ATP-sensitive K+ channels in the kidney

Molecular nociceptive mechanisms in migraine: The migraine cascade

OBJECTIVE

This review will explore the categorization of migraine-provoking molecules, their cellular actions, site of action and potential drug targets based on the migraine cascade model.

METHODS

Personal experience and literature.

RESULTS

Migraine impacts over 1 billion people worldwide but is underfunded in research. Recent progress, particularly through the human and animal provocation model, has deepened our understanding of its mechanisms. This model have identified endogenous neuropeptides such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating peptide (PACAP) that induces controlled migraine-like attacks leading to significant discoveries of their role in migraine. This knowledge led to the development of CGRP-inhibiting drugs; a groundbreaking migraine treatment now accessible globally. Also a PACAP-inhibiting drug was effective in a recent phase II trial. Notably, rodent studies have shed light on pain pathways and the mechanisms of various migraine-inducing substances identifying novel drug targets. This is primarily done by using selective inhibitors that target specific signaling pathways of the known migraine triggers leading to the hypothesized cellular cascade model of migraine.

CONCLUSION

The model of migraine presents numerous opportunities for innovative drug development. The future of new migraine treatments is limited only by the investment from pharmaceutical companies.

The ATP sensitive potassium channel (KATP) is a novel target for migraine drug development

Migraine is one of the leading causes of disability worldwide, affecting work and social life. It has been estimated that sales of migraine medicines will reach 12.9 billion USD in 2027. To reduce social impact, migraine treatments must improve, and the ATP-sensitive potassium (K_ATP) channel is a promising target because of the growing evidence of its implications in the pathogenesis of migraine. Strong human data show that opening of the K_ATP channel using levcromakalim is the most potent headache and migraine trigger ever tested as it induces headache in almost all healthy subjects and migraine attacks in 100% of migraine sufferers. This review will address the basics of the K_ATP channel together with clinical and preclinical data on migraine implications. We argue that K_ATP channel blocking, especially the Kir6.1/SUR2B subtype, may be a target for migraine drug development, however translational issues remain. There are no human data on the closure of the K_ATP channel, although blocking the channel is effective in animal models of migraine. We believe there is a good likelihood that an antagonist of the Kir6.1/SUR2B subtype of the K_ATP channel will be effective in the treatment of migraine. The side effects of such a blocker may be an issue for clinical use, but the risk is likely only moderate. Future clinical trials of a selective Kir6.1/SUR2B blocker will answer these questions.

Kir6.1 and SUR2B in Cantú syndrome

Kir6.1 and SUR2 are subunits of ATP-sensitive potassium (KATP) channels expressed in a wide range of tissues. Extensive study has implicated roles of these channel subunits in diverse physiological functions. Together they generate the predominant KATP conductance in vascular smooth muscle and are the target of vasodilatory drugs. Roles for Kir6.1/SUR2 dysfunction in disease have been suggested based on studies of animal models and human genetic discoveries. In recent years, it has become clear that gain-of-function (GoF) mutations in both genes result in Cantú syndrome (CS)-a complex, multisystem disorder. There is currently no targeted therapy for CS, but studies of mouse models of the disease reveal that pharmacological reversibility of cardiovascular and gastrointestinal pathologies can be achieved by administration of the KATP channel inhibitor, glibenclamide. Here we review the function, structure, and physiological and pathological roles of Kir6.1/SUR2B channels, with a focus on CS. Recent studies have led to much improved understanding of the underlying pathologies and the potential for treatment, but important questions remain: Can the study of genetically defined CS reveal new insights into Kir6.1/SUR2 function? Do these reveal new pathophysiological mechanisms that may be important in more common diseases? And is our pharmacological armory adequately stocked?

Impact of Membrane Voltage on Formation and Stability of Human Renal Proximal Tubules in Vitro

More than 15% of adults in the United States suffer from some form of chronic kidney disease (CKD). Current strategies for CKD consist of dialysis or kidney transplant, which, however, can take several years. In this light, tissue engineering and regenerative medicine approaches are the key to improving people's living conditions by advancing previous tissue engineering approaches and seeking new targets as intervention methods for kidney repair or replacement. The membrane voltage (V_m) dynamics of a cell have been associated with cell migration, cell cycle progression, differentiation, and pattern formation. Furthermore, bioelectrical stimuli have been used as a means in the treatment of diseases and wound healing. Here, we investigated the role of V_m as a novel target to guide and manipulate in vitro renal tissue models. Human-immortalized renal proximal tubule epithelial cells (RPTECs-TERT1) were cultured on Matrigel to support the formation of 3D proximal tubular-like structures with the incorporation of a voltage-sensitive dye indicator─bis-(1,3-dibutylbarbituric acid)timethine oxonol (DiBAC). The results demonstrated a correlation between the depolarization and the reorganization of human renal proximal tubule cells, indicating V_m as a candidate variable to control these events. Accordingly, V_m was pharmacologically manipulated using glibenclamide and pinacidil, K_ATP channel modulators, and proximal tubule formation and tubule stability over 21 days were assessed. Chronic manipulation of K_ATP channels induced changes in the tubular network topology without affecting lumen formation. Thus, a relationship was found between the preluminal tubulogenesis phase and K_ATP channels. This relationship may provide future options as a control point during kidney tissue development, treatment, and regeneration goals.

