Recombinant cardiac ATP-sensitive K+ channel subunits confer resistance to chemical hypoxia-reoxygenation injury.

Opening of cardiac ATP-sensitive K+ (KATP) channels has emerged as a promising but still controversial cardioprotective mechanism. Defining KATP channel function at the level of recombinant channel proteins is a necessary step toward further evaluation of the cardioprotective significance of this ion conductance.



KATP channel deficient COS-7 cells were found to be vulnerable to chemical hypoxia-reoxygenation injury that induced significant cytosolic Ca2+ loading (from 97+/-3 to 236+/-11 nmol/L). In these cells, the potassium channel opener pinacidil (10 micromol/L) did not prevent Ca2+ loading (from 96+/-3 nmol/L before to 233+/-12 nmol/L after reoxygenation) or evoked membrane current. Cotransfection with Kir6.2/SUR2A genes, which encode cardiac KATP channel subunits, resulted in a cellular phenotype that, in the presence of pinacidil (10 micromol/L), expressed K+ current and gained resistance to hypoxia-reoxygenation (Ca2+ concentration from 99+/-7 to 127+/-11 nmol/L; P>0.05). Both properties were abolished by the KATP channel blocker glyburide (1 micromol/L). In COS-7 cells transfected with individual channel subunits Kir6.2 or SUR2A, which alone do not form functional cardiac KATP channels, pinacidil did not protect against hypoxia-reoxygenation.



The fact that transfer of cardiac KATP channel subunits protected natively KATP channel deficient cells provides direct evidence that the cardiac KATP channel protein complex harbors intrinsic cytoprotective properties. These findings validate the concept that targeting cardiac KATP channels should be considered a valuable approach to protect the myocardium against injury.

Related papers:

title: ATP-sensitive potassium channels and myocardial preconditioning
abstract: The ATP-sensitive potassium channel (KATP) has been shown to serve an endogenous cardioprotective role in a number of experimental models of myocardial stunning and infarction. More importantly, a majority of evidence has also been obtained which suggests that the KATP channel may be intimately involved in both triggering and maintaining the cardioprotection afforded by the phenomenon of ischemic preconditioning particularly in large animal models such as dogs and pigs. Although the evidence for an involvement of KATP in ischemic pre-conditioning is equivocal in smaller animal species such as rabbits and rats, activation of this channel by KATP channel openers produces cardioprotection in all species studied. Whether this channel is an important mediator of ischemic preconditioning in all animal species including man and the mechanism by which this cardio-protective effect is obtained await further experimental studies. Nevertheless, the use of selective potassium channel openers to mimic preconditioning in selected clinical settings may be a desirable future therapeutic goal.

title: Activation of ATP-sensitive potassium channels: a novel pharmacological approach to myocardial protection?
abstract: A number of compounds of widely differing chemical structure have been developed in an attempt to alter the open probability of what has become known as the adenosine triphosphate-sensitive potassium channel (KATP channel). They include channel openers (such as pinacidil, aprikalim, nicorandil, bimakalim, cromakalim and its active enantiomer levcromakalim) and channel blockers such as glibenclamide, tolbutamide and 5-hydroxydecanoate. They all are able to override the actions of an array of biochemical regulators of channel activity including ATP (an inhibitor of channel opening), adenosine diphosphate, adenosine, fatty acids, lactate, pyruvate and pH (for detailed reviews on the nature of the KATP channel and its biological and pharmacological control see (1–6)

title: Pharmacology of ATP-sensitive potassium channel (KATP) openers in models of myocardial ischemia and reperfusion.
abstract: Recently, much interest has been focused on the pharmacology of ATP-sensitive potassium channels (KATP) in myocardial ischemia. KATP are thought to be involved with the mechanism of myocardial preconditioning, therefore further increasing the level of interest in these channels. Pharmacologic KATP openers have been shown by numerous investigators to protect ischemic-reperfused myocardium. These agents reduce necrosis, improve postischemic functional recovery, and inhibit contracture formation. These protective effects are abolished by KATP blockers. The cardioprotective effects of KATP openers are independent of vasodilator and cardiodepressant effects, but seem to be mediated by energy conservation (reduced ATP hydrolysis). Action potential shortening is also not correlated with cardioprotection, suggesting a role for intracellular (mitochondrial) KATP. Agents have been developed that retain the glyburide-reversible cardioprotective effects of cromakalim but are devoid of vasodilator and action potential shortening activity. Currently, studies are underway to determine the mechanism of cardioprotection. The potential role of mitochondrial KATP in the pathogenesis of ischemia is discussed in this review article.

title: Ischemic preconditioning.
abstract: It has been shown that repeated brief coronary occlusions increase myocardial resistance towards prolonged episodes of ischemia. This phenomenon, which renders the heart more tolerant to ischemia with subsequent limitation of infarct size, has been termed ischemic preconditioning and has been described in a variety of species. Preconditioning may also protect the heart against postischemic dysfunction and ventricular arrhythmias. Although the beneficial effects seem to be transient, they re-appear at 24 hours, representing a "second window of protection." Ischemia-induced activation of adenosine receptors and opening of ATP-sensitive potassium channels appear to play a role in the acute cardioprotection. For the late protection, stress protein synthesis may play a role. There is experimental and clinical evidence that preconditioning effects may also exist in the human heart. If so, the mechanisms of ischemic preconditioning might be applied to future therapy.

