Regulation of K(ATP) channel by 17β-estradiol in pancreatic β-cells

ATP-sensitive potassium channels (K_ATP) regulate electrical activity and insulin secretion in pancreatic β-cells. When glucose concentration increases, the [ATP]/[ADP] ratio rises closing K_ATP channels, and the membrane potential depolarizes, triggering insulin secretion. This pivotal role of K_ATP channels is used not only by glucose but also by neurotransmitters, hormones and other physiological agents to modulate electrical and secretory β-cell response.In recent years, it has been demonstrated that estrogens and estrogen receptors are involved in glucose homeostasis, and that they can modulate the electrical activity and insulin secretion of pancreatic β-cells. The hormone 17β-estradiol (E2), at physiological levels, is implicated in maintaining normal insulin sensitivity for β-cell function. Long term exposure to E2 increases insulin content, insulin gene expression and insulin release via the estrogen receptor α (ERα), while rapid responses to E2 can regulate K_ATP channels increasing cGMP levels through the estrogen receptor β (ERβ) and type A guanylate cyclase receptor (GC-A). This review summarizes the main actions of 17β-estradiol on K_ATP channels and the subsequent insulin release in pancreatic β-cells.     

Carbamazepine inhibits ATP-sensitive potassium channel activity by disrupting channel response to MgADP

In pancreatic β-cells, K_ATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. Sulfonylureas, insulin secretagogues used to treat type II diabetes, inhibit K_ATP channel activity primarily by abolishing the stimulatory effect of MgADP endowed by SUR1. In addition, sulfonylureas have been shown to function as pharmacological chaperones to correct channel biogenesis and trafficking defects. Recently, we reported that carbamazepine, an anticonvulsant known to inhibit voltage-gated sodium channels, has profound effects on K_ATP channels. Like sulfonylureas, carbamazepine corrects trafficking defects in channels bearing mutations in the first transmembrane domain of SUR1. Moreover, carbamazepine inhibits the activity of K_ATP channels such that rescued mutant channels are unable to open when the intracellular ATP/ADP ratio is lowered by metabolic inhibition. Here, we investigated the mechanism by which carbamazepine inhibits K_ATP channel activity. We show that carbamazepine specifically blocks channel response to MgADP. This gating effect resembles that of sulfonylureas. Our results reveal striking similarities between carbamazepine and sulfonylureas in their effects on K_ATP channel biogenesis and gating and suggest that the 2 classes of drugs may act via a converging mechanism.     

Activation of an ATP-dependent K+ conductance in Xenopus oocytes by expression of adenylate kinase cloned from renal proximal tubules

In rabbit proximal convoluted tubules, an ATP-sensitive K⁺ (K_ATP) channel has been shown to be involved in membrane cross-talk, i.e. the coupling (most likely mediated through intracellular ATP) between transepithelial Na⁺ transport and basolateral K⁺ conductance. This K⁺ conductance is inhibited by taurine. We sought to isolate this K⁺ channel by expression cloning in Xenopus oocytes. Injection of renal cortex mRNA into oocytes induced a K⁺ conductance, largely inhibited by extracellular Ba²⁺ and intracellular taurine. Using this functional test, we isolated from our proximal tubule cDNA library a unique clone, which induced a large K⁺ current which was Ba²⁺-, taurine- and glibenclamide-sensitive. Surprisingly, this clone is not a K⁺ channel but an adenylate kinase protein (AK3), known to convert NTP+AMP into NDP+ADP (N could be G, I or A). AK3 expression resulted in a large ATP decrease and activation of the whole-cell currents including a previously unknown, endogenous K⁺ current. To verify whether ATP decrease was responsible for the current activation, we demonstrated that inhibition of glycolysis greatly reduces oocyte ATP levels and increases an inwardly rectifying K⁺ current. The possible involvement of AK in the K_ATP channel’s regulation provides a means of explaining their observed activity in cytosolic environments characterized by high ATP concentrations.     

