Potassium Channelopathies and Gastrointestinal Ulceration.

Potassium channels and transporters maintain potassium homeostasis and play significant roles in several different biological actions via potassium ion regulation. In previous decades, the key revelations that potassium channels and transporters are involved in the production of gastric acid and the regulation of secretion in the stomach have been recognized. Drugs used to treat peptic ulceration are often potassium transporter inhibitors. It has also been reported that potassium channels are involved in ulcerative colitis. Direct toxicity to the intestines from nonsteroidal anti-inflammatory drugs has been associated with altered potassium channel activities. Several reports have indicated that the long-term use of the antianginal drug Nicorandil, an adenosine triphosphate-sensitive potassium channel opener, increases the chances of ulceration and perforation from the oral to anal regions throughout the gastrointestinal (GI) tract. Several of these drug features provide further insights into the role of potassium channels in the occurrence of ulceration in the GI tract. The purpose of this review is to investigate whether potassium channelopathies are involved in the mechanisms responsible for ulceration that occurs throughout the GI tract.

Romk1 Knockout Mice Do Not Produce Bartter Phenotype but Exhibit Impaired K Excretion.

Romk knock-out mice show a similar phenotype to Bartter syndrome of salt wasting and dehydration due to reduced Na-K-2Cl-cotransporter activity. At least three ROMK isoforms have been identified in the kidney; however, unique functions of any of the isoforms in nephron segments are still poorly understood. We have generated a mouse deficient only in Romk1 by selective deletion of the Romk1-specific first exon using an ES cell Cre-LoxP strategy and examined the renal phenotypes, ion transporter expression, ROMK channel activity, and localization under normal and high K intake. Unlike Romk(-/-) mice, there was no Bartter phenotype with reduced NKCC2 activity and increased NCC expression in Romk1(-/-) mice. The small conductance K channel (SK) activity showed no difference of channel properties or gating in the collecting tubule between Romk1(+/+) and Romk1(-/-) mice. High K intake increased SK channel number per patch and increased the ROMK channel intensity in the apical membrane of the collecting tubule in Romk1(+/+), but such regulation by high K intake was diminished with significant hyperkalemia in Romk1(-/-) mice. We conclude that 1) animal knockouts of ROMK1 do not produce Bartter phenotype. 2) There is no functional linking of ROMK1 and NKCC2 in the TAL. 3) ROMK1 is critical in response to high K intake-stimulated K(+) secretion in the collecting tubule.

Src-family protein tyrosine kinase phosphorylates WNK4 and modulates its inhibitory effect on KCNJ1 (ROMK).

With-no-lysine kinase 4 (WNK4) inhibits the activity of the potassium channel KCNJ1 (ROMK) in the distal nephron, thereby contributing to the maintenance of potassium homeostasis. This effect is inhibited via phosphorylation at Ser1196 by serum/glucocorticoid-induced kinase 1 (SGK1), and this inhibition is attenuated by the Src-family protein tyrosine kinase (SFK). Using Western blot and mass spectrometry, we now identify three sites in WNK4 that are phosphorylated by c-Src: Tyr(1092), Tyr(1094), and Tyr(1143), and show that both c-Src and protein tyrosine phosphatase type 1D (PTP-1D) coimmunoprecipitate with WNK4. Mutation of Tyr(1092) or Tyr(1143) to phenylalanine decreased the association of c-Src or PTP-1D with WNK4, respectively. Moreover, the Tyr1092Phe mutation markedly reduced ROMK inhibition by WNK4; this inhibition was completely absent in the double mutant WNK4(Y1092/1094F). Similarly, c-Src prevented SGK1-induced phosphorylation of WNK4 at Ser(1196), an effect that was abrogated in the double mutant. WNK4(Y1143F) inhibited ROMK activity as potently as wild-type (WT) WNK4, but unlike WT, the inhibitory effect of WNK4(Y1143F) could not be reversed by SGK1. The failure to reverse WNK4(Y1143F)-induced inhibition of ROMK by SGK1 was possibly due to enhancing endogenous SFK effect on WNK4 by decreasing the WNK4-PTP-1D association because inhibition of SFK enabled SGK1 to reverse WNK4(Y1143F)-induced inhibition of ROMK. We conclude that WNK4 is a substrate of SFKs and that the association of c-Src and PTP-1D with WNK4 at Tyr(1092) and Tyr(1143) plays an important role in modulating the inhibitory effect of WNK4 on ROMK.