title: Ischemic cardioprotection by ATP-sensitive K+ channels involves high-energy phosphate preservation.
abstract: We previously demonstrated that ATP-sensitive K+ channels (KATP) protect the guinea pig myocardium against ischemia-reperfusion injury (Cole et al.,  1991), but the cellular alterations leading to ischemic injury affected by KATP remain to be defined. This study investigates the relationship between activation of KATP and preservation of high-energy phosphates during global no-flow ischemia in arterially perfused guinea pig right ventricular walls. Electrical and mechanical activity were recorded via intracellular microelectrodes and a force transducer. Glibenclamide (10 and 50 microM) and pinacidil (10 microM) were used to modulate KATP. ATP and creatine phosphate (CP) levels were determined at the end of no-flow ischemia by enzymatic analysis. Preparations were subjected to 1) 20 min no-flow +/- glibenclamide (10 or 50 microM), 2) 30 min no-flow +/- pinacidil (10 microM) or pinacidil (10 microM) and glibenclamide (50 microM), or 3) 40 or 50 min of control perfusion before rapid freezing in liquid nitrogen. Pinacidil (10 microM) enhanced ischemic shortening of action potential duration (APD) and early contractile failure, prevented ischemic contracture, and inhibited high-energy phosphate depletion during ischemia. Glibenclamide (50 microM) inhibited the effects of pinacidil (10 microM) on electromechanical function and preservation of ATP and CP. Glibenclamide (10 microM) alone inhibited the early decline in APD and produced earlier ischemic contracture but did not enhance ATP or CP depletion compared with untreated tissues during 20 min of no-flow. Glibenclamide (50 microM) produced a greater inhibition of APD shortening in early ischemia, further decreased the latency to ischemic contracture, and caused enhanced ischemic depletion of ATP. The data indicate the changes in electrical activity induced by KATP indirectly preserve high-energy phosphates and reduce injury associated with ischemia. However, the data also suggest the possible presence of additional mechanisms for cardioprotection by KATP.

title: Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs.
abstract: ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells where they regulate cardiac function during cellular metabolic impairment. KATP channels have been implicated in the shortening of the action potential duration and the cellular loss of K+ that occurs during metabolic inhibition. KATP channels have been associated with the cardioprotective mechanism of ischemia-related preconditioning. Intracellular ATP (ATPi) is the main regulator of KATP channels. ATPi has two functions: 1) to close the channel (ligand function) and 2) in the presence of Mg2+, to maintain the activity of KATP channels (presumably through an enzymatic reaction). KATP channel activity is modulated by intracellular nucleoside diphosphates that antagonize the ATPi-induced inhibition of channel opening or induce KATP channels to open. How nucleotides will affect KATP channels depends on the state of the channel. K+ channel-opening drugs are pharmacological agents that enhance KATP channel activity through different mechanisms and have great potential in the management of cardiovascular conditions. KATP channel activity is also modulated by neurohormones. Adenosine, through the activation of a GTP-binding protein, antagonizes the ATPi-induced channel closure. Understanding the molecular mechanisms that underlie KATP channel regulation should prove essential to further define the function of KATP channels and to elucidate the pharmacological regulation of this channel protein. Since the molecular structure of the KATP channel has now become available, it is anticipated that major progress in the KATP channel field will be achieved.

title: Anti-ischemic effects of the potassium channel activators pinacidil and cromakalim and the reversal of these effects with the potassium channel blocker glyburide.
abstract: The direct cardioprotective efficacy of the potassium channel activators pinacidil and cromakalim was determined in isolated globally ischemic rat hearts. Isolated buffer-perfused rat hearts were subjected to 25 min of ischemia followed by 30 min of reperfusion. These hearts were pretreated with 1 to 100 microM pinacidil, 1 to 7 microM cromakalim or vehicle. Pinacidil resulted in significant improvements in reperfusion function and cardiac compliance, though it did not significantly reduce lactate dehydrogenase release at any concentration. The protective effects of pinacidil were greatest at a 10 microns concentration and were slightly diminished at higher concentrations (30 and 100 microns). Although not affecting the severity of ischemia alone, 10 microM glyburide (potassium channel blocker) completely reversed the protective effects of pinacidil on reperfusion function and compliance. Cromakalim (7 microM) resulted in a greater than 50% improvement in reperfusion function and compliance and unlike pinacidil significantly reduced lactate dehydrogenase release by approximately 50%. At 1 microM, glyburide alone did not significantly affect the severity of ischemia but reversed the protective effects of cromakalim. Not only did glyburide reverse the protective effects of cromakalim, it resulted in a worsening of ischemia compared to vehicle, an effect not seen with glyburide alone. Thus, both pinacidil and cromakalim appear to have direct cardioprotective efficacy, though some differences between them may be possible. The mechanism of their protective effects appears to be via potassium channel opening as the potassium channel blocker glyburide reverses the protective effect of these compounds. Intracellular electrophysiological studies showed that ischemia-induced depolarization was reversed with cromakalim, which increased the resting potential nearly back to preischemic levels.(ABSTRACT TRUNCATED AT 250 WORDS)