Effects of ATP-sensitive potassium channel blockers on vascular hyporeactivity, mesenteric blood flow, and survival in lipopolysaccharide-induced septic shock model

In this study, the possible therapeutic effects of various ATP-sensitive potassium channel (KATP) blockers (glibenclamide, repaglinide, 5-HD, HMR-1098) have been tested in experimental septic shock model. Rats were given lipopolysaccharide (1 mg·kg−1) to create experimental shock model and 4 h later, under 400 mg·kg−1 chloral hydrate anesthesia, parameters such as blood pressure, mesenteric blood flow, the response of mesenteric circulation to phenylephrine (vasoconstrictor stimulation), and organ and oxidative damage were analyzed. Also 75 mg·kg−1 lethal dose of lipopolysaccharide was given to mice and effects of KATP blockers on survival have been tested. Non-selective blocker glibenclamide with sulphonylurea structure and sarcolemmal KATP channel blocker HMR-1098, which have the similar chemical structure, have improved the pathological parameters such as decrease in mesenteric blood flow, vascular hyporeactivity, but could not prevent the decrease in blood pressure, and oxidative and organ damage that were observed in the shock model. Also, both blockers have decreased the mortality rate from 80% to 40%–50%. Similar (preventive) therapeutic effects were not observed with non-selective blocker repaglinide and mitochondrial KATP channel blocker 5-HD, which were non-sulphonylurea structure. As a result, only KATP channel blockers that have sulphonylurea structure can be a new therapeutic approach in septic shock.

Nicorandil ameliorates ischaemia-reperfusion injury in the rat kidney

BACKGROUND AND PURPOSE

Nicorandil, an ATP-sensitive potassium (K(ATP) ) channel opener and nitric oxide donor, is used in the treatment of angina and acute heart failure. Here we investigated the effects of two K(ATP) channel openers, nicorandil and cromakalim on ischaemia reperfusion (I-R) injury in the kidney.

EXPERIMENTAL APPROACH

Right nephrectomy was performed in 8-week-old male Sprague-Dawley rats and they were then divided into six groups: control group; I-R, including 30 min of left renal ischaemia followed by 24 h of reperfusion; I-R groups plus nicorandil 3 or 10 mg·kg⁻¹ i.p.; and I-R groups plus cromakalim 100 or 300 µg·kg⁻¹ i.p. After reperfusion, renal function was estimated by serum creatinine (SCr), urinary albumin:creatinine ratio (ACR) and urinary β2-microglobulin (β2-MG). Levels of K(ATP) channel subtypes were investigated by Western blot. Kidney sections were stained for 4-hydroxy-2-nonenal and 8-hydroxy-2'-deoxyguanosine.

KEY RESULTS

Renal I-R induced significant increases in SCr, ACR and β2-MG levels compared with the control animals. Treatment with K(ATP) channel openers reduced urinary β2-MG levels, raised by I-R. Both K(IR) 6.1 and K(IR) 6.2 channels were expressed. Expression of K(IR) 6.2 channels in the I-R group was lower than in the control group, which was restored to normal by treatment with K(ATP) channel openers. Histologically, severe acute tubular damage was observed in the I-R kidney and this damage was ameliorated by K(ATP) channel openers, dose-dependently.

CONCLUSIONS AND IMPLICATIONS

ATP-sensitive potassium channel openers protected against proximal tubule damage after I-R injury. Nicorandil could represent a powerful additional component in the treatment of patients undergoing partial nephrectomy or renal transplantation.

