Remote preconditioning reduces ischemic injury in the explanted heart by a KATP channel-dependent mechanism

Local and remote ischemic preconditioning (IPC) reduce ischemia-reperfusion (I/R) injury and preserve cardiac function. In this study, we tested the hypothesis that remote preconditioning is memorized by the explanted heart and yields protection from subsequent I/R injury and that the underlying mechanism involves sarcolemmal and mitochondrial ATP-sensitive K(+) (K(ATP)) channels. Male Wistar rats (300-350 g) were randomized to a control (n = 10), a remote IPC (n = 10), and a local IPC group (n = 10). Remote IPC was induced by four cycles of 5 min of limb ischemia, followed by 5 min of reperfusion. Local IPC was induced by four cycles of 2 min of regional myocardial ischemia, followed by 3 min of reperfusion. The heart was excised within 5 min after the final cycle of preconditioning, mounted in a perfused Langendorff preparation for 40 min of stabilization, and subjected to 45 min of sustained ischemia by occluding the left coronary artery and 120 min of reperfusion. I/R injury was assessed as infarct size by triphenyltetrazolium staining. The influence of sarcolemmal and mitochondrial K(ATP) channels on remote preconditioning was assessed by the addition of glibenclamide (10 microM, a nonselective K(ATP) blocker), 5-hydroxydecanoic acid (5-HD; 100 microM, a mitochondrial K(ATP) blocker), and HMR-1098 (30 microM, a sarcolemmal K(ATP) blocker) to the Langendorff preparation before I/R. The role of mitochondrial K(ATP) channels as an effector mechanism for memorizing remote preconditioning was further studied by the effect of the specific mitochondrial K(ATP) activator diaxozide (10 mg/kg) on myocardial infarct size. Remote preconditioning reduced I/R injury in the explanted heart (0.17 +/- 0.03 vs. 0.39 +/- 0.05, P < 0.05) and improved left ventricular function during reperfusion compared with control (P < 0.05). Similar effects were obtained with diazoxide. Remote preconditioning was abolished by the addition of 5-HD and glibenclamide but not by HMR-1098. In conclusion, the protective effect of remote preconditioning is memorized in the explanted heart by a mechanism that involves mitochondrial K(ATP) channels.     

Modulation of ATP-sensitive potassium channels by cGMP-dependent protein kinase in rabbit ventricular myocytes

This investigation used a patch clamp technique to test the hypothesis that protein kinase G (PKG) contributes to the phosphorylation and activation of ATP-sensitive K(+) (K(ATP)) channels in rabbit ventricular myocytes. Nitric oxide donors and PKG activators facilitated pinacidil-induced K(ATP) channel activities in a concentration-dependent manner, and a selective PKG inhibitor abrogated these effects. In contrast, neither a selective protein kinase A (PKA) activator nor inhibitor had any effect on K(ATP) channels at concentrations up to 100 and 10 microm, respectively. Exogenous PKG, in the presence of both cGMP and ATP, increased channel activity, while the catalytic subunit of PKA had no effect. PKG activity was prevented by heat inactivation, replacing ATP with adenosine 5'-O-(thiotriphosphate) (a nonhydrolyzable analog of ATP), removing Mg(2+) from the internal solution, applying a PKG inhibitor, or by adding exogenous protein phosphatase 2A. The effects of cGMP analogs and PKG were observed under conditions in which PKA was repressed by a selective PKA inhibitor. The results suggest that K(ATP) channels are regulated by a PKG-signaling pathway that acts via PKG-dependent phosphorylation. This mechanism may, at least in part, contribute to a signaling pathway that induces ischemic preconditioning in rabbit ventricular myocytes.     

H(2)O(2) opens mitochondrial K(ATP) channels and inhibits GABA receptors via protein kinase C-epsilon in cardiomyocytes

