Renal K+ channels: structure and function

Regulation of ROMK1 channels by protein-tyrosine kinase and -tyrosine phosphatase

We have used the two-electrode voltage clamp technique and the patch clamp technique to investigate the regulation of ROMK1 channels by protein-tyrosine phosphatase (PTP) and protein-tyrosine kinase (PTK) in oocytes coexpressing ROMK1 and cSrc. Western blot analysis detected the presence of the endogenous PTP-1D isoform in the oocytes. Addition of phenylarsine oxide (PAO), an inhibitor of PTP, reversibly reduced K(+) current by 55% in oocytes coinjected with ROMK1 and cSrc. In contrast, PAO had no significant effect on K(+) current in oocytes injected with ROMK1 alone. Moreover, application of herbimycin A, an inhibitor of PTK, increased K(+) current by 120% and completely abolished the effect of PAO in oocytes coexpressing ROMK1 and cSrc. The effects of herbimycin A and PAO were absent in oocytes expressing the ROMK1 mutant R1Y337A in which the tyrosine residue at position 337 was mutated to alanine. However, addition of exogenous cSrc had no significant effect on the activity of ROMK1 channels in inside-out patches. Moreover, the effect of PAO was completely abolished by treatment of oocytes with 20% sucrose and 250 microg/ml concanavalin A, agents that inhibit the endocytosis of ROMK1 channels. Furthermore, the effect of herbimycin A is absent in the oocytes pretreated with either colchicine, an inhibitor of microtubules, or taxol, an agent that freezes microtubules. We conclude that PTP and PTK play an important role in regulating ROMK1 channels. Inhibiting PTP increases the internalization of ROMK1 channels, whereas blocking PTK stimulates the insertion of ROMK1 channels.

Role of 20-HETE in mediating the effect of dietary K intake on the apical K channels in the mTAL

We have used the patch-clamp technique to study the effect of dietary K intake on the apical K channels in the medullary thick ascending limb (mTAL) of rat kidneys. The channel activity, defined by the number of channels in a patch and the open probability (NPo), of the 30- and 70-pS K channels, was 0.18 and 0.11, respectively, in the mTAL from rats on a K-deficient diet. In contrast, NPo of the 30- and 70-pS K channels increased to 0.60 and 0.80, respectively, in the tubules from animals on a high-K diet. The concentration of 20-hydroxyeicosatetraenoic acid (20-HETE) measured with gas chromatography-mass spectrometry was 0.8 pg/microg protein in the mTAL from rats on a high-K diet and increased significantly to 4.6 pg/microg protein in the tubules from rats on a K-deficient diet. Addition of N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or 17-octadecynoic acid (17-ODYA), agents that inhibit the formation of 20-HETE, had no significant effect on the activity of the 30-pS K channels. However, DDMS/17-ODYA significantly increased the activity of the apical 70-pS K channel from 0.11 to 0.91 in the mTAL from rats on a K-deficient diet. In contrast, inhibition of the cytochrome P-450 metabolism of arachidonic acid increased NPo from 0.64 to 0.81 in the tubules from animals on a high-K diet. Furthermore, the sensitivity of the 70-pS K channel to 20-HETE was the same between rats on a high-K diet and on a K-deficient diet. Finally, the pretreatment of the tubules with DDMS increased NPo of the 70-pS K channels in the mTAL from rats on a K-deficient diet to 0.76. We conclude that an increase in 20-HETE production is involved in reducing the activity of the apical 70-pS K channels in the mTAL from rats on a K-deficient diet.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Tracing the origin of the RACTK1 K(+) channel

Potassium secretion by the kidney is vital for the maintenance of K(+) homeostasis. RACTK1, a putative inwardly rectifying potassium channel cloned from cultured rabbit collecting duct cells, has been proposed to play a role in this process. However, the lack of homology with any other cloned potassium channel and the inability to reproduce the results across different laboratories has brought into question the existence of RACTK1. Recently, it has been suggested that RACTK1 is a contamination from Escherichia coli. In this work we add conclusive evidence supporting the bacterial origin of RACTK1. Using both genomic PCR and RT-PCR we were unable to detect RACTK1 in a number of mammalian species. In addition sequencing of RACTK1 cDNA confirmed a complete homology between RACTK1 and a region of E. coli genomic DNA. Finally, a hypothesis on how RACTK1 could have been generated from a contamination by E. coli genomic DNA is presented.

Stable, polarised, functional expression of Kir1.1b channel protein in Madin-Darby canine kidney cell line

1. The family of Kir1.1 (ROMK) channel proteins constitute a secretory pathway for potassium in principal cells of cortical collecting duct and thick ascending limb of Henle's loop. Mutations in Kir1.1 account for some types of Bartter's syndrome. 2. Here we report that stable transfection of Kir1.1b (ROMK2) in Madin-Darby canine kidney (MDCK) cell line results in expression of inwardly rectifying K+ currents and transmonolayer electrical and transport properties appropriate to Kir1.1 function. When grown on permeable supports, transfected monolayers secreted K+ into the apical solution. This secretion was inhibited by application of barium to the apical membrane, or by reduction in expression temperature from 37 to 26 C. However, whole-cell voltage clamp electrophysiology showed that K+ conductance was higher in cells expressing Kir1.1b at 26C. 3. To investigate this further, Kir1.1b was tagged with (EGFP), a modification that did not affect channel activity. Protein synthesis was inhibited with cycloheximide. Spectrofluorimetry was used to compare protein degradation at 37 and 26 C. The increased level of Kir1.1b at the plasma membrane at 26 C was due to an increase in protein stability. 4. Confocal microscopic investigation of EGFP-Kir1. 1b fluorescence in transfected cells showed that the channel protein was targeted to the apical domain of the cell. 5. These results demonstrate that Kir1.1b is capable of appropriate trafficking and function in MDCK cell lines at physiological temperatures. In addition, expression of Kir1.1b in MDCK cell lines provides a useful and convenient tool for the study of functional activity and targeting of secretory K+ channels.