title: Blockade of ATP-sensitive potassium channels prevents myocardial preconditioning in dogs.
abstract: Single or multiple brief periods of ischemia (preconditioning) have been shown to protect the myocardium from infarction after a subsequent more prolonged ischemic insult. To test the hypothesis that preconditioning is the result of opening ATP-sensitive potassium (KATP) channels, a selective KATP channel antagonist, glibenclamide, was administered before or immediately after preconditioning in barbital-anesthetized open-chest dogs subjected to 60 minutes of left circumflex coronary artery (LCX) occlusion followed by 5 hours of reperfusion. Preconditioning was elicited by 5 minutes of LCX occlusion followed by 10 minutes of reperfusion before the 60-minute occlusion period. Glibenclamide (0.3 mg/kg i.v.) or vehicle was given 10 minutes before the initial ischemic insult in each of four groups. In a fifth group, glibenclamide was administered immediately after preconditioning. In a final series (group 6), a selective potassium channel opener, RP 52891 (10 micrograms/kg bolus and 0.1 micrograms/mg/min i.v.) was started 10 minutes before occlusion and continued throughout reperfusion. Transmural myocardial blood flow was measured at 30 minutes of occlusion, and infarct size was determined by triphenyltetrazolium staining and expressed as a percent of the area at risk. There were no significant differences in hemodynamics, collateral blood flow, or area at risk between groups. The ratio of infarct size to area at risk in the control group (28 +/- 6%) was not different from the group pretreated with glibenclamide in the absence of preconditioning (31 +/- 6%). Preconditioning produced a marked reduction (p less than 0.002) in infarct size (28 +/- 6% to 6 +/- 2%), whereas glibenclamide administered before or immediately after preconditioning completely abolished the protective effect (28 +/- 6% and 30 +/- 8%, respectively). RP 52891 also produced a significant (p less than 0.03) reduction (28 +/- 6% to 13 +/- 3%) in infarct size. These results suggest that myocardial preconditioning in the canine heart is mediated by activation of KATP channels and that these channels may serve an endogenous myocardial protective role.

title: Metabolic regulation of cardiac ATP-sensitive K+ channels
abstract: SummaryActivation of ATP-sensitive K+ (KATP) channels has been implicated as a cause of increased cellular K+ efflux and action potential duration (APD) shortening during myocardial ischemia, hypoxia, and selective glycolytic inhibition, since selective KATP channel antagonists partially or completely block increased cellular K+ efflux and APD shortening under these conditions. During substrate-free hypoxia or myocardial ischemia in intact rabbit ventricle, unidirectional K+ efflux rate during systole approximately doubled and APD decreased by ≈40% after 10 minutes. In patch-clamped guinea pig ventricular myocytes, similar changes could be produced by activation of <0.5% of the maximal KATP channel conductance. Furthermore, from studying the desensitizing effects of ADPi on the ATP sensitivity of KATP channels in excised inside-out patches, it was estimated that the rapid changes in the cytosolic ATP/ADP ratio during ischemia and hypoxia were of sufficient magnitude to activate KATP channels to this degree. During selective glycolytic inhibition, however, the global cytosolic ATP/ADP ratio in intact heart remained normal despite an increase in cellular K+ efflux comparable to ischemia and hypoxia. In patch-clamped saponin-permeabilized ventricular myocytes, KATP channels were preferentially suppressed by glycolytic ATP production compared to ATP generated by mitochondria or by the creatinine kinase reaction, and functional glycolytic enzymes were found to be associated with KATP channels in excised membrane patches. We hypothesize that sarcolemma-associated glycolytic enzymes may be important in maintaining a high local cytosolic ATP/ADP ratio in the vicinity of KATP channels, where sarcolemmal ATPases are tending to depress the local ATP/ADP ratio.

title: Preconditioning the human myocardium: Recent advances and aspirations for the development of a new means of cardioprotection in clinical practice
abstract: SummaryIschemic preconditioning has been shown to be one of the most powerful means of protecting the myocardium from ischemic injury in experimental animal models, although the mechanism is incompletely understood. In this review we discuss the evidence for preconditioning occurring in ischemic syndromes in humans, whether the human myocardium can be preconditioned, and whether preconditioning would have a place as a therapeutic tool in clinical practice. Some studies evaluating patients after acute myocardial infarction have shown a better outcome in patients reporting angina before the onset of the infarction, but this is not a universal finding, and it is difficult to exclude other confounding factors, such as collateral flow, from influencing the results. More controlled prospective studies have evaluated patients undergoing percutaneous transluminal coronary angioplasty and have found less ST-segment change and less reported angina during the second balloon inflation when compared with the first. Again, it is impossible to completely exclude other causes for this effect, but the dependence on mechanisms that are known to be important for preconditioning in animal models does suggest the phenomena are the same. Further experiments using isolated human atrial muscle have shown that human myocardium can be preconditioned and that the mechanisms involved are similar to those elucidated in animal models (adenosine, protein kinase C, and ATP-dependent potassium channels). In clinical medicine preconditioning is most likely to benefit patients when it is used to protect against the ischemia induced by cardiac surgery. In this respect, a study has shown that in patients undergoing coronary artery bypass grafts, the reduction in ATP occurring during the first ischemic period is attenuated in those given an ischemic preconditioning protocol beforehand. Despite these advances, it is likely that the full potential of preconditioning in clinical practice will not be realized until the whole mechanism of protection is understood and a safe pharmacological “preconditioning” agent becomes available.