Leptin Regulates KATP Channel Trafficking in Pancreatic β-Cells by a Signaling Mechanism Involving AMP-activated Protein Kinase (AMPK) and cAMP-dependent Protein Kinase (PKA)

Pancreatic β-cells secrete insulin in response to metabolic and hormonal signals to maintain glucose homeostasis. Insulin secretion is under the control of ATP-sensitive potassium (K_ATP) channels that play key roles in setting β-cell membrane potential. Leptin, a hormone secreted by adipocytes, inhibits insulin secretion by increasing K_ATP channel conductance in β-cells. We investigated the mechanism by which leptin increases K_ATP channel conductance. We show that leptin causes a transient increase in surface expression of K_ATP channels without affecting channel gating properties. This increase results primarily from increased channel trafficking to the plasma membrane rather than reduced endocytosis of surface channels. The effect of leptin on K_ATP channels is dependent on the protein kinases AMP-activated protein kinase (AMPK) and PKA. Activation of AMPK or PKA mimics and inhibition of AMPK or PKA abrogates the effect of leptin. Leptin activates AMPK directly by increasing AMPK phosphorylation at threonine 172. Activation of PKA leads to increased channel surface expression even in the presence of AMPK inhibitors, suggesting AMPK lies upstream of PKA in the leptin signaling pathway. Leptin signaling also leads to F-actin depolymerization. Stabilization of F-actin pharmacologically occludes, whereas destabilization of F-actin simulates, the effect of leptin on K_ATP channel trafficking, indicating that leptin-induced actin reorganization underlies enhanced channel trafficking to the plasma membrane. Our study uncovers the signaling and cellular mechanism by which leptin regulates K_ATP channel trafficking to modulate β-cell function and insulin secretion.     

Adenylate kinase: Kinetic behavior in intact cells indicates it is integral to multiple cellular processes

Monitoring the kinetic behavior of adenylate kinase (AK) and creatine kinase (CK) in intact cells by ¹⁸O-phosphoryl oxygen exchange analysis has provided new perspectives from which to more fully define the involvement of these phosphotransferases in cellular bioenergetics. A primary function attributable to both AK and CK is their apparent capability to couple ATP utilization with its generation by glycolytic and/or oxidative processes depending on cell metabolic status. This is evidenced by the observation that the sum of the net AK- plus CK-catalyzed phosphoryl transfer is equivalent to about 95% of the total ATP metabolic flux in non-contracting rat diaphragm; under basal conditions almost every newly generated ATP molecule appears to be processed by one or the other of these phosphotransferases prior to its utilization. Although CK accounts for the transfer of a majority of the ATP molecules generated/consumed in the basal state there is a progressive, apparently compensatory, shift in phosphotransfer catalysis from the CK to the AK system with increasing muscle contraction or graded chemical inhibition of CK activity. AK and CK appear therefore to provide similar and interrelated functions. Evidence that high energy phosphoryl transfer in some cell types or metabolic states can also be provided by specific nucleoside mono- and diphosphate kinases and by the phosphotransfer capability inherent to the glycolytic system has been obtained. Measurements by ¹⁸O-exchange analyses of net AK- and CK-catalyzed phosphoryl transfer in conjunction with 31P NMR analyses of total unidirectional phosphoryl flux show that each new energy-bearing molecule CK or AK generates subsequently undergoes about 50 or more unidirectional CK-or AK-catalyzed phosphotransfers en route to an ATP consumption site in intact muscle. This evidence of multiple enzyme catalyzed exchanges coincides with the mechanism of vectorial ligand conduction suggested for accomplishing intracellular high energy phosphoryl transfer by the AK and CK systems. AK-catalyzed phosphotransfer also appears to be integral to the transduction of metabolic signals influencing the operation of ion channels regulated by adenine nucleotides such as ATP-inhibitable K⁺ channels in insulin secreting cells; transition from the ATP to ADP liganded states closely coincides with the rate AK-catalyzes phosphotransfer transforming ATP (+AMP) to (2) ADP.     

