ATP-sensitive K channel channel/enzyme multimer: Metabolic gating in the heart (original) (raw)
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ATP-sensitive potassium channels: metabolic sensing and cardioprotection
Journal of Applied Physiology, 2007
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 (KATP) channels serve a critical role in the orchestration of myocardial energetic well-being. KATP 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 KATP channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This KATP channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varyi...
Regulation of Atp-Sensitive Potassiumchannels in the Heart
2009
Indeed, K ATP channels have become the therapeutic targets for a variety of diseases including angina, hypertension ATP-sensitive potassium (K ATP) channels regulate insuand diabetes (Nichols and Lopatin, 1997). lin secretion, vascular tone, heart rate and neuronal The K ATP channel is an octamer (Clement et al., 1997; excitability by responding to transmitters as well as the Inagaki et al., 1997; Shyng and Nichols, 1997; Babenko internal metabolic state. K ATP channels are composed et al., 1998). It is composed of four pore-forming of four pore-forming α-subunits (Kir6.2) and four α-subunits (Kir6.2) (Inagaki et al., 1995; Sakura et al., regulatory β-subunits, the sulfonylurea receptor 1995) and four regulatory β-subunits (SUR1 or SUR2) (SUR1, SUR2A or SUR2B). Whereas protein kinase A (Aguilar-Bryan et al., 1995; Inagaki et al., 1996; Isomoto (PKA) phosphorylation of serine 372 of Kir6.2 has et al., 1996). Partial complexes with fewer than eight been shown biochemically by others, we found that the subunits do not reach the cell membrane because of the phosphorylation of T224 rather than S372 of Kir6.2 exposure of an endoplasmic reticulum (ER) retention/ underlies the catalytic subunits of PKA (c-PKA)-and retrieval signal that is present in each subunit (Zerangue the D1 dopamine receptor-mediated stimulation of et al., 1999). Deleting the last 36 amino acids containing K ATP channels expressed in HEK293 cells. Specific this ER retention/retrieval signal therefore permits funcchanges in the kinetic properties of channels treated tional expression of Kir6.2∆C36 channels in the absence with c-PKA, as revealed by single-channel analysis, of SUR (Tucker et al., 1997). were mimicked by aspartate substitution of T224. The Native K ATP channels in different tissues comprise the T224D mutation also reduced the sensitivity to ATP same Kir6.2 (α) subunits, which form a weakly inwardly inhibition. Alteration of channel gating and a decrease rectifying potassium channel, but different SUR (β) subin the apparent affinity for ATP inhibition thus underlie units. K ATP channels in pancreatic β-cells and some central the positive regulation of K ATP channels by PKA neurons contain Kir6.2 and SUR1 (Inagaki et al., 1995; phosphorylation of T224 in Kir6.2, which may repres-Aguilar-Bryan et al., 1998). Mutations of either Kir6.2 or ent a general mechanism for K ATP channel regulation SUR1 cause persistent hyperinsulinemic hypoglycemia of in different tissues. infancy (PHHI), a disease associated with unregulated Keywords: phosphorylation/PKA/potassium channel/ insulin secretion (Thomas et al., 1995). Cardiac and single-channel/transfection skeletal muscle K ATP channels are composed of Kir6.2 and SUR2A (Inagaki et al., 1996; Okuyama et al., 1998). K ATP channels in smooth muscle and certain central neurons in the substantia nigra (SN) comprise Kir6.2 and 942 © European Molecular Biology Organization K ATP channel activation by PKA
Reconstituted Human Cardiac K ATP Channels
Circulation Research, 1998