title: Is calcium a mediator of infarct size reduction with preconditioning in canine myocardium?
abstract: The cellular mechanisms by which brief episodes of ischemia protect or "precondition" the heart and limit infarct size caused by a later period of sustained coronary artery occlusion remain unresolved. We propose that calcium may be an important mediator in eliciting this cardioprotection.



To test this hypothesis, anesthetized dogs received a 15-minute intracoronary infusion of 20 mmol/L CaCl2 or saline before undergoing 1 hour of coronary occlusion and 4 hours of reperfusion (protocol 1). Collateral blood flow during occlusion was measured with radiolabeled microspheres, area at risk of infarction (AR) was delineated by injection of blue dye, and area of necrosis (AN) was determined by tetrazolium staining. AN/AR was reduced from 20+/-5% in the saline-treated controls to 9+/-3% in CaCl2-treated dogs (P<.05). Additional animals underwent 10 minutes of preconditioning ischemia or a comparable waiting period before the 1-hour test occlusion (protocol 2). Administration of 5-(N,N-dimethyl)-amiloride (an inhibitor of calcium influx via Na+-H+ and Na+-Ca2+ exchange) before the preconditioning stimulus attenuated the protective effect of ischemic preconditioning: AN/AR was 12+/-1%, larger than the value of 4+/-1% observed in preconditioned dogs that received saline (P<.05) and comparable to the values of 12+/-3% and 14+/-3% seen in saline- and dimethylamiloride-treated controls.



Brief intracoronary infusion of CaCl2 mimicked, whereas treatment with dimethylamiloride blocked, infarct size reduction with preconditioning, thereby implicating calcium as a mediator of preconditioning in this canine model.

title: Signal Transduction in Ischemic Preconditioning:
abstract: Signaling In Preconditioning. Ischemic preconditioning is a phenomenon whereby exposure of the myocardium to a brief episode of ischemia and reperfusion markedly reduces tissue necrosis induced by a subsequent prolonged ischemia. Therefore, it is hoped that elucidation of the mechanism of preconditioning will yield therapeutic strategies capable of reducing myocardial infarction. In the rabbit, the brief period of preconditioning ischemia and reperfusion releases adenosine, bradykinin, opioids, and oxygen radicals that summate to induce the translocation and activation of protein kinase C (PKC). PKC appears to be the first element of a complex kinase cascade that is activated during the prolonged ischemia in preconditioned hearts. Current evidence indicates that PKC activates a tyrosine kinase that leads to the activation ofp38 mitogen‐activated protein (MAP) kinase or JNK, or possibly both. The stimulation of these stress‐activated protein kinases ultimately induces the opening of mitochondrial KATP channels that may be the final mediator of protection by ischemic preconditioning.

title: Intracellular diadenosine polyphosphates: a novel family of inhibitory ligands of the ATP-sensitive K+ channel.
abstract: Intracellular diadenosine polyphosphates (Ap(n)A) are signal molecules that alert the cell under stress conditions. Herein, we review evidence that has recently identified a novel target for Ap(n)A, namely the ATP-sensitive K+ (K(ATP)) channel. These channels are abundant in pancreatic beta-cells and cardiac myocytes where they are essential in coupling the cellular metabolic state with membrane excitability. The potency and efficacy of Ap(n)A to inhibit K(ATP) channel activity were first described in cardiac K(ATP) channels, and appear similar to those of intracellular ATP, the primary ligand of K(ATP) channels. Also, the inhibitory ligand action of Ap(n)A is dependent upon the operative condition of K(ATP) channels and the presence of nucleotide diphosphates. In addition to a direct antagonism of channel opening, an indirect effect of Ap(n)A on K(ATP) channel activity has been associated with inhibition of adenylate kinase, a catalytic system believed essential for the regulation of channel opening. At present, the precise role for Ap(n)A-induced K(ATP) channel inhibition remains to be established. Yet, it is known that, under glucose challenge of pancreatic beta-cells, intracellular concentrations of Ap(n)A do increase to micromolar levels necessary to block K(ATP) channels, leading to insulin secretion. Thus, the Ap(n)A-mediated K(ATP) channel gating represents a previously unrecognized pathway of channel regulation.