Extracellular ATP inhibits the small-conductance K channel on the apical membrane of the cortical collecting duct from mouse kidney

We have used the patch-clamp technique to study the effects of changing extracellular ATP concentration on the activity of the small-conductance potassium channel (SK) on the apical membrane of the mouse cortical collecting duct. In cell-attached patches, the channel conductance and kinetics were similar to its rat homologue. Addition of ATP to the bathing solution of split-open single cortical collecting ducts inhibited SK activity. The inhibition of the channel by ATP was reversible, concentration dependent (K(i) = 64 microM), and could be completely prevented by pretreatment with suramin, a specific purinergic receptor (P(2)) blocker. Ranking of the inhibitory potency of several nucleotides showed strong inhibition by ATP, UTP, and ATP-gamma-S, whereas alpha, beta-Me ATP, and 2-Mes ATP failed to affect channel activity. This nucleotide sensitivity is consistent with P(2)Y(2) purinergic receptors mediating the inhibition of SK by ATP. Single channel analysis further demonstrated that the inhibitory effects of ATP could be elicited through activation of apical receptors. Moreover, the observation that fluoride mimicked the inhibitory action of ATP suggests the activation of G proteins during purinergic receptor stimulation. Channel inhibition by ATP was not affected by blocking phospholipase C and protein kinase C. However, whereas cAMP prevented channel blocking by ATP, blocking protein kinase A failed to abolish the inhibitory effects of ATP. The reduction of K channel activity by ATP could be prevented by okadaic acid, an inhibitor of protein phosphatases, and KT5823, an agent that blocks protein kinase G. Moreover, the effect of ATP was mimicked by cGMP and blocked by L-NAME (N(G)-nitro-l-arginine methyl ester). We conclude that the inhibitory effect of ATP on the apical K channel is mediated by stimulation of P(2)Y(2) receptors and results from increasing dephosphorylation by enhancing PKG-sensitive phosphatase activity.

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.

Biophysical and molecular mechanisms underlying the modulation of heteromeric Kir4.1-Kir5.1 channels by CO2 and pH

CO2 chemoreception may be related to modulation of inward rectifier K+ channels (Kir channels) in brainstem neurons. Kir4.1 is expressed predominantly in the brainstem and inhibited during hypercapnia. Although the homomeric Kir4.1 only responds to severe intracellular acidification, coexpression of Kir4.1 with Kir5.1 greatly enhances channel sensitivities to CO2 and pH. To understand the biophysical and molecular mechanisms underlying the modulation of these currents by CO2 and pH, heteromeric Kir4. 1-Kir5.1 were studied in inside-out patches. These Kir4.1-Kir5.1 currents showed a single channel conductance of 59 pS with open-state probability (P(open)) approximately 0.4 at pH 7.4. Channel activity reached the maximum at pH 8.5 and was completely suppressed at pH 6.5 with pKa 7.45. The effect of low pH on these currents was due to selective suppression of P(open) without evident effects on single channel conductance, leading to a decrease in the channel mean open time and an increase in the mean closed time. At pH 8.5, single-channel currents showed two sublevels of conductance at approximately 1/4 and 3/4 of the maximal openings. None of them was affected by lowering pH. The Kir4.1-Kir5.1 currents were modulated by phosphatidylinositol-4,5-bisphosphate (PIP2) that enhanced baseline P(open) and reduced channel sensitivity to intracellular protons. In the presence of 10 microM PIP2, the Kir4.1-Kir5.1 showed a pKa value of 7.22. The effect of PIP2, however, was not seen in homomeric Kir4.1 currents. The CO2/pH sensitivities were related to a lysine residue in the NH2 terminus of Kir4.1. Mutation of this residue (K67M, K67Q) completely eliminated the CO2 sensitivity of both homomeric Kir4.1 and heteromeric Kir4.1-Kir5.1. In excised patches, interestingly, the Kir4.1-Kir5.1 carrying K67M mutation remained sensitive to low pHi. Such pH sensitivity, however, disappeared in the presence of PIP2. The effect of PIP2 on shifting the titration curve of wild-type and mutant channels was totally abolished when Arg178 in Kir5.1 was mutated. Thus, these studies demonstrate a heteromeric Kir channel that can be modulated by both acidic and alkaline pH, show the modulation of pH sensitivity of Kir channels by PIP2, and provide information of the biophysical and molecular mechanisms underlying the Kir modulation by intracellular protons.

Sodium reabsorption in thick ascending limb of Henle's loop: effect of potassium channel blockade in vivo

1. Based on previous in vitro studies, inhibition of K(+) recycling in thick ascending limb (TAL) is expected to lower Na(+) reabsorption through (i) reducing the luminal availability of K(+) to reload the Na(+)-2Cl(-)-K(+) cotransporter and (ii) diminishing the lumen positive transepithelial potential difference which drives paracellular cation transport. 2. This issue was investigated in anaesthetized rats employing microperfusion of Henle's loop downstream from late proximal tubular site with K(+)-free artificial tubular fluid in nephrons with superficial glomeruli. 3. The unselective K(+) channel blocker Cs(+) (5 - 40 mM) dose-dependently increased early distal tubular delivery of fluid and Na(+) with a maximum increase of approximately 20 and 185%, respectively, indicating predominant effects on water-impermeable TAL. 4. The modest inhibition of Na(+) reabsorption in response to the 15 mM of Cs(+) but not the enhanced inhibition by 20 mM Cs(+) was prevented by luminal K(+) supplementation. Furthermore, pretreatment with 20 mM Cs(+) did not attenuate the inhibitory effect of furosemide (100 microM) on Na(+)-2Cl(-)-K(+) cotransport. 5. Neither inhibitors of large (charybdotoxin 1 microM) nor low (glibenclamide 250 microM; U37883A 100 microM) conductance K(+) channels altered loop of Henle fluid or Na(+) reabsorption. 6. The intermediate conductance K(+) channel blockers verapamil and quinine (100 microM) modestly increased early distal tubular Na(+) but not fluid delivery, indicating a role for this K(+) channel in Na(+) reabsorption in TAL. As observed for equieffective concentrations of Cs(+) (15 mM), Na(+) reabsorption was preserved by K(+) supplementation. 7. The results indicate that modest inhibition of K(+) channels lowers the luminal availability of K(+) and thus transcellular Na(+) reabsorption in TAL. More complete inhibition lowers paracellular Na(+) transport probably by reducing or even abolishing the lumen positive transepithelial potential difference. Under the latter conditions, transcellular Na(+) transport may be restored by paracellular K(+) backleak.

