Ligand-insensitive state of cardiac ATP-sensitive K+ channels. Basis for channel opening

Inward and outward currents of native and cloned K(ATP) channels (Kir6.2/SUR1) share single-channel kinetic properties

Background

Methods

Results

Conclusions

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Outward K(ATP) channel currents are not grouped in bursts regardless of membrane potential.

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No differences in the number of kinetic states could be seen for single channels between native K(ATP) channels, Kir6.2ΔC26 alone and co-expressed with SUR1 for outward currents.

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K(ATP) channel open time for outward currents corresponds to burst duration for inward currents.

Uridine treatment prevents myocardial injury in rat models of acute ischemia and ischemia/reperfusion by activating the mitochondrial ATP-dependent potassium channel

The effect of uridine on the myocardial ischemic and reperfusion injury was investigated. A possible mechanism of its cardioprotective action was established. Two rat models were used: (1) acute myocardial ischemia induced by occlusion of the left coronary artery for 60 min; and (2) myocardial ischemia/reperfusion with 30-min ischemia and 120-min reperfusion. In both models, treatment with uridine (30 mg/kg) prevented a decrease in cell energy supply and in the activity of the antioxidant system, as well as an increase in the level of lipid hydroperoxides and diene conjugates. This led to a reduction of the necrosis zone in the myocardium and disturbances in the heart rhythm. The blocker of the mitochondrial ATP-dependent potassium (mitoKATP) channel 5-hydroxydecanoate limited the positive effects of uridine. The data indicate that the cardioprotective action of uridine may be related to the activation of the mitoKATP channel. Intravenously injected uridine was more rapidly eliminated from the blood in hypoxia than in normoxia, and the level of the mitoKATP channel activator UDP in the myocardium after uridine administration increased. The results suggest that the use of uridine can be a potentially effective approach to the management of cardiovascular diseases.

Control of Kir channel gating by cytoplasmic domain interface interactions

The pore-forming unit of ATP-sensitive K channels is composed of four Kir6.2 subunits. Borschel et al. show that salt bridges between the cytoplasmic domain of adjacent Kir6.2 subunits determine the degree to which channels inactivate after removal of ATP.

Inward rectifier potassium (Kir) channels are expressed in almost all mammalian tissues and play critical roles in the control of excitability. Pancreatic ATP-sensitive K (K_ATP) channels are key regulators of insulin secretion and comprise Kir6.2 subunits coupled to sulfonylurea receptors. Because these channels are reversibly inhibited by cytoplasmic ATP, they link cellular metabolism with membrane excitability. Loss-of-function mutations in the pore-forming Kir6.2 subunit cause congenital hyperinsulinism as a result of diminished channel activity. Here, we show that several disease mutations, which disrupt intersubunit salt bridges at the interface of the cytoplasmic domains (CD-I) of adjacent subunits, induce loss of channel activity via a novel channel behavior: after ATP removal, channels open but then rapidly inactivate. Re-exposure to inhibitory ATP causes recovery from this inactivation. Inactivation can be abolished by application of phosphatidylinositol-4,5-bisphosphate (PIP₂) to the cytoplasmic face of the membrane, an effect that can be explained by a simple kinetic model in which PIP₂ binding competes with the inactivation process. Kir2.1 channels contain homologous salt bridges, and we find that mutations that disrupt CD-I interactions in Kir2.1 also reduce channel activity and PIP₂ sensitivity. Kir2.1 channels also contain an additional CD-I salt bridge that is not present in Kir6.2 channels. Introduction of this salt bridge into Kir6.2 partially rescues inactivating mutants from the phenotype. These results indicate that the stability of the intersubunit CD-I is a major determinant of the inactivation process in Kir6.2 and may control gating in other Kir channels.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Sulfonylureas suppress the stimulatory action of Mg-nucleotides on Kir6.2/SUR1 but not Kir6.2/SUR2A KATP channels: a mechanistic study

Sulfonylureas suppress the stimulatory effect of Mg-nucleotides on recombinant β-cell (Kir6.2/SUR1) but not cardiac (Kir6.2/SUR2A) K_ATP channels.

Sulfonylureas, which stimulate insulin secretion from pancreatic β-cells, are widely used to treat both type 2 diabetes and neonatal diabetes. These drugs mediate their effects by binding to the sulfonylurea receptor subunit (SUR) of the ATP-sensitive K⁺ (K_ATP) channel and inducing channel closure. The mechanism of channel inhibition is unusually complex. First, sulfonylureas act as partial antagonists of channel activity, and second, their effect is modulated by MgADP. We analyzed the molecular basis of the interactions between the sulfonylurea gliclazide and Mg-nucleotides on β-cell and cardiac types of K_ATP channel (Kir6.2/SUR1 and Kir6.2/SUR2A, respectively) heterologously expressed in Xenopus laevis oocytes. The SUR2A-Y1206S mutation was used to confer gliclazide sensitivity on SUR2A. We found that both MgATP and MgADP increased gliclazide inhibition of Kir6.2/SUR1 channels and reduced inhibition of Kir6.2/SUR2A-Y1206S. The latter effect can be attributed to stabilization of the cardiac channel open state by Mg-nucleotides. Using a Kir6.2 mutation that renders the K_ATP channel insensitive to nucleotide inhibition (Kir6.2-G334D), we showed that gliclazide abolishes the stimulatory effects of MgADP and MgATP on β-cell K_ATP channels. Detailed analysis suggests that the drug both reduces nucleotide binding to SUR1 and impairs the efficacy with which nucleotide binding is translated into pore opening. Mutation of one (or both) of the Walker A lysines in the catalytic site of the nucleotide-binding domains of SUR1 may have a similar effect to gliclazide on MgADP binding and transduction, but it does not appear to impair MgATP binding. Our results have implications for the therapeutic use of sulfonylureas.

Activation of the K(ATP) channel by Mg-nucleotide interaction with SUR1

The mechanism of adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channel activation by Mg-nucleotides was studied using a mutation (G334D) in the Kir6.2 subunit of the channel that renders K_ATP channels insensitive to nucleotide inhibition and has no apparent effect on their gating. K_ATP channels carrying this mutation (Kir6.2-G334D/SUR1 channels) were activated by MgATP and MgADP with an EC₅₀ of 112 and 8 µM, respectively. This activation was largely suppressed by mutation of the Walker A lysines in the nucleotide-binding domains of SUR1: the remaining small (∼10%), slowly developing component of MgATP activation was fully inhibited by the lipid kinase inhibitor LY294002. The EC₅₀ for activation of Kir6.2-G334D/SUR1 currents by MgADP was lower than that for MgATP, and the time course of activation was faster. The poorly hydrolyzable analogue MgATPγS also activated Kir6.2-G334D/SUR1. AMPPCP both failed to activate Kir6.2-G334D/SUR1 and to prevent its activation by MgATP. Maximal stimulatory concentrations of MgATP (10 mM) and MgADP (1 mM) exerted identical effects on the single-channel kinetics: they dramatically elevated the open probability (P_O > 0.8), increased the mean open time and the mean burst duration, reduced the frequency and number of interburst closed states, and eliminated the short burst states. By comparing our results with those obtained for wild-type K_ATP channels, we conclude that the MgADP sensitivity of the wild-type K_ATP channel can be described quantitatively by a combination of inhibition at Kir6.2 (measured for wild-type channels in the absence of Mg²⁺) and activation via SUR1 (determined for Kir6.2-G334D/SUR1 channels). However, this is not the case for the effects of MgATP.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Modulation of K(ATP) currents in rat ventricular myocytes by hypoxia and a redox reaction

AIM

The present study investigated the possible regulatory mechanisms of redox agents and hypoxia on the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

METHODS

Single-channel and whole-cell patch-clamp techniques were used to record the K(ATP) current (I(KATP)) in acutely isolated rat ventricular myocytes.