Carbamazepine inhibits ATP-sensitive potassium channel activity by disrupting channel response to MgADP

In pancreatic β-cells, K_ATP channels consisting of Kir6.2 and SUR1 couple cell metabolism to membrane excitability and regulate insulin secretion. Sulfonylureas, insulin secretagogues used to treat type II diabetes, inhibit K_ATP channel activity primarily by abolishing the stimulatory effect of MgADP endowed by SUR1. In addition, sulfonylureas have been shown to function as pharmacological chaperones to correct channel biogenesis and trafficking defects. Recently, we reported that carbamazepine, an anticonvulsant known to inhibit voltage-gated sodium channels, has profound effects on K_ATP channels. Like sulfonylureas, carbamazepine corrects trafficking defects in channels bearing mutations in the first transmembrane domain of SUR1. Moreover, carbamazepine inhibits the activity of K_ATP channels such that rescued mutant channels are unable to open when the intracellular ATP/ADP ratio is lowered by metabolic inhibition. Here, we investigated the mechanism by which carbamazepine inhibits K_ATP channel activity. We show that carbamazepine specifically blocks channel response to MgADP. This gating effect resembles that of sulfonylureas. Our results reveal striking similarities between carbamazepine and sulfonylureas in their effects on K_ATP channel biogenesis and gating and suggest that the 2 classes of drugs may act via a converging mechanism.     

Inhibition of ATP-regulated K+-channels by a photoactivatable ATP-analogue in mouse pancreatic β-cells

The effects of a photoactivatable (DMNPE-caged) ATP-analogue on ATP-regulated K⁺-channels (K_ATP-channel) in mouse pancreatic β-cells were investigated using the inside-out patch configuration of the patch-clamp technique. The caged precursor caused a concentration-dependent reduction of channel activity with a K_i of 17 μM; similar to the 11 μM obtained for standard Mg-ATP. The small difference in the blocking capacity between the precursor and ATP is probably the reason why no change in channel activity was observed upon photolysis of the caged molecule and liberation of ATP. It is suggested that the part of the ATP molecule involved in the blocking reaction of the K_ATP-channel is not sufficiently protected in DMNPE-caged ATP making this compound unsuitable for studying the rapid kinetics of ATP-induced K_ATP-channel inhibition.     

Impaired intracellular energetic communication in muscles from creatine kinase and adenylate kinase (M-CK/AK1) double knock-out mice

Previously we demonstrated that efficient coupling between cellular sites of ATP production and ATP utilization, required for optimal muscle performance, is mainly mediated by the combined activities of creatine kinase (CK)- and adenylate kinase (AK)-catalyzed phosphotransfer reactions. Herein, we show that simultaneous disruption of the genes for the cytosolic M-CK- and AK1 isoenzymes compromises intracellular energetic communication and severely reduces the cellular capability to maintain total ATP turnover under muscle functional load. M-CK/AK1 (MAK=/=) mutant skeletal muscle displayed aberrant ATP/ADP, ADP/AMP and ATP/GTP ratios, reduced intracellular phosphotransfer communication, and increased ATP supply capacity as assessed by 18O labeling of [Pi] and [ATP]. An analysis of actomyosin complexes in vitro demonstrated that one of the consequences of M-CK and AK1 deficiency is hampered phosphoryl delivery to the actomyosin ATPase, resulting in a loss of contractile performance. These results suggest that MAK=/= muscles are energetically less efficient than wild-type muscles, but an apparent compensatory redistribution of high-energy phosphoryl flux through glycolytic and guanylate phosphotransfer pathways limited the overall energetic deficit. Thus, this study suggests a coordinated network of complementary enzymatic pathways that serve in the maintenance of energetic homeostasis and physiological efficiency.     

Critical Role of Gap Junction Coupled KATP Channel Activity for Regulated Insulin Secretion

Pancreatic β-cells secrete insulin in response to closure of ATP-sensitive K⁺ (K_ATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca²⁺ (Ca_i). In intact islets, β-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Ca_i oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and β-cells from transgenic mice that express zero functional K_ATP channels in approximately 70% of their β-cells, but normal K_ATP channel density in the remainder. We found that K_ATP channel activity from approximately 30% of the β-cells is sufficient to maintain strong glucose dependence of metabolism, Ca_i, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of K_ATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell–cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.     