ATP-sensitive potassium (K ATP) channels in striated myocytes are heteromultimers of K IR 6.2, a weak potassium inward rectifier, plus SUR2A, a low-affinity sulfonylurea receptor. We have cloned human K IR 6.2 (huK IR 6.2) and a huSUR2A that corresponds to the major, full-length splice variant identified by polymerase chain reaction analysis of human cardiac poly A ϩ mRNA. ATP-and glibenclamide-sensitive K ϩ channels were produced when both subunits were coexpressed in COSm6 and Chinese hamster ovary cells lacking endogenous K ATP channels, but not when huSUR2A or huK IR 6.2 were transfected alone. Recombinant channels activated by metabolic inhibition in cell-attached configuration or in inside-out patches with ATP-free internal solution were compared with sarcolemmal K ATP channels in human ventricular cells. The single-channel conductance of Ϸ80 pS measured at Ϫ40 mV in quasi-symmetrical Ϸ150 mmol/L K ϩ solutions, the intraburst kinetics that were dependent on K ϩ driving force, and the weak inward rectification were indistinguishable for both channels. Similar to the native channels, huSUR2A/huK IR 6.2 recombinant channels were inhibited by ATP at quasi-physiological free Mg 2ϩ (Ϸ0.7 mmol/L) or in the absence of Mg 2ϩ , with an apparent IC 50 of Ϸ20 mol/L and a pseudo-Hill coefficient of Ϸ1. They were "refreshed" by MgATP and stimulated by ADP in the presence of Mg 2ϩ when inhibited by ATP. The huSUR2A/huK IR 6.2 channels were stimulated by cromakalim and pinacidil in the presence of ATP and Mg 2ϩ but were insensitive to diazoxide. The results suggest that reconstituted huSUR2A/huK IR 6.2 channels represent K ATP channels in sarcolemma of human cardiomyocytes and are an adequate experimental model with which to examine structure-function relationships, molecular physiology, and pharmacology of these channels from human heart.
Ligand-insensitive State of Cardiac ATP-sensitive K+ Channels: Basis for Channel Opening
The Journal of General Physiology, 1998
The mechanism by which ATP-sensitive K ϩ (K ATP ) channels open in the presence of inhibitory concentrations of ATP remains unknown. Herein, using a four-state kinetic model, we found that the nucleotide diphosphate UDP directed cardiac K ATP channels to operate within intraburst transitions. These transitions are not targeted by ATP, nor the structurally unrelated sulfonylurea glyburide, which inhibit channel opening by acting on interburst transitions. Therefore, the channel remained insensitive to ATP and glyburide in the presence of UDP. "Rundown" of channel activity decreased the efficacy with which UDP could direct and maintain the channel to operate within intraburst transitions. Under this condition, the channel was sensitive to inhibition by ATP and glyburide despite the presence of UDP. This behavior of the K ATP channel could be accounted for by an allosteric model of ligand-channel interaction. Thus, the response of cardiac K ATP channels towards inhibitory ligands is determined by the relative lifetime the channel spends in a ligand-sensitive versus -insensitive state. Interconversion between these two conformational states represents a novel basis for K ATP channel opening in the presence of inhibitory concentrations of ATP in a cardiac cell.
The Journal of physiology, 1993
1. ATP-sensitive K+ (K+ATP) channels are believed to make an important contribution to the increased cellular K+ efflux and shortening of the action potential duration (APD) during metabolic inhibition, hypoxia, and ischaemia in the heart. The mechanisms by which the activity of the K+ATP channel is regulated during conditions of metabolic impairment are not completely clear. Extrinsic factors such as increased [ADP]i, acidosis, and stimulation of adenosine receptors appear to decrease the K+ATP channel's sensitivity to closure by [ATP]i. The purpose of this study was to determine whether the K+ATP channel itself is intrinsically altered by the processes associated with metabolic impairment. 2. Isolated guinea-pig ventricular myocytes were metabolically inhibited in glucose-free 1.8 mM Ca2+ Tyrode solution containing 9 microM rotenone and 0.9 microM carbonyl cyanide-p-trifluoromethoxyphenylhydrazone (FCCP) while recording unitary currents through K+ATP channels in cell-attached ...