RESULTS

Oxidized glutathione (GSSG, 1 mmol/L) increased the I(KATP), while reduced glutathione (GSH, 1 mmol/L) could reverse the increased I(KATP) during normoxia. To further corroborate the effect of the redox agent on the K(ATP) channel, we employed the redox couple DTT (1 mmol/L)/H2O2 (0.3, 0.6, and 1 mmol/L) and repeated the previous processes, which produced results similar to the previous redox couple GSH/GSSG during normoxia. H2O2 increased the I(KATP) in a concentration dependent manner, which was reversed by DTT (1 mmol/L). In addition, our results have shown that 15 min of hypoxia increased the I(KATP), while GSH (1 mmol/L) could reverse the increased I(KATP). Furthermore, in order to study the signaling pathways of the I(KATP) augmented by hypoxia and the redox agent, we applied a protein kinase C(PKC) inhibitor bisindolylmaleimide VI (BIM), a protein kinase G(PKG) inhibitor KT5823, a protein kinase A (PKA) inhibitor H-89, and Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitors KN-62 and KN-93. The results indicated that BIM, KT5823, KN-62, and KN-93, but not H-89, inhibited the I(KATP) augmented by hypoxia and GSSG; in addition, these results suggest that the effects of both GSSG and hypoxia on K(ATP) channels involve the activation of the PKC, PKG, and CaMK II pathways, but not the PKA pathway.

CONCLUSION

The present study provides electrophysiological evidence that hypoxia and the oxidizing reaction are closely related to the modulation of I(KATP).

Cholesterol regulates glucose-stimulated insulin secretion through phosphatidylinositol 4,5-bisphosphate

Membrane cholesterol modulates the ability of glucose to stimulate insulin secretion from pancreatic beta-cells. The molecular mechanism by which this occurs is not understood. Here, we show that in cultured beta-cells, cholesterol acts through phosphatidylinositol 4,5-bisphosphate (PIP(2)) to regulate actin dynamics, plasma membrane potential, and glucose-stimulated insulin secretion. Cholesterol-overloaded beta-cells exhibited decreased PIP(2) hydrolysis, with diminished glucose-induced actin reorganization, membrane depolarization, and insulin secretion. The converse findings were observed in cholesterol-depleted cells. These results support a model in which cholesterol depletion is coupled through PIP(2) to enhance both plasma membrane Ca2+ influx from the extracellular space, as well as inositol 1,4,5-triphosphate-stimulated Ca2+ efflux from intracellular stores. The inability to increase cytosolic Ca2+ may be the main underlying factor to account for impaired glucose-stimulated insulin secretion in cholesterol-overloaded beta-cells.

Modeling K(ATP) channel gating and its regulation

ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.

How ATP inhibits the open K(ATP) channel

ATP-sensitive potassium (K_ATP) channels are composed of four pore-forming Kir6.2 subunits and four regulatory SUR1 subunits. Binding of ATP to Kir6.2 leads to inhibition of channel activity. Because there are four subunits and thus four ATP-binding sites, four binding events are possible. ATP binds to both the open and closed states of the channel and produces a decrease in the mean open time, a reduction in the mean burst duration, and an increase in the frequency and duration of the interburst closed states. Here, we investigate the mechanism of interaction of ATP with the open state of the channel by analyzing the single-channel kinetics of concatenated Kir6.2 tetramers containing from zero to four mutated Kir6.2 subunits that possess an impaired ATP-binding site. We show that the ATP-dependent decrease in the mean burst duration is well described by a Monod-Wyman-Changeux model in which channel closing is produced by all four subunits acting in a single concerted step. The data are inconsistent with a Hodgkin-Huxley model (four independent steps) or a dimer model (two independent dimers). When the channel is open, ATP binds to a single ATP-binding site with a dissociation constant of 300 μM.

Role for SUR2A ED domain in allosteric coupling within the K(ATP) channel complex

Allosteric regulation of heteromultimeric ATP-sensitive potassium (K_ATP) channels is unique among protein systems as it implies transmission of ligand-induced structural adaptation at the regulatory SUR subunit, a member of ATP-binding cassette ABCC family, to the distinct pore-forming K⁺ (Kir6.x) channel module. Cooperative interaction between nucleotide binding domains (NBDs) of SUR is a prerequisite for K_ATP channel gating, yet pathways of allosteric intersubunit communication remain uncertain. Here, we analyzed the role of the ED domain, a stretch of 15 negatively charged aspartate/glutamate amino acid residues (948–962) of the SUR2A isoform, in the regulation of cardiac K_ATP channels. Disruption of the ED domain impeded cooperative NBDs interaction and interrupted the regulation of K_ATP channel complexes by MgADP, potassium channel openers, and sulfonylurea drugs. Thus, the ED domain is a structural component of the allosteric pathway within the K_ATP channel complex integrating transduction of diverse nucleotide-dependent states in the regulatory SUR subunit to the open/closed states of the K⁺-conducting channel pore.

A mutation in the ATP-binding site of the Kir6.2 subunit of the KATP channel alters coupling with the SUR2A subunit

Mutations in the pore-forming subunit of the ATP-sensitive K(+) (K(ATP)) channel Kir6.2 cause neonatal diabetes. Understanding the molecular mechanism of action of these mutations has provided valuable insight into the relationship between the structure and function of the K(ATP) channel. When Kir6.2 containing a mutation (F333I) in the putative ATP-binding site is coexpressed with the cardiac type of regulatory K(ATP) channel subunit, SUR2A, the channel sensitivity to ATP inhibition is reduced and the intrinsic open probability (P(o)) is increased. However, the extent of macroscopic current activation by MgADP was unaffected. Here we examine rundown and MgADP activation of wild-type and Kir6.2-F333I/SUR2A channels using single-channel recording, noise analysis and spectral analysis. We also compare the effect of mutating the adjacent residue, G334, on rundown and MgADP activation. All three approaches indicated that rundown of Kir6.2-F333I/SUR2A channels is due to a reduction in the number of active channels in the patch and that MgADP reactivation involves recruitment of inactive channels. In contrast, rundown and MgADP reactivation of wild-type and Kir6.2-G334D/SUR2A channels, and of Kir6.2-F333I/SUR1 channels, involve a gradual change in P(o). Our results suggest that F333 in Kir6.2 interacts functionally with SUR2A to modulate channel rundown and MgADP activation. This interaction is fairly specific as it is not disturbed when the adjacent residue (G334) is mutated. It is also not a consequence of the enhanced P(o) of Kir6.2-F333I/SUR2A channels, as it is not found for other mutant channels with high P(o) (Kir6.2-I296L/SUR2A).

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

The cardiovascular system operates under a wide scale of demands, ranging from conditions of rest to extreme stress. How the heart muscle matches rates of ATP production with utilization is an area of active investigation. ATP-sensitive potassium (K(ATP)) channels serve a critical role in the orchestration of myocardial energetic well-being. K(ATP) channel heteromultimers consist of inwardly-rectifying K(+) channel 6.2 and ATP-binding cassette sulfonylurea receptor 2A that translates local ATP/ADP levels, set by ATPases and phosphotransfer reactions, to the channel pore function. In cells in which the mobility of metabolites between intracellular microdomains is limited, coupling of phosphotransfer pathways with K(ATP) channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This K(ATP) channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varying workloads. Mutations of K(ATP) channels result in disruption of the nucleotide signaling network and generate a stress-vulnerable phenotype with excessive susceptibility to injury, development of cardiomyopathy, and arrhythmia. Solving the mechanisms underlying the integration of K(ATP) channels into the cellular energy network will advance the understanding of endogenous cardioprotection and the development of strategies for the management of cardiovascular injury and disease progression.