Cytosolic adenylate kinases regulate K-ATP channel activity in human beta-cells

The role of adenylate kinase (AK) as a determinant of K-ATP channel activity in human pancreatic β-cells was investigated. We have identified that two cytosolic isoforms of AK, AK1 and AK5 are expressed in human islets and INS-1 cells. Elevated concentrations of glucose inhibit AK1 expression and AK1 immunoprecipitates with the Kir6.2 subunit of K-ATP. AK activation by ATP + AMP stimulates K-ATP channel activity and this stimulation is abolished by AK inhibitors. We propose that glucose stimulation of β-cells inhibits AK through glycolysis and also through the elevation of diadenosine polyphosphate levels. Glucose-dependent inhibition of AK increases the ATP/ADP ratio in the microenvironment of the K-ATP channel promoting channel closure and insulin secretion. The down-regulation of AK1 expression by hyperglycemia may contribute to the defective coupling of glucose metabolism to K-ATP channel activity in type 2 diabetes.     

Chronic palmitic acid-induced lipotoxicity correlates with defective trafficking of ATP sensitive potassium channels in pancreatic β cells

Lipotoxicity is associated with a high level of fatty acid accumulation in pancreatic β-cells. An overload of free fatty acids contributes to pancreatic β-cell apoptosis and dysfunction. Insulin secretion involves sequential ionic events upon glucose stimulation. ATP sensitive potassium (K_ATP) channels serve as glucose sensors and effectively initiate glucose-stimulated insulin secretion. This study investigated the effects of lipotoxicity on the trafficking of K_ATP channels in pancreatic β cells using chronic palmitic acid –injected mice and treated insulinoma cells. The chronic palmitic acid -injected mice displayed type II diabetic characteristics. The pancreatic sections of these mice exhibited a decrease in the expression of K_ATP channels. We then tested the time and dose effects of palmitic acid on the cell viability of INS-1 cells. We observed a significant decrease in the surface expression of K_ATP channels after 72 h of treatment with 0.4 mM palmitic acid. In addition, this treatment induced pancreatic β-cell apoptosis by increasing cleaved caspase 3 protein level. Our results demonstrated cotreatment with glibenclamide, the sulfonylurea compounds for type II diabetes mellitus, in palmitic acid -treated cells reduces cell death and recovers the glucose stimulated insulin secretion through increasing the surface expression of K_ATP channels. Importantly, glibenclamide also improved glucose tolerance, triglyceride concentration, and insulin sensitivity in the palmitic acid-injected mice. In conclusion, an increase in the surface expression of K_ATP channels restores insulin secretion, reduces pancreatic β-cell’s apoptosis, highlighting correct trafficking of K_ATP channels is important in survival of β-cells during lipotoxicity.     

Critical role of gap junction coupled KATP channel activity for regulated insulin secretion

Pancreatic β-cells secrete insulin in response to closure of ATP-sensitive K⁺ (K_ATP) channels, which causes membrane depolarization and a concomitant rise in intracellular Ca²⁺ (Ca_i). In intact islets, β-cells are coupled by gap junctions, which are proposed to synchronize electrical activity and Ca_i oscillations after exposure to stimulatory glucose (>7 mM). To determine the significance of this coupling in regulating insulin secretion, we examined islets and β-cells from transgenic mice that express zero functional K_ATP channels in approximately 70% of their β-cells, but normal K_ATP channel density in the remainder. We found that K_ATP channel activity from approximately 30% of the β-cells is sufficient to maintain strong glucose dependence of metabolism, Ca_i, membrane potential, and insulin secretion from intact islets, but that glucose dependence is lost in isolated transgenic cells. Further, inhibition of gap junctions caused loss of glucose sensitivity specifically in transgenic islets. These data demonstrate a critical role of gap junctional coupling of K_ATP channel activity in control of membrane potential across the islet. Control via coupling lessens the effects of cell–cell variation and provides resistance to defects in excitability that would otherwise lead to a profound diabetic state, such as occurs in persistent neonatal diabetes mellitus.     

