ATP is a coupling modulator of parallel Na,K-ATPase-K-channel activity in the renal proximal tubule

Adenosine stimulates the basolateral 50 pS K+ channel in renal proximal tubule via adenosine-A1 receptor

Background: The basolateral potassium channels play an important role in maintaining the membrane transport in the renal proximal tubules (PT) and adenosine receptors have been shown to regulate the trans-epithelial Na+ absorption in the PT. The aim of the present study is to explore whether adenosine also regulates the basolateral K+ channel of the PT and to determine the adenosine receptor type and the signaling pathway which mediates the effect of adenosine on the K+ channel. Methods: We have used the single channel recording to examine the basolateral K+ channel activity in the proximal tubules of the mouse kidney. All experiments were performed in cell-attached patches. Results: Single channel recording has detected a 50 pS inwardly-rectifying K+ channel with high channel open probability and this 50 pS K+ channel is a predominant type K+ channel in the basolateral membrane of the mouse PT. Adding adenosine increased 50 pS K+ channel activity in cell-attached patches, defined by NPo (a product of channel Numbers and Open Probability). The adenosine-induced stimulation of the 50 pS K+ channel was absent in the PT pretreated with DPCPX, a selective inhibitor of adenosine A1 receptor. In contrast, adenosine was still able to stimulate the 50 pS K+ channel in the PT pretreated with CP-66713, a selective adenosine A2 receptor antagonist. This suggests that the stimulatory effect of adenosine on the 50 pS K+ channel of the PT was mediated by adenosine-A1 receptor. Moreover, the effect of adenosine on the 50 pS K+ channel was blocked in the PT pretreated with U-73122 or Calphostin C, suggesting that adenosine-induced stimulation of the 50 pS K+ channels of the PT was due to the activation of phospholipase C (PLC) and protein kinase C (PKC) pathway. In contrast, the inhibition of phospholipase A2 (PLA2) with AACOCF3 or inhibition of protein kinase A (PKA) with H8 failed to block the adenosine-induced stimulation of the 50 pS K+ channel of the PT. Conclusion: We conclude that adenosine activates the 50 pS K+ channels in the basolateral membrane of PT via adenosine-A1 receptor. Furthermore, the effect of adenosine on the 50 pS K+ channel is mediated by PLC-PKC signaling pathway.

Connexin30-Deficiency Causes Mild Hearing Loss With the Reduction of Endocochlear Potential and ATP Release

GJB2 and GJB6 are adjacent genes encoding connexin 26 (Cx26) and connexin 30 (Cx30), respectively, with overlapping expressions in the inner ear. Both genes are associated with the commonest monogenic hearing disorder, recessive isolated deafness DFNB1. Cx26 plays an important role in auditory development, while the role of Cx30 in hearing remains controversial. Previous studies found that Cx30 knockout mice had severe hearing loss along with a 90% reduction in Cx26, while another Cx30 knockout mouse model showed normal hearing with nearly half of Cx26 preserved. In this study, we used CRISPR/Cas9 technology to establish a new Cx30 knockout mouse model (Cx30^−/−), which preserves approximately 70% of Cx26. We found that the 1, 3, and 6-month-old Cx30^−/− mice showed mild hearing loss at full frequency. Immunofluorescence and HE staining suggested no significant differences in microstructure of the cochlea between Cx30^−/− mice and wild-type mice. However, transmission electron microscopy showed slight cavity-like damage in the stria vascularis of Cx30^−/− mice. And Cx30 deficiency reduced the production of endocochlear potential (EP) and the release of ATP, which may have induced hearing loss. Taken together, this study showed that lack of Cx30 can lead to hearing loss with an approximately 30% reduction of Cx26 in the present Cx30 knockout model. Hence, Cx30 may play an important rather than redundant role in hearing development.

Pannexin1 channels dominate ATP release in the cochlea ensuring endocochlear potential and auditory receptor potential generation and hearing

Pannexin1 (Panx1) is a gap junction gene in vertebrates whose proteins mainly function as non-junctional channels on the cell surface. Panx1 channels can release ATP under physiological conditions and play critical roles in many physiological and pathological processes. Here, we report that Panx1 deficiency can reduce ATP release and endocochlear potential (EP) generation in the cochlea inducing hearing loss. Panx1 extensively expresses in the cochlea, including the cochlear lateral wall. We found that deletion of Panx1 in the cochlear lateral wall almost abolished ATP release under physiological conditions. Positive EP is a driving force for current through hair cells to produce auditory receptor potential. EP generation requires ATP. In the Panx1 deficient mice, EP and auditory receptor potential as measured by cochlear microphonics (CM) were significantly reduced. However, no apparent hair cell loss was detected. Moreover, defect of connexin hemichannels by deletion of connexin26 (Cx26) and Cx30, which are predominant connexin isoforms in the cochlea, did not reduce ATP release under physiological conditions. These data demonstrate that Panx1 channels dominate ATP release in the cochlea ensuring EP and auditory receptor potential generation and hearing. Panx1 deficiency can reduce ATP release and EP generation causing hearing loss.

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.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

Evidence of K+ channel function in epithelial cell migration, proliferation, and repair

Efficient repair of epithelial tissue, which is frequently exposed to insults, is necessary to maintain its functional integrity. It is therefore necessary to better understand the biological and molecular determinants of tissue regeneration and to develop new strategies to promote epithelial repair. Interestingly, a growing body of evidence indicates that many members of the large and widely expressed family of K(+) channels are involved in regulation of cell migration and proliferation, key processes of epithelial repair. First, we briefly summarize the complex mechanisms, including cell migration, proliferation, and differentiation, engaged after epithelial injury. We then present evidence implicating K(+) channels in the regulation of these key repair processes. We also describe the mechanisms whereby K(+) channels may control epithelial repair processes. In particular, changes in membrane potential, K(+) concentration, cell volume, intracellular Ca(2+), and signaling pathways following modulation of K(+) channel activity, as well as physical interaction of K(+) channels with the cytoskeleton or integrins are presented. Finally, we discuss the challenges to efficient, specific, and safe targeting of K(+) channels for therapeutic applications to improve epithelial repair in vivo.

Lysophosphatidic acid promotes secretion of VEGF by increasing expression of 150-kD Oxygen-regulated protein (ORP150) in mesenchymal stem cells

We previously reported that transplantation of lysophosphatidic acid (LPA) treated mesenchymal stem cells (MSCs) enhances capillary density in the myocardium and improves myocardial function in the ischemic heart. This effect may be mediated through the release of paracrine factors by MSC and potentially involves pro-angiogenic molecules such as vascular endothelial growth factor (VEGF). In this study, we examined the pharmacological and molecular regulation of VEGF secretion induced by LPA in rat MSCs. We showed that LPA stimulated VEGF secretion in MSCs but not in cardiomyocytes or cardiac fibroblasts. LPA-induced VEGF secretion occurred at the post-transcriptional levels and was mediated through the classical ER/Golgi-dependent protein secretory route. LPA also increased ORP150 protein expression. Inhibition of ORP150 upregulation by siRNA knockdown attenuated LPA-induced VEGF secretion. On the other hand, diazoxide, an activator of KATP channel, markedly inhibited LPA-induced ORP150 expression and VEGF secretion. Meanwhile, ATP concentration dependently increased VEGF secretion. In addition, l-Glutamate and NH4Cl significantly reduced VEGF secretion. Furthermore, inhibition of two major subtypes of LPA receptors by Ki16425 and specific siRNA for LPA receptors prevented LPA-induced VEGF secretion and ORP150 expression. Lastly, inhibition of Gi protein that couples with LPA receptors by PTX and siRNA knockdown had no effect on LPA-induced VEGF secretion. Taken together, our findings demonstrate that LPA promotes VEGF secretion at the post-translation level by up-regulating ORP150 expression. Both LPA1 and LPA3 are involved in the LPA-induced VEGF secretion that is independent of Gi protein coupling but associated with the inactivation of KATP channels and inhibition of Na(+)/K(+)-ATPase activity.