title: Operative condition-dependent response of cardiac ATP-sensitive K+ channels toward sulfonylureas.
abstract: A defining property of ATP-sensitive K+ (K[ATP]) channels is inhibition by sulfonylurea drugs, yet the response of cardiac K[ATP] channels toward sulfonylureas during myocardial ischemia is not consistent. Altered channel sensitivity toward sulfonylureas has, in part, been ascribed to antagonism by cytosolic nucleotide diphosphates, although the mechanism of interaction remains unclear. Herein, in inside-out patches excised from cardiomyocytes, we observed a dual response of K[ATP] channels toward the sulfonylurea drug, glyburide, in the presence of cytosolic UDP. Specifically, glyburide failed to inhibit spontaneous K[ATP] channel activity in the presence of UDP but inhibited UDP-induced channel activity after rundown of spontaneous channel openings. Such behavior of K[ATP] channels cannot be explained by differences in the level of channel activity or by UDP-induced displacement of glyburide. Rather, the dual response toward the sulfonylurea could be attributed to a property of K[ATP] channels to switch between operative conditions (spontaneous versus UDP-induced) each associated with a distinct responsiveness toward ligands. Conversion of post-rundown K[ATP] channels to the spontaneously operative channel condition, by Mg-ATP, restored the ability of UDP to antagonize the inhibitory action of glyburide lost after rundown, suggesting that the response of the channel to glyburide is phosphorylation dependent. The existence of distinct operative conditions of cardiac K[ATP] channels could be the basis for the inconsistent response of the channel toward sulfonylurea drugs and should be considered when sulfonylureas are used to implicate the opening of K[ATP] channels in the myocardium.

title: Adenosine prevents K+-induced Ca2+ loading: insight into cardioprotection during cardioplegia.
abstract: In clinical practice, hyperkalemic cardioplegia induces sarcolemmic depolarization, and therefore is used to arrest the heart during open heart operations. However, the elevated concentration of K+ that is present in cardioplegic solutions promotes intracellular Ca2+ loading, which could aggravate ventricular dysfunction after cardiac operations. This review highlights recent findings that have established, at the single cell level, the protective action of adenosine against hyperkalemia-induced Ca2+ loading. When it was added to hyperkalemic cardioplegic solutions, adenosine, at millimolar concentrations and through a direct action on ventricular cardiomyocytes, prevented K+-induced Ca2+ loading. This action of adenosine required the activation of protein kinase C, and it was effective only in cardiomyocytes with low diastolic Ca2+ levels. Of importance, adenosine did not diminish the magnitude of K+-induced membrane depolarization, allowing unimpeded cardiac arrest. Taken together, these findings provide direct support for the idea that adenosine is valuable when used as an adjunct to hyperkalemic cardioplegia. This idea has emerged from previous clinical studies that have shown improvement of the clinical outcome after cardiac operations when adenosine or related substances were used to supplement cardioplegic solutions. Further studies are required to define more precisely the mechanism of action of adenosine, and the conditions that may determine the efficacy of adenosine as a cytoprotective supplement to cardioplegia.

title: Cloning of the beta cell high-affinity sulfonylurea receptor: a regulator of insulin secretion.
abstract: Sulfonylureas are a class of drugs widely used to promote insulin secretion in the treatment of non-insulin-dependent diabetes mellitus. These drugs interact with the sulfonylurea receptor of pancreatic beta cells and inhibit the conductance of adenosine triphosphate (ATP)-dependent potassium (KATP) channels. Cloning of complementary DNAs for the high-affinity sulfonylurea receptor indicates that it is a member of the ATP-binding cassette or traffic ATPase superfamily with multiple membrane-spanning domains and two nucleotide binding folds. The results suggest that the sulfonylurea receptor may sense changes in ATP and ADP concentration, affect KATP channel activity, and thereby modulate insulin release.

title: Reconstitution of IKATP: An Inward Rectifier Subunit Plus the Sulfonylurea Receptor
abstract: A member of the inwardly rectifying potassium channel family was cloned here. The channel, called BIR (Kir6.2), was expressed in large amounts in rat pancreatic islets and glucose-responsive insulin-secreting cell lines. Coexpression with the sulfonylurea receptor SUR reconstituted an inwardly rectifying potassium conductance of 76 picosiemens that was sensitive to adenosine triphosphate (ATP) (IKATP) and was inhibited by sulfonylureas and activated by diazoxide. The data indicate that these pancreatic β cell potassium channels are a complex composed of at least two subunits-BIR, a member of the inward rectifier potassium channel family, and SUR, a member of the ATP-binding cassette superfamily. Gene mapping data show that these two potassium channel subunit genes are clustered on human chromosome 11 at position 11p15.1.