Investigating the role of endogenous opioids and KATP channels in glycerol-induced acute renal failure

The present study was designed to investigate the possible role of endogenous opioids and K(ATP) channels in glycerol-induced acute renal failure (ARF) in rats. The rats were subjected to rhabdomyolytic ARF by single intramuscular injection of hypertonic glycerol (50% v/v; 8 mL/kg), and the animals were sacrificed after 24 h of glycerol injection. The plasma creatinine, blood urea nitrogen, creatinine clearance, and histopathological studies were performed to assess the degree of renal injury. Naltrexone (2.5, 5.0 and 10.0 mg/kg s.c.), glibenclamide (5.0 and 10.0 mg/kg i.p.), and minoxidil (25 and 50 mg/kg) were employed to explore the role of endogenous opioids and K(ATP) channels in rhabdomyolysis-induced ARF. Pretreatment with naltrexone and glibenclamide attenuated hypertonic glycerol-induced renal dysfunction in a dose-dependent manner, suggesting the role of endogenous opioids and K(ATP) channels in the pathogenesis of myoglobuniric renal failure. However, the simultaneous pretreatment with naltrexone (10 mg/kg s.c.) and glibenclamide (10 mg/kg i.p.) did not enhance the reno-protective effects of individual drugs, suggesting that release of endogenous opioids and opening of K(ATP) channels constitute a single pathway in acute renal injury triggered by hypertonic glycerol-induced rhabdomyolysis. Furthermore, administration of minoxidil abolished the attenuating effects of naltrexone in glycerol-induced renal failure, suggesting that opening of K(ATP) channels is downstream to opioid receptor activation. It is concluded that hypertonic glycerol-induced rhabdomyolysis may involve release of endogenous opioids that in turn modulate K(ATP) channels to contribute in the pathogenesis of ARF.

Insulin-induced electrophysiology changes in human pleura are mediated via its receptor

Background. Insulin directly changes the sheep pleural electrophysiology. The aim of this study was to investigate whether insulin induces similar effects in human pleura, to clarify insulin receptor's involvement, and to demonstrate if glibenclamide (hypoglycemic agent) reverses this effect. Methods. Human parietal pleural specimens were mounted in Ussing chambers. Solutions containing insulin or glibenclamide and insulin with anti-insulin antibody, anti-insulin receptor antibody, and glibenclamide were used. The transmesothelial resistance (R_TM) was determined. Immunohistochemistry for the presence of Insulin Receptors (IRa, IRb) was also performed. Results. Insulin increased R_TM within 1st min (P = .016), when added mesothelially which was inhibited by the anti-insulin and anti-insulin receptor antibodies. Glibenclamide also eliminated the insulin-induced changes. Immunohistochemistry verified the presence of IRa and IRb. Conclusion. Insulin induces electrochemical changes in humans as in sheep via interaction with its receptor. This effect is abolished by glibenclamide.

Arginine-Vasopressin Modulates Intracellular pH via V1 and V2 Receptors in Renal Collecting Duct Cells

Arginine-vasopressin (AVP) has been proposed to be involved in the modulation of acid-base transporters; however, the nature of the mechanisms underlying AVP direct action on intracellular pH (pH(i)) in the cortical collecting duct (CCD) is not yet clearly defined. The aim of the present study was to elucidate which are the proteins implicated in AVP modulation of pH(i), as well as the receptors involved in these responses using a CCD cell line (RCCD(1)); pH(i) was monitored with the fluorescent dye BCECF in basal conditions and after stimulation with basolateral 10(-8) M AVP. Specific V1- or V2-receptor antagonists were also used. RT-PCR studies demonstrated that RCCD(1) cells express V1a and V2 receptors. Functional studies showed that while V2-receptor activation induced a biphasic response (alkalinization-acidification), V1-receptor activation resulted in an intracellular acidification. The V2-mediated alkalinization phase involves the activation of basolateral NHE-1 isoform of the Na(+)/H(+) exchanger while in the acidification phase CFTR is probably implicated. On the other hand, V1-mediated acidification was due to activation of a Cl(-)/HCO(3)(-) exchanger. We conclude that in RCCD(1) cells AVP selectively activates, via a complex of V1 and V2 receptor-mediated actions, different ion transporters linked to pH(i) regulation which might have physiological implications.

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.

Characterization of K(ATP)-channels in rat basilar and middle cerebral arteries: studies of vasomotor responses and mRNA expression