Regulation of glomerulotubular balance: II: impact of angiotensin II on flow-dependent transport

Underlying glomerulotubular balance (GTB) is the impact of axial flow to regulate Na(+) and HCO(3)(-) transport by modulating Na(+)-H(+) exchanger 3 (NHE3) and H-ATPase activity. It is not known whether the cascade of events following a change in flow relies on local angiotensin (ANG II) generation or receptor availability. Mouse tubules were microperfused in vitro at flows of 5 and 20 nl/min, and net fluid (J(v)) and HCO(3)(-) (J(HCO3)) absorption and cell height were measured. Na(+) (J(Na)) and Cl(-) (J(Cl)) absorption and changes in microvillous torque were estimated. Raising flow increased Na(+) and HCO(3)(-) reabsorption but did not change either Cl(-) transport or cell volume. Losartan reduced absolute Na(+) and HCO(3)(-) absorption at both low and high flows but did not affect fractional flow-stimulated transport. Compared with controls, in AT(1a) knockout (KO) mouse tubules, 53% of flow-stimulated Na(+) absorption was abolished, but flow-stimulated HCO(3)(-) absorption was retained at similar levels. The remaining flow-stimulated J(HCO3) was eliminated by the H-ATPase inhibitor bafilomycin. Inhibition of the AT(2) receptor by PD123319 increased both J(Na) and J(HCO3) but did not affect flow-mediated fractional changes. NHE3 expression at the protein level was reduced in AT(1a) KO mice kidneys. We conclude that 1) although the AT(1a) receptor is necessary for flow to impact NHE3, the effect on H(+)-ATPase is independent of AT(1a); 2) the small flow-mediated changes in cell volume suggest a coordinate flow effect on both luminal and basolateral transporters; and 3) there is no evidence of flow-dependent Cl(-) transport, and thus no evidence for convective paracellular Cl(-) transport in mouse tubules.

Basolateral membrane K+ channels in renal epithelial cells

The major function of epithelial tissues is to maintain proper ion, solute, and water homeostasis. The tubule of the renal nephron has an amazingly simple structure, lined by epithelial cells, yet the segments (i.e., proximal tubule vs. collecting duct) of the nephron have unique transport functions. The functional differences are because epithelial cells are polarized and thus possess different patterns (distributions) of membrane transport proteins in the apical and basolateral membranes of the cell. K(+) channels play critical roles in normal physiology. Over 90 different genes for K(+) channels have been identified in the human genome. Epithelial K(+) channels can be located within either or both the apical and basolateral membranes of the cell. One of the primary functions of basolateral K(+) channels is to recycle K(+) across the basolateral membrane for proper function of the Na(+)-K(+)-ATPase, among other functions. Mutations of these channels can cause significant disease. The focus of this review is to provide an overview of the basolateral K(+) channels of the nephron, providing potential physiological functions and pathophysiology of these channels, where appropriate. We have taken a "K(+) channel gene family" approach in presenting the representative basolateral K(+) channels of the nephron. The basolateral K(+) channels of the renal epithelia are represented by members of the KCNK, KCNJ, KCNQ, KCNE, and SLO gene families.

Renal phenotype in mice lacking the Kir5.1 (Kcnj16) K+ channel subunit contrasts with that observed in SeSAME/EAST syndrome

The heteromeric inwardly rectifying Kir4.1/Kir5.1 K(+) channel underlies the basolateral K(+) conductance in the distal nephron and is extremely sensitive to inhibition by intracellular pH. The functional importance of Kir4.1/Kir5.1 in renal ion transport has recently been highlighted by mutations in the human Kir4.1 gene (KCNJ10) that result in seizures, sensorineural deafness, ataxia, mental retardation, and electrolyte imbalance (SeSAME)/epilepsy, ataxia, sensorineural deafness, and renal tubulopathy (EAST) syndrome, a complex disorder that includes salt wasting and hypokalemic alkalosis. Here, we investigated the role of the Kir5.1 subunit in mice with a targeted disruption of the Kir5.1 gene (Kcnj16). The Kir5.1(-/-) mice displayed hypokalemic, hyperchloremic metabolic acidosis with hypercalciuria. The short-term responses to hydrochlorothiazide, an inhibitor of ion transport in the distal convoluted tubule (DCT), were also exaggerated, indicating excessive renal Na(+) absorption in this segment. Furthermore, chronic treatment with hydrochlorothiazide normalized urinary excretion of Na(+) and Ca(2+), and abolished acidosis in Kir5.1(-/-) mice. Finally, in contrast to WT mice, electrophysiological recording of K(+) channels in the DCT basolateral membrane of Kir5.1(-/-) mice revealed that, even though Kir5.1 is absent, there is an increased K(+) conductance caused by the decreased pH sensitivity of the remaining homomeric Kir4.1 channels. In conclusion, disruption of Kcnj16 induces a severe renal phenotype that, apart from hypokalemia, is the opposite of the phenotype seen in SeSAME/EAST syndrome. These results highlight the important role that Kir5.1 plays as a pH-sensitive regulator of salt transport in the DCT, and the implication of these results for the correct genetic diagnosis of renal tubulopathies is discussed.

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.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Mechanotransduction in the renal tubule

The role of mechanical forces in the regulation of glomerulotubular balance in the proximal tubule (PT) and Ca(2+) signaling in the distal nephron was first recognized a decade ago, when it was proposed that the microvilli in the PT and the primary cilium in the cortical collecting duct (CCD) acted as sensors of local tubular flow. In this review, we present a summary of the theoretical models and experiments that have been conducted to elucidate the structure and function of these unique apical structures in the modulation of Na(+), HCO(3)(-), and water reabsorption in the PT and Ca(2+) signaling in the CCD. We also contrast the mechanotransduction mechanisms in renal epithelium with those in other cells in which fluid shear stresses have been recognized to play a key role in initiating intracellular signaling, most notably endothelial cells, hair cells in the inner ear, and bone cells. In each case, small hydrodynamic forces need to be greatly amplified before they can be sensed by the cell's intracellular cytoskeleton to enable the cell to regulate its membrane transporters or stretch-activated ion channels in maintaining homeostasis in response to changing flow conditions.