title: A Novel Sulfonylurea Receptor Forms with BIR (Kir6.2) a Smooth Muscle Type ATP-sensitive K+ Channel*
abstract: We have isolated a cDNA encoding a novel isoform of the sulfonylurea receptor from a mouse heart cDNA library. Coexpression of this isoform and BIR (Kir6.2) in a mammalian cell line elicited ATP-sensitive K+ (KATP) channel currents. The channel was effectively activated by both diazoxide and pinacidil, which is the feature of smooth muscle KATP channels. Sequence analysis indicated that this clone is a variant of cardiac type sulfonylurea receptor (SUR2). The 42 amino acid residues located in the carboxyl-terminal end of this novel sulfonylurea receptor is, however, divergent from that of SUR2 but highly homologous to that of the pancreatic one (SUR1). Therefore, this short part of the carboxyl terminus may be important for diazoxide activation of KATP channels. The reverse transcription-polymerase chain reaction analysis showed that mRNA of this clone was ubiquitously expressed in diverse tissues, including brain, heart, liver, urinary bladder, and skeletal muscle. These results suggest that this novel isoform of sulfonylurea receptor is a subunit reconstituting the smooth muscle KATP channel.

title: Burst Kinetics of Co-expressed Kir6.2/SUR1 Clones: Comparison of Recombinant with Native ATP-sensitive K+ Channel Behavior

abstract: Co-expression of clones encoding Kir6.2, a K+ inward rectifier, and SUR1, a sulfonylurea receptor, reconstitutes elementary features of ATP-sensitive K+ (KATP) channels. However, the precise kinetic properties of Kir6.2/SUR1 clones remain unknown. Herein, intraburst kinetics of Kir6.2/SUR1 channel activity, heterologously co-expressed in COS cells, displayed mean closed times from 0.7 ± 0.1 to 0.4 ± 0.03 msec, and from 0.4 ± 0.1 to 2.0 ± 0.2 msec, and mean open times from 1.9 ± 0.4 to 4.5 ± 0.8 msec, and from 12.1 ± 2.4 to 5.0 ± 0.2 msec between −100 and −20 mV, and +20 to +80 mV, respectively. Burst duration for Kir6.2/SUR1 activity was 17.9 ± 1.8 msec with 5.6 ± 1.5 closings per burst. Burst kinetics of the Kir6.2/SUR1 activity could be fitted by a four-state kinetic model defining transitions between one open and three closed states with forward and backward rate constants of 1905 ± 77 and 322 ± 27 sec−1 for intraburst, 61.8 ± 6.6 and 23.9 ± 5.8 sec−1 for interburst, 12.4 ± 6.0 and 13.6 ± 2.9 sec−1 for intercluster events, respectively. Intraburst kinetic properties of Kir6.2/SUR1 clones were essentially indistinguishable from pancreatic or cardiac KATP channel phenotypes, indicating that intraburst kinetics per se were insufficient to classify recombinant Kir6.2/SUR1 amongst native KATP channels. Yet, burst kinetic behavior of Kir6.2/SUR1 although similar to pancreatic, was different from that of cardiac KATP channels. Thus, expression of Kir6.2/SUR1 proteins away from the pancreatic micro-environment, confers the burst kinetic identity of pancreatic, but not cardiac KATP channels. This study reports the kinetic properties of Kir6.2/SUR1 clones which could serve in the further characterization of novel KATP channel clones.

title: Evidence for Direct Physical Association between a K+Channel (Kir6.2) and an ATP-Binding Cassette Protein (SUR1) Which Affects Cellular Distribution and Kinetic Behavior of an ATP-Sensitive K+ Channel
abstract: Structurally unique among ion channels, ATP-sensitive K+ (KATP) channels are essential in coupling cellular metabolism with membrane excitability, and their activity can be reconstituted by coexpression of an inwardly rectifying K+ channel, Kir6.2, with an ATP-binding cassette protein, SUR1. To determine if constitutive channel subunits form a physical complex, we developed antibodies to specifically label and immunoprecipitate Kir6.2. From a mixture of Kir6.2 and SUR1 in vitro-translated proteins, and from COS cells transfected with both channel subunits, the Kir6.2-specific antibody coimmunoprecipitated 38- and 140-kDa proteins corresponding to Kir6.2 and SUR1, respectively. Since previous reports suggest that the carboxy-truncated Kir6.2 can form a channel independent of SUR, we deleted 114 nucleotides from the carboxy terminus of the Kir6.2 open reading frame (Kir6.2ΔC37). Kir6.2ΔC37 still coimmunoprecipitated with SUR1, suggesting that the distal carboxy terminus of Kir6.2 is unnecessary for subunit association. Confocal microscopic images of COS cells transfected with Kir6.2 or Kir6.2ΔC37 and labeled with fluorescent antibodies revealed unique honeycomb patterns unlike the diffuse immunostaining observed when cells were cotransfected with Kir6.2-SUR1 or Kir6.2ΔC37-SUR1. Membrane patches excised from COS cells cotransfected with Kir6.2-SUR1 or Kir6.2ΔC37-SUR1 exhibited single-channel activity characteristic of pancreatic KATPchannels. Kir6.2ΔC37 alone formed functional channels with single-channel conductance and intraburst kinetic properties similar to those of Kir6.2-SUR1 or Kir6.2ΔC37-SUR1 but with reduced burst duration. This study provides direct evidence that an inwardly rectifying K+ channel and an ATP-binding cassette protein physically associate, which affects the cellular distribution and kinetic behavior of a KATP channel.