Oxygen radicals and protein kinase C (PKC) mediate ischemic preconditioning. Using a cultured chick embryonic cardiomyocyte model of hypoxia and reoxygenation, we found that the oxygen radicals generated by ischemic preconditioning were H(2)O(2). Like preconditioning, H(2)O(2) selectively activated the epsilon-isoform of PKC in the particulate compartment and increased cell viability after 1 h of hypoxia and 3 h of reoxygenation. The glutathione peroxidase ebselen (converting H(2)O(2) to H(2)O) and the superoxide dismutase inhibitor diethyldithiocarbamic acid abolished the increased H(2)O(2) and the protection of preconditioning. PKC activation with phorbol 12-myristate 13-acetate increased cell survival; the protection of preconditioning was blocked by epsilonV(1-2), a selective PKC-epsilon antagonist. Similar to preconditioning, the protection of PKC activation was abolished by mitochondrial K(ATP) channel blockade with 5-hydroxydecanoate or by GABA receptor stimulation with midazolam or diazepam. In addition, PKC, mitochondrial ATP-sensitive K(+) (K(ATP)) channels, and GABA receptors had no effects on H(2)O(2) generated by ischemic preconditioning before prolonged hypoxia and reoxygenation. We conclude that H(2)O(2) opens mitochondrial K(ATP) channels and inhibits GABA receptors via activating PKC-epsilon. Through this signal transduction, preconditioning protects ischemic cardiomyocytes.     

Preconditioning blocks cardiocyte apoptosis: role of K(ATP) channels and PKC-epsilon

The aims of this study were to determine whether preconditioning blocks cardiocyte apoptosis and to determine the role of mitochondrial ATP-sensitive K(+) (K(ATP)) channels and the protein kinase C epsilon-isoform (PKC-epsilon) in this effect. Ventricular myocytes from 10-day-old chick embryos were used. In the control series, 10 h of simulated ischemia followed by 12 h of reoxygenation resulted in 42 +/- 3% apoptosis (n = 8). These results were consistent with DNA laddering and TdT-mediated dUTP nick-end labeling (TUNEL) assay. Preconditioning, elicited with three cycles of 1 min of ischemia separated by 5 min of reoxygenation before subjection to prolonged simulated ischemia, markedly attenuated the apoptotic process (28 +/- 4%, n = 8). The selective mitochondrial K(ATP) channel opener diazoxide (400 micromol/l), given before ischemia, mimicked preconditioning effects to prevent apoptosis (22 +/- 4%, n = 6). Pretreatment with 5-hydroxydecanoate (100 micromol/l), a selective mitochondrial K(ATP) channel blocker, abolished preconditioning (42 +/- 2%, n = 6). In addition, the effects of preconditioning and diazoxide were blocked with the specific PKC inhibitors G?-6976 (0.1 micromol/l) or chelerythrine (4 micromol/l), given at simulated ischemia and reoxygenation. Furthermore, preconditioning and diazoxide selectively activated PKC-epsilon in the particulate fraction before simulated ischemia without effect on the total fraction, cytosolic fraction, and PKC delta-isoform. The specific PKC activator phorbol 12-myristate 13-acetate (0.2 micromol/l), added during simulated ischemia and reoxygenation, mimicked preconditioning to block apoptosis. Opening mitochondrial K(ATP) channels blocks cardiocyte apoptosis via activating PKC-epsilon in cultured ventricular myocytes. Through this signal transduction, preconditioning blocks apoptosis and preserves cardiac function in ischemia-reperfusion.     

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.     

Noninvasive delayed limb ischemic preconditioning attenuates myocardial ischemia-reperfusion injury in rats by a mitochondrial K(ATP) channel-dependent mechanism

We previously demonstrated in rats that noninvasive delayed limb ischemic preconditioning (LIPC) induced by three cycles of 5-min occlusion and 5-min reperfusion of the left hind limb per day for three days confers the same cardioprotective effect as local ischemic preconditioning of the heart, but the mechanism has not been studied in depth. The aim of this project was to test the hypothesis that delayed LIPC enhances myocardial antioxidative ability during ischemia-reperfusion by a mitochondrial K(ATP) channel (mito K(ATP))-dependent mechanism. Rats were randomized to five groups: ischemia-reperfusion (IR)-control group, myocardial ischemic preconditioning (MIPC) group, LIPC group, IR-5HD group and LIPC-5HD group. The MIPC group underwent local ischemic preconditioning induced by three cycles of 5-min occlusion and 5-min reperfusion of the left anterior descending coronary arteries. The LIPC and LIPC-5HD groups underwent LIPC induced by three cycles of 5-min occlusion and 5-min reperfusion of the left hind limb using a modified blood pressure aerocyst per day for three days. All rats were subjected to myocardial ischemia-reperfusion injury. The IR-5HD and LIPC-5HD groups received the mito K(ATP) channel blocker 5-hydroxydecanoate Na (5-HD) before and during the myocardial ischemia-reperfusion injury. Compared with the IR-control group, both the LIPC and MIPC groups showed an amelioration of ventricular arrhythmia, reduced myocardial infarct size, increased activities of total superoxide dismutase, manganese-superoxide dismutase (Mn-SOD) and glutathione peroxidase, increased expression of Mn-SOD mRNA and decreased xanthine oxidase activity and malondialdehyde concentration. These beneficial effects of LIPC were prevented by 5-HD. In conclusion, delayed LIPC offers similar cardioprotection as local IPC. These results support the hypothesis that the activation of mito K(ATP) channels enhances myocardial antioxidative ability during ischemia-reperfusion, thereby contributing, at least in part, to the anti-arrhythmic and anti-infarct effects of delayed LIPC.     