Changes in the activity of K+ channels represent a major mechanism that regulates vascular tone. Cerebrovascular adenosine 5'-triphosphate-sensitive K+(K(ATP)) channels were characterized in studies of the molecular expression and vasomotor reactivity to different K(ATP) channel openers in rat basilar and middle cerebral arteries. Both arteries showed strong mRNA expression of the subunits of the pore-forming inward-rectifying K+ channel type 6.1 (Kir6.1), Kir6.2 and the connected sulfonylurea receptor (SUR) subunits, SUR1 and SUR2B, while only weak bands for SUR2A were seen. The K(ATP) channel openers induced relaxation of prostaglalndin F2alpha-precontracted isolated basilar and middle cerebral arteries with the order of potency N-Cyano-N-(1,1-dimethylpropyl)-N''-3pyridylguanidine (P-1075)>levcromakalim>N-(4-Phenylsulfonylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropanamide (ZM226600)>pinacidil>diazoxide. The responses induced by levcromakalim, ZM226600 and diazoxide were significantly more potent in basilar arteries than in middle cerebral arteries, while pinacidil and P-1075 were equipotent. Endothelium removal decreased (P<0.05) the sensitivity (pIC50) of basilar arteries, but not of middle cerebral arteries, to pinacidil, levcromakalim, P-1075 and ZM226600. The maximum relaxant response to P-1075 was stronger (P<0.005) in basilar arteries with endothelium than without endothelium. Correlation of the relaxant potency of K(ATP) channel openers in rat basilar and middle cerebral arteries with historical measurements of affinity obtained in COS-7 cell lines expressing either SUR1, SUR2A or SUR2B showed that vasodilatation by K(ATP) channel openers correlated with binding to either the SUR2A or the SUR2B subunit. Glibenclamide was a blocker of relaxation induced by pinacidil, levcromakalim, P-1075 and ZM226600 in basilar arteries. Only a weak antagonistic effect of glibenclamide on pinacidil-, levcromakalim- and ZM226600-induced relaxations was found in middle cerebral arteries. The subunit profile and the observed pharmacological properties suggest that the K(ATP) channels expressed in rat basilar and middle cerebral artery are likely to be composed of SUR2B co-associated with Kir6.1 or Kir6.2. In basilar arteries, but not in middle cerebral arteries, endothelial K(ATP) channels may be involved.

The ATP-sensitive potassium channel blocker glibenclamide prevents renal ischemia/reperfusion injury in rats

BACKGROUND

Renal ischemia/reperfusion (I/R) is a complex neutrophil-mediated syndrome. Adenosine-triphosphate (ATP)-sensitive potassium (K(ATP)) channels are involved in neutrophil migration in vivo. In the present study, we have investigated the effects of glibenclamide, a K(ATP) channel blocker, in renal I/R injury in rats.

METHODS

The left kidney of the rats was excised through a flank incision and ischemia was performed in the contralateral kidney by total interruption of renal artery flow for 45 minutes. Renal perfusion was reestablished, and the kidney and lungs were removed for analysis of vascular permeability, neutrophil accumulation, and content of cytokines [tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, and IL-10] 4 and 24 hours later. Renal function was assessed by measuring creatinine, Na(+), and K(+) levels in the plasma and by determination of creatinine clearance. Drugs were administered subcutaneously after the onset of ischemia.

RESULTS

Reperfusion of the ischemic kidney induced local (kidney) and remote (lung) inflammatory injury and marked renal dysfunction. Glibenclamide (20 mg/kg) significantly inhibited the reperfusion-associated increase in vascular permeability, neutrophil accumulation, increase in TNF-alpha levels and nuclear factor-kappaB (NF-kappaB) translocation. These inhibitory effects were noticed in the kidney and lungs. Moreover, glibenclamide markedly ameliorated the renal dysfunction at 4 and 24 hours.

CONCLUSION

Treatment with glibenclamide is associated with inhibition of neutrophil recruitment and amelioration of renal dysfunction following renal I/R. Glibenclamide may have a therapeutic role in the treatment of renal I/R injury, such as after renal transplantation.

3-Trifluoromethyl-4-nitro-5-arylpyrazoles are novel KATP channel agonists

This communication describes the discovery and synthesis of a series of 3-trifluoromethyl-4-nitro-5-arylpyrazoles as potent K(ATP) channel agonists. The most potent compound reported is ca. 100-fold more potent than diazoxide and exhibits selectivity for the SUR1 K(ATP) channel subtype. The 4-nitro substitutent on the pyrazole ring was required for activity, and limited SAR suggests that the de-protonated pyrazole maybe the active species.

Multiparametric monitoring of ischemia-reperfusion in rat kidney: effect of ischemic preconditioning

BACKGROUND

Microelectrode technology is a promising tool for monitoring kidney ischemia and the changes induced by its therapeutic management. Ischemic preconditioning, that is, brief ischemic periods before sustained ischemia, has been shown to protect several organs, including the kidney, from ischemia-reperfusion injury. We tested whether the effect of preconditioning could be appraised by real-time measurement of parameters representative of tissue hypoxia.