KCNJ10 determines the expression of the apical Na-Cl cotransporter (NCC) in the early distal convoluted tubule (DCT1).

The renal phenotype induced by loss-of-function mutations of inwardly rectifying potassium channel (Kir), Kcnj10 (Kir4.1), includes salt wasting, hypomagnesemia, metabolic alkalosis and hypokalemia. However, the mechanism by which Kir.4.1 mutations cause the tubulopathy is not completely understood. Here we demonstrate that Kcnj10 is a main contributor to the basolateral K conductance in the early distal convoluted tubule (DCT1) and determines the expression of the apical Na-Cl cotransporter (NCC) in the DCT. Immunostaining demonstrated Kcnj10 and Kcnj16 were expressed in the basolateral membrane of DCT, and patch-clamp studies detected a 40-pS K channel in the basolateral membrane of the DCT1 of p8/p10 wild-type Kcnj10(+/+) mice (WT). This 40-pS K channel is absent in homozygous Kcnj10(-/-) (knockout) mice. The disruption of Kcnj10 almost completely eliminated the basolateral K conductance and decreased the negativity of the cell membrane potential in DCT1. Moreover, the lack of Kcnj10 decreased the basolateral Cl conductance, inhibited the expression of Ste20-related proline-alanine-rich kinase and diminished the apical NCC expression in DCT. We conclude that Kcnj10 plays a dominant role in determining the basolateral K conductance and membrane potential of DCT1 and that the basolateral K channel activity in the DCT determines the apical NCC expression possibly through a Ste20-related proline-alanine-rich kinase-dependent mechanism.

Renal potassium homeostasis: a short historical perspective.

During the past century, investigators have increased our understanding of renal potassium excretion significantly using many techniques. Notable among these were renal clearance experiments, renal micropuncture, isolated tubule microperfusion, and electrophysiological and patch clamp analysis. These experiments have been made possible by technical advances that have allowed the measurement of potassium in progressively smaller quantities. Initially, the kidney was viewed as controlling potassium excretion by the regulated absorption of potassium from the glomerular filtrate, predominantly in the proximal tubule. This concept was supplanted when clearance experiments deduced and subsequent micropuncture studies directly identified the importance of the distal nephron and collecting duct as the principal site responsible for the regulation of potassium excretion. Additional micropuncture and microperfusion studies showed that a component of potassium secreted by the distal cortical nephron and cortical collecting duct is reabsorbed in the medullary collecting duct, which results in renal medullary potassium recycling. Studies have defined the cellular and molecular mechanisms responsible for potassium secretion and potassium reabsorption in the collecting duct. Further understanding of renal potassium handling will require integrated investigation of the renal and extrarenal signaling systems that control these transport mechanisms.

Angiotensin II stimulates H⁺-ATPase activity in intercalated cells from isolated mouse connecting tubules and cortical collecting ducts.

Intercalated cells in the collecting duct system express V-type H(+)-ATPases which participate in acid extrusion, bicarbonate secretion, and chloride absorption depending on the specific subtype. The activity of H(+)-ATPases is regulated by acid-base status and several hormones, including angiotensin II and aldosterone. Angiotensin II stimulates chloride absorption mediated by pendrin in type B intercalated cells and this process is energized by the activity of H(+)-ATPases. Moreover, angiotensin II stimulates bicarbonate secretion by the connecting tubule (CNT) and early cortical collecting duct (CCD). In the present study we examined the effect of angiotensin II (10 nM) on H(+)-ATPase activity and localization in isolated mouse connecting tubules and cortical collecting ducts. Angiotensin II stimulated Na(+)-independent intracellular pH recovery about 2-3 fold, and this was abolished by the specific H(+)-ATPase inhibitor concanamycin. The effect of angiotensin II was mediated through type 1 angiotensin II receptors (AT(1)-receptors) because it could be blocked by saralasin. Stimulation of H(+)-ATPase activity required an intact microtubular network--it was completely inhibited by colchicine. Immunocytochemistry of isolated CNT/CCDs incubated in vitro with angiotensin II suggests enhanced membrane associated staining of H(+)-ATPases in pendrin expressing intercalated cells. In summary, angiotensin II stimulates H(+)-ATPases in CNT/CCD intercalated cells, and may contribute to the regulation of chloride absorption and bicarbonate secretion in this nephron segment.