title: The effects of nucleotides and potassium channel openers on the SUR2A/Kir6.2 complex K+ channel expressed in a mammalian cell line, HEK293T cells
abstract: The effects of potassium channel opening drugs and intracellular nucleotides on the ATP-sensitive K+ (KATP) channel composed of SUR2A and Kir6.2 in HEK293T cells were examined using the patch-clamp technique. The SUR2A/Kir6.2 channel was activated effectively by pinacidil, marginally by nicorandil but not by diazoxide. The pinacidil-activated channel currents were inhibited by glibenclamide with a Ki value of 160 nM. Upon formation of inside-out (I-O) patches, spontaneous openings of the channels appeared, which were inhibited by intracellular ATP (ATPi) equipotently in the presence and in the absence of intracellular Mg2+ (Mg2+i). The channel activity ran-down gradually in I-O patches. The run-down channels could be reactivated by ATPi only in the presence of Mg2+i. Uridine 5’-diphosphate (UDP) antagonized the ATPi-mediated inhibition of the channel activity before run-down. After run-down, UDP activated the channel without antagonizing ATPi-mediated channel inhibition. Thus, the SUR2A/Kir6.2 reproduced the major properties of the native cardiac KATP channel well in terms of nucleotide regulation and pharmacology, and therefore can be a useful tool with which to elucidate the molecular mechanisms characterizing the KATP channel.

title: Adenosine prevents hyperkalemia-induced calcium loading in cardiac cells: relevance for cardioplegia.
abstract: Hyperkalemic cardioplegic solutions effectively arrest the heart but also induce membrane depolarization, which could lead to intracellular Ca2+ loading and contribute to ventricular dysfunction associated with cardiac operations. Adenosine, which possesses cardioprotective properties, has been proposed as an adjunct to conventional cardioplegic solutions. However, it is not known whether adenosine supplementation enables cardiac cells to withstand hyperkalemia-induced Ca2+ loading.



Single ventricular cardiomyocytes were isolated from guinea pig hearts, loaded with a Ca(2+)-sensitive fluorescent probe, and imaged by digital epifluorescent microscopy. The emitted fluorescence of the probe, a measure of the intracellular Ca2+ concentration, was recorded from single myocytes during hyperkalemic challenges in the absence and the presence of adenosine to assess the protective effectiveness of this agent.



Hyperkalemic solutions induced intracellular Ca2+ loading (estimated intracellular Ca2+ concentration, 88 +/- 5 nmol/L before and 1,825 +/- 112 nmol/L after addition of 16 mmol/L KCl). Adenosine (1 mmol/L) prevented K(+)-induced Ca2+ loading (intracellular Ca2+ concentration, 86 +/- 6 nmol/L before and 85 +/- 8 nmol/L after exposure to K+). Whereas glyburide (3 mumol/L), an antagonist of adenosine triphosphate-sensitive K+ channels, had no effect, staurosporine (200 nmol/L) and chelerythrine (5 mumol/L), two inhibitors of protein kinase C, did abolish the action of adenosine.



Adenosine prevents hyperkalemia-induced Ca2+ loading in cardiomyocytes. This effect is due to a direct action on ventricular cells, as the preparation employed was free from atrial, neuronal, and vascular elements, and appears to be mediated through a protein kinase C-dependent mechanism. The property of adenosine to prevent hyperkalemia-induced Ca2+ loading may contribute to the cytoprotective efficacy of this agent as an adjunct to conventional hyperkalemic cardioplegic solutions.

title: Physiologic mechanisms of postischemic tissue injury.
abstract: Early restitution of blood flow to ischemic tissues is essential to halt the progression of cellular injury associated with decreased oxygen and nutrient delivery. Recognition of this fact provides the basis for the traditional view that minimizing ischemic time is the only important intervention for diminish­ ing the extent of ischemic injury. However. it is now clear that reperfusion of ischemic tissues initiates a complex series of reactions that paradoxically injures tissues. Although several mechanisms have been proposed to explain the pathogenesis of ischemialreperfusion (IIR) injury. most attention has fo­ cused on a role for reactive oxygen metabolites and inflammatory leukocytes. This work has led to the proposal that' free radical ablation or inhibition of postischemic neutrophil infiltration may prove useful for therapeutic interven­ tion in I1R. In addition to these mechanisms. recent evidence suggests that prior exposure to brief periods of ischemia (ischemic preconditioning) prevents the development of cellular injury induced by a subsequent prolonged inter­ ruption in blood flow. By elucidating the cellular events underlying precondi­ tioning. additional mechanisms contributing to the pathogenesis of I/R may become apparent. This review summarizes the evidence supporting the view that oxidants and leukocytes contribute to the pathogenesis of reperfusion injury and discusses the mechanisms underlying the protective actions of ischemic preconditioning.