METHODS

In a sample of pentobarbital-anesthetized and mechanically ventilated rats, we studied the effect of renal ischemic preconditioning (10-min ischemia and 10-min reflow interval) on subsequent ischemia-reperfusion (45 min and 60 min). Renal tissue electrical impedance, extracellular pH, and potassium concentration [K+] were measured continuously by implanted microelectrodes.

RESULTS

Ischemia induced an early, rapid rise in extracellular potassium and impedance module, followed by a phase of slower increase, whereas pH decreased rapidly, reaching a plateau. Preconditioning treatment did not cause significant changes in interstitial pH and [K+] but increased ischemic tissue impedance. During reperfusion, the three variables recovered progressively; however, after a decline, electrical impedance showed a clear postischemic increase. This rise was suppressed by preconditioning.

CONCLUSIONS

Real-time measurement of any of the three parameters showed capability for early detection of ischemia. In contrast with findings in myocardial tissue, preconditioning in the kidney did not increase potassium cell loss during ischemia or improve ischemic acidosis or tissue impedance. Electrical impedance increased for a second time during reperfusion, indicating the presence of a postischemic cellular edema; concealing this episode was the most noticeable effect of the preconditioning treatment.

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.

Synthesis and evaluation of 7-substituted-3-cyclobutylamino-4H-1,2,4-benzothiadiazine-1,1-dioxide derivatives as KATP channel agonists

A series of 7-substituted-3-cyclobutylamino-4H-1,2,4-benzothiadiazine-1,1-dioxide derivatives has been synthesized and evaluated as K(ATP) channel agonists using the inside-out excised patch clamp technique. The most active compounds were approximately 20-fold more potent than diazoxide in opening K(ATP) channels. A linear relationship exists between the potency of the compound and the sigma value of the 7-substituent with electron-withdrawing groups exhibiting higher activity. These compounds may be useful in modulating insulin release from pancreatic beta-cells and in diseases associated with hyperinsulinemia.

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.

Sulphonylurea drugs reduce hypoxic damage in the isolated perfused rat kidney

Sulphonylurea drugs have been shown to protect against hypoxic damage in isolated proximal tubules of the kidney. In the present study we investigated whether these drugs can protect against hypoxic damage in a whole kidney preparation. Tolbutamide (200 microM) and glibenclamide (10 microM) were applied to the isolated perfused rat kidney prior to changing the gassing from oxygen to nitrogen for 30 min. Hypoxic perfusions resulted in an increased fractional excretion of glucose (FE % glucose 14.3+/-1.5 for hypoxic perfusions vs 4.9+/-1.6 for normoxic perfusions, mean +/- s.e. mean, P<0.05), which could be completely restored by 200 microM tolbutamide (5.7+/-0.4 for tolbutamide vs 14.3+/-1.5 for untreated hypoxic kidneys, P<0.01). Furthermore, tolbutamide reduced the total amount of LDH excreted in the urine (220+/-100 mU for tolbutamide vs. 1220+/-160 mU for untreated hypoxic kidneys, P<0.01). Comparable results were obtained with glibenclamide (10 microM). In agreement with the effect on functional parameters, ultrastructural analysis of proximal tubules showed increased brush border preservation in tolbutamide treated kidneys compared to untreated hypoxic kidneys. We conclude that glibenclamide and tolbutamide are both able to reduce hypoxic damage to proximal tubules in the isolated perfused rat kidney when applied in the appropriate concentrations.

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

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

Regulation of the ROMK channel: interaction of the ROMK with associate proteins

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

Eukaliuric diuresis and natriuresis in response to the KATP channel blocker U37883A: micropuncture studies on the tubular site of action

1. Systemic application of U37883A, a blocker of ATP sensitive potassium (KATP) channels, elicits diuresis and natriuresis without significantly altering urinary potassium excretion. 2. To elucidate tubular sites of action upstream to the distal nephron, micropuncture experiments were performed in nephrons with superficial glomeruli of anaesthetized Munich-Wistar-Frömter rats during systemic application of U37883A (1, 5 or 15 mg kg-1 i.v.). 3. The observed eukaliuric diuresis and natriuresis in response to U37883A at 15 mg kg-1 was accompanied by an increase in early distal tubular flow rate (VED) from 10 - 18 nl min(-1) reflecting a reduction in fractional reabsorption of fluid up to this site (FR-fluid) of 13%. The latter proposed an effect on water-permeable segments such as the proximal tubule which could fully account for the observed reduction in fractional reabsorption of Na+ up to the early distal tubule (FR-Na+) of 8% and the increase in early distal tubular Na+ concentration ([Na+]ED) from 35 - 51 mM whereas [K+]ED was left unaltered. 4. In comparison, furosemide (3 mg kg-1 i.v.), which acts in the water-impermeable thick ascending limb, elicited diuresis, natriuresis and kaliuresis which were associated with a fall in FR-Na+ of 10% with no change in FR-fluid, and a rise in [Na+]ED from 42 - 117 mM and [K+]ED from 1.2 - 5.7 mM with no change in VED. 5. Direct late proximal tubular fluid collections confirmed a significant inhibition of fluid reabsorption in proximal convoluted tubule in response to systemic application of U37883A. 6. These findings suggest that the diuretic and natriuretic effect upstream to the distal tubule in response to systemic application of U37883A involves actions on water-permeable segments such as the proximal convoluted tubule.