Variable effects of the mitoK(ATP) channel modulators diazoxide and 5-HD in ATP-depleted renal epithelial cells

The role of mitochondrial K(ATP) (mitoK(ATP)) channels in renal ischemia-reperfusion injury is controversial with studies showing both protective and deleterious effects. In this study, we compared the effects of the putative mitoK(ATP) opener, diazoxide, and the mitoK(ATP) blocker, 5-hydroxydecanoate (5-HD) on cytotoxicity and apoptosis in tubular epithelial cells derived from rat (NRK-52E) and pig (LLC-PK1) following in vitro ischemic injury. Following ATP depletion-recovery, there was a significant increase in cytotoxicity in both NRK cells and LLC-PK1 cells although NRK cells were more sensitive to the injury. Diazoxide treatment attenuated cytotoxicity in both cell types and 5-HD treatment-increased cytotoxicity in the sensitive NRK cells in a superoxide-dependant manner. The protective effect of diazoxide was also reversed in the presence of 5-HD in ATP-depleted NRK cells. The ATP depletion-mediated increase in superoxide was enhanced by both diazoxide and 5-HD with the effect being more pronounced in the cells undergoing 5-HD treatment. Further, ATP depletion-induced activation of caspase-3 was decreased by diazoxide in NRK cells. In order to determine the signaling pathways involved in apoptosis, we examined the activation of Erk and JNK in ATP-depleted NRK cells. Diazoxide-activated Erk in ATP-depleted cells, but did not have any effect on JNK activation. In contrast, 5-HD did not impact Erk levels but increased JNK activation even under controlled conditions. Further, the use of a JNK inhibitor with 5-HD reversed the deleterious effects of 5-HD. This study demonstrates that in cells that are sensitive to ATP depletion-recovery, mitoK(ATP) channels protect against ATP depletion-mediated cytotoxicity and apoptosis through Erk- and JNK-dependant mechanisms.

Epilepsy, ataxia, sensorineural deafness, tubulopathy, and KCNJ10 mutations

BACKGROUND

Five children from two consanguineous families presented with epilepsy beginning in infancy and severe ataxia, moderate sensorineural deafness, and a renal salt-losing tubulopathy with normotensive hypokalemic metabolic alkalosis. We investigated the genetic basis of this autosomal recessive disease, which we call the EAST syndrome (the presence of epilepsy, ataxia, sensorineural deafness, and tubulopathy).

METHODS

Whole-genome linkage analysis was performed in the four affected children in one of the families. Newly identified mutations in a potassium-channel gene were evaluated with the use of a heterologous expression system. Protein expression and function were further investigated in genetically modified mice.

RESULTS

Linkage analysis identified a single significant locus on chromosome 1q23.2 with a lod score of 4.98. This region contained the KCNJ10 gene, which encodes a potassium channel expressed in the brain, inner ear, and kidney. Sequencing of this candidate gene revealed homozygous missense mutations in affected persons in both families. These mutations, when expressed heterologously in xenopus oocytes, caused significant and specific decreases in potassium currents. Mice with Kcnj10 deletions became dehydrated, with definitive evidence of renal salt wasting.

CONCLUSIONS

Mutations in KCNJ10 cause a specific disorder, consisting of epilepsy, ataxia, sensorineural deafness, and tubulopathy. Our findings indicate that KCNJ10 plays a major role in renal salt handling and, hence, possibly also in blood-pressure maintenance and its regulation.

Physiology and pathophysiology of potassium channels in gastrointestinal epithelia

Epithelial cells of the gastrointestinal tract are an important barrier between the "milieu interne" and the luminal content of the gut. They perform transport of nutrients, salts, and water, which is essential for the maintenance of body homeostasis. In these epithelia, a variety of K(+) channels are expressed, allowing adaptation to different needs. This review provides an overview of the current literature that has led to a better understanding of the multifaceted function of gastrointestinal K(+) channels, thereby shedding light on pathophysiological implications of impaired channel function. For instance, in gastric mucosa, K(+) channel function is a prerequisite for acid secretion of parietal cells. In epithelial cells of small intestine, K(+) channels provide the driving force for electrogenic transport processes across the plasma membrane, and they are involved in cell volume regulation. Fine tuning of salt and water transport and of K(+) homeostasis occurs in colonic epithelia cells, where K(+) channels are involved in secretory and reabsorptive processes. Furthermore, there is growing evidence for changes in epithelial K(+) channel expression during cell proliferation, differentiation, apoptosis, and, under pathological conditions, carcinogenesis. In the future, integrative approaches using functional and postgenomic/proteomic techniques will help us to gain comprehensive insights into the role of K(+) channels of the gastrointestinal tract.

Interactions of K+ATP channel blockers with Na+/K+-ATPase

Two K(+) (ATP) channel blockers, 5-hydroxydecanoate (5-HD) and glyburide, are often used to study cross-talk between Na(+)/K(+)-ATPase and these channels. The aim of this work was to characterize the effects of these blockers on purified Na(+)/K(+)-ATPase as an aid to appropriate use of these drugs in studies on this cross-talk. In contrast to known dual effects (activating and inhibitory) of other fatty acids on Na(+)/K(+)-ATPase, 5-HD only inhibited the enzyme at concentrations exceeding those that block mitochondrial K(+) (ATP) channels. 5-HD did not affect the ouabain sensitivity of Na(+)/K(+)-ATPase. Glyburide had both activating and inhibitory effects on Na(+)/K(+)-ATPase at concentrations used to block plasma membrane K(+) (ATP) channels. The findings justify the use of 5-HD as specific mitochondrial channel blocker in studies on the relation of this channel to Na(+)/K(+)-ATPase, but question the use of glyburide as a specific blocker of plasma membrane K(+) (ATP) channels, when the relation of this channel to Na(+)/K(+)-ATPase is being studied.

The effect of shear stress on the basolateral membrane potential of proximal convoluted tubule of the rat kidney

As consequence of glomerular filtration the viscosity of blood flowing through the efferent arteriole increases. Recently, we found that shear stress modulates proximal bicarbonate reabsorption and nitric oxide (NO.) was the chemical mediator of this effect. In the present work, we found that agonists of NO. production affected basolateral membrane potential (V (blm)) of the proximal convoluted tubule (PCT) epithelium. Using paired micropuncture experiments, we perfused peritubular capillaries with solutions with different viscosity while registering the V (blm). Our results showed that a 50% increment in the viscosity, or the addition of bradykinin (10(-5) M) to the peritubular perfusion solution, induced a significant and similar hyperpolarization of the V (blm) at the PCT epithelium of 6 +/- 0.7 mV (p < 0.05). Both hyperpolarizations were reverted by L-NAME (10(-4) M). Addition of 2,2'-(hydroxynitrosohydrazino) bis-ethanamine (NOC-18) 3 x 10(-4) M to the peritubular perfusion solution induced a hyperpolarization of the same magnitude of that high viscosity or bradykinin. These results strongly suggest the involvement of NO. in the effect of high viscosity solutions. This effect seems to be mediated by activation of K+(ATP) channels as glybenclamide (5 x 10(-5) M) added to peritubular solutions induced a larger depolarization of the V (blm) with high viscosity solutions. Acetazolamide (5 x 10(-5) M) added to high viscosity solutions induced a larger hyperpolarization (8 +/- 1 mV; p < 0.05), suggesting that depolarizing current due to HCO(-)3 exit across the basolateral membrane damps the hyperpolarizing effect of high viscosity. Considering that Na(+) and consequently water reabsorption is highly dependent on electrical gradient, the present data suggest that the endothelium of kidney vascular bed interacts in paracrine fashion with the epithelia, affecting V (blm) and thus modulating PCT reabsorption.