Concerted Trafficking Regulation of Kv2.1 and KATP Channels by Leptin in Pancreatic β-Cells

In pancreatic β-cells, voltage-gated potassium 2.1 (Kv2.1) channels are the dominant delayed rectifier potassium channels responsible for action potential repolarization. Here, we report that leptin, a hormone secreted by adipocytes known to inhibit insulin secretion, causes a transient increase in surface expression of Kv2.1 channels in rodent and human β-cells. The effect of leptin on Kv2.1 surface expression is mediated by the AMP-activated protein kinase (AMPK). Activation of AMPK mimics whereas inhibition of AMPK occludes the effect of leptin. Inhibition of Ca²⁺/calmodulin-dependent protein kinase kinase β, a known upstream kinase of AMPK, also blocks the effect of leptin. In addition, the cAMP-dependent protein kinase (PKA) is involved in Kv2.1 channel trafficking regulation. Inhibition of PKA prevents leptin or AMPK activators from increasing Kv2.1 channel density, whereas stimulation of PKA is sufficient to promote Kv2.1 channel surface expression. The increased Kv2.1 surface expression by leptin is dependent on actin depolymerization, and pharmacologically induced actin depolymerization is sufficient to enhance Kv2.1 surface expression. The signaling and cellular mechanisms underlying Kv2.1 channel trafficking regulation by leptin mirror those reported recently for ATP-sensitive potassium (K_ATP) channels, which are critical for coupling glucose stimulation with membrane depolarization. We show that the leptin-induced increase in surface K_ATP channels results in more hyperpolarized membrane potentials than control cells at stimulating glucose concentrations, and the increase in Kv2.1 channels leads to a more rapid repolarization of membrane potential in cells firing action potentials. This study supports a model in which leptin exerts concerted trafficking regulation of K_ATP and Kv2.1 channels to coordinately inhibit insulin secretion.     

Monovalent cation requirement for ADP inhibition of pyruvate dehydrogenase kinase

ADP is a competitive inhibitor with respect to ATP for pyruvate dehydrogenase kinase. Evidence is presented that K⁺ or NH₄⁺ ions are required for inhibition of the kinase by ADP. K⁺ at 30–90 mM and NH₄⁺ at 1–5 mM decrease markedly the apparent K_i of bovine kidney pyruvate dehydrogenase kinase for ADP and also decrease, to a lesser extent, the apparent K_m for ATP. Na⁺ is less effective and, in addition, inhibits kinase activity. Since K⁺ and NH₄⁺ are not required for kinase activity, their effect appears to be primarily of regulatory significance. K⁺ and NH₄⁺ have little effect, if any, on pyruvate dehydrogenase phosphatase activity. When both the kinase and the phosphatase are present and functional, the near steady state activity of the pyruvate dehydrogenase complex is affected significantly by varying the concentration of K⁺ or NH₄⁺ at a fixed ADP/ATP concentration ratio and by varying the $\text{ADP}\text{ATP}$ ratio at a fixed concentration of monovalent cation.     

Electrophysiology of pancreatic β-cells in intact mouse islets of Langerhans

When exposed to intermediate glucose concentrations (6–16 mol/l), pancreatic β-cells in intact islets generate bursts of action potentials (superimposed on depolarised plateaux) separated by repolarised electrically silent intervals. First described more than 40 years ago, these oscillations have continued to intrigue β-cell electrophysiologists. To date, most studies of β-cell ion channels have been performed on isolated cells maintained in tissue culture (that do not burst). Here we will review the electrophysiological properties of β-cells in intact, freshly isolated, mouse pancreatic islets. We will consider the role of ATP-regulated K⁺-channels (K_ATP-channels), small-conductance Ca²⁺-activated K⁺-channels and voltage-gated Ca²⁺-channels in the generation of the bursts. Our data indicate that K_ATP-channels not only constitute the glucose-regulated resting conductance in the β-cell but also provide a variable K⁺-conductance that influence the duration of the bursts of action potentials and the silent intervals. We show that inactivation of the voltage-gated Ca²⁺-current is negligible at voltages corresponding to the plateau potential and consequently unlikely to play a major role in the termination of the burst. Finally, we propose a model for glucose-induced β-cell electrical activity based on observations made in intact pancreatic islets.     

The Nucleotide-Binding Sites of SUR1: A Mechanistic Model

ATP-sensitive potassium (K_ATP) channels comprise four pore-forming Kir6.2 subunits and four modulatory sulfonylurea receptor (SUR) subunits. The latter belong to the ATP-binding cassette family of transporters. K_ATP channels are inhibited by ATP (or ADP) binding to Kir6.2 and activated by Mg-nucleotide interactions with SUR. This dual regulation enables the K_ATP channel to couple the metabolic state of a cell to its electrical excitability and is crucial for the K_ATP channel’s role in regulating insulin secretion, cardiac and neuronal excitability, and vascular tone. Here, we review the regulation of the K_ATP channel by adenine nucleotides and present an equilibrium allosteric model for nucleotide activation and inhibition. The model can account for many experimental observations in the literature and provides testable predictions for future experiments.     