Pharmacological evidence for a KATP channel in renin-secreting cells from rat kidney

1. Openers of the ATP-sensitive potassium channel (KATP channel) increase and blockers decrease renin secretion. Here we report the effects of levcromakalim (LCRK, a channel opener) and glibenclamide (GBC, a blocker) on membrane potential, whole-cell current and the cytoplasmic Ca2+ concentration of renin-secreting cells (RSC). Studies were performed on afferent arterioles from the kidney of Na+-depleted rats. 2. As monitored with the fluorescent oxonol dye DiBAC4(3), LCRK (0.3 and 1 microM) induced a hyperpolarization of approximately 15 mV which was abolished by GBC (1 microM). 3. Whole-cell current-clamp experiments showed that RSC had a membrane potential of -61 +/- 1 mV (n = 16). LCRK (1 microM) induced a hyperpolarization of 9.9 +/- 0.2 mV (n = 16) which, in the majority of cells, decreased slowly with time. 4. Capacitance measurements showed a strong electrical coupling of the cells in the preparation. 5. At -60 mV, LCRK induced a hyperpolarizing current in a concentration-dependent manner with an EC50 of 152 +/- 31 nM and a maximum current of about 200 pA. 6. Application of GBC (1 microM) produced no effect; however, when applied after LCRK (300 nM), GBC inhibited the opener-induced hyperpolarizing current with an IC50 of 103 +/- 36 nM. 7. LCRK (0.3 and 1 microM) did not significantly affect the cytoplasmic Ca2+ concentration either at rest or after stimulation by angiotensin II. 8. The data show that LCRK induces a GBC-sensitive hyperpolarizing current in rat RSC. This current presumably originates from the activation of KATP channels which pharmacologically resemble those in vascular smooth muscle cells. The stimulatory effect of KATP channel opening on renin secretion is not mediated by a decrease in intracellular Ca2+ concentration.

Alternative splicing of sur2 Exon 17 regulates nucleotide sensitivity of the ATP-sensitive potassium channel

ATP-sensitive potassium channels (KATP) are implicated in a diverse array of physiological functions. Previous work has shown that alternative usage of exons 14, 39, and 40 of the muscle-specific KATP channel regulatory subunit, sur2, occurs in tissue-specific patterns. Here, we show that exon 17 of the first nucleotide binding fold of sur2 is also alternatively spliced. RNase protection demonstrates that SUR2(Delta17) predominates in skeletal muscle and gut and is also expressed in bladder, fat, heart, lung, liver, and kidney. Polymerase chain reaction and restriction digest analysis of sur2 cDNA demonstrate the existence of at least five sur2 splice variants as follows: SUR2(39), SUR2(40), SUR2(Delta17/39), SUR2(Delta17/40), and SUR2(Delta14/39). Electrophysiological recordings of excised, inside-out patches from COS cells cotransfected with Kir6.2 and the sur2 variants demonstrated that exon 17 splicing alters KATP sensitivity to ATP block by 2-fold from approximately 40 to approximately 90 microM for exon 17 and Delta17, respectively. Single channel kinetic analysis of SUR2(39) and SUR2(Delta17/39) demonstrated that both exhibited characteristic KATP kinetics but that SUR2(Delta17/39) exhibited longer mean burst durations and shorter mean interburst dwell times. In sum, alternative splicing of sur2 enhances the observed diversity of KATP and may contribute to tissue-specific modulation of ATP sensitivity.