Effects of pH on potassium: new explanations for old observations.

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

A long affair with renal tubules.

This essay provides a summary of my professional activities. My interest in renal physiology started as a medical student in Vienna, when I became acquainted with Homer Smith's essays on kidney function. After moving to the United States in 1951, I was fortunate to be mentored by Robert Pitts, in whose Department of Physiology at Cornell Medical College in New York I was given early independence, intellectual stimulation, and the opportunity to pursue experiments on single renal tubules. The problem of how the nephron manages its myriad of transport functions has never lost its fascination for me, and I am profoundly grateful to the many colleagues at Cornell Medical College and at Yale University School of Medicine who shared my passion for the kidney.

Potassium transport--an update.

Potassium homeostasis depends on the coordinated interaction between tightly regulated potassium transfer in and out of the extracellular fluid compartment, and renal excretion or retention of potassium. Potassium transport along the nephron involves extensive proximal tubule reabsorption of potassium. Potassium is also reabsorbed along the thick ascending limb of Henle's loop. Regulated potassium secretion, or potassium reabsorption in exchange for hydrogen ions along the connecting tubule and collecting tubule, is responsible for potassium excretion. Renal potassium transport is modulated by potassium intake, several hormones, acid-base factors and distal nephron sodium delivery. WNK family kinases have also emerged as factors regulating sodium and potassium transport in the distal nephron.

Mouse cystic fibrosis transmembrane conductance regulator forms cAMP-PKA-regulated apical chloride channels in cortical collecting duct.

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in many segments of the mammalian nephron, where it may interact with and modulate the activity of a variety of apical membrane proteins, including the renal outer medullary potassium (ROMK) K(+) channel. However, the expression of CFTR in apical cell membranes or its function as a Cl(-) channel in native renal epithelia has not been demonstrated. Here, we establish that CFTR forms protein kinase A (PKA)-activated Cl(-) channels in the apical membrane of principal cells from the cortical collecting duct obtained from mice. These Cl(-) channels were observed in cell-attached apical patches of principal cells after stimulation by forskolin/3-isobutyl-1-methylxanthine. Quiescent Cl(-) channels were present in patches excised from untreated tubules because they could be activated after exposure to Mg-ATP and the catalytic subunit of PKA. The single-channel conductance, kinetics, and anion selectivity of these Cl(-) channels were the same as those of recombinant mouse CFTR channels expressed in Xenopus laevis oocytes. The CFTR-specific closed-channel blocker CFTR(inh)-172 abolished apical Cl(-) channel activity in excised patches. Moreover, apical Cl(-) channel activity was completely absent in principal cells from transgenic mice expressing the DeltaF508 CFTR mutation but was present and unaltered in ROMK-null mice. We discuss the physiologic implications of open CFTR Cl(-) channels on salt handling by the collecting duct and on the functional CFTR-ROMK interactions in modulating the metabolic ATP-sensing of ROMK.

Src family protein tyrosine kinase (PTK) modulates the effect of SGK1 and WNK4 on ROMK channels.