Flow-dependent transport in a mathematical model of rat proximal tubule

The mathematical model of rat proximal tubule has been extended to include calculation of microvillous torque and to incorporate torque-dependent solute transport in a compliant tubule. The torque calculation follows that of Du Z, Yan Q, Duan Y, Weinbaum S, Weinstein AM, and Wang T (Am J Physiol 290: F289-F296, 2006). In the model calculations, torque-dependent scaling of luminal membrane transporter density [either as an ensemble or just type 3 Na(+)/H(+) exchanger (NHE3) alone] had a relatively small impact on overall Na(+) reabsorption and could produce a lethal derangement of cell volume; coordinated regulation of luminal and peritubular transporters was required to represent the overall impact of luminal flow on Na(+) reabsorption. When the magnitude of torque-dependent Na(+) reabsorption in the model agrees with that observed in mouse proximal tubules, the model tubule shows nearly perfect perfusion-absorption balance at high luminal perfusion rates, but enhanced sensitivity of reabsorption at low flow. With a slightly lower coefficient for torque-sensitive transporter insertion, perfusion-absorption balance in the model tubule is closer to observations in the rat over a broader range of inlet flows. In simulation of hyperglycemia, torque-dependent transport attenuated the diuretic effect and brought the model tubule into closer agreement with experimental observation in the rat. The model was also extended to represent finite rates of hydration and dehydration of CO(2) and H(2)CO(3). With carbonic anhydrase inhibition, torque-dependent transport blunted the diuretic effect and enhanced the shift from paracellular to transcellular NaCl reabsorption. The new features of this model tubule are an important step toward simulation of glomerulotubular balance.

Effect of Renal Ischemia/Reperfusion on Gene Expression of a pH-Sensitive K+ Channel

BACKGROUND

Sodium reabsorption depends on the Na/K/ATPase activity coupled to basolateral K+ recycling through K+ channels. ATP depletion reduces pump activity and increases K+ leak resulting in transport dysfunction. Kir4.1 is a pH-sensitive K+ channel expressed in the basolateral membrane of distal tubules. In this study, we evaluated whether Kir4.1 is also expressed in proximal tubules (PTs) and whether renal ischemia alters Kir4.1 mRNA expression levels.

METHODS

The presence of Kir4.1 mRNA was evaluated in PTs microdissected from collagenase-treated rat kidneys. Kir4.1 expression levels were estimated in the renal cortex by multiplex RT-PCR after 30 or 60 min of renal ischemia followed by 1, 24, 48 or 72 h of reperfusion.

RESULTS

The PCR product obtained from isolated tubules was sequenced and showed approximately 98% homology with rat Kir4.1 cDNA. Ischemia/reperfusion for 30 min induced a time-dependent reduction in Kir4.1 mRNA expression in parallel with plasma creatinine, however recovery was delayed after 60 min of ischemia, remaining reduced after 72 h of reperfusion when plasma creatinine was already normalized.

CONCLUSION

Kir4.1 mRNA expression was decreased by renal ischemia. The ischemia-induced cellular K+ loss may be minimized by Kir4.1 downregulation and may contribute to the mechanism by which cellular acidification induces cell protection against ATP depletion.

Ion regulation in invertebrates: molecular and integrative aspects

The subject of ion regulation in invertebrates is discussed, using a variety of invertebrate model species and approaches that range from the whole-organism level to tissue, subcellular, and molecular levels to illustrate the future direction of the field. These organisms inhabit a variety of aquatic, freshwater, and terrestrial environments, showing specific adaptations to each environment. This overview discusses mechanisms of metal detoxification and the presence of Cl-ATPase in marine organisms to avoid excess intracellular Cl(-); Ca(2+) regulation and endocrine aspects of adaptations to transitional (semiterrestrial) environments; adaptations to Ca(2+)-poor freshwater, particularly the reabsorption of Ca(2+) through specific transporters found in the urine; and finally, ionoregulatory mechanisms for life on land, such as Ca(2+) conservation during molting in isopods and the presence of K(+) channels in insect Malpighian tubules. Convergent mechanisms for dealing with similar problems in dissimilar habitats are discussed, taking into consideration that invertebrates will continue to serve as model systems for the evolution of ionoregulation in different habitats.

Glibenclamide depletes ATP in renal proximal tubular cells by interfering with mitochondrial metabolism

Sulfonylurea drugs, like glibenclamide, stimulate insulin secretion by blocking ATP-sensitive potassium channels on pancreatic beta cells. Renal tubular epithelial cells possess a different class of K(ATP) channels with much lower affinities for sulfonylurea drugs, necessitating the use of micromolar glibenclamide concentrations to study these channels. Here, we describe the toxic effects of these concentrations on mitochondrial energy metabolism in freshly isolated renal proximal tubular cells. Glibenclamide, at concentrations of 50 and 100 microM, reduced intracellular ATP levels by 28+/-4 and 53+/-5%, respectively (P<0.01). This decline in ATP could be attributed to a dose-dependent inhibition of mitochondrial respiration. Glibenclamide (10-500 microM) inhibited ADP-stimulated mitochondrial oxygen consumption. In addition to this toxic effect, specific association of radiolabeled and fluorescent glibenclamide to renal mitochondria was found. Association of [(3)H]glibenclamide with renal mitochondria revealed a low-affinity site with a K(D) of 16+/-6 microM and a B(max) of 167+/-29 pmol mg(-1). We conclude that micromolar concentrations of glibenclamide interfere with mitochondrial bioenergetics and, therefore, should be used with care for inhibition of epithelial K(ATP) channels.

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.

Mechanisms underlying regulation of a barium-sensitive K+ conductance by ATP in single proximal tubule cells isolated from frog kidney

K(+) channels play an important role in pump-leak coupling and volume regulation in the renal proximal tubule. Previous experiments have identified a barium-sensitive K(+) conductance (G(Ba)) in proximal tubule cells isolated from frog kidneys. In this paper we examine the regulation of G(Ba) by ATP. G(Ba) was measured in single cells isolated from frog kidney using the whole-cell patch-clamp technique. G(Ba) was activated by 2 mM: intracellular ATP. This activation was enhanced by inhibition of protein kinase C and attenuated by inhibition of protein kinase A, indicating reciprocal regulation by these kinases. Activation by ATP was reduced in the presence of a hypertonic bath solution, suggesting that cell swelling was required. However, after activation to steady-state, G(Ba )was not sensitive to cell-volume changes. Hypotonic shock-induced volume regulation was inhibited by barium and quinidine, inhibitors of G(Ba). The effect of maximal inhibitory concentrations of barium and quinidine on volume regulation was similar and addition of both blockers together did not augment the inhibitory response. G(Ba) was also activated by ADP, via a mechanism dependent on the presence of Mg(2+). However, the responses to ADP and ATP were not additive, suggesting that these nucleotides may share a common mechanism of activation. The regulation of G(Ba) by ATP was biphasic, with a half-maximal activating concentration of 0.89 mM and a half maximal inhibitory concentration of 6.71 mM. The sensitivity to nucleotides suggests that G(Ba) may be regulated by the metabolic state of the cell. Furthermore, the sensitivity to solution osmolality, coupled with the blocker profile of inhibition of volume regulation, suggests that G(Ba) could play a role in volume regulation.

