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

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

Angiotensin II type 2 receptor regulates ROMK-like K⁺ channel activity in the renal cortical collecting duct during high dietary K⁺ adaptation

The kidney adjusts K⁺ excretion to match intake in part by regulation of the activity of apical K⁺ secretory channels, including renal outer medullary K⁺ (ROMK)-like K⁺ channels, in the cortical collecting duct (CCD). ANG II inhibits ROMK channels via the ANG II type 1 receptor (AT1R) during dietary K⁺ restriction. Because AT1Rs and ANG II type 2 receptors (AT2Rs) generally function in an antagonistic manner, we sought to characterize the regulation of ROMK channels by the AT2R. Patch-clamp experiments revealed that ANG II increased ROMK channel activity in CCDs isolated from high-K⁺ (HK)-fed but not normal K⁺ (NK)-fed rats. This response was blocked by PD-123319, an AT2R antagonist, but not by losartan, an AT1R antagonist, and was mimicked by the AT2R agonist CGP-42112. Nitric oxide (NO) synthase is present in CCD cells that express ROMK channels. Blockade of NO synthase with N-nitro-l-arginine methyl ester and free NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt completely abolished ANG II-stimulated ROMK channel activity. NO enhances the synthesis of cGMP, which inhibits phosphodiesterases (PDEs) that normally degrade cAMP; cAMP increases ROMK channel activity. Pretreatment of CCDs with IBMX, a broad-spectrum PDE inhibitor, or cilostamide, a PDE3 inhibitor, abolished the stimulatory effect of ANG II on ROMK channels. Furthermore, PKA inhibitor peptide, but not an activator of the exchange protein directly activated by cAMP (Epac), also prevented the stimulatory effect of ANG II. We conclude that ANG II acts at the AT2R to stimulate ROMK channel activity in CCDs from HK-fed rats, a response opposite to that mediated by the AT1R in dietary K⁺-restricted animals, via a NO/cGMP pathway linked to a cAMP-PKA pathway.

Function of cGMP-dependent protein kinase II in volume load-induced diuresis

Atrial natriuretic peptide (ANP)/cGMPs cause diuresis and natriuresis. Their downstream effectors beyond cGMP remain unclear. To elucidate a probable function of cGMP-dependent protein kinase II (cGKII), we investigated renal parameters in different conditions (basal, salt diets, starving, water load) using a genetically modified mouse model (cGKII-KO), but did not detect any striking differences between WT and cGKII-KO. Thus, cGKII is proposed to play only a marginal role in the adjustment of renal concentration ability to varying salt loads without water restriction or starving conditions. When WT mice were subjected to a volume load (performed by application of a 10-mM glucose solution (3% of BW) via feeding needle), they exhibited a potent diuresis. In contrast, urine volume was decreased significantly in cGKII-KO. We showed that AQP2 plasma membrane (PM) abundance was reduced for about 50% in WT upon volume load, therefore, this might be a main cause for the enhanced diuresis. In contrast, cGKII-KO mice almost completely failed to decrease AQP2-PM distribution. This significant difference between both genotypes is not induced by an altered p-Ser256-AQP2 phosphorylation, as phosphorylation at this site decreases similarly in WT and KO. Furthermore, sodium excretion was lowered in cGKII-KO mice during volume load. In summary, cGKII is only involved to a minor extent in the regulation of basal renal concentration ability. By contrast, cGKII-KO mice are not able to handle an acute volume load. Our results suggest that membrane insertion of AQP2 is inhibited by cGMP/cGKII.