WNK4 (with no lysine kinase 4) inhibits ROMK channel activity in the distal nephron by stimulating clathrin-dependent endocytosis, an effect attenuated by SGK1 (serum-glucocorticoids-induced kinase)-mediated phosphorylation. It has been suggested that increased ROMK activity because of SGK1-mediated inhibition of WNK4 plays a role in promoting renal K secretion in response to elevated serum K or high K (HK) intake. In contrast, intravascular volume depletion also increases SGK1 activity but fails to stimulate ROMK channels and K secretion. Because HK intake decreases Src family protein tyrosine kinase (PTK) activity an inhibitor of ROMK channels, it is possible that Src family PTK may modulate the effects of SGK1 on WNK4. Here, we show that c-Src prevents SGK1 from attenuating WNK4's inhibition of ROMK activity. This effect of c-Src was WNK4-dependent because c-Src had no effect on ROMK harboring mutation at the site of c-Src phosphorylation (R1Y337A) in the absence of WNK4. Moreover, expression c-Src diminished the SGK1-mediated increase in serine phosphorylation of WNK4, suggesting that c-Src enhances WNK4-mediated inhibition of ROMK channels by suppressing the SGK1-induced phosphorylation. This notion is also supported by the observation that c-Src was not able to modulate the interaction between SGK1 and WNK4 mutants (WNK4(S1169A) or WNK4(S1169D)) in which an SGK1-phosphorylation site (serine 1169) was mutated by alanine or aspartate. We conclude that c-Src inhibits SGK1-mediated phosphorylation hereby restoring the WNK4-mediated inhibition of ROMK channels thus suppressing K secretion.

POSH stimulates the ubiquitination and the clathrin-independent endocytosis of ROMK1 channels.

POSH (plenty of SH3) is a scaffold protein that has been shown to act as an E3 ubiquitin ligase. Here we report that POSH stimulates the ubiquitination of Kir1.1 (ROMK) and enhances the internalization of this potassium channel. Immunostaining reveals the expression of POSH in the renal cortical collecting duct. Immunoprecipitation of renal tissue lysate with ROMK antibody and glutathione S-transferase pulldown experiments demonstrated the association between ROMK and POSH. Moreover, immunoprecipitation of lysates of HEK293T cells transfected with ROMK1 or with constructs encoding the ROMK-N terminus or ROMK1-C-Terminus demonstrated that POSH binds to ROMK1 on its N terminus. To study the effect of POSH on ROMK1 channels, we measured potassium currents with electrophysiological methods in HEK293T cells and in oocytes transfected or injected with ROMK1 and POSH. POSH decreased potassium currents, and the inhibitory effect of POSH on ROMK channels was dose-dependent. Biotinylation assay further showed that POSH decreased surface expression of ROMK channels in HEK293T cells transfected with ROMK1 and POSH. The effect of POSH on ROMK1 channels was specific because POSH did not inhibit sodium current in oocytes injected with ENaC-alpha, beta, and gamma subunits. Moreover, POSH still decreased the potassium current in oocytes injected with a ROMK1 mutant (R1Delta373-378), in which a clathrin-dependent tyrosine-based internalization signal residing between amino acid residues 373 and 378 is deleted. However, the inhibitory effect of POSH on ROMK channels was absent in cells expressing with dominant negative dynamin and POSHDeltaRING, in which the RING domain was deleted. Expression of POSH also increased the ubiquitination of ROMK1, whereas expression of POSHDeltaRING diminished its ubiquitination in HEK293T cells. The notion that POSH may serve as an E3 ubiquitin ligase is also supported by in vitro ubiquitination assays in which adding POSH increased the ROMK ubiquitination. We conclude that POSH inhibits ROMK channels by enhancing dynamin-dependent and clathrin-independent endocytosis and by stimulating ubiquitination of ROMK channels.

Basolateral Na+/H+ exchange maintains potassium secretion during diminished sodium transport in the rabbit cortical collecting duct.

Stimulation of the basolateral Na(+)/K(+)-ATPase in the isolated perfused rabbit cortical collecting duct by raising either bath potassium or lumen sodium increases potassium secretion, sodium absorption and their apical conductances. Here we determined the effect of stimulating Na(+)/K(+)-ATPase on potassium secretion without luminal sodium transport. Acutely raising bath potassium concentrations from 2.5 to 8.5 mM, without luminal sodium, depolarized the basolateral membrane and transepithelial voltages while increasing the transepithelial, basolateral and apical membrane conductances of principal cells. Fractional apical membrane resistance and cell pH were elevated. Net potassium secretion was maintained albeit diminished and was still enhanced by raising bath potassium, but was reduced by basolateral ethylisopropylamiloride, an inhibitor of Na(+)/H(+) exchange. Luminal iberitoxin, a specific inhibitor of the calcium-activated big-conductance potassium (BK) channel, impaired potassium secretion both in the presence and absence of luminal sodium. In contrast, iberitoxin did not affect luminal sodium transport. We conclude that basolateral Na(+)/H(+) exchange in the cortical collecting duct plays an important role in maintaining potassium secretion during compromised sodium supplies and that BK channels contribute to potassium secretion.