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.

KATP-channel-induced vasodilation is modulated by the Na,K-pump activity in rabbit coronary small arteries

The purpose of the study was to evaluate the importance of the Na,K-pump in relaxations induced by K(ATP)-channel openers in rabbit coronary small arteries. Arterial segments were mounted in myographs for recording of isometric tension. Whole-cell patch clamp was used to assess K(ATP)-channel currents in isolated smooth muscle cells from the arteries. In arteries preconstricted with the thromboxane A(2) analogue U46619 pinacidil and cromakalim induced concentration-dependent relaxations. In arteries preconstricted with potassium (124 mM) only high concentrations of pinacidil had a small relaxant effect. In arteries preconstricted with U46619 pinacidil-induced relaxations were unaffected by pretreatment with N(omega)-nitro-L-arginine (L-NNA) and only slightly reduced after mechanical removal of the endothelium. Pinacidil induced relaxations were not significantly affected by 1 microM glibenclamide. However, the relaxations were partly inhibited in potassium-free media and by 1 microM ouabain. In contrast, the concentration-dependent relaxation to cromakalim was partly blocked by 1 microM glibenclamide and partly by 1 microM ouabain and when both drugs were present the inhibition increased. Ouabain (1 microM) and glibenclamide (1 microM) each partly inhibited an ATP-sensitive current induced by pinacidil and cromakalim. In the presence of both inhibitors a greater inhibition was seen. When the solution in the patch pipette was sodium-free the current was reduced and ouabain had no effect. The study suggests that the relaxation to cromakalim and most likely pinacidil is mediated through opening of K(ATP) channels. Inhibition of the Na,K-pump, however, may change the local environment for the K(ATP) channels (i.e. increases the ATP/ADPratio and/or decreases the transmembrane potassium gradient), which partly prevents the activation of the K(ATP)-channel current.

Challenges to potassium metabolism: internal distribution and external balance

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. Net K secretion occurs mainly in principal cells while K absorption takes place in intercalated cells. Studies on single tubules and principal and intercalated cells have defined the determinants of K secretion and reabsorption including the electrochemical driving forces, specific carriers, ATPases, and K channels. Recent studies on the properties and molecular identity of renal K channels have also contributed significantly to understanding the renal mechanisms that transport and regulate K excretion.

Modulation of Na+,K+-ATPase activity is of importance for RVD

AIM

This study was performed to examine the role of Na+,K+-ATPase activity for the adaptive response to cell swelling induced by hypoosmoticity, i.e. the regulatory volume decrease (RVD).

METHODS

The studies were performed on COS-7 cells transfected with rat Na+,K+-ATPase. To study changes in cell volume, cells were loaded with the fluorescent dye calcein and the intensity of the dye, following exposure to a hypoosmotic medium, was recorded with confocal microscopy.

RESULTS

Ouabain-mediated inhibition of Na+,K+-ATPase resulted in a dose dependent decrease in the rate of RVD. Total 86Rb+ uptake as well as ouabain dependent 86Rb+ uptake, used as an index of Na+,K+-ATPase dependent K+ uptake, was significantly increased during the first 2 min following exposure to hypoosmoticity. Since protein kinase C (PKC) plays an important role in the modulation of RVD, a study was carried out on COS-7 cells expressing rat Na+,K+-ATPase, where Ser23 in the catalytic alpha1 subunit of rat Na+,K+-ATPase had been mutated to Ala (S23A), abolishing a known PKC phosphorylation site. Cells expressing S23A rat Na+,K+-ATPase exhibited a significantly lower rate of RVD and showed no increase in 86Rb+ uptake during RVD.

CONCLUSION

Taken together, these results suggest that a PKC-mediated transient increase in Na+,K+-ATPase activity plays an important role in RVD.

Regulation of renal K transport by dietary K intake

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

ATP-dependent K+ channels in renal ischemia reperfusion injury

ATP-dependent K+ channels (KATP) account for most of the recycling of K+ which enters the proximal tubules cell via Na, K-ATPase. In the mitochondrial membrane, opening of these channels preserves mitochondrial viability and matrix volume during ischemia. We examined KATP channel modulation in renal ischemia-reperfusion injury (IRI), using an isolated perfused rat kidney (IPRK) model, in control, IRI, IRI+200 microM diazoxide (a KATP opener), IRI + 10 microM glibenclamide (a KATP blocker) and IRI + 200 microM diazoxide + 10 microM glibenclamide groups. IRI was induced by 2 periods of warm ischemia, followed by 45 min of reperfusion. IRI significantly decreased glomerular filtration rate (GFR) and increased fractional excretion of sodium (FENa) (p < 0.01). Neither diazoxide nor glibenclamide had an effect on control kidney function other than an increase in renal vascular resistance produced by glibenclamide. Pretreatment with 200 microM diazoxide reduced the postischemic increase in FENa (p < 0.05). Adding 10 microM glibenclamide inhibited the diazoxide effect on postischemic FENa (p < 0.01). Histology showed that kidneys pretreated with glibenclamide demonstrated an increase in injury in the thick ascending limb of outer medulla (p < 0.05). Glibenclamide significantly decreased post ischemic renal vascular resistance (p < 0.05), but had no significant effect on other renal function parameters. Our results suggest that sodium reabsorption is improved by KATP activation and blockade of KATP channels during IRI has an injury enhancing effect on renal epithelial function and histology. This may be mediated through KATP modulation in cell and/or mitochondrial inner membrane.

Activity of the basolateral K+ channels is coupled to the Na+-K+-ATPase in the cortical collecting duct

Microelectrode and patch-clamp techniques were used in the isolated cortical collecting duct to study the effects of stimulating Na+-K+-ATPase by raising bath K+ (Fujii Y and Katz AI. Am J Physiol Renal Fluid Electrolyte Physiol 257: F595-F601, 1989 and Muto S, Asano Y, Seldin D, and Giebisch. Am J Physiol Renal Physiol 276: F143-F158, 1999) on the transepithelial (VT) and basolateral membrane (VB) voltages and basolateral K+ channel activity. Increasing bath K+ from 2.5 to 8.5 mM resulted in an initial hyperpolarization of both VT and VB followed by a delayed depolarization. The effects of raising bath K+ on VT and VB were attenuated by decreasing luminal Na+ from 146.8 to 14.0 mM and were abolished by removal of luminal Na+, whereas those were magnified in desoxycorticosterone acetate (DOCA)-treated rabbits. Increasing bath K+ also led to a significant reduction of the intracellular Na+ and Ca2+ concentrations. The transepithelial conductance (GT) or fractional apical membrane resistance (fRA) were unaltered during the initial hyperpolarization phase, whereas, in the late depolarization phase, there were an increase in GT and a decrease in fRA, both of which were attenuated in the presence of low luminal Na+ (14.0 mM). In tubules from DOCA-treated animals, bath Ba2+ not only caused a significantly larger initial hyperpolarization of VT and VB but also blunted the late depolarization by high bath K+. Nomega-nitro-l-arginine methyl ester (l-NAME) partially mimicked the effect of Ba2+ and decreased the amplitude of the late depolarization. Patch-clamp experiments showed that raising bath K+ from 2.5 to 8.5 mM resulted in an increased activity of the basolateral K+ channel, which was absent in the presence of l-NAME. We conclude that stimulation of Na+-K+-ATPase increases the basolateral K+ conductance and that this effect involves suppression of nitric oxide-dependent inhibition of K+ channels.