An ATP-binding mutation (G334D) in KCNJ11 is associated with a sulfonylurea-insensitive form of developmental delay, epilepsy, and neonatal diabetes

Mutations in the pancreatic ATP-sensitive K(+) channel (K(ATP) channel) cause permanent neonatal diabetes mellitus (PNDM) in humans. All of the K(ATP) channel mutations examined result in decreased ATP inhibition, which in turn is predicted to suppress insulin secretion. Here we describe a patient with severe PNDM, which includes developmental delay and epilepsy, in addition to neonatal diabetes (developmental delay, epilepsy, and neonatal diabetes [DEND]), due to a G334D mutation in the Kir6.2 subunit of K(ATP) channel. The patient was wholly unresponsive to sulfonylurea therapy (up to 1.14 mg . kg(-1) . day(-1)) and remained insulin dependent. Consistent with the putative role of G334 as an ATP-binding residue, reconstituted homomeric and mixed WT+G334D channels exhibit absent or reduced ATP sensitivity but normal gating behavior in the absence of ATP. In disagreement with the sulfonylurea insensitivity of the affected patient, the G334D mutation has no effect on the sulfonylurea inhibition of reconstituted channels in excised patches. However, in macroscopic rubidium-efflux assays in intact cells, reconstituted mutant channels do exhibit a decreased, but still present, sulfonylurea response. The results demonstrate that ATP-binding site mutations can indeed cause DEND and suggest the possibility that sulfonylurea insensitivity of such patients may be a secondary reflection of the presence of DEND rather than a simple reflection of the underlying molecular basis.

The cardioprotective effect of uridine and uridine-5'-monophosphate: the role of the mitochondrial ATP-dependent potassium channel

The activity of mitochondrial ATP-dependent potassium channel (mitoKATP) of rat heart and liver mitochondria was shown to decrease during aging. This partially explains the increase of risk of ischemia at a mature age since mitoKATP activation provides cardioprotection. We demonstrated that uridine-5'-diphosphate (UDP) possesses the property to activate mitoKATP. At a concentration of 30 microM, it reactivated mitoKATP in mitochondria, and 5-hydroxydecanoate (5-HD) eliminated this effect. In experimental animals, UDP precursors uridine and uridine-5'-monophosphate (UMP) (both 30 mg/kg, administered intravenously 5 min before coronary occlusion) decreased the myocardium ischemic alteration index (1.9 and 3.5 times, respectively) and the T-wave amplitude within 60 min after occlusion. Both effects were inhibited by Glibenclamide (Glib) and 5-HD. UMP and uridine decreased the number of premature ventricular beats 5.6 and 1.9 times and the duration of ventricular tachycardia 9.4 and 4.1 times, respectively. Glib and 5-HD inhibited the anti-arrhythmic parameters, 5-HD being less effective. Uridine and UMP decreased the duration of fibrillation 10.8 and 3.6 times, respectively, and this effect was not abolished by Glib and 5-HD. Thus, uridine and UMP, which are the precursors of UDP in the cell, possess cardioprotective properties. MitoKATP prevents mainly ischemic injuries and partially rhythm disorders.

ATP-sensitive K+ channels: regulation of bursting by the sulphonylurea receptor, PIP2 and regions of Kir6.2

ATP-sensitive K+ channels composed of the pore-forming protein Kir6.2 and the sulphonylurea receptor SUR1 are inhibited by ATP and activated by Phosphatidylinositol Bisphosphate (PIP2). Residues involved in binding of these ligands to the Kir6.2 cytoplasmic domain have been identified, and it has been hypothesized that gating mechanisms involve conformational changes in the regions of the bundle crossing and/or the selectivity filter of Kir6.2. Regulation of Kir6.2 by SUR1, however, is not well-understood, even though this process is ATP and PIP2 dependent. In this study, we investigated the relationship between channel regulation by SUR1 and PIP2 by comparing a number of single and double mutants known to affect open probability (P(o)), PIP2 affinity, and sulphonylurea and MgADP sensitivity. When coexpressed with SUR1, the Kir6.2 mutant C166A, which is characterized by a P(o) value close to 0.8, exhibits no sulphonylurea or MgADP sensitivity. However, when P(o) was reduced by combining mutations at the PIP2-sensitive residues R176 and R177 with C166A, sulphonylurea and MgADP sensitivities were restored. These effects correlated with a dramatic decrease in PIP2 affinity, as assessed by PIP2-induced channel reactivation and inhibition by neomycin, an antagonist of PIP2 binding. Based on macroscopic and single-channel data, we propose a model in which entry into the high-P(o) bursting state by the C166A mutation or by SUR1 depends on the interaction of PIP2 with R176 and R177 and, to a lesser extent, R54. In conjunction with this PIP2-dependent process, SUR1 also regulates channel activity via a PIP2-independent, but MgADP-dependent process.

Ligand-dependent linkage of the ATP site to inhibition gate closure in the KATP channel

Major advances have been made on the inhibition gate and ATP site of the K(ir)6.2 subunit of the K(ATP) channel, but little is known about conformational coupling between the two. ATP site mutations dramatically disrupt ATP-dependent gating without effect on ligand-independent gating, observed as interconversions between active burst and inactive interburst conformations in the absence of ATP. This suggests that linkage between site and gate is conditionally dependent on ATP occupancy. We studied all substitutions at position 334 of the ATP site in K(ir)6.2deltaC26 that express in Xenopus oocytes. All substitutions disrupted ATP-dependent gating by 10-fold or more. Only positive-charged arginine or lysine at 334, however, slowed ligand-independent gating from the burst, and this was in some but not all patches. Moreover, the polycationic peptide protamine reversed the slowed gating from the burst of 334R mutant channels, and speeded the slow gating from the burst of wild-type SUR1/K(ir)6.2 in the absence of ATP. Our results support a two-step ligand-dependent linkage mechanism for K(ir)6.2 channels in which ATP-occupied sites function to electrostatically dissociate COOH-terminal domains from the membrane, then as in all K(ir) channels, free COOH-terminal domains and inner M2 helices transit to a lower energy state for gate closure.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

Two structurally distinct and spatially compartmentalized adenylate kinases are expressed from the AK1 gene in mouse brain

Adenylate kinases (AK, EC 2.7.4.3) have been considered important enzymes for energy homeostasis and metabolic signaling. To gain a better understanding of their cell-specific significance we studied the structural and functional aspects of products of one adenylate kinase gene, AK1, in mouse tissues. By combined computer database comparison and Northern analysis of mRNAs, we identified transcripts of 0.7 and 2.0 kilobases with different 5' and 3' non-coding regions which result from alternative use of promoters and polyadenylation sites. These mRNAs specify two distinct proteins, AK1 and a membrane-bound AK1 isoform (AK1beta), which differ in their N-terminal end and are co-expressed in several tissues with high-energy demand, including the brain. Immunohistochemical analysis of brain tissue and primary neurons and astrocytes in culture demonstrated that AK1 isoforms are expressed predominantly in neurons. AK1beta, when tested in transfected COS-1 and N2a neuroblastoma cells, located at the cellular membrane and was able to catalyze phosphorylation of ADP in vitro. In addition, AK1beta mediated AMP-induced activation of recombinant ATP-sensitive potassium channels in the presence of ATP. Thus, two structurally distinct AK1 isoforms co-exist in the mouse brain within distinct cellular locations. These enzymes may function in promoting energy homeostasis in the compartmentalized cytosol and in translating cellular energetic signals to membrane metabolic sensors.

Concerted gating mechanism underlying KATP channel inhibition by ATP

KATP channels assemble from four regulatory SUR1 and four pore-forming Kir6.2 subunits. At the single-channel current level, ATP-dependent gating transitions between the active burst and the inactive interburst conformations underlie inhibition of the KATP channel by intracellular ATP. Previously, we identified a slow gating mutation, T171A in the Kir6.2 subunit, which dramatically reduces rates of burst to interburst transitions in Kir6.2DeltaC26 channels without SUR1 in the absence of ATP. Here, we constructed all possible mutations at position 171 in Kir6.2DeltaC26 channels without SUR1. Only four substitutions, 171A, 171F, 171H, and 171S, gave rise to functional channels, each increasing Ki,ATP for ATP inhibition by >55-fold and slowing gating to the interburst by >35-fold. Moreover, we investigated the role of individual Kir6.2 subunits in the gating by comparing burst to interburst transition rates of channels constructed from different combinations of slow 171A and fast T171 "wild-type" subunits. The relationship between gating transition rate and number of slow subunits is exponential, which excludes independent gating models where any one subunit is sufficient for inhibition gating. Rather, our results support mechanisms where four ATP sites independently can control a single gate formed by the concerted action of all four Kir6.2 subunit inner helices of the KATP channel.