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.

Female ROMK null mice manifest more severe Bartter II phenotype on renal function and higher PGE2 production.

ROMK null mice with a high survival rate and varying severity of hydronephrosis provide a good model to study type II Bartter syndrome pathophysiology (26). During the development of such a colony, we found that more male than female null mice survived, 58.7% vs. 33.3%. To investigate the possible mechanism of this difference, we compared the survival rates, renal functions, degree of hydronephrosis, as well as PGE(2) and TXB(2) production between male and female ROMK wild-type and null mice. We observed that female ROMK Bartter's mice exhibited lower GFR (0.37 vs. 0.54 ml.min(-1).100 g BW(-1), P < 0.05) and higher fractional Na(+) excretion (0.66% vs. 0.48%, P < 0.05) than male Bartter's. No significant differences in acid-base parameters, urinary K(+) excretion, and plasma electrolyte concentrations were observed between sexes. In addition, we assessed the liquid retention rate in the kidney to evaluate the extent of hydronephrosis and observed that 67% of male and 90% of female ROMK null mice were hydronephrotic mice. Urinary PGE(2) excretion was higher in both sexes of ROMK null mice: 1.35 vs. 1.10 ng/24 h in males and 2.90 vs. 0.87 ng/24 h in females. TXB(2) excretion was higher in female mice in both wild-type and ROMK null mice. The increments of urinary PGE(2) and TXB(2) were significantly higher in female null mice than males, 233.33% vs. 22.74% of PGE(2) and 85.67% vs. 20.36% of TXB(2). These data demonstrate a more severe Bartter phenotype in female ROMK null mice, and higher PGE(2) and TXB(2) production may be one of the mechanisms of this manifestation.

Mouse model of type II Bartter's syndrome. I. Upregulation of thiazide-sensitive Na-Cl cotransport activity.

ROMK-deficient (Romk(-/-)) mice exhibit polyuria, natriuresis, and kaliuresis similar to individuals with type II Bartter's form of hyperprostaglandin E syndrome (HPS; antenatal Bartter's syndrome). In the present study, we utilized both metabolic and clearance studies to define the contributions of specific distal nephron segments to the renal salt wasting in these mice. The effects of furosemide, hydrochlorothiazide, and benzamil on urinary Na(+) and K(+) excretion in both wild-type (Romk(+/+)) and Romk(-/-) mice were used to assess and compare salt transport by the Na(+)-K(+)-2Cl(-) cotransporter (NKCC2)-expressing thick ascending limb (TAL), the Na(+)-Cl(-) cotransporter (NCC)-expressing distal convoluted tubule (DCT1/DCT2), and the epithelial Na(+) channel (ENaC)-expressing connecting segment (CNT) and collecting duct (CD), respectively. Whole kidney glomerular filtration rate was reduced by 47% in Romk(-/-) mice. Furosemide-induced increments in the fractional excretion rate of Na(+) and K(+) and absolute excretion of Na(+) and K(+) were significantly blunted in Romk(-/-) mice, consistent with a major salt transport defect in the TAL. In contrast, hydrochlorothiazide produced an exaggerated natriuresis in Romk(-/-) mice, indicating upregulation of salt absorption by the DCT. Benzamil resulted in a similar increment in absolute Na excretion in both Romk(-/-) and Romk(+/+), indicating no significant upregulation of Na(+) transport by ENaC in ROMK null mice. Moreover, hydrochlorothiazide increased the fractional K(+) excretion rate in Romk(-/-) mice, confirming our recent observation that maxi-K channels contribute to distal K(+) secretion in the absence of ROMK.

Mouse model of type II Bartter's syndrome. II. Altered expression of renal sodium- and water-transporting proteins.