Two types of K(+) channels are present in the apical membrane of the thick ascending limb of the mouse kidney

We used the patch-clamp technique to study apical K(+) channels in the split-opened thick ascending limb (TAL) of mouse kidneys. We observed a low-conductance K(+) channel in 8 patches and an intermediate-conductance K(+) channel in 11 patches from a total of 135 cell-attached patches with GOmega seals in the apical membrane of the mouse TAL. Open probability (P(o)) of the low-conductance K(+) channel was 0.85 and was not voltage-dependent between -60 mV and 0 mV. P(o) of the intermediate-conductance was 0.6 at a spontaneous cell membrane potential and decreased to 0.4 by 40 mV hyperpolarization. Both the low-conductance and intermediate-conductance K(+) channels were an inwardly rectifying K(+) channel with inward slope conductances of 26 pS and 74 pS between -20 and 20 mV, respectively. The 74-pS K(+) channel was inhibited by 1 mM Ba(2+) and 10 mM tetraethylammonium when they were applied to the bath facing the cytosolic surface of inside-out patches. Furthermore, addition of 1 mM Mg-ATP reversibly reduced the activity of the 74-pS K(+) channel by 90% within 1 min in inside-out patches. A decrease in bath pH from 7.4 to 6.5 completely blocked the 74-pS K(+) channel in inside-out patches. We conclude that two types of K+ channels are present in the apical membrane of the mouse TAL and that the biophysical properties of the apical 74-pS K(+) channel are identical to those in the rat TAL.

pH dependence of the inwardly rectifying potassium channel, Kir5.1, and localization in renal tubular epithelia

The physiological role of the inwardly rectifying potassium channel, Kir5.1, is poorly understood, as is the molecular identity of many renal potassium channels. In this study we have used Kir5.1-specific antibodies to reveal abundant expression of Kir5.1 in renal tubular epithelial cells, where Kir4.1 is also expressed. Moreover, we also show that Kir5.1/Kir4.1 heteromeric channel activity is extremely sensitive to inhibition by intracellular acidification and that this novel property is conferred predominantly by the Kir5.1 subunit. These findings suggest that Kir5.1/Kir4.1 heteromeric channels are likely to exist in vivo and implicate an important and novel functional role for the Kir5.1 subunit.

The cGMP-dependent protein kinase stimulates the basolateral 18-pS K channel of the rat CCD

We used the patch-clamp technique to study the effect of cGMP on the 18-pS K channel in the basolateral membrane of the rat cortical collecting duct. Addition of 100 microM 8-bromoguanosine 3', 5'-cyclic monophosphate (8-Br-cGMP) increased the activity of the 18-pS K channel, defined by NP(o), by 95%. In contrast, applying 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) has no effect on channel activity. The effect of 8-Br-cGMP was observed only in cell-attached but not in inside-out patches. Application of 1 microM KT-5823, an inhibitor of the cGMP-dependent protein kinase (PKG), not only reduced the channel activity, but also completely abolished the stimulatory effect of 8-Br-cGMP, suggesting that the 18-pS K channel is not a cGMP-gated K channel. Addition of H-89, an agent that also blocks the PKG, mimicked the effect of KT-5823. To examine the possibility that the effect of 8-Br-cGMP is the result of inhibiting cGMP-dependent phosphodiesterase (PDE) and, accordingly, increasing cAMP or cGMP levels, we explored the effect on the 18-pS K channel of IBMX, an agent that inhibits the PDE. The addition of 100 microM IBMX had no significant effect on channel activity in cell-attached patches. Moreover, in the presence of IBMX, 8-Br-cGMP increased the channel activity to the same extent as that observed in the absence of IBMX, suggesting that the effect of cGMP is not mediated by inhibiting the cGMP-dependent PDE. That the effect of cGMP is mediated by stimulating PKG was further indicated by experiments in which application of exogenous PKG restored the channel activity when it decreased after the excision of the patches. In contrast, adding exogenous cAMP-dependent protein kinase catalytic subunit failed to reactivate the run-down channels. We conclude that cGMP stimulates the 18-pS channel, and the effect of cGMP is mediated by PKG.

Vasopressin and PGE(2) regulate activity of apical 70 pS K(+) channel in thick ascending limb of rat kidney

Vasopressin and prostaglandin E(2) (PGE(2)) are involved in regulating NaCl reabsorption in the thick ascending limb (TAL) of the rat kidney. In the present study, we used the patch-clamp technique to study the effects of vasopressin and PGE(2) on the apical 70 pS K(+) channel in the rat TAL. Addition of vasopressin increased the channel activity, defined as NP(o), from 1.11 to 1.52 (200 pM) and 1.80 (500 pM), respectively. The effect of vasopressin can be mimicked by either forskolin (1-5 microM) or 8-bromo-cAMP/dibutyryl-cAMP (8-Br-cAMP/DBcAMP) (200-500 microM). Moreover, the effects of cAMP and vasopressin were not additive and application of 10 microM H-89 abolished the effect of vasopressin. This suggests that the effect of vasopressin is mediated by a cAMP-dependent pathway. Applying 10 nM PGE(2) alone had no significant effect on the channel activity. However, PGE(2) (10 nM) abolished the stimulatory effect of vasopressin. The PGE(2)-induced inhibition of the vasopressin effect was the result of decreasing cAMP production because addition of 200 microM 8-Br-cAMP/DBcAMP reversed the PGE(2)-induced inhibition. In addition to antagonizing the vasopressin effect, high concentrations of PGE(2) reduced channel activity in the absence of vasopressin by 33% (500 nM) and 51% (1 microM), respectively. The inhibitory effect of high concentrations of PGE(2) was not the result of decreasing cAMP production because adding the membrane-permeant cAMP analog failed to restore the channel activity. In contrast, inhibiting protein kinase C (PKC) with calphostin C (100 nM) abolished the effect of 1 microM PGE(2). We conclude that PGE(2) inhibits apical K(+) channels by two mechanisms: 1) low concentrations of PGE(2) attenuate the vasopressin-induced stimulation mainly by reducing cAMP generation, and 2) high concentrations of PGE(2) inhibit the channel activity by a PKC-dependent pathway.

Aldosterone and potassium secretion by the cortical collecting duct

BACKGROUND

: Aldosterone has been implicated in the regulation of both Na and K concentrations in the plasma. Release of the hormone is known to be stimulated by high plasma K, and infusion of aldosterone lowers plasma K. However, the correlation between changes in mineralocorticoid levels and rates of K secretion is not perfect, suggesting that other factors may be involved.

METHODS

: Patch-clamp recordings were made of K-channel activity in the split-open cortical collecting tubule of the rat. Estimates of channel density were made in cell-attached patches on the luminal membrane of principal cells of this segment.