Intracellular Na concentration and Rb uptake in proximal convoluted tubule cells and abundance of Na/K-ATPase alpha1-subunit in NHE3-/- mice

Proximal solute and fluid absorption is greatly reduced in mice in which the gene encoding the Na/H exchanger isoform 3 has been ablated (NHE3-/-). To obtain information on the intracellular functional consequences of such selective NHE3 deficiency, Na, Cl and K concentrations and cell Rb uptake were measured using electron microprobe analysis after a 30-s infusion of Rb (an index of basolateral Na/K-ATPase activity) in proximal convoluted tubule (PCT) cells of NHE3-/- and wild-type (NHE3+/+) mice. In addition, the relative abundance of the alpha1-subunit of the Na/K-ATPase in the outer cortex was determined by Western blot analysis. PCT cell Na concentration in NHE3-/- mice was slightly but significantly lower than in NHE3+/+ [13.1+/-0.6 ( n=64) vs. 14.9+/-0.6 ( n=62) mmol/kg wet wt.; means +/-SEM]. The lower intracellular Na concentration was associated with significantly reduced Rb uptake rates [9.7+/-0.6 ( n=59) vs. 14.8+/-0.8 ( n=50) mmol/kg wet wt./30 s], but the abundance of the alpha1-subunit of the Na/K-ATPase was not different between NHE3-/- and NHE3+/+ mice. Intracellular Cl concentration was higher (14.2+/-0.4 vs. 12.8+/-0.4 mmol/kg wet wt.) and K concentration unchanged (122.7+/-2.7 vs. 121.6+/-2.5 mmol/kg wet wt.) in PCT cells in NHE3-/- compared with NHE3+/+ mice. These findings suggest that the elimination of apical NHE3 in PCT cells of NHE3-/- mice reduces apical Na entry and, due to lower cell Na concentrations, Na/K-ATPase activity. The observed changes in intracellular Na concentration did not affect the expression of Na/K-ATPase in the renal cortex of NHE3-/- mice. There were no significant changes of cell Na concentration and Rb uptake in distal convoluted tubule, connecting tubule, principal and intercalated cells.

Dopamine D2-like receptor-mediated opening of K+ channels in opossum kidney cells

(1) This study examined the effects of dopamine D(1)- and D(2)-like receptor activation upon basolateral K(+) (I(K)) currents and changes in membrane potential in opossum kidney (OK) cells. (2) The addition of amphotericin B (3 micro g ml(-1)) to the apical side resulted in a rapid increase in I(K), this effect being markedly inhibited by the addition of the K(+) channel blockers barium chloride (1 mM) or glibenclamide (10 micro M), but not apamin (1 micro M). The K(+) channel opener pinacidil increased the amphotericin B-induced I(K). The selective D(2)-like receptor agonist quinerolane increased, in a concentration dependent manner (EC(50)=136 nM), I(K) across the basolateral membrane, this effect being abolished by pre-treatment with pertussis toxin (PTX), S-sulpiride (selective D(2)-like receptor antagonist) and glibenclamide. The selective D(1)-like receptor agonist SKF 38393 did not change I(K). Both H-89 (PKA inhibitor) and chelerythrine (PKC inhibitor) failed to prevent the stimulatory effect of quinerolane upon I(K). (3) Quinerolane did not change basal levels of cyclic AMP and also failed to affect the forskolin-induced increase in cyclic AMP levels. (4) The stimulation of D(2)-like receptor was associated with a rapid hyperpolarizing effect, whereas D(1)-like receptor activation was accompanied by increases in cell membrane potential. The hyperpolarizing effect of quinerolane (EC(50)=129 nM) was prevented by pre-treatment with PTX, S-sulpiride and glibenclamide. (5) It is concluded that stimulation of dopamine D(2)-like, but not D(1)-like, receptors coupled to PTX-sensitive G proteins of the G(i/o) class produce membrane hyperpolarization through opening of K(ATP) channels.

K(+) transport in Malpighian tubules of Tenebrio molitor L: is a K(ATP) channel involved?

The presence of ATP-regulated K(+) (K(ATP)) channels in Tenebrio molitor Malpighian tubules was investigated by examining the effect of glibenclamide on both fluid secretion and basolateral membrane potentials (V(bl)). Glibenclamide, a K(ATP) channel blocker, slowed fluid secretion of Tenebrio tubules. In low bath K(+) concentration (5 mmol l(-1)), glibenclamide either hyperpolarized or depolarized V(bl), resembling the effect seen with Ba(2+). Subsequent addition of 6 mmol l(-1) Ba(2+) caused a further hyper- or depolarization of V(bl). In control Ringer (50 mmol l(-1) KCl, 90 mmol l(-1) NaCl), glibenclamide had no visible effect on V(bl). The effect of ouabain was investigated in low bath [K(+)] in the presence of Ba(2+). V(bl) responded by a small but significant hyperpolarization from -51+/-4 mV to -56+/-4 mV (n=16, P<0.001) in response to 1 mmol l(-1) ouabain. Repeating the experiments in the presence of both glibenclamide and Ba(2+) resulted in a depolarization of V(bl) when ouabain was added. In low bath [K(+)] (high Na(+)), the Na(+)/K(+)-ATPase is expected to function at a high rate. In the presence of Ba(2+), replacing Na(+) by K(+) rapidly depolarized V(bl), but this was followed by a repolarization. Repeating the experiments in the presence of glibenclamide markedly reduced the depolarizing effect and abolished the repolarization, with a gradual decrease in the sensitivity of V(bl) to the surrounding [K(+)]. These results suggest the presence of K(ATP) channels in the basolateral membrane. Glibenclamide had no visible effect on V(bl) in high K(+) or in the absence of Ba(2+), indicating that other highly conductive K(+) channels may mask the effect on K(ATP) channels. This is the first demonstration of the presence of K(ATP) channels in an insect epithelium.

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.