Bartter's syndrome represents a group of hereditary salt- and water-losing renal tubulopathies caused by loss-of-function mutations in proteins mediating or regulating salt transport in the thick ascending limb (TAL) of Henle's loop. Mutations in the ROMK channel cause type II antenatal Bartter's syndrome that presents with maternal polyhydramnios and postnatal life-threatening volume depletion. We have developed a colony of Romk null mice showing a Bartter-like phenotype and with increased survival to adulthood, suggesting the activation of compensatory mechanisms. To test the hypothesis that upregulation of Na(+)-transporting proteins in segments distal to the TAL contributes to compensation, we studied expression of salt-transporting proteins in ROMK-deficient (Romk(-/-)) mice. Plasma aldosterone was 40% higher and urinary PGE(2) excretion was 1.5-fold higher in Romk(-/-) compared with wild-type littermates. Semiquantitative immunoblotting of kidney homogenates revealed decreased abundances of proximal tubule Na(+)/H(+) exchanger (NHE3) and Na(+)-P(i) cotransporter (NaPi-IIa) and TAL-specific Na(+)-K(+)-2Cl(-)-cotransporter (NKCC2/BSC1) in Romk(-/-) mice, while the distal convoluted tubule (DCT)-specific Na(+)-Cl(-) cotransporter (NCC/TSC) was markedly increased. The abundance of the alpha-,beta-, and gamma-subunits of the epithelial Na(+) channel (ENaC) was slightly increased, although only differences for gamma-ENaC reached statistical significance. Morphometry revealed a fourfold increase in the fractional volume of DCT but not of connecting tubule (CNT) and collecting duct (CCD). Consistently, CNT and CD of Romk(-/-) mice revealed no apparent increase in the luminal abundance of the ENaC compared with those of wild-type mice. These data suggest that the loss of ROMK-dependent Na(+) absorption in the TAL is compensated predominately by upregulation of Na(+) transport in downstream DCT cells. These adaptive changes in Romk(-/-) mice may help to limit renal Na(+) loss, and thereby, contribute to survival of these mice.

A novel protein kinase signaling pathway essential for blood pressure regulation in humans.

The discovery that mutations in WNK4 [encoding a member of the WNK family - so named because of the unique substitution of cysteine for lysine at a nearly invariant residue within subdomain II of its catalytic core: with no K (lysine)] cause pseudohypoaldosteronism type II, an autosomal dominant form of human hypertension, provided the initial clue that this serine/threonine kinase is a crucial part of a complex renal salt regulatory system. Recent findings from physiological studies of WNK4 in Xenopus laevis oocytes, mammalian cell systems and in vivo in mouse models have provided novel insights into the mechanisms by which the kidney regulates salt homeostasis, and therefore blood pressure, downstream of aldosterone signaling in mammals. The current evidence supports a model in which WNK4 coordinates the activities of diverse aldosterone-sensitive mediators of ion transport in the distal nephron to promote normal homeostasis in response to physiological perturbation.

Inhibitor of growth 4 (ING4) is up-regulated by a low K intake and suppresses renal outer medullary K channels (ROMK) by MAPK stimulation.

Dietary K intake plays an important role in the regulation of renal K secretion: a high K intake stimulates whereas low K intake suppresses renal K secretion. Our previous studies demonstrated that the Src family protein-tyrosine kinase and mitogen-activated protein kinase (MAPK) are involved in mediating the effect of low K intake on renal K channels and K secretion. However, the molecular mechanism by which low K intake stimulates MAPK is not completely understood. Here we show that inhibitor of growth 4 (ING4), a protein with a highly conserved plant homeodomain finger motif, is involved in mediating the effect of low K intake on MAPK. K restriction stimulates the expression of ING4 in the kidney and superoxide anions, and its related products are involved in mediating the effect of low K intake on ING4 expression. We used HEK293 cells to express ING4 and observed that expression of ING4 increased the phosphorylation of p38 and ERK MAPK, whereas down-regulation of ING4 with small interfering RNA decreased the phosphorylation of p38 and ERK. Immunocytochemistry showed that ING4 was expressed in the renal outer medullary potassium (ROMK)-positive tubules. Moreover, ING4 decreased K currents in Xenopus oocytes injected with ROMK channel cRNA. This inhibitory effect was reversed by blocking p38 and ERK MAPK. These data provide evidence for the role of ING4 in mediating the effect of low K intake on ROMK channel activity by stimulation of p38 and ERK MAPK.