ATP-sensitive potassium channels and myocardial preconditioning

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

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.     

Nicorandil inhibits oxidative stress-induced apoptosis in cardiac myocytes through activation of mitochondrial ATP-sensitive potassium channels and a nitrate-like effect

The anti-anginal drug nicorandil has been shown to inhibit apoptosis by activating mitochondrial ATP-sensitive potassium (K(ATP)) channels. The possible contribution of the nitrate moiety of this drug to its anti-apoptotic effect has now been investigated in neonatal rat ventricular myocytes subjected to oxidative stress. Exposure of cultured myocytes to 100 micromol/l hydrogen peroxide (H(2)O(2)) increased the number of nuclei stained by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling technique as well as induced internucleosomal DNA fragmentation, loss of mitochondrial membrane potential, cytochrome c release into the cytosol, and activation of caspases-3 and -9, all of which are characteristics of apoptosis. Pretreatment of cells with nicorandil (100 micromol/l) inhibited these effects of H(2)O(2). Both the mitochondrial K(ATP) channel antagonist 5-hydroxydecanoate (5-HD) and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of soluble guanylyl cyclase, attenuated the anti-apoptotic effect of nicorandil in concentration-dependent manners. Coapplication of ODQ (10 micromol/l) and 5-HD (500 micromol/l) completely abolished nicorandil-induced cytoprotection. The effect of nicorandil was also reduced by an inhibitor of cGMP-dependent protein kinase (KT5823, 1 micromol/l). The nitric oxide donor (+/-)-S-nitroso-N-acetylpenicillamine (SNAP, 50 micromol/l) mimicked the protective effect of nicorandil in a manner sensitive to ODQ but not to 5-HD. A cell-permeable cGMP analog, 8-bromo-cGMP, also reduced H(2)O(2)-induced apoptosis. The inhibition of the H(2)O(2)-induced activation of caspase-3, but not that of caspase-9, by nicorandil in the presence of 5-HD or by SNAP was reversed by the addition of dithiothreitol to the enzyme assay. Nicorandil inhibits oxidative stress-induced apoptosis in cardiac myocytes through a nitric oxide/cGMP-dependent mechanism as well as by activating mitochondrial K(ATP) channels.     

ATP敏感性钾通道与低氧性肺动脉高压

研究证明肺血管上存在ATP敏感性钾通道（KATP通道），该通道参与了膜静息电位和血管紧张性的调节。在低氧性诱导的肺动脉高压中，KATP通道参与了血管收缩和重构的病理过程。了解KATP通道的结构和其生物学特性，对认识低氧性肺动脉高压的形成机制和发展新的临床治疗手段有重要意义。     

肾上腺髓质素与KATP通道的研究进展

肾上腺髓质素(ADM)是一种心血管活性多肽,具有扩血管、抑制血管平滑肌细胞增殖等广泛的生物学效应,特别是扩张血管作用更为突出.ATP敏感钾通道(KATP)是由一类较广泛分布的内向整流钾通道Kir和硫脲类受体SUR亚基共同组成,在生理及某些病理条件下KATP参与血管张力的调节,并受多种因素的调控,胞内二磷酸核苷酸(NDPs)、钾通道开放剂(KCOs)等可激活通道,而ATP和硫脲类药物则特异性抑制通道的开放.ADM的扩血管作用多与KATP有关,具体作用机制目前尚不清楚.     

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K_ATP) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K_ATP channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K_ATP channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K_ATP channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K_ATP channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.     

