = 8) reversed CSA-induced channel inhibition. Apical membrane protein kinase C (PKC) activation with 0.1 microM phorbol ester, PMA (N = 8) or 10 microM synthetic diacylglycerol, OAG (N = 7), mimicked (mean NPo = 0.99 +/- 0.40) the inhibitory effect of CsA. Apical PKC inhibition by prolonged apical exposure to PMA (N = 10) or 100 microM D-sphingosine (N = 6) blocked CsA's effect. Cyclic AMP increasing maneuvers, 10 microM forskolin (N = 5) or 0.5 mM db-cAMP (N = 8), stimulated basal K+ channel activity in the absence of CsA.

IN CONCLUSION

(1) basolateral exposure to CsA inhibits the activity of apical membrane 13 pS channels responsible for physiologic K+ secretion in rabbit CCT principal cells. (2) The inhibition is mediated by changes in intracellular Ca2+ and activation of apical PKC. (3) Pharmacologic AVP (nM) augments CsA-induced inhibition by releasing intracellular Ca2+ stores; more physiologic AVP (pM) attenuates channel inhibition, probably through cAMP generation. (4) Inhibition of apical secretory K+ channels by CsA likely contributes to decreased kaliuresis and clinical hyperkalemia observed in patients on CsA therapy.

Mechanism of apical K+ channel modulation in principal renal tubule cells. Effect of inhibition of basolateral Na(+)-K(+)-ATPase

The effects of inhibition of the basolateral Na(+)-K(+)-ATPase (pump) on the apical low-conductance K+ channel of principal cells in rat cortical collecting duct (CCD) were studied with patch-clamp techniques. Inhibition of pump activity by removal of K+ from the bath solution or addition of strophanthidin reversibly reduced K+ channel activity in cell-attached patches to 36% of the control value. The effect of pump inhibition on K+ channel activity was dependent on the presence of extracellular Ca2+, since removal of Ca2+ in the bath solution abolished the inhibitory effect of 0 mM K+ bath. The intracellular [Ca2+] (measured with fura-2) was significantly increased, from 125 nM (control) to 335 nM (0 mM K+ bath) or 408 nM (0.2 mM strophanthidin), during inhibition of pump activity. In contrast, cell pH decreased only moderately, from 7.45 to 7.35. Raising intracellular Ca2+ by addition of 2 microM ionomycin mimicked the effect of pump inhibition on K+ channel activity. 0.1 mM amiloride also significantly reduced the inhibitory effect of the K+ removal. Because the apical low-conductance K channel in inside-out patches is not sensitive to Ca2+ (Wang, W., A. Schwab, and G. Giebisch, 1990. American Journal of Physiology. 259:F494-F502), it is suggested that the inhibitory effect of Ca2+ is mediated by a Ca(2+)-dependent signal transduction pathway. This view was supported in experiments in which application of 200 nM staurosporine, a potent inhibitor of Ca(2+)-dependent protein kinase C (PKC), markedly diminished the effect of the pump inhibition on channel activity. We conclude that a Ca(2+)-dependent protein kinase such as PKC plays a key role in the downregulation of apical low-conductance K+ channel activity during inhibition of the basolateral Na(+)-K(+)-ATPase.

Cation specificity and pharmacological properties of the Ca(2+)-dependent K+ channel of rat cortical collecting ducts

The luminal membrane of principal cells of rat cortical collecting duct (CCD) is dominated by a K+ conductance. Two different K+ channels are described for this membrane. K+ secretion probably occurs via a small-conductance Ca(2+)-independent channel. The function of the second, large-conductance Ca(2+)-dependent channel is unclear. This study examines properties of this channel to allow a comparison of this K+ channel with the macroscopic K+ conductance of the CCD and with similar K+ channels from other preparations. The channel is poorly active on the cell. It has a conductance of 263 +/- 11 pS (n = 36, symmetrical K+ concentrations) and of 139 +/- 3 pS (n = 91) with 145 mmol/l K+ on one side and 3.6 mmol/l K+ on the other side of the membrane. Its open probability is high after excision (0.71 +/- 0.03, n = 85). The channel flickers rapidly between open and closed states. Its permeability in the cell-free configuration was 7.0 +/- 0.2 x 10(-13) cm3/s (n = 85). It is inhibited by several typical blockers of K+ channels such as Ba2+, tetraethylammonium, quinine, and quinidine and high concentrations of Mg2+. The Ca2+ antagonist verapamil and diltiazem also inhibit this K+ channel. As is typical for the maxi K+ channel, it is inhibited by charybdotoxin but not by apamin. The selectivity of this large-conductance K+ channel demonstrates significant differences between the permeability sequence (pK > pRb > pNH4 > pCs = pLi = pNa = pcholine = 0) and the conductance sequence (gK > gNH4 > gRb > gLi = gcholine > gCs = gNa = 0). The only other cations that are significantly conducted by this channel besides K+ (gK at Vc = infinity is 279 +/- 8 pS, n = 88) re NH+4 (gNH4 = 127 +/- 22 pS, n = 10) and Rb+ (gRb = 36 +/- 5 pS, n = 6). The K+ currents through this channel are reduced by high concentrations of choline+, Cs+, Rb+, and NH+4. These properties and the dependence of this channel on Ca2+ and voltage classify it as a "maxi" K+ channel. A possible physiological function of this channel is discussed in the accompanying paper.