Opening of mitochondrial ATP-sensitive potassium channels evokes oxygen radical generation in rabbit heart slices

The purpose of this study was to determine whether ATP-sensitive potassium channel (K(ATP) channel) activation generates oxygen free radicals in the rabbit heart. We assayed malondialdehyde (MDA) in rabbit heart slices in vitro as an indicator of oxygen free radical generation. The K(ATP) channel openers, pinacidil and cromakalim, significantly increased MDA production in a concentration-dependent manner. MDA formation also increased linearly with incubation time in the presence of K(ATP) channel openers. The K(ATP) channel blockers, glibenclamide and 5-hydroxydecanoate (5-HD), decreased K(ATP) channel opener-induced MDA formation in a concentration-dependent manner. When Fe(2+) was administered to heart slices that had been pretreated with K(ATP) channel openers, a marked elevation in MDA was observed, compared to heart slices that were treated with Fe(2+) alone. A positive linear correlation between Fe(2+) and MDA level was observed. The MDA levels of heart slices subjected to anoxia for 15 min remained unchanged until reperfusion. When the heart slices were reoxygenated for 30 min, a marked increase in MDA formation was observed. However, in the presence of glibenclamide and 5-HD, reperfusion following anoxia did not result in increased MDA. These results suggest that the opening of mitochondrial K(ATP) channels in rabbit heart slices evokes oxygen free radical generation via a Fenton-type reaction.     

AMP-activated protein kinase mediates preconditioning in cardiomyocytes by regulating activity and trafficking of sarcolemmal ATP-sensitive K(+) channels

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that is complex and yet to be fully defined. AMP-activated kinase (AMPK) is activated in cells under conditions associated with ATP depletion and increased AMP/ATP ratio. In the present study, we have taken advantage of a cardiac phenotype overexpressing a dominant negative form of the alpha2 subunit of AMPK to analyze the role, if any, that AMPK plays in preconditioning the heart. We have found that myocardial preconditioning activates AMPK in wild type, but not transgenic mice. Cardiac cells from transgenic mice could not be preconditioned, as opposed to cells from the wild type. The cytoprotective effect of AMPK was not related to the effect that preconditioning has on mitochondrial membrane potential as revealed by JC-1, a mitochondrial membrane potential-sensitive dye, and laser confocal microscopy. In contrast, experiments with di-8-ANEPPS, a sarcolemmal-potential sensitive dye, has demonstrated that intact AMPK activity is required for preconditioning-induced shortening of the action membrane potential. The preconditioning-induced activation of sarcolemmal K(ATP) channels was observed in wild type, but not in transgenic mice. HMR 1098, a selective inhibitor of sarcolemmal K(ATP) channels opening, inhibited preconditioning-induced shortening of action membrane potential as well as cardioprotection afforded by AMPK. Immunoprecipitation followed by Western blotting has shown that AMPK is essential for preconditioning-induced recruitment of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that AMPK mediates preconditioning in cardiac cells by regulating the activity and recruitment of sarcolemmal K(ATP) channels without being a part of signaling pathway that regulates mitochondrial membrane potential.     

mitoKATP通道参与心肌缺血预处理保护作用的机制

目的：探讨血管紧张素转换酶抑制剂（ACEI）和阈下缺血预处理联合预处理诱导的心肌保护作用中mi-toKatp通道激动后的作用机制：方法：采用离体大鼠心脏Langendorff灌流模型,观察心脏电脱耦联发生时间、细胞膜Na^＋／K^＋-ATPase和Ca^2＋/Mg^2＋-ATPase活性的改变：结果：单独使用卡托普利、或给予大鼠心脏2min缺血／10min复灌作为阈下缺血预处理,均不能改善长时间缺血／复灌引起的心脏收缩功能下降？而卡托普利和阂下缺血预处理联合使用可增高心脏收缩功能。mitoKatp通道特异性阻断剂5-HD可取消这一联合预处理的作用一联合预处理可引起缺血后电脱耦联发生时间延长,缺血心肌细胞膜Na^＋／K^＋-ATPase和Ca^2＋/Mg^2＋-ATPase活性增高;5-HD可取消此作用结论：mitoKatp通道参与了联合预处理延迟缺血引起的细胞间脱耦联和促进细胞膜离子通道稳定性维持的作用。     