Involvement of ATP-sensitive potassium channels in preconditioning protection

Summary Single or multiple brief periods of ischemia (preconditioning, PC) have been shown to protect the myocardium from infarction during a subsequent more prolonged ischemic insult. To test the hypothesis that opening of ATP-sensitive potassium channels (K_ATP) is involved in this mechanism, either bimakalim, a K_ATP channel opener, or glibenclamide, a K_ATP channel blocker, were administered to mimic or to block preconditioning protection in barbital-anesthetized pigs. PC was elicited by a single period of 10 min left anterior descending coronary artery (LADCA) occlusion followed by 15 min of reperfusion before the LADCA was reoccluded for 60 min. Instead of PC, bimakalim infusion was started 15 min before the 60 min LADCA occlusion (TCO) and stopped with the onset of ischemia. Glibenclamide was administered either for 10 min prior to the PC protocol, before bimakakim infusion, or before TCO. Regional wall function was quantified with ultrasonic crystals aligned to measure wall thickening (%WT). At the end of the protocol, infarct size was determined by incubating myocardium with p-nitrobluetetrazolium.In seven preconditioned pigs, infarct size was 9.9±5.1% of the risk region compared with 65.9±6.0% in the seven control pigs subjected to 60 min of ischemia only (p<0.001). In seven pigs treated with bimakalim, infarct size was reduced to 35.3±6.6 (p<0.05 vs. controls). Blocking ATP-sensitive potassium channels with glibenclamide prior to PC abolished its protective effect (infarct size, 62.2±4.5%;p<0.001 vs. PC alone). Glibenclamide also antagonized the protective effect of bimakalim (infarct size, 55.2±4.0%), but did not affect infarct size, when solely administered prior to the prolonged ischemic period (62.2±4.3%). We conclude that in swine myocardium K_ATP channels are involved in the protective effect of ischemic preconditioning, since glibenclamide completely abolished the protective effect of preconditioning, while bimakalim could — at least in part — mimic it.Dedicated to Prof. Dr. H.-J. Langmann on the occassion of his 70th birthday.     

腺苷三磷酸敏感性钾通道与婴幼儿先天性胰岛素过多症

婴幼儿先天性胰岛素过多症(congenital hyperinsulinism in infancy,CHI)又称婴幼儿持续性高胰岛素血症性低血糖症(persistent hyperinsulinemic hypoglycemia of infancy,PHHI)或婴幼儿高胰岛素血症(hyperinsulinism in infancy,HI),是一种罕见疾病,也是婴幼儿持续性低血糖最常见的原因~([1]).     

Functional roles of cardiac and vascular ATP-sensitive potassium channels clarified by Kir6.2-knockout mice

-ATP-sensitive potassium (K(ATP)) channels were discovered in ventricular cells, but their roles in the heart remain mysterious. K(ATP) channels have also been found in numerous other tissues, including vascular smooth muscle. Two pore-forming subunits, Kir6.1 and Kir6.2, contribute to the diversity of K(ATP) channels. To determine which subunits are operative in the cardiovascular system and their functional roles, we characterized the effects of pharmacological K(+) channel openers (KCOs, ie, pinacidil, P-1075, and diazoxide) in Kir6.2-deficient mice. Sarcolemmal K(ATP) channels could be recorded electrophysiologically in ventricular cells from Kir6.2(+/+) (wild-type [WT]) but not from Kir6.2(-/-) (knockout [KO]) mice. In WT ventricular cells, pinacidil induced an outward current and action potential shortening, effects that were blocked by glibenclamide, a K(ATP) channel blocker. KO ventricular cells exhibited no response to KCOs, but gene transfer of Kir6.2 into neonatal ventricular cells rescued the electrophysiological response to P-1075. In terms of contractile function, pinacidil decreased force generation in WT but not KO hearts. Pinacidil and diazoxide produced concentration-dependent relaxation in both WT and KO aortas precontracted with norepinephrine. In addition, pinacidil induced a glibenclamide-sensitive current of similar magnitude in WT and KO aortic smooth muscle cells and comparable levels of hypotension in anesthetized WT and KO mice. In both WT and KO aortas, only Kir6.1 mRNA was expressed. These findings indicate that the Kir6.2 subunit mediates the depression of cardiac excitability and contractility induced by KCOs; in contrast, Kir6.2 plays no discernible role in the arterial tree.     