Renal epithelial cells show nonselective cation channel activity and express a gene related to the cGMP-gated photoreceptor channel

Nonselective cation channels have been found in various parts of the nephron and represent a heterogeneous group of channels. We briefly review their putative physiological function. Renal epithelial nonselective cation channels may play a role in volume regulation, calcium entry, cell proliferation, and sodium reabsorption. In some renal epithelia cGMP seems to be involved in the regulation of nonselective cation channels. Furthermore, there is evidence that a gene related to the cGMP-gated photoreceptor channel, a well-characterized, nonselective cation channel, is also expressed in whole rat kidney tissue. In the context of these observations, we review recent findings from our own work on a nonselective cation channel in the M-1 mouse cortical collecting duct cell line. We could demonstrate that M-1 cells show nonselective cation channel activity in inside-out patches and express a gene related to the cGMP-gated photoreceptor channel (Proc. Natl. Acad. Sci. USA 89:10262-10266, 1992). The possibility of a relation between the kidney channel and the photoreceptor channel is discussed.

Mouse cortical collecting duct cells show nonselective cation channel activity and express a gene related to the cGMP-gated rod photoreceptor channel

Apical nonselective cation channels with an average single-channel conductance of 34 +/- 2.3 pS were found in M-1 mouse cortical collecting duct cells. Channel activity is increased by depolarization and abolished by cytoplasmic calcium removal. Cytoplasmic application of 0.1 mM cGMP decreases channel open probability by 27%. cDNAs corresponding to approximately 40% of the coding region of the photoreceptor channel were isolated by the polymerase chain reaction from M-1 cells and a rat kidney cDNA library. The rat kidney-derived sequence differs by a single base, and the M-1-cell-derived sequence differs by only two bases, from the photoreceptor sequence. A second clone from M-1 cells differs by 20 out of 426 bases from the photoreceptor sequence. In all three clones, the deduced amino acid sequence is identical to that of the rat photoreceptor channel. Northern blot analysis of poly(A)+ RNA from M-1 cells reveals the presence of a 3.2-kilobase band hybridizing with a retinal cGMP-gated cation channel probe. The results suggest the expression in M-1 cells of more than one gene coding for nonselective cation channels or channel subunits, one of which is identical to the cGMP-gated cation channel gene of rod photoreceptors.

Inhibition of apical Na+ channels in rabbit cortical collecting tubules by basolateral prostaglandin E2 is modulated by protein kinase C

We used the cell-attached patch clamp technique to investigate the interaction of exogenous prostaglandins (PG), intracellular [Ca2+]i, and protein kinase C (PKC) on the high selectivity, 4 pS Na+ channel found in the principal cell apical membrane of rabbit cortical collecting tubule (CCT) cultures grown on collagen supports with 1.5 microM aldosterone. Application of 0.5 microM PGE2 to the basolateral membrane decreased mean NP0 (number of channels times the open probability) for apical Na+ channels by 46.5% (n = 9). There was no consistent change in NP0 after apical 0.5 microM PGE2 (n = 12) or after apical or basolateral 0.5 microM PGF2 alpha (n = 8). Release of [Ca2+]i stores with 0.25 microM thapsigargin (n = 7), or activation of apical membrane PKC with apical 0.1 microM 4 beta-phorbol-12-myristate-13-acetate (n = 5) or 10 microM 1-oleyl-2-acetylglycerol (n = 4) also decreased NP0. Depletion of [Ca2+]i stores (0.25 microM thapsigargin pretreatment) (n = 7) or inhibition of apical PKC (100 microM D-sphingosine pretreatment) (n = 8) abolished the inhibitory effects of basolateral PGE2.

CONCLUSIONS

(a) apical Na+ transport in rabbit CCT principal cells is modulated by basolateral PGE2; (b) the mechanism involves release of IP3-sensitive, [Ca2+]i stores; and (c) Ca(2+)-dependent activation of apical membrane PKC, which then inhibits apical Na+ channels.

Ion conductances of isolated cortical collecting duct cells

The study of ion conductances in the intact cortical collecting duct (CCD) with the patch-clamp method is rather difficult. An optimized method to isolate CCD cells from rat kidneys using an in vivo followed by an in vitro enzyme digestion is described. Individual CCD segments were collected after this digestion and incubated in EGTA-buffered medium. This procedure resulted in single cells or cell clusters. These freshly isolated CCD cells were studied with different modifications of the patch-clamp method. Membrane voltages measured in the cell-attached-nystatin configuration were -74 +/- 1 mV (n = 13) and -68 +/- 3 mV (n = 22) in cells isolated from normal and mineralocorticoid-treated rats respectively. These values and those measured with the nystatin-perforated slow-whole-cell configuration (-79 +/- 1 mV, n = 23) are comparable to those measured in principal cells of isolated CCD segments. The cells hyperpolarized after the addition of amiloride and depolarized with the addition of adiuretin to the bath. The amiloride effect was enhanced when cells were isolated from deoxycorticosterone-acetate-treated rats. The cells were strongly depolarized upon elevation of the extracellular K(+)-concentration and did not demonstrate a measurable Cl- conductance. A large-conductance K+ channel (174 pS, n = 5, cell-attached, 145 mmol/l K+ in the pipette; 140 pS, n = 12, cell-free, 3.6 mmol/l K+ in the bath) was seen. It had a very low activity on the cell, but a high open probability when excised into a solution with 1 mmol/l Ca2+ on the cytosolic side.(ABSTRACT TRUNCATED AT 250 WORDS)