Cellular remodeling in heart failure disrupts K(ATP) channel-dependent stress tolerance

ATP-sensitive potassium (K(ATP)) channels are required for maintenance of homeostasis during the metabolically demanding adaptive response to stress. However, in disease, the effect of cellular remodeling on K(ATP) channel behavior and associated tolerance to metabolic insult is unknown. Here, transgenic expression of tumor necrosis factor alpha induced heart failure with typical cardiac structural and energetic alterations. In this paradigm of disease remodeling, K(ATP) channels responded aberrantly to metabolic signals despite intact intrinsic channel properties, implicating defects proximal to the channel. Indeed, cardiomyocytes from failing hearts exhibited mitochondrial and creatine kinase deficits, and thus a reduced potential for metabolic signal generation and transmission. Consequently, K(ATP) channels failed to properly translate cellular distress under metabolic challenge into a protective membrane response. Failing hearts were excessively vulnerable to metabolic insult, demonstrating cardiomyocyte calcium loading and myofibrillar contraction banding, with tolerance improved by K(ATP) channel openers. Thus, disease-induced K(ATP) channel metabolic dysregulation is a contributor to the pathobiology of heart failure, illustrating a mechanism for acquired channelopathy.

Analysis of the differential modulation of sulphonylurea block of beta-cell and cardiac ATP-sensitive K+ (K(ATP)) channels by Mg-nucleotides

Sulphonylureas stimulate insulin secretion by binding with high-affinity to the sulphonylurea receptor (SUR) subunit of the ATP-sensitive potassium (K(ATP)) channel and thereby closing the channel pore (formed by four Kir6.2 subunits). In the absence of added nucleotides, the maximal block is around 60-80 %, indicating that sulphonylureas act as partial antagonists. Intracellular MgADP modulated sulphonylurea block, enhancing inhibition of Kir6.2/SUR1 (beta-cell type) and decreasing that of Kir6.2/SUR2A (cardiac-type) channels. We examined the molecular basis of the different response of channels containing SUR1 and SUR2A, by recording currents from inside-out patches excised from Xenopus oocytes heterologously expressing wild-type or chimeric channels. We used the benzamido derivative meglitinide as this drug blocks Kir6.2/SUR1 and Kir6.2/SUR2A currents, reversibly and with similar potency. Our results indicate that transfer of the region containing transmembrane helices (TMs) 8-11 and the following 65 residues of SUR1 into SUR2A largely confers a SUR1-like response to MgADP and meglitinide, whereas the reverse chimera (SUR128) largely endows SUR1 with a SUR2A-type response. This effect was not specific for meglitinide, as tolbutamide was also unable to prevent MgADP activation of Kir6.2/SUR128 currents. The data favour the idea that meglitinide binding to SUR1 impairs either MgADP binding or the transduction pathway between the NBDs and Kir6.2, and that TMs 8-11 are involved in this modulatory response. The results provide a basis for understanding how beta-cell K(ATP) channels show enhanced sulphonylurea inhibition under physiological conditions, whereas cardiac K(ATP) channels exhibit reduced block in intact cells, especially during metabolic inhibition.

Phosphatidic acid stimulates cardiac KATP channels like phosphatidylinositols, but with novel gating kinetics

Membrane-bound anionic phospholipids such as phosphatidylinositols have the capacity to modulate ATP-sensitive potassium (K(ATP)) channels through a mechanism involving long-range electrostatic interaction between the lipid headgroup and channel. However, it has not yet been determined whether the multiple effects of phosphatidylinositols reported in the literature all result from this general electrostatic interaction or require a specific headgroup structure. The present study investigated whether phosphatidic acid (PA), an anionic phospholipid substantially different in structure from phosphatidylinositols, evokes effects similar to phosphatidylinositols on native K(ATP) channels of rat heart and heterogeneous Kir6.2/SUR2A channels. Channels treated with PA (0.2-1 mg/ml applied to the cytoplasmic side of the membrane) exhibited higher activity, lower sensitivity to ATP inhibition, less Mg(2+)-dependent nucleotide stimulation, and poor sulfonylurea inhibition. These effects match the spectrum of phosphatidylinositols' effects, but, in addition, PA also induced a novel pattern in gating kinetics, represented by a decreased mean open time (from 12.2 +/- 2.0 to 3.3 +/- 0.7 ms). This impact on gating kinetics clearly distinguishes PA's effects from those of phosphatidylinositols. Results indicate that multiple effects of anionic phospholipids on K(ATP) channels are related phenomena and can likely be attributed to a common mechanism, but additional specific effects due to other mechanisms may also coincide.

Effect of metabolic inhibition on glimepiride block of native and cloned cardiac sarcolemmal K(ATP) channels

1. We have investigated the effects of the sulphonylurea, glimepiride, currently used to treat type 2 diabetes, on ATP-sensitive K(+) (K(ATP)) currents of rat cardiac myocytes and on their cloned constituents Kir6.2 and SUR2A expressed in HEK 293 cells. 2. Glimepiride blocked pinacidil-activated whole-cell K(ATP) currents of cardiac myocytes with an IC(50) of 6.8 nM, comparable to the potency of glibenclamide in these cells. Glimepiride blocked K(ATP) channels formed by co-expression of Kir6.2/SUR2A subunits in HEK 293 cells in outside-out excised patches with a similar IC(50) of 6.2 nM. 3. Glimepiride was much less effective at blocking K(ATP) currents activated by either metabolic inhibition (MI) with CN(-) and iodoacetate or by the K(ATP) channel opener diazoxide in the presence of inhibitors of F(0)/F(1)-ATPase (oligomycin) and creatine kinase (DNFB). Thus 10 microM glimepiride blocked pinacidil-activated currents by >99%, MI-activated currents by 70% and diazoxide-activated currents by 82%. 4. In inside-out patches from HEK 293 cells expressing the cloned K(ATP) channel subunits Kir6.2/SUR2A, increasing the concentration of ADP (1 - 100 microM), in the presence of 100 nM glimepiride, lead to significant increases in Kir6.2/SUR2A channel activity. However, over the range tested, ADP did not affect cloned K(ATP) channel activity in the presence of 100 nM glibenclamide. These results are consistent with the suggestion that ADP reduces glimepiride block of K(ATP) channels. 5. Our results show that glimepiride is a potent blocker of native cardiac K(ATP) channels activated by pinacidil and blocks cloned Kir6.2/SUR2A channels activated by ATP depletion with similar potency. However, glimepiride is much less effective when K(ATP) channels are activated by MI and this may reflect a reduction in glimepiride block by increased intracellular ADP.

Tandem function of nucleotide binding domains confers competence to sulfonylurea receptor in gating ATP-sensitive K+ channels

Fundamental to the metabolic sensor function of ATP-sensitive K(+) (K(ATP)) channels is the sulfonylurea receptor. This ATP-binding cassette protein, which contains nucleotide binding domains (NBD1 and NBD2) with conserved Walker motifs, regulates the ATP sensitivity of the pore-forming Kir6.2 subunit. Although NBD2 hydrolyzes ATP, a property essential in K(ATP) channel gating, the role of NBD1, which has limited catalytic activity, if at all, remains less understood. Here, we provide functional evidence that cooperative interaction, rather than the independent contribution of each NBD, is critical for K(ATP) channel regulation. Gating of cardiac K(ATP) channels by distinct conformations in the NBD2 ATPase cycle, induced by gamma-phosphate analogs, was disrupted by point mutation not only of the Walker motif in NBD2 but also in NBD1. Cooling membrane patches to decelerate the intrinsic ATPase activity counteracted ATP-induced K(ATP) channel inhibition, an effect that mimicked stabilization of the MgADP-bound posthydrolytic state at NBD2 by the gamma-phosphate analog orthovanadate. Temperature-induced channel activation was abolished by mutations that either prevent stabilization of MgADP at NBD2 or ATP at NBD1. These findings provide a paradigm of K(ATP) channel gating based on integration of both NBDs into a functional unit within the multimeric channel complex.