Sulfonylurea-sensitive K(+) transport is involved in Cl(-) secretion and cyst trowth by cultured ADPKD cells

Transepithelial chloride and fluid secretion by many types of epithelia involves activation of a conductive K(+) pathway that serves to support the electrochemical driving force for Cl(-) secretion. This study sought to determine if such a pathway is involved in Cl(-) and fluid secretion by the cystic epithelia in autosomal dominant polycystic kidney disease (ADPKD). Primary cultures of cells derived from the cysts of patients with ADPKD were used. Confluent monolayers of these cells, mounted in Ussing chambers, were stimulated to secrete Cl(-) by application of the adenylyl cyclase agonist, forskolin. The effects of various K(+) channel blockers on the increase in short-circuit current (I(sc)) generated by active Cl(-) secretion were determined. Charybdotoxin, an inhibitor of Ca(2+)-sensitive K(+) channels exerted no effect. Similarly, the chromanole 293B, an inhibitor of cAMP-induced K(+) conductance, exerted no effect on cAMP-dependent anion secretion. Glibenclamide, an inhibitor of ATP-sensitive K(+) channels and the cystic fibrosis transmembrane conductance regulator (CFTR), modestly inhibited the forskolin-stimulated current when applied to the apical surface of the monolayers, suggesting a relatively weak effect on CFTR. Basolateral application of glibenclamide inhibited I(sc) to a greater extent. This latter effect may be due to inhibition of a K(+)-conductive transport step. Glibenclamide exerted little effect on the I(sc) of nonstimulated monolayers. Cyst growth in ADPKD is driven by cell proliferation and Cl(-) and fluid secretion. The effect of glibenclamide on the growth of cysts formed within a collagen gel by cultured ADPKD cells was tested. Addition of glibenclamide to the media bathing the cysts inhibited their growth. Glibenclamide also blocked the formation of cysts when it was added to the media at the time the cells were seeded within the collagen gel. Glibenclamide was also found to inhibit the proliferation of ADPKD cells. RT-PCR analysis demonstrated that the ATP-sensitive K(+) channel, K(ir) 6.2, is expressed in cultured ADPKD cells and in normal human kidney. These results suggest that ATP-sensitive K(+) channel blockers should be investigated as possible therapeutic agents to inhibit cyst growth in ADPKD.

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.

D2-like receptor-mediated inhibition of Na+-K+-ATPase activity is dependent on the opening of K+ channels

This study examined the effects of D2-like dopamine receptor activation on Na+-K+-ATPase activity while apical-to-basal, ouabain-sensitive, amphotericin B-induced increases in short-circuit current and basolateral K+ (I(K)) currents in opossum kidney cells were measured. The inhibitory effect of dopamin on Na+-K+-ATPase activity was completely abolished by either D1- or D2-like receptor antagonists and mimicked by D1- and D2-like receptor agonists SKF-38393 and quinerolane, respectively. Blockade of basolateral K+ channels with BaCl2 (1 mM) or glibenclamide (10 microM), but not apamin (1 microM), totally prevented the inhibitory effects of quinerolane. The K+ channel opener pinacidil decreased Na+-K+-ATPase activity. The inhibitory effect of quinerolane on Na+-K+- ATPase activity was abolished by pretreatment of opossum kidney cells with pertussis toxin (PTX). Quinerolane increased I(K) across the basolateral membrane in a concentration-dependent manner; this effect was abolished by pretreatment with PTX, S-sulpiride, and glibenclamide. SKF-38393 did not change I(K). Both H-89 (protein kinase A inhibitor) and chelerythrine (protein kinase C inhibitor) failed to prevent the stimulatory effect of quinerolane on I(K). The stimulation of the D2-like receptor was associated with a rapid hyperpolarizing effect, whereas D1-like receptor activation was accompanied by increases in cell membrane potential. It is concluded that stimulation of D2-like receptors leads to inhibition of Na+-K+-ATPase activity and hyperpolarization; both effects are associated with the opening of K+ channels.

A mechanogated nonselective cation channel in proximal tubule that is ATP sensitive

Ion channels that are gated in response to membrane deformation or "stretch" are empirically designated stretch-activated channels. Here we describe a stretch-activated nonselective cation channel in the basolateral membrane (BLM) of the proximal tubule (PT) that is nucleotide sensitive. Single channels were studied in cell-intact and cell-free patches from the BLM of PT cells that maintain their epithelial polarity. The limiting inward Cs+ conductance is ~28 pS, and channel activity persists after excision into a Ca2+- and ATP-free bath. The stretch-dose response is sigmoidal, with half-maximal activation of about -19 mmHg at -40 mV, and the channel is activated by depolarization. The inward conductance sequence is: NH ~ Cs+ ~ Rb+ > K+ ~ Na+ ~ Li+ > Ca2+ ~ Ba2+ > N-methyl-D-glucamine ~ tetraethylammonium. The venom of the common Chilean tarantula, Grammostola spatulata, completely blocks channel activity in cell-attached patches. Hypotonic swelling reversibly activates the channel. Intracellular ATP concentration ([ATP]i) reversibly blocks the channel (inhibitory constant approximately 0.48 mM), suggesting that channel function is coupled to the metabolic state of the cell. We conclude that this channel may function as a Ca2+ entry pathway and/or be involved in regulation of cell volume. We speculate this channel may be important when [ATP]i is depleted, as occurs during periods of increased transepithelial transport or with ischemic injury.

Properties of an inwardly rectifying K(+) channel in the basolateral membrane of mouse TAL

We investigated the properties of K(+) channels in the basolateral membrane of the cortical thick ascending limb (CTAL) using the patch-clamp technique. Approximately 34% of cell-attached patches contained an inwardly rectifying K(+) channel (K(+)-to-Na(+) permeability ratio approximately 22), having an inward conductance (G(in)) of 44 pS and an outward conductance (G(out)) of approximately 10 pS (G(in)/G(out) approximately 4). Channel activity (NP(o)) increased with depolarization. When the cytosolic sides of inside-out patches were exposed to an Mg(2+)-free medium, the channel had a G(in) of 50 pS and was weakly inwardly rectifying (G(in)/G(out) approximately 1). Cytosolic Mg(2+) reduced G(out), yielding a G(in)/G(out) of 3.8 at 1.3 mM Mg(2+). Internal Na(+) also yielded a G(in)/G(out) of 1.6 at 20 mM Na(+). Spermine reduced NP(o) on inside-out membrane patches. Sensitivity to spermine at depolarizing voltages [half-maximal inhibitory concentration (K(i)) = 0.2 microM] was much greater than at hyperpolarizing voltages (K(i) = 26 microM). Half-inactivation by 0.5 microM spermine occurred at a clamp potential of 43 mV, with an effective valence of 1.25. A sigmoid relationship between bath pH and NP(o) of inside-out membrane patches was observed, with a pK of 7.6 and a Hill coefficient of 1.8. Intracellular acidification also reduced the NP(o) of cell-attached patches. This channel is probably a major component of K(+) conductance in the CTAL basolateral membrane.