三磷酸腺苷敏感性钾通道在心肌缺血预适应中的作用

采用结扎家兔左冠状动脉左室支复制心肌缺血预适应（ＩＰ）模型，研究三磷酸腺苷敏感性钾（ＫＡＴＰ）通道在ＩＰ中的作用。结果表明：ＩＰ能缩小梗塞面积和恢复再灌注期心脏功能；活化ＫＡＴＰ通道显著加强ＩＰ的作用，尤以恢复心功能为著；阻滞ＫＡＴＰ通道则抑制ＩＰ的作用。提示ＫＡＴＰ通道活化与ＩＰ作用有关。     

大鼠心房肌细胞三磷酸腺苷敏感钾通道的牵张敏感性

目的研究成年大鼠心房肌细胞三磷酸腺苷敏感钾通道(KATP)的牵张敏感性。方法应用单通道膜片钳技术在开放细胞贴附模式下记录大鼠心房肌细胞上KATP通道的电活动,并研究KATP通道对机械刺激的反应。结果根据通道的激活方式、药理学特性、I-V曲线、反转电位及整流特性,确定所记录的通道为KATP通道。进一步研究表明该通道的开放概率对负压刺激呈现强度依赖性。结论大鼠心房肌细胞上的KATP通道对牵张刺激敏感。     

Ca(2+)-activated potassium channels and ATP-sensitive potassium channels as modulators of vascular tone

Membrane hyperpolarization through activation of potassium channels in arterial smooth muscle appears to be an effective mechanism to dilate arteries. Conversely, membrane depolarization through inhibition of potassium channels can lead to vasoconstriction. Here, I briefly review the roles of Ca(2+)-activated K(+) (K(Ca)) channels and ATP-sensitive K(+) (K(ATP)) channels in the control of arterial smooth muscle function. K(Ca) channels regulate arterial tone in response to changes in intravascular pressure and possibly to a variety of vasoconstrictors. K(ATP) channels respond to changes in the cellular metabolic state and are targets of a variety of synthetic and endogenous vasodilators.     

Pharmacology of cardiac potassium channels

Cardiac K+ channels are membrane-spanning proteins that allow the passive movement of K+ ions across the cell membrane along its electrochemical gradient. They regulate the resting membrane potential, the frequency of pacemaker cells and the shape and duration of the cardiac action potential. Additionally, they have been recognized as potential targets for the actions of neurotransmitters and hormones and class III antiarrhythmic drugs that prolong the action potential duration (APD) and refractoriness and have been found effective to prevent/suppress cardiac arrhythmias. In the human heart, K+ channels include voltage-gated channels, such as the rapidly activating and inactivating transient outward current (Ito1), the ultrarapid (IKur), rapid (IKr) and slow (IKs) components of the delayed rectifier current and the inward rectifier current (IK1), the ligand-gated channels, including the adenosine triphosphate-sensitive (IKATP) and the acetylcholine-activated (IKAch) currents and the leak channels. Changes in the expression of K+ channels explain the regional variations in the morphology and duration of the cardiac action potential among different cardiac regions and are influenced by heart rate, intracellular signalling pathways, drugs and cardiovascular disorders. A progressive number of cardiac and noncardiac drugs block cardiac K+ channels and can cause a marked prolongation of the action potential duration (i.e. an acquired long QT syndrome, LQTS) and a distinct polymorphic ventricular tachycardia termed torsades de pointes. In addition, mutations in the genes encoding IKr (KCNH2/KCNE2) and IKs (KCNQ1/KCNE1) channels have been identified in some types of the congenital long QT syndrome. This review concentrates on the function, molecular determinants, regulation and, particularly, on the mechanism of action of drugs modulating the K+ channels present in the sarcolemma of human cardiac myocytes that contribute to the different phases of the cardiac action potential under physiological and pathological conditions.     