Genetic modifiers of hypertension in soluble guanylate cyclase α1-deficient mice

Nitric oxide (NO) plays an essential role in regulating hypertension and blood flow by inducing relaxation of vascular smooth muscle. Male mice deficient in a NO receptor component, the α1 subunit of soluble guanylate cyclase (sGCα1), are prone to hypertension in some, but not all, mouse strains, suggesting that additional genetic factors contribute to the onset of hypertension. Using linkage analyses, we discovered a quantitative trait locus (QTL) on chromosome 1 that was linked to mean arterial pressure (MAP) in the context of sGCα1 deficiency. This region is syntenic with previously identified blood pressure-related QTLs in the human and rat genome and contains the genes coding for renin. Hypertension was associated with increased activity of the renin-angiotensin-aldosterone system (RAAS). Further, we found that RAAS inhibition normalized MAP and improved endothelium-dependent vasorelaxation in sGCα1-deficient mice. These data identify the RAAS as a blood pressure-modifying mechanism in a setting of impaired NO/cGMP signaling.

Nitric oxide inhibits the expression of AT1 receptors in neurons

We have previously observed an increased of angiotensin II (ANG II) type 1 receptor (AT(1)R) with enhanced AT(1)R-mediated sympathetic outflow and concomitant downregulation of neuronal nitric oxide (NO) synthase (nNOS) with reduced NO-mediated inhibition from the paraventricular nucleus (PVN) in rats with heart failure. To test the hypothesis that NO exerts an inhibitory effect on AT(1)R expression in the PVN, we used primary cultured hypothalamic cells of neonatal rats and neuronal cell line NG108-15 as in vitro models. In hypothalamic primary culture, NO donor sodium nitroprusside (SNP) induced dose-dependent decreases in mRNA and protein of AT(1)R (10(-5) M SNP, AT(1)R protein was 10 ± 2% of control level) while NOS inhibitor N(G)-monomethyl-l-arginine (l-NMMA) induced dose-dependent increases in mRNA and protein levels of AT(1)R (10(-5) M l-NMMA, AT(1)R protein was 148 ± 8% of control level). Similar effects of SNP and l-NMMA on AT(1)R expression were also observed in NG108-15 cell line (10(-6) M SNP, AT(1)R protein was 30 ± 4% of control level while at the dose of 10(-6) M l-NMMA, AT(1)R protein was 171 ± 15% of the control level). Specific inhibition of nNOS, using antisense, caused an increase in AT(1)R expression while overexpression of nNOS, using adenoviral gene transfer (Ad.nNOS), caused an inhibition of AT(1)R expression in NG108 cells. Antisense nNOS transfection augmented the increase while Ad.nNOS infection blunted the increase in intracellular calcium concentration in response to ANG II treatment in NG108 cells. In addition, downregulation of AT(1)R mRNA as well as protein level in neuronal cell line in response to S-nitroso-N-acetyl pencillamine (SNAP) treatment was blocked by protein kinase G (PKG) inhibitor, while the peroxynitrite scavenger deforxamine had no effect. These results suggest that NO acts as an inhibitory regulator of AT(1)R expression and the activation of PKG is the required step in the regulation of AT(1)R gene expression via cGMP-dependent signaling pathway.

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.

Delayed and acute effects of interferon-gamma on activity of an inwardly rectifying K+ channel in cultured human proximal tubule cells

The activity of an inwardly rectifying K(+) channel in cultured human renal proximal tubule cells (RPTECs) is stimulated and inhibited by nitric oxide (NO) at low and high concentrations, respectively. In this study, we investigated the effects of IFN-gamma, one of the cytokines which affect the expression of inducible NO synthase (iNOS), on intracellular NO and channel activity of RPTECs, using RT-PCR, NO imaging, and the cell-attached mode of the patch-clamp technique. Prolonged incubation (24 h) of cells with IFN-gamma (20 ng/ml) enhanced iNOS mRNA expression and NO production. In these cells, a NOS inhibitor, N(omega)-nitro-l-arginine methyl ester (l-NAME; 100 microM), elevated channel activity, suggesting that NO production was so high as to suppress the channel. This indicated that IFN-gamma would chronically suppress channel activity by enhancing NO production. Acute effects of IFN-gamma was also examined in control cells. Simple addition of IFN-gamma (20 ng/ml) to the bath acutely stimulated channel activity, which was abolished by inhibitors of IFN-gamma receptor-associated Janus-activated kinase [P6 (1 microM) and AG490 (10 microM)]. However, l-NAME did not block the acute effect of IFN-gamma. Indeed, IFN-gamma did not acutely affect NO production. Moreover, the acute effect was not blocked by inhibition of PKA, PKG, and phosphatidylinositol 3-kinase (PI3K). We conclude that IFN-gamma exerted a delayed suppressive effect on K(+) channel activity by enhancing iNOS expression and an acute stimulatory effect, which was independent of either NO pathways or phosphorylation processes mediated by PKA, PKG, and PI3K in RPTECs.