RESULTS

: Most of the K conductance of the apical membrane is mediated through low-conductance "SK" channels. The number of conducting SK channels is increased when animals are placed on a high-K diet. However, increasing plasma aldosterone levels by infusion of the hormone or by sodium restriction failed to change the number of active channels.

CONCLUSIONS

: At least two circulating factors are required for the regulation of renal K secretion by the cortical collecting tubule. Aldosterone mainly stimulates secretion by increasing the driving force for K movement through apical channels. A second, as yet unidentified, factor increases the number of conducting K channels.

Involvement of histidine residues in proton sensing of ROMK1 channel

ROMK channels are inhibited by intracellular acidification. This pH sensitivity is related to several amino acid residues in the channel proteins such as Lys-61, Thr-51, and His-206 (in ROMK2). Unlike all other amino acids, histidine is titratable at pH 6-7 carrying a positive charge below pH 6. To test the hypothesis that certain histidine residues are engaged in CO(2) and pH sensing of ROMK1, we performed experiments by systematic mutations of all histidine residues in the channel using the site-directed mutagenesis. There are two histidine residues in the N terminus. Mutations of His-23, His-31, or both together did not affect channel sensitivity to CO(2). Six histidine residues are located in the C terminus. His-225, His-274, His-342, and His-354 were critical in CO(2) and pH sensing. Mutation of either of them reduced CO(2) and pH sensitivities by 20-50% and approximately 0.2 pH units, respectively. Simultaneous mutations of all of them eliminated the CO(2) sensitivity and caused this mutant channel to respond to only extremely acidic pH. Similar mutations of His-280 had no effect. The role of His-270 in CO(2) and pH sensing is unclear, because substitutions of this residue with either a neutral, negative, or positive amino acid did not produce any functional channel. These results therefore indicate that histidine residues contribute to the sensitivity of the ROMK1 channel to hypercapnia and intracellular acidosis.

Regulation of ATP-sensitive potassium channel mRNA expression in rat kidney following ischemic injury

ATP-sensitive potassium (K(ATP)) channels are involved in the regulation of potassium homeostasis in kidneys. In the event of renal ischemia, they are thought to contribute to the important intracellular potassium loss observed in proximal tubules and thus to hypoxic injury. We have analyzed the transcriptional regulation of K(ATP) genes in rat kidney following transient renal ischemia. We observed that mRNA expression level was down-regulated for Kir1.1 and Kir4.1 potassium channels between 24 and 120 h after ischemia. In contrast, a strong increase in mRNA expression was observed for Kir6.1 shortly (2-6 h) after ischemia. Thus, renal ischemia followed by reperfusion provokes differential regulation of K(ATP) channel gene expression.

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 the ROMK channel: interaction of the ROMK with associate proteins

The ROMK channel plays an important role in K recycling in the thick ascending limb (TAL) and K secretion in the cortical collecting duct (CCD). A large body of evidence indicates that the ROMK channel is a key component of the native K secretory channel identified in the apical membrane of the TAL and the CCD. Although the ROMK channel shares several key regulatory mechanisms with the native K secretory channel in a variety of respects, differences in the channel modulatory mechanism are clearly present between the ROMK channel and the native K secretory channel. Therefore, it is possible that additional associate proteins are required to interact with the ROMK channel to assemble the native K secretory channel. This notion is supported by recent reports showing that cystic fibrosis transmembrane conductance regulator (CFTR) and A kinase anchoring proteins (AKAP) interact with the ROMK channels to restore the response to ATP sensitivity and protein kinase A stimulation. This review is an attempt to summarize the up-to-date progress regarding the interaction between the ROMK channel and the associate proteins in forming the native K secretory channel.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

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.

Channelopathies of inwardly rectifying potassium channels

Mutations in genes encoding ion channels have increasingly been identified to cause disease conditions collectively termed channelopathies. Recognizing the molecular basis of an ion channel disease has provided new opportunities for screening, early diagnosis, and therapy of such conditions. This synopsis provides an overview of progress in the identification of molecular defects in inwardly rectifying potassium (Kir) channels. Structurally and functionally distinct from other channel families, Kir channels are ubiquitously expressed and serve functions as diverse as regulation of resting membrane potential, maintenance of K(+) homeostasis, control of heart rate, and hormone secretion. In humans, persistent hyperinsulinemic hypoglycemia of infancy, a disorder affecting the function of pancreatic beta cells, and Bartter's syndrome, characterized by hypokalemic alkalosis, hypercalciuria, increased serum aldosterone, and plasma renin activity, are the two major diseases linked so far to mutations in a Kir channel or associated protein. In addition, the weaver phenotype, a neurological disorder in mice, has also been associated with mutations in a Kir channel subtype. Further genetic linkage analysis and full understanding of the consequence that a defect in a Kir channel would have on disease pathogenesis are among the priorities in this emerging field of molecular medicine.

A secretory mechanism of renal kallikrein by a high potassium ion; a possible involvement of ATP-sensitive potassium channel

A relatively rapid excretion of urinary kallikrein into urine was observed by an intravenous infusion of high potassium in anesthetized rats. Superfusion of sliced cortex isolated from rat kidney with an isotonic solution containing more than 20 mM of KCl significantly stimulated the release of kallikrein. The latter in vitro result supported another mechanism for the release of renal kallikrein from kidney other than biosynthesis of kallikrein by aldosterone released from adrenal cortex after loading of high potassium and the mechanism was elucidated. ATP-sensitive potassium channel blockers, glibenclamide, 4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexylhydr ochloride (U37883A), and barium chloride, which inhibit an efflux of intracellular potassium to block the channel, showed a significant increase of the kallikrein release from the slice of kidney cortex. Cytochalasin B, which inhibits a polymerization of actin, also showed a stimulation of the release. Enhanced release of kallikrein by a high potassium or ATP-sensitive potassium channel blocker was reduced by the absence of calcium ion and the presence of voltage-dependent calcium channel blocker in the superfused solution. These results indicate the ATP-sensitive potassium channel which couples to voltage-dependent calcium channel and cytoskeletal protein could be involved in a rapid secretory mechanism of renal kallikrein by high potassium.