Open state destabilization by ATP occupancy is mechanism speeding burst exit underlying KATP channel inhibition by ATP

The ATP-sensitive potassium (K(ATP)) channel is named after its characteristic inhibition by intracellular ATP. The inhibition is a centerpiece of how the K(ATP) channel sets electrical signaling to the energy state of the cell. In the beta cell of the endocrine pancreas, for example, ATP inhibition results from high blood glucose levels and turns on electrical activity leading to insulin release. The underlying gating mechanism (ATP inhibition gating) includes ATP stabilization of closed states, but the action of ATP on the open state of the channel is disputed. The original models of ATP inhibition gating proposed that ATP directly binds the open state, whereas recent models indicate a prerequisite transition from the open to a closed state before ATP binds and inhibits activity. We tested these two classes of models by using kinetic analysis of single-channel currents from the cloned mouse pancreatic K(ATP) channel expressed in Xenopus oocytes. In particular, we combined gating models based on fundamental rate law and burst gating kinetic considerations. The results demonstrate open-state ATP dependence as the major mechanism by which ATP speeds exit from the active burst state underlying inhibition of the K(ATP) channel by ATP.

Activation of ATP-sensitive K(+) channels by epoxyeicosatrienoic acids in rat cardiac ventricular myocytes

1. We examined the effects of epoxyeicosatrienoic acids (EETs), which are cytochrome P450 metabolites of arachidonic acid (AA), on the activities of the ATP-sensitive K(+) (K(ATP)) channels of rat cardiac myocytes, using the inside-out patch-clamp technique. 2. In the presence of 100 microM cytoplasmic ATP, the K(ATP) channel open probability (P(o)) was increased by 240 +/- 60 % with 0.1 microM 11,12-EET and by 400 +/- 54 % with 5 microM 11,12-EET (n = 5-10, P < 0.05 vs. control), whereas neither 5 microM AA nor 5 microM 11,12-dihydroxyeicosatrienoic acid (DHET), which is the epoxide hydrolysis product of 11,12-EET, had any effect on P(o). 3. The half-maximal activating concentration (EC(50)) was 18.9 +/- 2.6 nM for 11,12-EET (n = 5) and 19.1 +/- 4.8 nM for 8,9-EET (n = 5, P = n.s. vs. 11,12-EET). Furthermore, 11,12-EET failed to alter the inhibition of K(ATP) channels by glyburide. 4. Application of 11,12-EET markedly decreased the channel sensitivity to cytoplasmic ATP. The half-maximal inhibitory concentration of ATP (IC(50)) was increased from 21.2 +/- 2.0 microM at baseline to 240 +/- 60 microM with 0.1 microM 11,12-EET (n = 5, P < 0.05 vs. control) and to 780 +/- 30 microM with 5 microM 11,12-EET (n = 11, P < 0.05 vs. control). 5. Increasing the ATP concentration increased the number of kinetically distinguishable closed states, promoting prolonged closure durations. 11,12-EET antagonized the effects of ATP on the kinetics of the K(ATP) channels in a dose- and voltage-dependent manner. 11,12-EET (1 microM) reduced the apparent association rate constant of ATP to the channel by 135-fold. 6. Application of 5 microM 11,12-EET resulted in hyperpolarization of the resting membrane potential in isolated cardiac myocytes, which could be blocked by glyburide. 7. These results suggest that EETs are potent activators of the cardiac K(ATP) channels, modulating channel behaviour by reducing the channel sensitivity to ATP. Thus, EETs could be important endogenous regulators of cardiac electrical excitability.

Mutations within the P-loop of Kir6.2 modulate the intraburst kinetics of the ATP-sensitive potassium channel

The ATP-sensitive potassium (K(ATP)) channel exhibits spontaneous bursts of rapid openings, which are separated by long closed intervals. Previous studies have shown that mutations at the internal mouth of the pore-forming (Kir6.2) subunit of this channel affect the burst duration and the long interburst closings, but do not alter the fast intraburst kinetics. In this study, we have investigated the nature of the intraburst kinetics by using recombinant Kir6.2/SUR1 K(ATP) channels heterologously expressed in Xenopus oocytes. Single-channel currents were studied in inside-out membrane patches. Mutations within the pore loop of Kir6.2 (V127T, G135F, and M137C) dramatically affected the mean open time (tau(o)) and the short closed time (tauC1) within a burst, and the number of openings per burst, but did not alter the burst duration, the interburst closed time, or the channel open probability. Thus, the V127T and M137C mutations produced longer tau(o), shorter tauC1, and fewer openings per burst, whereas the G135F mutation had the opposite effect. All three mutations also reduced the single-channel conductance: from 70 pS for the wild-type channel to 62 pS (G135F), 50 pS (M137C), and 38 pS (V127T). These results are consistent with the idea that the K(ATP) channel possesses a gate that governs the intraburst kinetics, which lies close to the selectivity filter. This gate appears to be able to operate independently of that which regulates the long interburst closings.

Signaling in channel/enzyme multimers: ATPase transitions in SUR module gate ATP-sensitive K+ conductance

ATP-sensitive potassium (K(ATP)) channels are bifunctional multimers assembled by an ion conductor and a sulfonylurea receptor (SUR) ATPase. Sensitive to ATP/ADP, K(ATP) channels are vital metabolic sensors. However, channel regulation by competitive ATP/ADP binding would require oscillations in intracellular nucleotides incompatible with cell survival. We found that channel behavior is determined by the ATPase-driven engagement of SUR into discrete conformations. Capture of the SUR catalytic cycle in prehydrolytic states facilitated pore closure, while recruitment of posthydrolytic intermediates translated in pore opening. In the cell, channel openers stabilized posthydrolytic states promoting K(ATP) channel activation. Nucleotide exchange between intrinsic ATPase and ATP/ADP-scavenging systems defined the lifetimes of specific SUR conformations gating K(ATP) channels. Signal transduction through the catalytic module provides a paradigm for channel/enzyme operation and integrates membrane excitability with metabolic cascades.

Increased calcium vulnerability of senescent cardiac mitochondria: protective role for a mitochondrial potassium channel opener

In senescence, endogenous mechanisms of cardioprotection are apparently attenuated resulting in increased vulnerability to ischemia-reperfusion. In particular, mitochondria, which are essential in maintaining cardiac energetic and ionic homeostasis, are susceptible to Ca2+ overload, a component of metabolic injury. However, effective means of protecting senescent mitochondria are lacking. Here, mitochondrial function and structure were assessed using ion-selective mini-electrodes, high-performance liquid chromatography and electron microscopy. Aging decreased ADP-induced oxygen consumption and prolonged the time associated with ADP to ATP conversion, which manifested as a reduced rate of oxidative phosphorylation. Aging also reduced mitochondrial Ca2+ handling, and increased Ca2+-induced mitochondrial damage. Diazoxide, a potassium channel opener, reduced Ca2+ loading and protected the functional and structural integrity of senescent mitochondria from Ca2+-induced injury. In this way, the present study identifies the potential usefulness for pharmacotherapy in protecting vulnerable senescent mitochondria from conditions of Ca2+ overload, such as ischemia-reperfusion.