Kir4.1/Kir5.1 channel forms the major K+ channel in the basolateral membrane of mouse renal collecting duct principal cells

K(+) channels in the basolateral membrane of mouse cortical collecting duct (CCD) principal cells were identified with patch-clamp technique, real-time PCR, and immunohistochemistry. In cell-attached membrane patches, three K(+) channels with conductances of approximately 75, 40, and 20 pS were observed, but the K(+) channel with the intermediate conductance (40 pS) predominated. In inside-out membrane patches exposed to an Mg(2+)-free medium, the current-voltage relationship of the intermediate-conductance channel was linear with a conductance of 38 pS. Addition of 1.3 mM internal Mg(2+) had no influence on the inward conductance (G(in) = 35 pS) but reduced outward conductance (G(out)) to 13 pS, yielding a G(in)/G(out) of 3.2. The polycation spermine (6 x 10(-7) M) reduced its activity on inside-out membrane patches by 50% at a clamp potential of 60 mV. Channel activity was also dependent on intracellular pH (pH(i)): a sigmoid relationship between pH(i) and channel normalized current (NP(o)) was observed with a pK of 7.24 and a Hill coefficient of 1.7. By real-time PCR on CCD extracts, inwardly rectifying K(+) (Kir)4.1 and Kir5.1, but not Kir4.2, mRNAs were detected. Kir4.1 and Kir5.1 proteins cellularly colocalized with aquaporin 2 (AQP2), a specific marker of CCD principal cells, while AQP2-negative cells (i.e., intercalated cells) showed no staining. Dietary K(+) had no influence on the properties of the intermediate-conductance channel, but a Na(+)-depleted diet increased its open probability by approximately 25%. We conclude that the Kir4.1/Kir5.1 channel is a major component of the K(+) conductance in the basolateral membrane of mouse CCD principal cells.

Arachidonic acid inhibits basolateral K channels in the cortical collecting duct via cytochrome P-450 epoxygenase-dependent metabolic pathways

We used the patch-clamp technique to study the effect of arachidonic acid (AA) on basolateral 18-pS K channels in the principal cell of the cortical collecting duct (CCD) of the rat kidney. Application of AA inhibited the 18-pS K channels in a dose-dependent manner and 10 microM AA caused a maximal inhibition. The effect of AA on the 18-pS K channel was specific because application of 11,14,17-eicosatrienoic acid had no effect on channel activity. Also, the inhibitory effect of AA on the 18-pS K channels was abolished by blocking cytochrome P-450 (CYP) epoxygenase with N-methylsulfonyl-6-(propargyloxyphenyl)hexanamide (MS-PPOH) but was not affected by inhibiting CYP omega-hydroxylase or cyclooxygenase. The notion that the inhibitory effect of AA was mediated by CYP epoxygenase-dependent metabolites was further supported by the observation that application of 100 nM 11,12-epoxyeicosatrienoic acid (EET) mimicked the effect of AA and inhibited the basolateral 18-pS K channels. In contrast, addition of either 5,6-, 8,9-, or 14,15-EET failed to inhibit the 18-pS K channels. Moreover, application of 11,12-EET was still able to inhibit the 18-pS K channels in the presence of MS-PPOH. This suggests that 11,12-EET is a mediator for the AA-induced inhibition of the 18-pS K channels. We conclude that AA inhibits basolateral 18-pS K channels by a CYP epoxygenase-dependent pathway and that 11,12-EET is a mediator for the effect of AA on basolateral K channels in the CCD.