title: Alteration of intracellular Ca2+ transients in COS-7 cells transfected with the cDNA encoding skeletal-muscle ryanodine receptor carrying a mutation associated with malignant hyperthermia.
abstract: Malignant hyperthermia (MH), an inherited neuromuscular disease triggered by halogenated inhalational anaesthetics and skeletal-muscle relaxants, appears to be due to an alteration of intracellular Ca2+ homoeostasis. MH occurs in 1 out of 20,000 anaesthetized adults and is characterized by hypermetabolism, skeletal-muscle rigidity and elevation in body temperature, which is frequently fatal [MacLennan and Phillips (1992) Science 256, 789-794]. The defect responsible for the disease may lie within the mechanism controlling the release of Ca2+ from sarcoplasmic reticulum via the ryanodine-receptor (RYR) Ca2+ channel; in fact a point mutation in the RYR has been associated with MH in some human families, as well as in the MH-susceptible pig. To date, however, no direct evidence has been obtained demonstrating that the point mutation is both necessary and sufficient to cause functional alterations in RYR-mediated Ca2+ release. In the present report we show that the presence of the Arg-to-Cys point mutation in the recombinant RYR expressed in COS-7 transfected cells causes abnormal cytosolic Ca2+ transients in response to 4-chloro-m-cresol, an agent capable of eliciting in vitro contracture of MH-susceptible muscles.

title: Essential role of adenosine, adenosine A1 receptors, and ATP-sensitive K+ channels in cerebral ischemic preconditioning.
abstract: Preconditioning with sublethal ischemia protects against neuronal damage after subsequent lethal ischemic insults in hippocampal neurons. A pharmacological approach using agonists and antagonists at the adenosine A1 receptor as well as openers and blockers of ATP-sensitive K+ channels has been combined with an analysis of neuronal death and gene expression of subunits of glutamate and gamma-aminobutyric acid receptors, HSP70, c-fos, c-jun, and growth factors. It indicates that the mechanism of ischemic tolerance involves a cascade of events including liberation of adenosine, stimulation of adenosine A1 receptors, and, via these receptors, opening of sulfonylurea-sensitive ATP-sensitive K+ channels.

title: Activation of ATP‐dependent K+ channels by metabolic poisoning in adult mouse skeletal muscle: role of intracellular Mg(2+) and pH.
abstract: 1. The effects of metabolic poisoning, intracellular Mg(2+) and pH on ATP‐dependent K+ (K+ATP) channels were examined in adult mouse isolated skeletal muscle fibres using the patch clamp technique. 2. In cell‐attached membrane patches, while openings of one kind of channel could only rarely be detected under control conditions, cell poisoning with fluorodinitrobenzene (FDNB), dinitrophenol (DNP) and cyanide (CN) induced a strong and partially reversible increase in channel activity. 3. Slope conductance and glibenclamide sensitivity of this outward current indicated that the channel activated during poisoning was the K+ATP channel. 4. Single channel current amplitude was reduced during poisoning, but remained unchanged when activation of the K+ATP channel was induced by cromakalim. 5. In inside‐out membrane patches, in the absence of intracellular ATP, intracellular application of Mg2+ decreased channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 6. In the absence of intracellular ATP, a decrease in intracellular pH induced a reduction in channel activity and single channel current amplitude. Inhibition of K+ATP channels by ATP was also reduced. 7. The reduction of single channel current amplitude during poisoning was attributed to an increase in intracellular Mg2+ concentration caused by a fall in intracellular ATP concentration. These results also show that metabolic poisoning causes direct activation of K+ATP channels in skeletal muscle, and that is activation is at least partially mediated through an increase in intracellular Mg(2+) concentration and a decrease in intracellular pH.

title: The ABCs of ATP-sensitive potassium channels: more pieces of the puzzle.
abstract: ATP-sensitive potassium channels, KATP channels are critical for the normal regulation of insulin secretion. The cloning of cDNAs encoding the subunits of these channels shows that they are a novel combination of an ATP-binding protein and a small inward rectifier. Loss of pancreatic beta-cell KATP channels has been shown to cause familial hyperinsulinism.

title: Abnormalities of pancreatic islets by targeted expression of a dominant-negative KATP channel.
abstract: ATP-sensitive K+ (KATP) channels are known to play important roles in various cellular functions, but the direct consequences of disruption of KATP channel function are largely unknown. We have generated transgenic mice expressing a dominant-negative form of the KATP channel subunit Kir6.2 (Kir6.2G132S, substitution of glycine with serine at position 132) in pancreatic beta cells. Kir6.2G132S transgenic mice develop hypoglycemia with hyperinsulinemia in neonates and hyperglycemia with hypoinsulinemia and decreased beta cell population in adults. KATP channel function is found to be impaired in the beta cells of transgenic mice with hyperglycemia. In addition, both resting membrane potential and basal calcium concentrations are shown to be significantly elevated in the beta cells of transgenic mice. We also found a high frequency of apoptotic beta cells before the appearance of hyperglycemia in the transgenic mice, suggesting that the KATP channel might play a significant role in beta cell survival in addition to its role in the regulation of insulin secretion.