Long-chain acyl-coenzyme A esters and fatty acids directly link metabolism to K(ATP) channels in the heart

ATP-sensitive K (K(ATP)) channels are inhibited by cytosolic ATP, a defining property that implicitly links these channels to cellular metabolism. Here we report a direct link between fatty acid metabolism and K(ATP) channels in cardiac muscle cells. Long-chain (LC) acyl-coenzyme A (CoA) esters are synthesized from fatty acids and serve as the principal metabolic substrates of the heart. We have studied the effects of LC acyl-CoA esters and LC fatty acids on K(ATP) channels of isolated guinea pig ventricular myocytes and compared them with the effects of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Application of oleoyl-CoA (0.2 or 1 micromol/L), a naturally occurring acyl-CoA ester, to the cytosolic side of excised patches completely prevented rundown of K(ATP) channels, but not of Kir2 channels. The open probability of K(ATP) channels measured in the presence of oleoyl-CoA or PIP(2) was voltage dependent, increasing with depolarization. Oleoyl-CoA greatly reduced the ATP sensitivity of K(ATP) channels. At a concentration of 2 micromol/L, oleoyl-CoA increased the half-maximal inhibitory concentration of ATP >200-fold. The time course of the decrease in ATP sensitivity was much faster during application of oleoyl-CoA than during application of PIP(2). The effects of PIP(2), but not of oleoyl-CoA, were inhibited by increasing Ca(2+) to 1 mmol/L. Oleate (C18:1; 10 micromol/L), the precursor of oleoyl-CoA, inhibited K(ATP) channels activated by oleoyl-COA: Palmitoleoyl-CoA and palmitoleate (C16:1) exerted similar reciprocal effects. These findings indicate that LC fatty acids and their CoA-linked derivatives may be key physiological modulators of K(ATP) channel activity in the heart.

Adenylate kinase phosphotransfer communicates cellular energetic signals to ATP-sensitive potassium channels

Transduction of energetic signals into membrane electrical events governs vital cellular functions, ranging from hormone secretion and cytoprotection to appetite control and hair growth. Central to the regulation of such diverse cellular processes are the metabolism sensing ATP-sensitive K+ (K(ATP)) channels. However, the mechanism that communicates metabolic signals and integrates cellular energetics with K(ATP) channel-dependent membrane excitability remains elusive. Here, we identify that the response of K(ATP) channels to metabolic challenge is regulated by adenylate kinase phosphotransfer. Adenylate kinase associates with the K(ATP) channel complex, anchoring cellular phosphotransfer networks and facilitating delivery of mitochondrial signals to the membrane environment. Deletion of the adenylate kinase gene compromised nucleotide exchange at the channel site and impeded communication between mitochondria and K(ATP) channels, rendering cellular metabolic sensing defective. Assigning a signal processing role to adenylate kinase identifies a phosphorelay mechanism essential for efficient coupling of cellular energetics with K(ATP) channels and associated functions.

ATP interaction with the open state of the K(ATP) channel

The mechanism of ATP-sensitive potassium (K(ATP)) channel closure by ATP is unclear, and various kinetic models in which ATP binds to open or to closed states have previously been presented. Effects of phosphatidylinositol bisphosphate (PIP2) and multiple Kir6.2 mutations on ATP inhibition and open probability in the absence of ATP are explainable in kinetic models where ATP stabilizes a closed state and interaction with an open state is not required. Evidence that ATP can in fact interact with the open state of the channel is presented here. The mutant Kir6.2[L164C] is very sensitive to Cd2+ block, but very insensitive to ATP, with no significant inhibition in 1 mM ATP. However, 1 mM ATP fully protects the channel from Cd2+ block. Allosteric kinetic models in which the channel can be in either open or closed states with or without ATP bound are considered. Such models predict a pedestal in the ATP inhibition, i.e., a maximal amount of inhibition at saturating ATP concentrations. This pedestal is predicted to occur at >50 mM ATP in the L164C mutant, but at >1 mM in the double mutant L164C/R176A. As predicted, ATP inhibits Kir6.2[L164C/R176A] to a maximum of approximately 40%, with a clear plateau beyond 2 mM. These results indicate that ATP acts as an allosteric ligand, interacting with both open and closed states of the channel.

Diazoxide protects mitochondria from anoxic injury: implications for myopreservation

BACKGROUND

Heart muscle primarily relies on adenosine triphosphate produced by oxidative phosphorylation and is highly vulnerable to anoxic insult. Although a number of strategies aimed at improving myopreservation are available, no effective means of preserving mitochondrial energetics under conditions of anoxic injury have been developed. Openers of mitochondrial adenosine triphosphate-sensitive potassium channels have emerged as powerful cardioprotective agents presumably capable of maintaining mitochondrial function under metabolic stress. Here, we evaluated the ability of a prototype mitochondrial adenosine triphosphate-sensitive potassium channel opener, diazoxide, to preserve oxidative phosphorylation in mitochondria subjected to anoxia and reoxygenation.

METHODS

Mitochondria were isolated from rat hearts and subjected to 20 minutes of anoxia, followed by reoxygenation. Mitochondrial respiration and oxidative phosphorylation, as well as mitochondrial integrity, were assessed by means of ion-selective minielectrodes, high-performance liquid chromatography, fluorometry, and electron microscopy.

RESULTS

Anoxia-reoxygenation decreased the rate of adenosine diphosphate-stimulated oxygen consumption, inhibited adenosine triphosphate production, and disrupted mitochondrial integrity. On average, anoxic stress reduced adenosine diphosphate-stimulated respiration from 291 +/- 14 to 141 +/- 15 ng-atoms O(2). min(-1). mg(-1) protein and decreased the rate of adenosine triphosphate production from 752 +/- 14 to 414 +/- 34 nmol adenosine triphosphate. min(-1). mg(-1) protein. After anoxia, the majority (88%) of mitochondria was damaged or swollen and released adenylate kinase, a marker of mitochondrial integrity. Diazoxide (100 micromol/L), present throughout anoxia, preserved adenosine diphosphate-stimulated respiration at 255 +/- 7 ng-atoms O(2). min(-1). mg(-1) protein and adenosine triphosphate production at 640 +/- 39 nmol adenosine triphosphate. min(-1). mg(-1) protein. Diazoxide also protected mitochondrial structure from anoxia-mediated damage, so that after anoxic stress, 67% of mitochondria remained intact and adenylate kinase was confined to the mitochondria.

CONCLUSIONS

The present study demonstrates that diazoxide diminishes anoxia-induced functional and structural deterioration of cardiac mitochondria. By protecting mitochondria and preserving myocardial energetics, diazoxide may be useful under conditions of reduced oxygen availability, including global surgical ischemia or storage of donor heart.

KATP channels gated by intracellular nucleotides and phospholipids

The KATP channel is a heterooctamer composed of two different subunits, four inwardly rectifying K+ channel subunits, either Kir6. 1 or Kir6.2, and four sulfonylurea receptors (SUR), which belong to the family of ABC transporters. This unusual molecular architecture is related to the complex gating behaviour of these channels. Intracellular ATP inhibits KATP channels by binding to the Kir6.x subunits, whereas Mg-ADP increases channel activity by a hydrolysis reaction at the SUR. This ATP/ADP dependence allows KATP channels to link metabolism to excitability, which is important for many physiological functions, such as insulin secretion and cell protection during periods of ischemic stress. Recent work has uncovered a new class of regulatory molecules for KATP channel gating. Membrane phospholipids such as phosphoinositol 4, 5-bisphosphate and phosphatidylinositiol 4-monophosphate were found to interact with KATP channels resulting in increased open probability and markedly reduced ATP sensitivity. The membrane concentration of these phospholipids is regulated by a set of enzymes comprising phospholipases, phospholipid phosphatases and phospholipid kinases providing a possible mechanism for control of cell excitability through signal transduction pathways that modulate activity of these enzymes. This review discusses the mechanisms and molecular determinants that underlie gating of KATP channel by nucleotides and phospholipids and their physiological implications.