Uroguanylin and guanylin regulate transport of mouse cortical collecting duct independent of guanylate cyclase C

BACKGROUND

Electrolyte and water homeostasis mostly depend on differentially regulated intestinal and renal transport. Guanylin and uroguanylin were proposed as first hormones linking intestinal with renal electrolyte and water transport, which is disturbed in pathophysiology. Guanylate cyclase C is the intestinal receptor for these peptides, but in guanylate cyclase C-deficient mice renal effects are retained. Unlike for the intestine the sites of renal actions and cellular mechanisms of guanylin peptides are still unclear.

METHODS

After first data on proximal tubular effects in this study their effects are examined in detail in mouse cortical collecting duct (CCD). Effects of guanylin peptides on principal cells of isolated mouse CCD were studied by slow whole-cell patch-clamp analysis, reverse transcription-polymerase chain reaction (RT-PCR), and microfluorimetric measurements of intracellular Ca2+.

RESULTS

Guanylin peptides depolarized or hyperpolarized principal cells. Whereas 8-Br-cyclic guanosine monophosphate (8-Br-cGMP) hyperpolarized, 8-Br-cyclic adenosine monophosphate (8-Br-cAMP) depolarized principal cells. All effects of guanylin peptides were inhibited by Ba2+. Hyperpolarizations were blocked by clotrimazole or protein kinase G (PKG) inhibition, suggesting an involvement of basolateral Ca2+- and cGMP-dependent K+ channels. Effects remained in CCD isolated from guanylate cyclase C-deficient mice. Depolarizations were inhibited by arachidonic acid or inhibition of phospholipase A2 (PLA2), but not by protein kinase A (PKA) inhibition. Conclusion. These results suggest the existence of two signaling pathways for guanylin peptides in principal cells of mouse CCD. One pathway is cGMP- and PKG-dependent but not mediated by guanylate cyclase C, the second involves PLA2 and arachidonic acid. The first pathway most likely leads to an activation of the basolateral K+-conductance while the latter probably results in decreased activity of ROMK channels in the luminal membrane.

Mineralocorticoids decrease the activity of the apical small-conductance K channel in the cortical collecting duct

We used the patch-clamp technique to examine the effect of DOCA treatment (2 mg/kg) on the apical small-conductance K (SK) channels, epithelial Na channels (ENaC), and the basolateral 18-pS K channels in the cortical collecting duct (CCD). Treatment of rats with DOCA for 6 days significantly decreased the plasma K from 3.8 to 3.1 meq and reduced the activity of the SK channel, defined as NP(o), from 1.3 in the CCD of control rats to 0.6. In contrast, DOCA treatment significantly increased ENaC activity from 0.01 to 0.53 and the basolateral 18-pS K channel activity from 0.67 to 1.63. Moreover, Western blot analysis revealed that DOCA treatment significantly increased the expression of the nonreceptor type of protein tyrosine kinase (PTK), cSrc, and the tyrosine phosphorylation of ROMK in the renal cortex and outer medulla. The possibility that decreases in apical SK channel activity induced by DOCA treatment were the result of stimulation of PTK activity was further supported by experiments in which inhibition of PTK with herbimycin A significantly increased NP(o) from 0.6 to 2.1 in the CCD from rats receiving DOCA. Also, when rats were fed a high-K (10%) diet, DOCA treatment did not increase the expression of c-Src and decrease the activity of the SK channel in the CCD. We conclude that DOCA treatment decreased the apical SK channel activity in rats on a normal-K diet and that an increase in PTK expression may be responsible for decreased channel activity in the CCD from DOCA-treated rats.