A novel cGMP-regulated K+ channel in immortalized human kidney epitheliall cells (IHKE-1)

1. K+ channels from the apical membrane of immortalized human kidney epithelial (IHKE-1) cells were investigated in the cell-attached membrane configuration as well as in excised membranes using the patch clamp technique. 2. In cell-attached membrane patches the open probability (Po) of the K+ channel was 0.42 +/- 0.06 (mean +/- s.e.m. , n = 22) and its conductance was 94 +/- 5 pS with 145 mM K+ in the pipette (n = 25). In excised membrane patches the Po of the channel was 0.55 +/- 0.03 (n = 86) and its conductance was 65 +/- 2 pS (n = 68) with 145 mM K+ on one side of the membrane and 3.6 mM K+ on the other. The I-V curve of the K+ channel was not rectifying. 3. The channel was inhibited by several blockers of K+ channels such as 1 mM Ba2+ (cell-attached membrane: 78 +/- 8 %, n = 9; excised: 80 +/- 4 %, n = 26), 10 mM TEA+ (excised inside-out: 48 +/- 5 %, n = 34; excised outside-out: 100 +/- 0 %, n = 26), 0.1 mM verapamil (excised: 73 +/- 9 %, n = 12), and 10 nM charybdotoxin (excised outside-out: 67 +/- 9 %, n = 9). 4. The K+ channel was activated by depolarization and rising cytosolic Ca2+. Half-maximal activity occurred at a cytosolic Ca2+ concentration of 200 nM. In the cell-attached membrane configuration the K+ channel was inhibited in a concentration-dependent manner by atrial natriuretic peptide (ANP). Powas blocked equally well by 10 nM ANP (52 +/- 7 %, n = 10), brain natriuretic peptide (BNP; 37 +/- 11 %, n = 6) and C-type natriuretic peptide (CNP; 44 +/- 13 %, n = 8). 8-Bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, 0.1 mM) also inhibited Poof this K+ channel, by 70 +/- 10 % (n = 5). 5. In excised membrane patches cGMP inhibited Po of this K+ channel in a concentration-dependent manner. The first significant effects were measured at a concentration of 1 microM (22 +/- 7 %, n = 6), and greatest effects were obtained at 0.1 mM (34 +/- 5 %, n = 15). cAMP (0.1 mM, n = 5) as well as GTP (0.1 mM, n = 5) had no significant effects on Po of this K+ channel. ATP (0.1 mM) had a weak inhibitory effect (17 +/- 5 %, n = 14). Addition of Mg-ATP to cGMP did not increase the inhibitory effect (30 +/- 4 %, n = 14). KT5823 (1 microM), a specific inhibitor of cGMP-dependent protein kinases, did not significantly alter the cGMP-induced reduction in Po of the K+ channel in three excised membrane patches. 6. The results present the first electrophysiological characterization of a mammalian K+ channel that is directly regulated by cGMP.

Antisense and kidney cell research

Antisense oligodeoxynucleotides offer the potential to block the expression of specific genes with the goal of altering the phenotypic behavior of the cell. Antisense technology has attracted special interest as potential therapeutic agents for the treatment of genetic disorders, viral infections, and most recently proliferative diseases such as glomerular kidney disease. This technique has recently been used for in vitro and in vivo studies in renal cells. The use of antisense technology has been applied in vitro to help define both the normal mechanisms of specific ion transport and function and the pathobiological processes leading to glomerular proliferation and matrix formation. Most promising are the recent uses of antisense technology in vivo that have been used to treat the damaged peritoneum and alter glomerular remodeling in experimental animal models. It is hoped that widespread use of antisense will not only provide new insight into the normal regulatory behavior of the kidney cells but also allow one to develop therapeutic strategies to treat kidney disease.

Regulation of the ROMK potassium channel in the kidney

ROMK is a gene encoding inwardly rectifying adenosine triphosphate regulated K+ channels. Alternative splicing of ROMK exons yields several different transcripts, ROMK 1-3, that are differentially expressed along the nephron. Cloned ROMK channels expressed in Xenopus oocytes exhibit properties similar to those of the native low-conductance K+ secretory channels in cortical collecting duct and medullary thick ascending limb, as manifested by use of the patch-clamp technique. These similarities between the cloned and native channels suggest that ROMK represents the low-conductance secretory K+ channels in the kidney. We studied the role of dietary K+ and aldosterone in the regulation of ROMK mRNA expression in the rat kidney. K+ deficiency downregulated ROMK mRNA in cortex and medulla. Adrenalectomy markedly downregulated cortical ROMK, while it increased it in the medulla. In adrenalectomized rats K+ deficiency decreased ROMK mRNA in cortex and medulla similarly to intact rats. Na-K-ATPase subunits alpha1 and beta1 were regulated in parallel to the regulation of ROMK. In the medulla ROMK mRNA correlated highly with serum K+ and with the alpha1 and beta1 subunits of Na-K-ATPase. These results show that cortical ROMK expression is regulated by aldosterone and K+, while the medullary ROMK mRNA is regulated by serum K+, irrespective of aldosterone.

Carbon monoxide stimulates the apical 70-pS K+ channel of the rat thick ascending limb

We have investigated the expression of heme oxygenase (HO) in the rat kidney and the effects of HO-dependent heme metabolites on the apical 70-pS K+ channel in the thick ascending limb (TAL). Reverse transcriptase-PCR (RT-PCR) and Western blot analyses indicate expression of the constitutive HO form, HO-2, in the rat cortex and outer medulla. Patch-clamping showed that application of 10 microM chromium mesoporphyrin (CrMP), an inhibitor of HO, reversibly reduced the activity of the apical 70-pS K+ channel, defined by NPo, to 26% of the control value. In contrast, addition of 10 microM magnesium protoporphyrin had no significant effect on channel activity. HO involvement in regulation of the apical 70-pS K+ channel of the TAL, was further indicated by the addition of 10 microM heme-L-lysinate, which significantly stimulated the channel activity in cell-attached patches by 98%. The stimulatory effect of heme on channel activity was also observed in inside-out patches in the presence of 0.5-1 mM reduced nicotinamide adenine dinucleotide phosphate. This was completely abolished by 10 microM CrMP, suggesting that a HO-dependent metabolite of heme mediated the effect. This was further supported by exposure of the cytosolic membrane of inside-out patches to a carbon monoxide-bubbled bath solution, which increased channel activity. Moreover, carbon monoxide completely abolished the effect of 10 microM CrMP on the channel activity. In contrast, 10 microM biliverdin, another HO-dependent metabolite of heme, had no effect. We conclude that carbon monoxide produced from heme via an HO-dependent metabolic pathway stimulates the apical 70-pS K+ channel in the rat TAL.