CFTR is a conductance regulator as well as a chloride channel

CFTR Is a Conductance Regulator as well as a Chloride Channel. Physiol. Rev. 79, Suppl.: S145-S166, 1999. - Cystic fibrosis transmembrane conductance regulator (CFTR) is a member of the ATP-binding cassette (ABC) transporter gene family. Although CFTR has the structure of a transporter that transports substrates across the membrane in a nonconductive manner, CFTR also has the intrinsic ability to conduct Cl- at much higher rates, a function unique to CFTR among this family of ABC transporters. Because Cl- transport was shown to be lost in cystic fibrosis (CF) epithelia long before the cloning of the CF gene and CFTR, CFTR Cl- channel function was considered to be paramount. Another equally valid perspective of CFTR, however, derives from its membership in a family of transporters that transports a multitude of different substances from chemotherapeutic drugs, to amino acids, to glutathione conjugates, to small peptides in a nonconductive manner. Moreover, at least two members of this ABC transporter family (mdr-1, SUR) can regulate other ion channels in the membrane. More simply, ABC transporters can regulate somehow the function of other cellular proteins or cellular functions. This review focuses on a plethora of studies showing that CFTR also regulates other ion channel proteins. It is the hope of the authors that the reader will take with him or her the message that CFTR is a conductance regulator as well as a Cl- channel.

The attenuated effect of ATP-sensitive K+ channel opener pinacidil on renal haemodynamics in spontaneously hypertensive rats

In hypertension, impairment of hyperpolarization by K+ efflux through ATP-sensitive K+ (K(ATP)) channels may contribute to the elevated renal vascular resistance. To elucidate such a role for K(ATP) channels in the renal vasculature, we used micropuncture techniques to examine the effect of K(ATP) channel opener, pinacidil (0.15 mg/h per kg body wt i.v.), on renal and glomerular haemodynamics in spontaneously hypertensive rats (SHR) and in normotensive controls (Wistar Kyoto, WKY). Since pinacidil reduced blood pressure significantly in both groups, the abdominal aorta was clamped before pinacidil administration to yield a renal perfusion pressure equivalent to that during pinacidil infusion. Pinacidil significantly decreased renal vascular resistance in both groups, but the relative change from baseline value was greater in WKY than in SHR. These effects of pinacidil were abolished by pretreatment with glibenclamide (3 mg/kg body wt i.v.). Proximal tubular stop-flow pressure (Psf), an index of glomerular capillary pressure, was significantly elevated by pinacidil infusion in WKY, a response abolished by pretreatment with glibenclamide, but not in SHR. The tubuloglomerular feedback response of Psf was not affected by pinacidil in either group. These data suggest that the activity of K(ATP) channels in SHR may be attenuated in the renal microvasculature. This may contribute to the elevated vascular tone in the renal preglomerular vasculature in SHR.

Mg2+ and ATP dependence of K(ATP) channel modulator binding to the recombinant sulphonylurea receptor, SUR2B

1. The binding of modulators of the ATP-sensitive K+ channel (KATP channel) to the murine sulphonylurea receptor, SUR2B, was investigated. SUR2B, a proposed subunit of the vascular KATP channel, was expressed in HEK 293 cells and binding assays were performed in membranes at 37 degrees C using the tritiated KATP channel opener, [3H]-P1075. 2. Binding of [3H]-P1075 required the presence of Mg2+ and ATP. MgATP activated binding with EC50 values of 10 and 3 microM at free Mg2+ concentrations of 3 microM and 1 mM, respectively. At 1 mM Mg2+, binding was lower than at 3 microM Mg2+. 3. [3H]-P1075 saturation binding experiments, performed at 3 mM ATP and free Mg2+ concentrations of 3 microM and 1 mM, gave KD values of 1.8 and 3.4 nM and BMAX values of 876 and 698 fmol mg(-1), respectively. 4. In competition experiments, openers inhibited [3H]-P1075 binding with potencies similar to those determined in rings of rat aorta. 5. Glibenclamide inhibited [3H]-P1075 binding with Ki values of 0.35 and 2.4 microM at 3 Mm and 1 mM free Mg2+, respectively. Glibenclamide enhanced the dissociation of the [3H]-P1075-SUR2B complex suggesting a negative allosteric coupling between the binding sites for P1075 and the sulphonylureas. 6. It is concluded that an MgATP site on SUR2B with microM affinity must be occupied to allow opener binding whereas Mg2+ concentrations > or = 10 microM decrease the affinities for openers and glibenclamide. The properties of the [3H]-P1075 site strongly suggest that SUR2B represents the drug receptor of the openers in vascular smooth muscle.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Binding and effects of KATP channel openers in the vascular smooth muscle cell line, A10