Renal potassium channels: recent developments

PURPOSE OF REVIEW

A variety of K+ channels have been identified with the patch-clamp technique and molecular cloning in the kidney. However, it is still a challenging task to determine the location and function of the cloned K+ channels in the corresponding nephron segment. The aim of the present review is to update the recent developments regarding the location and function of the cloned K+ channels in the native tubule. Also, the review describes the new regulatory mechanism of renal outer-medullary K (ROMK) channels and the role of Ca(2+)-activated maxi K+ channels in flow-dependent K+ secretion.

RECENT FINDINGS

Several types of voltage-gated K+ (Kv) channel, such as KCNQ1, KCNA10 and Kv1.3, are highly expressed at the apical membrane of proximal tubules and distal tubules. They may participate in stabilizing the cell membrane potential. Moreover, studies performed in ROMK-knockout mice have shown that the apical 70 pS K+ channel is absent in the thick ascending limb in these mice, suggesting that the ROMK channel is also involved in forming the apical 70 pS K+ channel in the thick ascending limb. Three important kinases, protein tyrosine kinase, serum- and glucocorticoid-inducible kinase and with-no-lysine kinase, have been suggested to regulate the ROMK channel density in the cortical collecting duct. Low K+ intake increases protein tyrosine kinase expression and tyrosine phosphorylation of ROMK channels. Coexpression of with-no-lysine kinase with the ROMK channel decreases K+ current whereas serum- and glucocorticoid-inducible kinase 1 stimulates the ROMK current in oocytes in the presence of Na/H exchanger regulatory factor 2. The Ca-activated maxi K+ channel has been shown to be activated by an increase in flow rate in the rabbit cortical collecting duct.

SUMMARY

The voltage-gated K+ channels are expressed in a variety of nephron segments and play a role in stabilization of cell membrane potential. With-no-lysine kinase and serum- and glucocorticoid-inducible kinase 1 have been shown to regulate ROMK1 channels. Protein tyrosine kinase mediates the effect of K+ intake on K+ secretion by stimulation of tyrosine phosphorylation of ROMK1 channels. The Ca-activated maxi K+ channel plays a role in flow-dependent K+ secretion in the distal nephron.

Protein kinase A affectsGalleria mellonella(Insecta: Lepidoptera) larval haemocyte non‐self responses

We used the protein kinase A (PKA) specific activator Sp-8-Br-cAMPS and type I inhibitor Rp-8-Br-cAMPS alone and in combination to define the role of PKA in the non-self responses of larval Galleria mellonella haemocytes in vitro and in vivo. Active PKA depressed haemocyte responses whereas PKA inhibition enhanced activities, including bacterial phagocytosis, the number of haemocytes with adherent bacteria, bacterial-induced haemocytic protein release and haemocyte adhesion to slides in vitro, as well as in vivo bacterial removal from the haemolymph. Non-attached haemocytes had more PKA activity than attached haemocytes; therefore, active PKA limited haemocyte response to foreign materials. We found that (i) PKA inhibitor alone induced non-self responses, including haemocyte protein discharge and lowered haemocyte counts in vivo, and induced nodulation; (ii) the enzyme activator produced effects opposite to those of the inhibitor; and (iii) together, the modulators offset each others' effects and influenced haemocyte lysate PKA activity. These findings establish PKA as a mediator of haemocytic non-self responses.

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.