Long-term (subacute) potassium treatment in congenital HERG-related long QT syndrome (LQTS2)

INTRODUCTION

Congenital long QT syndrome (LQTS) is subdivided according to the underlying gene defect. In LQTS2, an aberrant HERG gene that encodes the potassium channel IKr leads to insufficient IKr activity and delayed repolarization, causing ECG abnormalities and torsades de pointes (TdP). Increasing serum potassium levels by potassium infusion normalizes the ECG in LQTS2 because IKr activity varies with serum potassium levels.

METHODS AND RESULTS

In an LQTS2 patient who presented with TdP, we attempted to achieve a long-term (subacute) elevation of serum potassium by increased potassium intake and potassium-sparing drugs. However, due to renal potassium homeostasis, it was impossible to achieve a long-lasting rise of serum potassium above 4.0 mmol/L.

CONCLUSION

Although raising serum potassium reverses the ECG abnormalities in LQTS2, a long-lasting rise of serum potassium is only partially achievable because in the presence of normal renal function, potassium homeostasis limits the amount of serum potassium increase.

Basolateral Na+ pump modulates apical Na+ and K+ conductances in rabbit cortical collecting ducts

Previous studies indicated that an acute elevation of peritubular K+ enhances K+ secretion and Na+ reabsorption in the isolated perfused cortical collecting duct (CCD) from rabbit kidneys [S. Muto, G. Giebisch, and S. Sansom. Am. J. Physiol. 255 (Renal Fluid Electrolyte Physiol. 24): F108-F114, 1988]. To determine the underlying cellular mechanisms, we used microelectrode techniques to assess the membrane properties of collecting duct cells in isolated perfused CCDs of control and desoxycorticosterone acetate (DOCA)-treated rabbits following acute stimulation of the basolateral Na+-K+ pump by rapidly increasing the bath solution from 2.5 to 8.5 mM K+. This induced in both groups of tubules, first, a short-lasting hyperpolarization and, second, a sustained phase of depolarization of transepithelial, basolateral, and apical membrane voltages. Whereas the transepithelial conductance (GT) and fractional apical membrane resistance (fRA) remained unchanged during the initial phase of hyperpolarization, during the depolarization, GT increased and fRA decreased. Perfusion of the lumen with solutions containing either amiloride or Ba2+ attenuated the high K+-induced apical electrical changes, and basolateral strophanthidin abolished both apical and basolateral electrical responses during elevation of K+ in the bath. From these results we conclude the following: 1) acute elevation of basolateral K+ activates the basolateral Na+-K+ pump, which secondarily elevates the apical Na+ and K+ conductances; 2) DOCA pretreatment increases the basolateral K+ conductance and augments the response to the rise of K+ of both basolateral Na+-K+ pump activity and apical cation conductances.

Activation of inwardly rectifying K+ channel in OK proximal tubule cells involves cGMP-dependent phosphorylation process

The inwardly rectifying K+ channel with an inward conductance of about 90 pS in the surface membrane of cultured opossum kidney proximal tubule (OKP) cell is activated by cyclic AMP-dependent protein kinase (PKA). In this study, we further examined the involvement of the guanosine 3',5'-cyclic monophosphate (cGMP)-dependent process in modulation of this K+ channel by using the patch-clamp technique. In cell-attached patches, channel activity was increased by the application of either N2, 2'-O-dibutyrylguanosine 3',5'-cyclic monophosphate (DBcGMP, 100 microM) or 8-bromoguanosine 3',5'-cyclic monophosphate (8BrcGMP, 100 microM), and it was inhibited by KT5823 (10 microM), a membrane-permeable specific inhibitor of cGMP-dependent protein kinase (PKG). The effect of DBcGMP on channel activity was abolished by the pretreatment of cells with KT5823 (10 microM), but it was observed in the presence of KT5720 (200 nM), a specific inhibitor of PKA. Furthermore, atrial natriuretic peptide (ANP, 10 nM) increased channel activity, which was also prevented by the application of KT5823 (10 microM). In inside-out patches, ATP (3 mM) was required to maintain channel activity, which was inhibited by KT5823 (10 microM), but it was not increased by cGMP (100 microM) alone. The channel activity was increased by the coapplication of PKG (500 U/ml) and cGMP (100 microM). These results suggest that cGMP activates the inwardly rectifying K+ channel in OKP cells through PKG-mediated phosphorylation processes independent of PKA-mediated processes, and that ANP is an agonist which stimulates PKG-mediated processes in the proximal tubule cell. Furthermore, it is suggested that the ATP-dependent channel activity in inside-out patches is maintained at least in part by PKG, which is the membrane-bound catalytic domain.

Functional demonstration of Na+-K+-2Cl- cotransporter activity in isolated, polarized choroid plexus cells

The function of the apical Na+-K+-2Cl- cotransporter in mammalian choroid plexus (CP) is uncertain and controversial. To investigate cotransporter function, we developed a novel dissociated rat CP cell preparation in which single, isolated cells maintain normal polarized morphology. Immunofluorescence demonstrated that in isolated cells the Na+-K+-ATPase, Na+-K+-2Cl- cotransporter, and aquaporin 1 water channel remained localized to the brush border, whereas the Cl-/HCO-3 (anion) exchanger type 2 was confined to the basolateral membrane. We utilized video-enhanced microscopy and cell volume measurement techniques to investigate cotransporter function. Application of 100 microM bumetanide caused CP cells to shrink rapidly. Elevation of extracellular K+ from 3 to 6 or 25 mM caused CP cells to swell 18 and 33%, respectively. Swelling was blocked completely by Na+ removal or by addition of 100 microM bumetanide. Exposure of CP cells to 5 mM BaCl2 induced rapid swelling that was inhibited by 100 microM bumetanide. We conclude that the CP cotransporter is constitutively active and propose that it functions in series with Ba2+-sensitive K+ channels to reabsorb K+ from cerebrospinal fluid to blood.