ATP‐Sensitive Potassium Channels and Their Physiological and Pathophysiological Roles
Comprehensive Physiology
ATP sensitive potassium channels (K ATP) are so named because they open as cellular ATP levels fall. This leads to membrane hyperpolarisation and thus links cellular metabolism to membrane excitability. They also respond to MgADP and are regulated by a number of cell signalling pathways. They have a rich and diverse pharmacology with a number of agents acting as specific inhibitors and activators. K ATP channels are formed of pore-forming subunits, Kir6.1 and Kir6.2, and a large auxiliary subunit, the sulphonylurea receptor (SUR1, SUR2A and SUR2B). The Kir6.0 subunits are a member of the inwardly rectifying family of potassium channels and the sulphonylurea receptor is part of the ATP binding cassette family of proteins. Four SURs and four Kir6.x form an octameric channel complex and the association of a particular SUR with a specific Kir6.x subunit constitutes the K ATP current in a particular tissue. A combination of mutagenesis work combined with structural studies has identified how these channels work as molecular machines. They have a variety of physiological roles including controlling the release of insulin from pancreatic β cells and regulating blood vessel tone and blood pressure. Furthermore, mutations in the genes underlie human diseases such as congenital diabetes and hyperinsulinism. Additionally, opening of these channels is protective in a number of pathological conditions such as myocardial ischaemia and stroke. Didactic Synopsis Major teaching points ATP-sensitive potassium channels (K ATP) are widely distributed and characteristically are activated by falling cellular ATP levels. K ATP channels link membrane excitability to cellular metabolism. K ATP channels have a rich and diverse pharmacology with specific inhibitors such as glibenclamide and openers such as diazoxide. The channel is an octamer formed of four inwardly rectifying potassium channels of the Kir6.0 family and four sulphonylurea receptor subunits, a member of the ATP binding cassette family of proteins. Extensive mutagenesis experiments and recent structural studies have defined many aspects of how the channel works as a molecular machine. K ATP channels are key to the release of insulin from pancreatic β cells. K ATP channels in the heart are involved in adaptation to exercise and cellular protection and in vascular smooth muscle controlling vascular tone and blood pressure. K ATP channels are present in the brain and may be involved in neuroprotection and nutrient sensing. Mutations in K ATP channel subunits can result in human disease and includes disorders of insulin handling, cardiac arrhythmia, cardiomyopathy and neurological abnormalities.
Cardiovascular Research, 2008
Time for primary review: 25 days Aims ATP sensitive K þ channels (K ATP) sense adenine nucleotide concentrations and thus couple the metabolic state of the cell to membrane potential. The hetero-octameric complex of a sulphonylurea receptor (SUR2B) and an inwardly rectifying K þ channel (Kir6.1) and the corresponding native channel in smooth muscle are relatively insensitive to variations in intracellular ATP. Activation of these channels in blood vessels during hypoxia/ischaemia is thought to be mediated via hormonal regulation such as cellular adenosine release or the release of mediators from the endothelium. In contrast, intracellular ATP prominently inhibits Kir6.2 containing complexes, such as those present in cardiac myocytes. Thus, we investigated differences in the mechanism of metabolic regulation of Kir6.1 and Kir6.2 containing K ATP channels. Methods and results We have heterologously expressed K ATP channel subunits in HEK293 and CHO cells and studied their function using 86 Rb efflux and patch clamping. We show that rodent Kir6.1/SUR2B has direct intrinsic metabolic sensitivity independent of any regulation by protein kinase A. In contrast to Kir6.2 containing complexes, this was not endowed by the ATP sensitivity of the pore forming subunit but was instead a property of the SUR2B subunit. Mutagenesis of key residues within the nucleotidebinding domains (NBD) implicated both domains in governing the metabolic sensitivity. Conclusion Kir6.1\SUR2B has intrinsic sensitivity to metabolism endowed by the likely processing of adenine nucleotides at the NBD of SUR2B.
Cardiac KATP Channels in Health and Diseases
Electrical Diseases of the Heart, 2013
Adenosine triphosphate (ATP)-sensitive K + (K ATP ) channels were discovered in the heart almost 30 years ago. They are present in multiple tissues and link membrane excitability to the metabolic state of the cell. Under physiological conditions, cardiac K ATP channels are predominantly closed, but they may open during exertion, stress, and ischemia. Experimental and modeling studies have shown that activation of sarcolemmal K ATP channels causes dramatic action potential shortening in vitro, which can be cardioprotective. Conversely, there is emerging evidence that K ATP channel mutations are linked to heart diseases, including congestive heart failure and arrhythmias. The debate regarding the role, and even the very existence, of mitochondrial K ATP channels is still ongoing. Filling these knowledge gaps will require further study, and integration of results from basic cellular electrophysiology, to animal models and clinical disease. This chapter will address the current understanding of cardiac K ATP channels regarding molecular composition, regulation of channel activity, and physiological and pathophysiological roles.