Role of ATP-sensitive potassium channels in coronary microvascular autoregulatory responses

The purpose of the present study was to test the hypothesis that ATP-sensitive potassium channels mediate autoregulatory vasodilatation of coronary arterioles in vivo. Experiments were performed in 23 open-chest anesthetized dogs. Coronary arterial microvascular diameters were directly measured with fluorescence microangiography using an intravital microscope and stroboscopic epi-illumination synchronized to the cardiac cycle. A mild coronary stenosis (perfusion pressure = 60 mm Hg), a critical coronary stenosis (perfusion pressure = 40 mm Hg), and complete coronary artery occlusion were produced with an occluder around the left anterior descending coronary artery in the presence or absence of glibenclamide (10(-5) M, topically), which inhibits ATP-sensitive potassium channels, or of vehicle. During topical application of vehicle (0.01% dimethyl sulfoxide), there was dilatation of small (less than 100 microns diameter) arterioles during reductions in perfusion pressure (percent change in diameter: 6.7 +/- 1.5%, 11.7 +/- 3.5%, and 10.4 +/- 5.1% during mild stenosis, critical stenosis, and complete occlusion, respectively). In the presence of glibenclamide, arteriolar dilatations during coronary stenoses and occlusions were abolished. Glibenclamide did not affect responses of arterioles greater than 100 microns. Glibenclamide did not alter microvascular responses to nitroprusside. These data suggest that ATP-sensitive potassium channels play an important role in determining the coronary microvascular response to reductions in perfusion pressure.     

Effect of repaglinide on cloned beta cell, cardiac and smooth muscle types of ATP-sensitive potassium channels

Aims/hypothesis. The carbamoylbenzoic acid derivative repaglinide is a potent short-acting insulin secretagogue that acts by closing ATP-sensitive potassium (K_ATP) channels in the plasma membrane of the pancreatic beta cell. In this paper we investigated the specificity of repaglinide for three types of cloned (K_ATP) channel composed of the inwardly rectifying potassium channel Kir6.2 and either the sulphonylurea receptor SUR1, SUR2A or SUR2B, corresponding to the beta cell, cardiac and either smooth muscle types of K_ATP channel, respectively. Methods. The action of the drug was studied by whole-cell current recordings of K_ATP channels expressed either in Xenopus oocytes or mammalian cells (HEK293). We also used inside-out macropatches excised from Xenopus oocytes for detailed analysis of repaglinide action. Results. The drug blocked all three types of K_ATP channel with similar potency, by interacting with a low-affinity site on the pore-forming subunit of the channel (Kir6.2: half-maximal inhibition 230 μmol/l) and with a high-affinity site on the regulatory subunit, the sulphonylurea receptor (SUR: half-maximal inhibition 2–8 nmol/l). There was no difference in potency between channels containing SUR1, SUR2A or SUR2B. MgADP potentiated the inhibitory effect of repaglinide on Kir6.2/SUR1 and (to a lesser extent) Kir6.2/SUR2B, but not on Kir6.2/SUR2A. Conclusion/interpretation. Repaglinide interacts with a site common to all three types of sulphonylurea receptor leading to inhibition of the K_ATP channel. The fact that MgADP potentiated this effect in the case of the beta cell, but not cardiac, type of channel could help explain why the drug shows no adverse cardiovascular side-effects in vivo. [Diabetologia (2001) 44: 747–756] Received: 13 December 2000 and in revised form: 14 February 2001     

对KATP通道介导缺血预适应心肌再灌注损伤的研究

目的探讨优降糖对KATP通道介导心肌缺血预适应作用的影响。方法将44只大鼠随机分为心肌缺血预适应组(IPC组)、优降糖组(GLI组)、优降糖 IPC组(G-P组)和对照组(C组)。心肌缺血预适应由3次10min缺血/10min再灌注组成。所有大鼠均接受30min缺血/60min再灌注。梗死范围由饱和曲利本蓝和红四氮唑蓝染色判定,并以坏死区占缺血区的百分率表示。Ⅱ导联记录心脏室性心律失常。结果IPC能显著缩小缺血/再灌注后的心肌梗死范围,且这种作用能被KATP通道阻滞剂优降糖完全取消。IPC可减少缺血/再灌注所致的室性心律失常的发生,但这种保护作用不能被优降糖所阻断。结论优降糖对KATP介导IPC的心肌保护作用有影响。