ATPase activity of the sulfonylurea receptor: a catalytic function for the KATP channel complex

ATP-sensitive K+ (KATP) channels are unique metabolic sensors formed by association of Kir6.2, an inwardly rectifying K+ channel, and the sulfonylurea receptor SUR, an ATP binding cassette protein. We identified an ATPase activity in immunoprecipitates of cardiac KATP channels and in purified fusion proteins containing nucleotide binding domains NBD1 and NBD2 of the cardiac SUR2A isoform. NBD2 hydrolyzed ATP with a twofold higher rate compared to NBD1. The ATPase required Mg2+ and was insensitive to ouabain, oligomycin, thapsigargin, or levamisole. K1348A and D1469N mutations in NBD2 reduced ATPase activity and produced channels with increased sensitivity to ATP. KATP channel openers, which bind to SUR, promoted ATPase activity in purified sarcolemma. At higher concentrations, openers reduced ATPase activity, possibly through stabilization of MgADP at the channel site. K1348A and D1469N mutations attenuated the effect of openers on KATP channel activity. Opener-induced channel activation was also inhibited by the creatine kinase/creatine phosphate system that removes ADP from the channel complex. Thus, the KATP channel complex functions not only as a K+ conductance, but also as an enzyme regulating nucleotide-dependent channel gating through an intrinsic ATPase activity of the SUR subunit. Modulation of the channel ATPase activity and/or scavenging the product of the ATPase reaction provide novel means to regulate cellular functions associated with KATP channel opening.

Regulation of cloned ATP-sensitive K channels by phosphorylation, MgADP, and phosphatidylinositol bisphosphate (PIP(2)): a study of channel rundown and reactivation

Kir6.2 channels linked to the green fluorescent protein (GFP) (Kir6. 2-GFP) have been expressed alone or with the sulfonylurea receptor SUR1 in HEK293 cells to study the regulation of K(ATP) channels by adenine nucleotides, phosphatidylinositol bisphosphate (PIP(2)), and phosphorylation. Upon excision of inside-out patches into a Ca(2+)- and MgATP-free solution, the activity of Kir6.2-GFP+SUR1 channels spontaneously ran down, first quickly within a minute, and then more slowly over tens of minutes. In contrast, under the same conditions, the activity of Kir6.2-GFP alone exhibited only slow rundown. Thus, fast rundown is specific to Kir6.2-GFP+SUR1 and involves SUR1, while slow rundown is a property of both Kir6.2-GFP and Kir6.2-GFP+SUR1 channels and is due, at least in part, to Kir6.2 alone. Kir6. 2-GFP+SUR1 fast phase of rundown was of variable amplitude and led to increased ATP sensitivity. Excising patches into a solution containing MgADP prevented this phenomenon, suggesting that fast rundown involves loss of MgADP-dependent stimulation conferred by SUR1. With both Kir6.2-GFP and Kir6.2-GFP+SUR1, the slow phase of rundown led to further increase in ATP sensitivity. Ca(2+) accelerated this process, suggesting a role for PIP(2) hydrolysis mediated by a Ca(2+)-dependent phospholipase C. PIP(2) could reactivate channel activity after a brief exposure to Ca(2+), but not after prolonged exposure. However, in both cases, PIP(2) reversed the increase in ATP sensitivity, indicating that PIP(2) lowers the ATP sensitivity by increasing P(o) as well as by decreasing the channel affinity for ATP. With Kir6.2-GFP+SUR1, slow rundown also caused loss of MgADP stimulation and sulfonylurea inhibition, suggesting functional uncoupling of SUR1 from Kir6.2-GFP. Ca(2+) facilitated the loss of sensitivity to MgADP, and thus uncoupling of the two subunits. The nonselective protein kinase inhibitor H-7 and the selective PKC inhibitor peptide 19-36 evoked, within 5-15 min, increased ATP sensitivity and loss of reactivation by PIP(2) and MgADP. Phosphorylation of Kir6.2 may thus be required for the channel to remain PIP(2) responsive, while phosphorylation of Kir6.2 and/or SUR1 is required for functional coupling. In summary, short-term regulation of Kir6.2+SUR1 channels involves MgADP, while long-term regulation requires PIP(2) and phosphorylation.

The kinetic and physical basis of K(ATP) channel gating: toward a unified molecular understanding

K(ATP) channels can be formed from Kir6.2 subunits with or without SUR1. The open-state stability of K(ATP) channels can be increased or reduced by mutations throughout the Kir6.2 subunit, and is increased by application of PIP(2) to the cytoplasmic membrane. Increase of open-state stability is manifested as an increase in the channel open probability in the absence of ATP (Po(zero)) and a correlated decrease in sensitivity to inhibition by ATP. Single channel lifetime analyses were performed on wild-type and I154C mutant channels expressed with, and without, SUR1. Channel kinetics include a single, invariant, open duration; an invariant, brief, closed duration; and longer closed events consisting of a "mixture of exponentials," which are prolonged in ATP and shortened after PIP(2) treatment. The steady-state and kinetic data cannot be accounted for by assuming that ATP binds to the channel and causes a gate to close. Rather, we show that they can be explained by models that assume the following regarding the gating behavior: 1) the channel undergoes ATP-insensitive transitions from the open state to a short closed state (C(f)) and to a longer-lived closed state (C(0)); 2) the C(0) state is destabilized in the presence of SUR1; and 3) ATP can access this C(0) state, stabilizing it and thereby inhibiting macroscopic currents. The effect of PIP(2) and mutations that stabilize the open state is then to shift the equilibrium of the "critical transition" from the open state to the ATP-accessible C(0) state toward the O state, reducing accessibility of the C(0) state, and hence reducing ATP sensitivity.

Comparative tolerability of sulphonylureas in diabetes mellitus

The sulphonylurea drugs have been the mainstay of oral treatment for patients with diabetes mellitus since they were introduced. In general, they are well tolerated, with a low incidence of adverse effects, although there are some differences between the drugs in the incidence of hypoglycaemia. Over the years, the drugs causing the most problems with hypoglycaemia have been chlorpropamide and glibenclamide (glyburide), although this is a potential problem with all sulphonylureas because of their action on the pancreatic beta cell, stimulating insulin release. Other specific problems have been reported with chlorpropamide that occur only rarely, if at all, with other sulphonylureas. Hyponatraemia secondary to inappropriate antidiuretic hormone activity, and increased flushing following the ingestion of alcohol, have been well described. The progressive beta cell failure with time results in eventual loss of efficacy, as these agents depend on a functioning beta cell and are ineffective in the absence of insulin-producing capacity. Differences in this secondary failure rate have been reported, with chlorpropamide and gliclazide having lower failure rates than glibenclamide or glipizide. The reasons for this are unclear, but the more abnormal pattern of insulin release produced by glibenclamide may be partly responsible and, indeed, may explain the increased risk of hypoglycaemia with this agent. Previously reported increased mortality associated with tolbutamide therapy has not been substantiated, and more recent data have shown no increased mortality from sulphonylurea treatment. Indeed, benefit from glycaemic control, regardless of the agent used--insulin or sulphonylurea--was reported by the United Kingdom Prospective Diabetes Study. Nevertheless, there is still ongoing controversy in view of the experimental evidence, mainly from animal studies, of potential adverse effects on the heart from sulphonylureas, but these are difficult to extrapolate into clinical situations. Most of these studies have been carried out with glibenclamide, which makes comparison of possible risk difficult. Other cardiovascular risk factors may be modified by gliclazide, which seems unique among the sulphonylureas in this respect. Its reported haemobiological and free radical scavenging activity probably resides in the azabicyclo-octyl ring structure in the side chain. Reduced progression or improvement in retinopathy has been reported in comparative trials with other sulphonylureas, and the effect is unrelated to improvements in glycaemia. There are differences between the sulphonylureas in some adverse effects, risk of hypoglycaemia, failure rates and actions on vascular risk factors. As a group of drugs, they are very well tolerated, but differences in overall tolerability can be identified.