Cloning of rabbit Kir6.1, SUR2A, and SUR2B: possible candidates for a renal K(ATP) channel

In rabbit proximal tubules, a basolateral ATP- and taurine-sensitive K(+) channel (K(ATP)) was shown to be involved in the regulation of the basolateral K(+) conductance as a function of the rate of apical Na(+) entry. To establish the molecular identity of this channel, we used degenerated primers to look for cDNA transcripts for an inwardly rectifying K(+) channel (Kir6.1 and Kir6.2) and sulfonylurea receptors (SUR1, SUR2A, and SUR2B) in a cDNA library obtained from rabbit proximal tubules. PCR products were found only for Kir6.1, SUR2A, and SUR2B. Expression of Kir6.1 in Xenopus oocytes generated an additional K(+) current that was found to be sensitive to external barium and intracellular taurine and to changes in intracellular ATP concentrations. To study the specificity of the taurine sensitivity, intracellular taurine was tested on several members of the Kir family expressed in Xenopus oocytes. K(+) currents induced by Kir1.1A, Kir2.1, Kir3.2, Kir4.1, or Kir5.1 were insensitive to taurine, but all tested combinations of Kir6.x with or without the SUR subunit were significantly inhibited by taurine. This study suggests that the taurine-sensitive K(ATP) channel of rabbit proximal tubules is formed by a combination of Kir6.1 plus SUR2A and/or SUR2B.

Rapid and non-genomic reduction of intracellular [Ca(2+)] induced by aldosterone in human bronchial epithelium

1. Using a Ca(2+) imaging system and fura-2 AM (5 microM) we showed that exposure of polarised monolayers of human bronchial epithelial cells (16HBE14o- cell line) to aldosterone produced a fast intracellular [Ca(2+)] ([Ca(2+)](i)) decrease, in 70 % of cells. Exposure to aldosterone (1 nM) reduced the [Ca(2+)](i) by 39 +/- 9 nM (n = 282, P < 0.0001) within 10 min, from a basal [Ca(2+)](i) of 131 +/- 19 nM (n = 282). 2. The effect of aldosterone on [Ca(2+)](i) was not affected by inhibitors of the classical genomic pathway, cycloheximide (1 microM) or spironolactone (10 microM). The aldosterone-induced [Ca(2+)](i) decrease was inhibited by thapsigargin (1 microM), pertussis toxin (24 h at 200 ng ml(-1)), the adenylate cyclase inhibitors 2',3'-dideoxyadenosine (200 microM) and MDL-12,330A hydrochloride (500 microM), and the protein kinase A inhibitor R(P)-adenosine 3',5'-cyclic monophosphorothioate (200 microM). In addition, treatment of 16HBE14o- monolayers with aldosterone (1 nM) inhibited by approximately 30 % the large and transient [Ca(2+)](i) increase induced by apical exposure to uridine triphosphate (UTP, 0.1 mM), a known secretagogue in airway epithelia. 3. Our results demonstrate for the first time that in human bronchial epithelial cells, aldosterone decreases [Ca(2+)](i) levels via a non-genomic mechanism. The hormone-induced changes to [Ca(2+)](i) involve stimulation of thapsigargin-sensitive Ca(2+)-ATPase, via G-protein-, adenylate cyclase- and protein kinase A-coupled signalling pathways.

Renal potassium channels: function, regulation, and structure

Many transport functions in renal tubules depend on potassium (K) channels. Not only does K secretion and the maintenance of external K balance depend on K channel activity in principal tubule cells, but K channels also regulate cell volume; they are an integral party of cell function in all tubule cells because of their key role in the generation of the cell-negative electrical potential that affects the transmembrane movement of many charged solutes. Moreover, the recycling of K across the apical membrane of the thick ascending limb (TAL) plays an important role in the control of NaCl reabsorption in this tubule segment. Significant progress in our understanding of the structure and function of renal K channels has become possible by combining several strategies. These include transport studies in single tubules, application of the patch-clamp technique for exploring the properties of single K channels in native tubules and the cloning, and expression of diverse K channels of renal origin. Insights from these investigations promise to provide a deeper understanding of the mechanism by which K channels participate in many diverse tubule functions.

An ATP-regulated and pH-sensitive inwardly rectifying K(+) channel in cultured human proximal tubule cells

Although renal K(+) channels along the nephron have been explored in various animal species, little is known about the K(+) channels in human proximal tubule cells. Using the patch-clamp technique, we investigated the properties of an inwardly rectifying K(+) channel present in the surface membrane of cultured human proximal tubule cells of normal kidney origin. This channel was the most frequently observed K(+) channel in cell-attached patches, and cytoplasmic ATP was required to maintain channel activity in inside-out patches. Its single channel conductance was about 42 pS for inward currents and 7 pS for outward currents under the symmetrical K(+) condition. The ATP effect on channel activity was dose-dependently stimulatory within a range of 0.1 to 10 mM, and a nonhydrolyzable ATP analog, AMP-PNP (3 mM), had no effect on channel activity in either the presence or absence of ATP (1 mM). The channel activity observed in cell-attached patches was reduced to 30 to 50% of controls by a membrane-permeable nonspecific protein kinase inhibitor, K252a (1 microM), or a potent protein kinase A inhibitor, KT5720 (500 nM). In contrast, a membrane-permeable cAMP analog, 8Br-cAMP (100 microM), induced a twofold increase in channel activity. The addition of a catalytic subunit of protein kinase A (PKA-CS, 100 U/ml) to the bath in inside-out patches stimulated channel activity in the presence of 1 mM ATP. Furthermore, the channel activity maintained with 1 mM ATP in inside-out patches was suppressed by internal acidification and enhanced by alkalization. These results suggest that the activity of the inwardly rectifying K(+) channel in cultured human proximal tubule cells was ATP-dependent and regulated at least in part by cAMP/PKA-mediated phosphorylation processes and intracellular pH.

Cellular localization of the potassium channel Kir7.1 in guinea pig and human kidney

BACKGROUND

K(+) channels have important functions in the kidney, such as maintenance of the membrane potential, volume regulation, recirculation, and secretion of potassium ions. The aim of this study was to obtain more information on the localization and possible functional role of the inwardly rectifying K(+) channel, Kir7.1.

METHODS

Kir7.1 cDNA (1114 bp) was isolated from guinea pig kidney (gpKir7.1), and its tissue distribution was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). In addition, a genomic DNA fragment (6153 bp) was isolated from a genomic library. cRNA was expressed in Xenopus laevis oocytes for functional studies. Immunohistochemistry and RT-PCR were used to localize Kir7.1 in guinea pig and human kidney.

RESULTS

The expression of gpKir7.1 in Xenopus laevis oocytes revealed inwardly rectifying K(+) currents. The reversal potential was strongly dependent on the extracellular K(+) concentration, shifting from -14 mV at 96 mmol/L K(+) to -90 mV at 1 mmol/L K(+). gpKir7.1 showed a low affinity for Ba(2+). Significant expression of gpKir7.1 was found in brain, kidney, and lung, but not in heart, skeletal muscle, liver, or spleen. Immunocytochemical detection in guinea pig identified the gpKir7.1 protein in the basolateral membrane of epithelial cells of the proximal tubule. RT-PCR analysis identified strong gpKir7.1 expression in the proximal tubule and weak expression in glomeruli and thick ascending limb. In isolated human tubule fragments, RT-PCR showed expression in proximal tubule and thick ascending limb.

CONCLUSION

Our results suggest that Kir7.1 may contribute to basolateral K(+) recycling in the proximal tubule and in the thick ascending limb.

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.