KATP channels in focus: Progress toward a structural understanding of ligand regulation

K_ATP channels are hetero-octameric complexes of four inward rectifying potassium channels, Kir6.1 or Kir6.2, and four sulfonylurea receptors, SUR1, SUR2A, or SUR2B from the ABC transporter family. This unique combination enables K_ATP channels to couple intracellular ATP/ADP ratios, through gating, with membrane excitability, thus regulating a broad range of cellular activities. The prominence of K_ATP channels in human physiology, disease, and pharmacology has long attracted research interest. Since 2017, a steady flow of high-resolution K_ATP cryoEM structures has revealed complex and dynamic interactions between channel subunits and their ligands. Here, we highlight insights from recent structures that begin to provide mechanistic explanations for decades of experimental data and discuss the remaining knowledge gaps in our understanding of K_ATP channel regulation.     

Opening of Cardiac Sarcolemmal KATP Channels by Dinitrophenol Separate from Metabolic Inhibition

Opening of ATP-sensitive K⁺ (K_ATP) channels by the uncoupler of oxidative phosphorylation, 2,4 dinitrophenol (DNP), has been assumed to be secondary to metabolic inhibition and reduced intracellular ATP levels. Herein, we present data which show that DNP (200 μm) can induce opening of cardiac K_ATP channels, under whole-cell and inside-out conditions, despite millimolar concentrations of ATP (1–2.5 mm). DNP-induced currents had a single channel conductance (71 pS), inward rectification, reversal potential, and intraburst kinetic properties (open time constant, τ_open: 4.8 msec; fast closed time constant, τ_closed(f): 0.33 msec) characteristic of K_ATP channels suggesting that DNP did not affect the pore region of the channel, but may have altered the functional coupling of the ATP-dependent channel gating. A DNP analogue, with the pH-titrable hydroxyl replaced by a methyl group, could not open K_ATP channels. The pH-dependence of the effect of DNP on channel opening under whole-cell, cell-attached, and inside-out conditions suggested that transfer of protonated DNP across the sarcolemma is essential for activation of K_ATP channels in the presence of ATP. We conclude that the use of DNP for metabolic stress-induced K_ATP channel opening should be reevaluated. Received: 10 September 1996/Revised: 27 December 1996     

Adenylate kinase AK1 knockout heart: energetics and functional performance under ischemia-reperfusion

Deletion of the major adenylate kinase AK1 isoform, which catalyzes adenine nucleotide exchange, disrupts cellular energetic economy and compromises metabolic signal transduction. However, the consequences of deleting the AK1 gene on cardiac energetic dynamics and performance in the setting of ischemia-reperfusion have not been determined. Here, at the onset of ischemia, AK1 knockout mice hearts displayed accelerated loss of contractile force compared with wild-type controls, indicating reduced tolerance to ischemic stress. On reperfusion, AK1 knockout hearts demonstrated reduced nucleotide salvage, resulting in lower ATP, GTP, ADP, and GDP levels and an altered metabolic steady state associated with diminished ATP-to-P(i) and creatine phosphate-to-P(i) ratios. Postischemic AK1 knockout hearts maintained approximately 40% of beta-phosphoryl turnover, suggesting increased phosphotransfer flux through remaining adenylate kinase isoforms. This was associated with sustained creatine kinase flux and elevated cellular glucose-6-phosphate levels as the cellular energetic system adapted to deletion of AK1. Such metabolic rearrangements, along with sustained ATP-to-ADP ratio and total ATP turnover rate, maintained postischemic contractile recovery of AK1 knockout hearts at wild-type levels. Thus deletion of the AK1 gene reveals that adenylate kinase phosphotransfer supports myocardial function on initiation of ischemic stress and safeguards intracellular nucleotide pools in postischemic recovery.     

Ligand-insensitive State of Cardiac ATP-sensitive K+ Channels : Basis for Channel Opening

The mechanism by which ATP-sensitive K⁺ (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.