Channelopathies of inwardly rectifying potassium channels

Mutations in genes encoding ion channels have increasingly been identified to cause disease conditions collectively termed channelopathies. Recognizing the molecular basis of an ion channel disease has provided new opportunities for screening, early diagnosis, and therapy of such conditions. This synopsis provides an overview of progress in the identification of molecular defects in inwardly rectifying potassium (Kir) channels. Structurally and functionally distinct from other channel families, Kir channels are ubiquitously expressed and serve functions as diverse as regulation of resting membrane potential, maintenance of K(+) homeostasis, control of heart rate, and hormone secretion. In humans, persistent hyperinsulinemic hypoglycemia of infancy, a disorder affecting the function of pancreatic beta cells, and Bartter's syndrome, characterized by hypokalemic alkalosis, hypercalciuria, increased serum aldosterone, and plasma renin activity, are the two major diseases linked so far to mutations in a Kir channel or associated protein. In addition, the weaver phenotype, a neurological disorder in mice, has also been associated with mutations in a Kir channel subtype. Further genetic linkage analysis and full understanding of the consequence that a defect in a Kir channel would have on disease pathogenesis are among the priorities in this emerging field of molecular medicine.

The tolbutamide site of SUR1 and a mechanism for its functional coupling to K(ATP) channel closure

Micromolar concentrations of tolbutamide will inhibit (SUR1/K(IR)6. 2)(4) channels in pancreatic beta-cells, but not (SUR2A/K(IR)6.2)(4) channels in cardiomyocytes. Inhibition does not require Mg(2+) or nucleotides and is enhanced by intracellular nucleotides. Using chimeras between SUR1 and SUR2A, we show that transmembrane domains 12-17 (TMD12-17) are required for high-affinity tolbutamide inhibition of K(ATP) channels. Deletions demonstrate involvement of the cytoplasmic N-terminus of K(IR)6.2 in coupling sulfonylurea-binding with SUR1 to the stabilization of an interburst closed configuration of the channel. The increased efficacy of tolbutamide by nucleotides results from an impairment of their stimulatory action on SUR1 which unmasks their inhibitory effects. The mechanism of inhibition of beta-cell K(ATP) channels by sulfonylureas during treatment of non-insulin-dependent diabetes mellitus thus involves two components, drug-binding and conformational changes within SUR1 which are coupled to the pore subunit through its N-terminus and the disruption of nucleotide-dependent stimulatory effects of the regulatory subunit on the pore. These findings uncover a molecular basis for an inhibitory influence of SUR1, an ATP-binding cassette (ABC) protein, on K(IR)6.2, a ion channel subunit.

Pharmacological plasticity of cardiac ATP-sensitive potassium channels toward diazoxide revealed by ADP

The pharmacological phenotype of ATP-sensitive potassium (K(ATP)) channels is defined by their tissue-specific regulatory subunit, the sulfonylurea receptor (SUR), which associates with the pore-forming channel core, Kir6.2. The potassium channel opener diazoxide has hyperglycemic and hypotensive properties that stem from its ability to open K(ATP) channels in pancreas and smooth muscle. Diazoxide is believed not to have any significant action on cardiac sarcolemmal K(ATP) channels. Yet, diazoxide can be cardioprotective in ischemia and has been found to bind to the presumed cardiac sarcolemmal K(ATP) channel-regulatory subunit, SUR2A. Here, in excised patches, diazoxide (300 microM) activated pancreatic SUR1/Kir6.2 currents and had little effect on native or recombinant cardiac SUR2A/Kir6.2 currents. However, in the presence of cytoplasmic ADP (100 microM), SUR2A/Kir6.2 channels became as sensitive to diazoxide as SUR1/Kir6. 2 channels. This effect involved specific interactions between MgADP and SUR, as it required Mg(2+), but not ATP, and was abolished by point mutations in the second nucleotide-binding domain of SUR, which impaired channel activation by MgADP. At the whole-cell level, in cardiomyocytes treated with oligomycin to block mitochondrial function, diazoxide could also activate K(ATP) currents only after cytosolic ADP had been raised by a creatine kinase inhibitor. Thus, ADP serves as a cofactor to define the responsiveness of cardiac K(ATP) channels toward diazoxide. The present demonstration of a pharmacological plasticity of K(ATP) channels identifies a mechanism for the control of channel activity in cardiac cells depending on the cellular ADP levels, which are elevated under ischemia.

ATP-sensitive K+ channel openers prevent Ca2+ overload in rat cardiac mitochondria

1. Mitochondrial dysfunction, secondary to excessive accumulation of Ca2+, has been implicated in cardiac injury. We here examined the action of potassium channel openers on mitochondrial Ca2+ homeostasis, as these cardioprotective ion channel modulators have recently been shown to target a mitochondrial ATP-sensitive K+ channel. 2. In isolated cardiac mitochondria, diazoxide and pinacidil decreased the rate and magnitude of Ca2+ uptake into the mitochondrial matrix with an IC50 of 65 and 128 microM, respectively. At all stages of Ca2+ uptake, the potassium channel openers depolarized the mitochondrial membrane thereby reducing Ca2+ influx through the potential-dependent mitochondrial uniporter. 3. Diazoxide and pinacidil, in a concentration-dependent manner, also activated release of Ca2+ from mitochondria. This was prevented by cyclosporin A, an inhibitor of Ca2+ release through the mitochondrial permeability transition pore. 4. Replacement of extramitochondrial K+ with mannitol abolished the effects of diazoxide and pinacidil on mitochondrial Ca2+, while the K+ ionophore valinomycin mimicked the effects of the potassium channel openers. 5. ATP and ADP, which block K+ flux through mitochondrial ATP-sensitive K+ channels, inhibited the effects of potassium channel openers, without preventing the action of valinomycin. 6. In intact cardiomyocytes, diazoxide also induced mitochondrial depolarization and decreased mitochondrial Ca2+ content. These effects were inhibited by the mitochondrial ATP-sensitive K+ channel blocker 5-hydroxydecanoic acid. 7. Thus, potassium channel openers prevent mitochondrial Ca2+ overload by reducing the driving force for Ca2+ uptake and by activating cyclosporin-sensitive Ca2+ release. In this regard, modulators of an ATP-sensitive mitochondrial K+ conductance may contribute to the maintenance of mitochondrial Ca2+ homeostasis.

Phosphoinositides decrease ATP sensitivity of the cardiac ATP-sensitive K(+) channel. A molecular probe for the mechanism of ATP-sensitive inhibition

Anionic phospholipids modulate the activity of inwardly rectifying potassium channels (Fan, Z., and J.C. Makielski. 1997. J. Biol. Chem. 272:5388-5395). The effect of phosphoinositides on adenosine triphosphate (ATP) inhibition of ATP-sensitive potassium channel (K(ATP)) currents was investigated using the inside-out patch clamp technique in cardiac myocytes and in COS-1 cells in which the cardiac isoform of the sulfonylurea receptor, SUR2, was coexpressed with the inwardly rectifying channel Kir6.2. Phosphoinositides (1 mg/ml) increased the open probability of K(ATP) in low [ATP] (1 microM) within 30 s. Phosphoinositides desensitized ATP inhibition with a longer onset period (>3 min), activating channels inhibited by ATP (1 mM). Phosphoinositides treatment for 10 min shifted the half-inhibitory [ATP] (K(i)) from 35 microM to 16 mM. At the single-channel level, increased [ATP] caused a shorter mean open time and a longer mean closed time. Phosphoinositides prolonged the mean open time, shortened the mean closed time, and weakened the [ATP] dependence of these parameters resulting in a higher open probability at any given [ATP]. The apparent rate constants for ATP binding were estimated to be 0.8 and 0.02 mM(-1) ms(-1) before and after 5-min treatment with phosphoinositides, which corresponds to a K(i) of 35 microM and 5.8 mM, respectively. Phosphoinositides failed to desensitize adenosine inhibition of K(ATP). In the presence of SUR2, phosphoinositides attenuated MgATP antagonism of ATP inhibition. Kir6.2DeltaC35, a truncated Kir6.2 that functions without SUR2, also exhibited phosphoinositide desensitization of ATP inhibition. These data suggest that (a) phosphoinositides strongly compete with ATP at a binding site residing on Kir6.2; (b) electrostatic interaction is a characteristic property of this competition; and (c) in conjunction with SUR2, phosphoinositides render additional, complex effects on ATP inhibition. We propose a model of the ATP binding site involving positively charged residues on the COOH-terminus of Kir6.2, with which phosphoinositides interact to desensitize ATP inhibition.

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.