1. The ATP-sensitive K+ channel (KATP channel) in A10 cells, a cell line derived from rat thoracic aorta, was characterized by binding studies with the tritiated KATP channel opener, [3H]-P1075, and by electrophysiological techniques. 2. Saturation binding experiments gave a KD value of 9.2 +/- 5.2 nM and a binding capacity (BMax) of 140 +/- 40 fmol mg-1 protein for [3H]-P1075 binding to A10 cells; from the BMax value a density of binding sites of 5-10 per microns2 plasmalemma was estimated. 3. KATP channel modulators such as the openers P1075, pinacidil, levcromakalim and minoxidil sulphate and the blocker glibenclamide inhibited [3H]-P1075 binding. The extent of inhibition at saturation depended on the compound, levcromakalim inhibiting specific [3H]-P1075 binding by 85%, minoxidil sulphate and glibenclamide by 70%. The inhibition constants were similar to those determined in strips of rat aorta. 4. Resting membrane potential, recorded with microelectrodes, was -51 +/- 1 mV. P1075 and levcromakalim produced a concentration-dependent hyperpolarization by up to -25 mV with EC50 values of 170 +/- 40 nM and 870 +/- 190 nM, respectively. The hyperpolarization induced by levcromakalim (3 microM) was completely reversed by glibenclamide with an IC50 value of 86 +/- 17 nM. 5. Voltage clamp experiments were performed in the whole cell configuration under a physiological K+ gradient. Levcromakalim (10 microM) induced a current which reversed around -80 mV; the current-voltage relationship showed considerable outward rectification. Glibenclamide (3 microM) abolished the effect of levcromakalim. 6. Analysis of the noise of the levcromakalim (10 microM)-induced current at -40 and -20 mV yielded estimates of the channel density, the single channel conductance and the probability of the channel to be open of 0.14 micron-2, 8.8 pS and 0.39, respectively. 7. The experiments showed that A10 cells are endowed with functional KATP channels which resemble those in vascular tissue; hence, these cells provide an easily accessible source of channels for biochemical and pharmacological studies. The density of binding sites for [3H]-P1075 was estimated to be one order of magnitude higher than the density of functional KATP channels; assuming a plasmalemmal localization of the binding sites this suggests a large receptor reserve for the openers in A10 cells.

Simultaneous recording of tissue ion content and blood flow in rat renal medulla: evidence on interdependence

The relationship of renal medullary tissue ion concentration and medullary blood flow (MBF) has never been closely evaluated because of limitations of available measuring methods. In an attempt to overcome this difficulty, an integrated probe was developed for simultaneous recording in rat renal medulla of tissue electrical admittance (Y), an index of interstitial ion concentration, and tissue perfusion with blood (laser-Doppler method). During spontaneous-selective MBF variations tissue Y showed inverse changes (r = -0.77, P < 0.001). The inverse correlation of the two variables was also seen after MBF has been reduced (-43%) by indomethacin, 5 mg/kg body wt iv (r = -0.77, P < 0.01). A modest selective MBF reduction (15%) induced by glibenclamide, an inhibitor of ATP-dependent K channels, did not alter medullary tissue admittance. The data support experimentally the concept that the rate of medullary tissue perfusion with blood is one determinant of interstitial solute concentration; however, changes in the latter were demonstrable only with major alterations of the MBF.

Binding of [3H]-P1075, an opener of ATP-sensitive K+ channels, to rat glomerular preparations

ATP-sensitive K+ channels (KATP channels) in the kidney have been found in the tubular system and in the afferent arteriole. In this study we have examined the binding of [3H]-P1075 ([3H]-N-cyano-N'-(1, 1-dimethylpropyl)-N"-3-pyridylguanidine), a selective opener of KATP channels, in rat glomerular preparations. Equilibrium (saturation, competition) and kinetic experiments indicated that [3H]-P1075 binds to a single class of sites with a dissociation constant of about 3 nM and a maximum binding capacity of 10 fmol mg-1 glomerular protein. The association rate constant of the complex was 6,5 x 10(7) M-1 min-1; dissociation occurred with a half-time of 6.2 min. Specific [3H]-P1075 binding was strongly reduced when the metabolic state of the glomerular preparation was impaired during the preparation procedure or the binding assay or when the preparation was subjected to mild collagenase treatment. In different metabolically competent preparations, the amount of specific [3H]-P1075 binding correlated well with the number of vascular endings adherent to the glomeruli; no specific binding was found in mesangial cells in culture. Specific [3H]-P1075 binding was inhibited by representatives of the different classes of KATP channel openers and by sulphonylurea-type blockers with inhibition constants similar to those obtained in rat aortic rings. It is concluded that rat glomerular preparations possess specific binding sites for KATP channel openers with vascular characteristics. The sensitivity of binding to mild collagenase treatment suggests that these sites are located on a membrane protein; in addition, the data suggest that these sites are localized on smooth muscle and/or renin secreting cells of the afferent vascular endings attached to some of the glomeruli. Their estimated density (1,500 microns-2) is much higher than that of KATP channels in smooth muscle.