The epithelial inward rectifier channel Kir7.1 displays unusual K+ permeation properties

Rat and human cDNAs were isolated that both encoded a 360 amino acid polypeptide with a tertiary structure typical of inwardly rectifying K+ channel (Kir) subunits. The new proteins, termed Kir7.1, were <37% identical to other Kir subunits and showed various unique residues at conserved sites, particularly near the pore region. High levels of Kir7.1 transcripts were detected in rat brain, lung, kidney, and testis. In situ hybridization of rat brain sections demonstrated that Kir7.1 mRNA was absent from neurons and glia but strongly expressed in the secretory epithelial cells of the choroid plexus (as confirmed by in situ patch-clamp measurements). In cRNA-injected Xenopus oocytes Kir7.1 generated macroscopic Kir currents that showed a very shallow dependence on external K+ ([K+]e), which is in marked contrast to all other Kir channels. At a holding potential of -100 mV, the inward current through Kir7.1 averaged -3.8 +/- 1.04 microA with 2 mM [K+]e and -4.82 +/- 1.87 microA with 96 mM [K+]e. Kir7.1 has a methionine at position 125 in the pore region where other Kir channels have an arginine. When this residue was replaced by the conserved arginine in mutant Kir7.1 channels, the pronounced dependence of K+ permeability on [K+]e, characteristic for other Kir channels, was restored and the Ba2+ sensitivity was increased by a factor of approximately 25 (Ki = 27 microM). These findings support the important role of this site in the regulation of K+ permeability in Kir channels by extracellular cations.

Cloning and expression of a novel pH-sensitive two pore domain K+ channel from human kidney

A complementary DNA encoding a novel K+ channel, called TASK-2, was isolated from human kidney and its gene was mapped to chromosome 6p21. TASK-2 has a low sequence similarity to other two pore domain K+ channels, such as TWIK-1, TREK-1, TASK-1, and TRAAK (18-22% of amino acid identity), but a similar topology consisting of four potential membrane-spanning domains. In transfected cells, TASK-2 produces noninactivating, outwardly rectifying K+ currents with activation potential thresholds that closely follow the K+ equilibrium potential. As for the related TASK-1 and TRAAK channels, the outward rectification is lost at high external K+ concentration. The conductance of TASK-2 was estimated to be 14.5 picosiemens in physiological conditions and 59.9 picosiemens in symmetrical conditions with 155 mM K+. TASK-2 currents are blocked by quinine (IC50 = 22 microM) and quinidine (65% of inhibition at 100 microM) but not by the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine, and Cs+. They are only slightly sensitive to Ba2+, with less than 17% of inhibition at 1 mM. As TASK-1, TASK-2 is highly sensitive to external pH in the physiological range. 10% of the maximum current was recorded at pH 6. 5 and 90% at pH 8.8. Unlike all other cloned channels with two pore-forming domains, TASK-2 is essentially absent in the brain. In human and mouse, TASK-2 is mainly expressed in the kidney, where in situ hybridization shows that it is localized in cortical distal tubules and collecting ducts. This localization, as well as its functional properties, suggest that TASK-2 could play an important role in renal K+ transport.

Contribution of Na+-H+ exchange to sodium reabsorption in the loop of henle: a microperfusion study in rats

1. The contribution of apical Na+-H+ exchange to sodium reabsorption in the thick ascending limb of the loop of Henle (TALH) in vivo was examined in anaesthetized rats by perfusing loops of Henle of superficial nephrons with solutions containing the Na+-H+ exchange inhibitor, ethyl isopropyl amiloride (EIPA). 2. Using a standard perfusate, no statistically significant effect of EIPA on net sodium reabsorption (JNa) was detected. However, when sodium reabsorption in the pars recta of the proximal tubule was minimized by using a low-sodium perfusate, EIPA reduced JNa from 828 +/- 41 to 726 +/- 37 pmol min-1 (P < 0.05), indicating that apical Na+-H+ exchange can make a small contribution to net sodium reabsorption in the TALH in vivo. This contribution appears to be dependent on the bicarbonate load, since an increase in the latter led to an enhancement of EIPA-sensitive sodium transport. 3. Addition of the Na+-K+-2Cl- cotransport inhibitor, bumetanide, to the low-sodium perfusate reduced baseline JNa to 86 +/- 27 pmol min-1. In this setting, EIPA reduced JNa further, to -24 +/- 18 pmol min-1 (P < 0.05), an effect similar to that seen in the absence of bumetanide. This finding argues against previous suggestions (based on in vitro evidence) that inhibition of the Na+-K+-2Cl- cotransporter leads to an increase in apical Na+-H+ exchange in the TALH.

Formation of intermediate-conductance calcium-activated potassium channels by interaction of Slack and Slo subunits

Large-conductance calcium-activated potassium channels (maxi-K channels) have an essential role in the control of excitability and secretion. Only one gene Slo is known to encode maxi-K channels, which are sensitive to both membrane potential and intracellular calcium. We have isolated a potassium channel gene called Slack that is abundantly expressed in the nervous system. Slack channels rectify outwardly with a unitary conductance of about 25-65 pS and are inhibited by intracellular calcium. However, when Slack is co-expressed with Slo, channels with pharmacological properties and single-channel conductances that do not match either Slack or Slo are formed. The Slack/Slo channels have intermediate conductances of about 60-180 pS and are activated by cytoplasmic calcium. Our findings indicate that some intermediate-conductance channels in the nervous system may result from an interaction between Slack and Slo channel subunits.

Partially active channels produced by PKA site mutation of the cloned renal K+ channel, ROMK2 (kir1.2)

The activity of the cloned renal K+ channel (ROMK2) is dependent on a balance between phosphorylation and dephosphorylation. There are only three protein kinase A (PKA) sites on ROMK2, with the phosphorylated residues being serine-25 (S25), serine-200 (S200), and serine-294 (S294) (Z.-C. Xu, Y. Yang, and S. C. Hebert. J. Biol. Chem. 271: 9313-9319, 1996). We previously mutated these sites from serine to alanine to study the contribution of each site to overall channel function. Here we have studied each of these single PKA site mutants using the single-channel configuration of the patch-clamp technique. Both COOH-terminal mutations at sites S200A and S294A showed a decreased open channel probability (Po), whereas the NH2-terminal mutation at site S25A showed no change in Po compared with wild-type ROMK2. The decrease in Po for the S200A and S294A mutants was caused by the additional presence of a long closed state. In contrast, the occurrence of the S25A channel was approximately 66% less, suggesting fewer active channels at the membrane. The S200A and S294A channels had different kinetics compared with wild-type ROMK2 channels, showing an increased occurrence of sublevels. Similar kinetics were observed when wild-type ROMK2 was excised and exposed to dephosphorylating conditions, indicating that these effects are specifically a property of the partially phosphorylated channel and not due to an unrelated effect of the mutation.