Basolateral K+ conductance in principal cells of rat CCD

Whole cell K+ current was measured by forming seals on the luminal membrane of principal cells in split-open rat cortical collecting ducts. The mean inward, Ba2+-sensitive conductance, with 40 mM extracellular K+, was 76 +/- 12 and 141 +/- 22 nS/cell for animals on control and high-K+ diets, respectively. The apical contribution to this was estimated to be 3 and 16 nS/cell on control and high-K+ diets, respectively. To isolate the basolateral component of whole cell current, we blocked ROMK channels with either tertiapin-Q or intracellular acidification to pH 6.6. The current was weakly inward rectifying when bath K+ was > or =40 mM but became more strongly rectified when bath K+ was lowered into the physiological range. Including 1 mM spermine in the pipette moderately increased rectification, but most of the outward current remained. The K+ current did not require intracellular Ca2+ and was not inhibited by 3 mM ATP in the pipette. The negative log of the acidic dissociation constant (pKa) was approximately 6.5. Block by extracellular Ba2+ was voltage dependent with apparent Ki at -40 and -80 mV of approximately 160 and approximately 80 microM, respectively. The conductance was TEA insensitive. Substitution of Rb+ or NH4+ for K+ led to permeability ratios of 0.65 +/- 0.07 and 0.15 +/- 0.02 and inward conductance ratios of 0.17 +/- 0.03 and 0.57 +/- 0.09, respectively. Analysis of Ba2+-induced noise, with 40 mM extracellular K+, yielded single-channel currents of 0.39 +/- 0.04 and -0.28 +/- 0.04 pA at voltages of 0 and -40 mV, respectively, and a single-channel conductance of 17 +/- 1 pS.

Angiotensin II stimulates basolateral K channels in rat cortical collecting ducts

We used the patch-clamp technique to study the effects of angiotensin II (ANG II) on basolateral K channels in cortical collecting ducts (CCDs). Application of ANG II (100 pM-100 nM) increased the activity of basolateral 18-pS K channels. This effect of ANG II was completely abolished by losartan, which is an antagonist of type 1 angiotensin (AT(1)) receptors. In contrast, inhibition of type 2 angiotensin (AT(2)) receptors did not block the stimulatory effect of ANG II. Also, application of ANG II significantly increased intracellular Ca(2+) concentrations, which were measured with fura 2 dye. To explore the role of Ca(2+)-dependent pathways in the regulation of basolateral K channels, the effects of ANG II on channel activity were examined in the presence of arachidonyltrifluoromethyl ketone to inhibit phospholipase A(2) (PLA(2)), GF-109203X [a protein kinase C (PKC) inhibitor], and N(G)-nitro-l-arginine methyl ester (l-NAME) to inhibit nitric oxide synthase. Inhibition of either PLA(2) or PKC did not block the effect of ANG II on basolateral K-channel activity. However, the stimulatory effect of ANG II was absent in the CCDs treated with l-NAME. Moreover, addition of the membrane-permeant 8-bromo-guanosine 3',5'-cyclic monophosphate (8-bromo-cGMP) not only increased channel activity but also abolished the stimulatory effect of ANG II on channel activity. We conclude that ANG II increases basolateral K-channel activity via the stimulation of AT(1) receptors, and the stimulatory effect of ANG II is mediated by a nitric oxide-dependent cGMP pathway.

A trail of research on potassium

A complex pump-leak system involving both active and passive transport mechanisms is responsible for the appropriate distribution of potassium (K) between the intra- and extracellular fluid compartments. In addition, the kidneys, and to a lesser extent the colon, safeguard maintenance of the narrow range of low K concentrations in the extracellular fluid. Early renal clearance studies showed that K is normally both reabsorbed and secreted by renal tubules, and that regulated secretion is the major source of K excretion. Studies at the tubule and cell level have localized secretion and reabsorption of K to principal and intercalated cells in the collecting ducts. Measurements of the electrochemical driving forces across individual cell membranes have permitted the characterization of specific ATPases, K channels and K cotransporters and also provided insights into the molecular structure of individual transporters that regulate K excretion.