Tubule function in transgenic mice

Many transporters involved in renal electrolyte transport have recently been identified and cloned. The availability of genetically manipulated mice, especially transgenic knockout and overexpression models, has made it possible to examine ion transport in kidney tubules in the absence of specific transporters whose function in defined tubule segments is well known. Such selective alterations in transport functions are also useful to investigate adaptive mechanisms by which the kidney compensates for specific transport lesions. Examples of mouse models displaying altered renal transport function include targeted disruption of genes encoding the Na-H exchanger isoforms NHE2 and NHE3, the thiazide-sensitive Na-Cl cotransporter TSC, CFTR, and the colonic isoform of the H,K-ATPase. Moreover, mice with null mutation in the Na-H exchanger isoform NHE1 have been also identified. In addition, a strain of mice with enhanced H,K-ATPase expression due to a defective endocytosis signal has been developed. Other transporter knockout models will soon become available. In this review we focus on the physiological characterization of renal tubule transport in animals with well-defined genetic transport lesions.

The A kinase anchoring protein is required for mediating the effect of protein kinase A on ROMK1 channels

In the present study, we have used the two-electrode voltage-clamp and patch-clamp techniques to study the effects of forskolin and cAMP on the ROMK1 channels, which are believed to be the native K+ secretory channels in the kidney. Addition of 1 microM forskolin or 100 microM 8-bromo-cAMP, within 10 min, has no significant effect on the current of ROMK1 channels expressed in Xenopus oocytes. In contrast, application of 1 microM forskolin, within 3 min, significantly increased whole-cell K+ current by 35%, when ROMK1 channels were coexpressed with the A kinase anchoring protein AKAP79, which was cloned from neuronal tissue. Two lines of evidence indicate that the effect of forskolin is mediated by a cAMP-dependent pathway: (i) Addition of 100 microM 8-bromo-cAMP mimics the effect of forskolin and (ii) the effect of forskolin and cAMP is not additive. That AKAP is required for the effect of cAMP is further supported by experiments in which addition of ATP (100 microM) and cAMP (100 microM) restored the activity of run-down ROMK1 channels in inside-out patches in oocytes that coexpressed ROMK1 and AKAP79 but not in those that expressed ROMK1 alone. Moreover, when we used RII, the regulatory subunit of type II protein kinase A, in an overlay assay, we identified a RII-binding protein in membranes obtained from the kidney cortex but not in membranes from oocytes. This suggests that the insensitivity of ROMK1 channels to forskolin and cAMP is due to the absence of AKAPs. We conclude that AKAP may be a critical component that mediates the effect of protein kinase A on the ROMK channels in the kidney.

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.

Molecular characterization of a cDNA that encodes six isoforms of a novel murine A kinase anchor protein

We have cloned cDNA that encodes six novel A kinase anchor proteins (collectively named AKAP-KL). AKAP-KL diversity is generated by alternative mRNA splicing and utilization of two translation initiation codons. AKAP-KL polypeptides are evident in lung, kidney, and cerebellum, but are absent from many tissues. Different isoforms predominate in different tissues. Thus, AKAP-KL expression is differentially regulated in vivo. All AKAP-KL isoforms contain a 20-residue domain that avidly binds (Kd approximately 10 nM) regulatory subunits (RII) of protein kinase AII and is highly homologous with the RII tethering site in neuronal AKAP75. The distribution of AKAP-KL is strikingly asymmetric (polarized) in situ. Anchor protein accumulates near the inner, apical surface of highly polarized epithelium in tubules of nephrons. Both RII and AKAP-KL are enriched at an intracellular site that lies just below the plasma membrane of alveolar epithelial cells in lung. AKAP-KL interacts with and modulates the structure of the actin cytoskeleton in transfected cells. We also demonstrate that the tethering domain of AKAP-KL avidly ligates RII subunits in intact cells. AKAP-KL may be involved in (a) establishing polarity in signaling systems and (b) physically and functionally integrating PKAII isoforms with downstream effectors to capture, amplify, and precisely focus diffuse, trans-cellular signals carried by cAMP.

Dissociation of K channel density and ROMK mRNA in rat cortical collecting tubule during K adaptation

The density of conducting K channels in the apical membrane of the rat cortical collecting tubule (CCT) is increased by a high-K diet. To see whether this involved increased abundance of mRNA coding for K channel protein, we measured the relative amounts of mRNA for ROMK, the clone of the gene thought to encode the secretory K channel in the CCT. Tubules were isolated and fixed for in situ hybridization with a probe based on the ROMK sequence. Radiolabeled probe associated with the tubule was quantified using densitometric analysis of the autoradiographic images of the tubules. The densitometry signal was shown to be proportional to the amount of radioactive probe in the sample and to the time of exposure of the film. The technique was able to detect an approximately twofold increase in the abundance of mRNA coding for the water channel aquaporin 3 (AQP3), in response to a 30-h dehydration period. Tubules from rats fed a normal diet or a high-K (10% KCl) diet had equal amounts of ROMK mRNA. This suggests that an increase in the abundance of mRNA does not underlie the increase in channel density observed under these conditions.

Basolateral pH-sensitive K+ channels mediate membrane potential of proximal tubule cells in bullfrog kidney

The present study investigated basolateral K+ channels and their pH-sensitivity in isolated bullfrog proximal tubule cells by using the patch-clamp technique, and compared channel activity with the basolateral membrane potential (EM) and intracellular pH (pHi) monitored by using double-barreled H+-selective microelectrodes in perfused bullfrog proximal tubules. In the patch-clamp experiments, K+ channels with inward slope conductance of about 50 pS were observed in the basolateral membrane of isolated proximal tubule cells in cell-attached patches. Raising pH of the bath with HCO3(-)-free HEPES Ringer solutions from 7.7 (control) to 8.2 in the presence of an H+ ionophore, FCCP (2 micro M), enhanced channel activity to 126.4% of controls; lowering bath pH to 7.2 and to 6.7 reduced channel activity to 26.4 and to 1.7% of controls. Microelectrode experiments in bullfrog proximal tubules perfused with HCO3(-) -free HEPES Ringer solutions showed that EM and pHi in control conditions of peritubular pH 7.7 without FCCP were -52.6 mV and 7.45. Raising peritubular pH to 8.2 in the presence of FCCP (2 micro M) increased EM and pHi to -61.8 mV and 7.73; lowering it to 7. 2 and to 6.7 decreased EM and pHi to -28.6 mV and 7.25 and to -10.6 mV and 6.95. These results suggest that changes in EM in response to cellular alkalinization or acidification made by HCO3(-)-free HEPES solutions are produced primarily by changes in activity of the pH-sensitive K+ channels.