Protein kinase G activates inwardly rectifying K(+) channel in cultured human proximal tubule cells

An ATP-regulated inwardly rectifying K(+) channel, whose activity is enhanced by PKA, is present in the plasma membrane of cultured human proximal tubule cells. In this study, we investigated the effects of PKG on this K(+) channel, using the patch-clamp technique. In cell-attached patches, bath application of a membrane-permeant cGMP analog, 8-bromoguanosine 3',5'-monophosphate (8-BrcGMP; 100 microM), stimulated channel activity, whereas application of a PKG-specific inhibitor, KT-5823 (1 microM), reduced the activity. Channel activation induced by 8-BrcGMP was observed even in the presence of a PKA-specific inhibitor, KT-5720 (500 nM), which was abolished by KT-5823. Direct effects of cGMP and PKG were examined with inside-out patches in the presence of 1 mM MgATP. Although cytoplasmic cGMP (100 microM) alone had little effect on channel activity, subsequent addition of PKG (500 U/ml) enhanced it. Furthermore, bath application of atrial natriuretic peptide (ANP; 20 nM) in cell-attached patches stimulated channel activity, which was blocked by KT-5823. In conclusion, cGMP/PKG-dependent processes participate in activating the ATP-regulated K(+) channel and producing the stimulatory effect of ANP on channel activity.

Cyclic GMP signaling in podocytes

Natriuretic peptides (NP), together with nitric oxide (NO) are powerful relaxing factors acting via a common second messenger, cyclic GMP (cGMP). Together with other vasoactive modulators, these vasorelaxing factors play an essential role in regulating the function of kidney glomeruli. The presence of NP receptors in podocytes has been well documented. Recently, also mRNA for soluble guanylate cyclase, the NO receptor, has been shown in these cells. Stimulation of podocytes with atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP), and NO donors results in considerable upregulation of cellular cGMP synthesis. The podocyte foot processes contain a highly organized network of microfilaments adhering to the glomerular basement membrane (GBM). Changes in podocyte cytoskeleton accompanied by detachment of the cells from the GBM are closely associated with many glomerulopathies. The contractile apparatus in the podocyte foot processes seems to be an obvious target for the cyclic GMP signaling cascade. However, little is known about implications of the cGMP synthesis in these cells. We briefly review the current art regarding generation and modulation of cyclic GMP levels in podocytes. We discuss also the possible targets for this secondary messenger as well as its functional role in podocytes.

Ca2+ mediates the effect of inhibition of Na+-K+-ATPase on the basolateral K+ channels in the rat CCD

We investigated the effect of inhibiting Na+-K+-ATPase on the basolateral 18-pS K+ channel in the cortical collecting duct (CCD) of the rat kidney. Inhibiting Na+-K+-ATPase with strophanthidin decreased the activity of the 18-pS K+ channel and increased the intracellular Ca2+ to 420 nM. Removal of extracellular Ca2+ abolished the effect of strophanthidin. When intracellular Ca2+ was raised with 5 microM ionomycin or A-23187 to 300, 400, and 500 nM, the activity of the 18-pS K+ channel in cell-attached patches fell by 40, 85, and 96%, respectively. To explore the mechanism of Ca2+-induced inhibition, the effect of 400 nM Ca2+ on channel activity was studied in the presence of calphostin C, an inhibitor of protein kinase C, or KN-93 and KN-62, inhibitors of calmodulin-dependent kinase II. Addition of calphostin C or KN-93 or KN-62 failed to block the inhibitory effect of high concentrations of Ca2+ . This suggested that the inhibitory effect of high concentrations of Ca2+ was not mediated by protein kinase C or calmodulin-dependent kinase II pathways. To examine the possibility that the inhibitory effect of high concentrations of Ca2+ was mediated by the interaction of nitric oxide with superoxide, we investigated the effect of 400 nM Ca2+ on channel activity in the presence of 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron) or N(omega)-nitro-L-arginine methyl ester. Pretreatment of the tubules with 4,5-dihydroxy-1,3-benzenedisulfonic acid or N(omega)-nitro-L-arginine methyl ester completely abolished the inhibitory effect of 400 nM Ca2+ on channel activity. Moreover, application of 4,5-dihydroxy-1,3-benzenedisulfonic acid reversed the inhibitory effect of strophanthidin. We conclude that the effect of inhibiting Na+-K+-ATPase is mediated by intracellular Ca2+ and the inhibitory effect of high concentrations of Ca2+ is the result of interaction of nitric oxide with superoxide.