K(ATP) channel activation reduces the severity of postresuscitation myocardial dysfunction

Postresuscitation myocardial dysfunction has been recognized as a leading cause of the high postresuscitation mortality rate. We investigated the effects of ischemic preconditioning and activation of ATP-sensitive K(+) (K(ATP)) channels on postresuscitation myocardial function. Ventricular fibrillation (VF) was induced in 25 Sprague-Dawley rats. Cardiopulmonary resuscitation (CPR), including mechanical ventilation and precordial compression, was initiated after 4 min of untreated VF. Defibrillation was attempted after 6 min of CPR. The animals were randomized to five groups treated with 1) ischemic preconditioning, 2) K(ATP) channel opener, 3) ischemic preconditioning with K(ATP) channel blocker administered 1 min after VF, 4) K(ATP) channel blocker administered 45 min before induction of ischemic preconditioning, and 5) placebo. Postresuscitation myocardial function, as measured by the rate of left ventricular pressure increase at 40 mmHg, the rate of left ventricular decline, cardiac index, and duration of survival, was significantly improved in both preconditioned and K(ATP) channel opener-treated animals. K(ATP) channel blocker administered 45 min before induction of ischemic preconditioning completely abolished the myocardial protective effects of preconditioning. We conclude that ischemic preconditioning significantly improved post-CPR myocardial function and survival. These results also provide evidence that the myocardial protective effects of ischemic preconditioning are mediated by K(ATP) channel activation.     

Contractility and ischemic response of hearts from transgenic mice with altered sarcolemmal K(ATP) channels

The functional significance of ATP-sensitive K(+) (K(ATP)) channels is controversial. In the present study, transgenic mice expressing a mutant Kir6.2, with reduced ATP sensitivity, were used to examine the role of sarcolemmal K(ATP) in normal cardiac function and after an ischemic or metabolic challenge. We found left ventricular developed pressure (LVDP) was 15-20% higher in hearts from transgenics in the absence of cardiac hypertrophy. beta-Adrenergic stimulation caused a positive inotropic response from nontransgenic hearts that was not observed in transgenic hearts. Decreasing extracellular Ca(2+) decreased LVDP in hearts from nontransgenics but not in those from transgenics. These data suggest an increase in intracellular [Ca(2+)] in transgenic hearts. Additional studies have demonstrated hearts from nontransgenics and transgenics have a similar postischemic LVDP. However, ischemic preconditioning does not improve postischemic recovery in transgenics. Transgenic hearts also demonstrate a poor recovery after metabolic inhibition. These data are consistent with the hypothesis that sarcolemmal K(ATP) channels are required for development of normal myocardial function, and perturbations of K(ATP) channels lead to hearts that respond poorly to ischemic or metabolic challenges.     

Brief antecedent anoxia preserves mitochondrial function after sustained undersupply: A subcellular correlate to ischemic preconditioning?

Background: There is increasing evidence that mitochondria – owning a high degree of autonomy within the cell – might represent the target organelles of the myocardial protection afforded by ischemic preconditioning. It was the aim of the study to investigate a possible subcellular correlate to ischemic preconditioning at the mitochondrial level. In addition, we tested whether this protection depends on mitochondrial ATP-dependent potassium channels (K _ATP) and an might involve an attenuation of mitochondrial ATP hydrolysis during sustained anoxia.Methods and Results: Sustained anoxia (A, 14 min) and reoxygenation (R) completely inhibited state 3 and state 4 respiration of isolated ventricular mitochondria from Wistar rats. An antecedent brief anoxic incubation (4 min) followed by reoxygenation (2 min) prevented this loss of mitochondrial function. The protection afforded by anoxic preconditioning could be mimicked by the K _ATP opener diazoxide (30 μmol/l) and was completely inhibited by the K _ATP blocker 5-hydroxydecanoic acid (300 μmol/l). Structural mitochondrial integrity, as estimated from externalization of the mitochondrial enzymes creatine kinase and glutamateoxalacetate transaminase, remained unchanged between the groups, as did mitochondrial ATP loss during anoxia.Conclusion: For the first time, we provide direct evidence for a subcellular preconditioning-like functional mitochondrial adaptation to sustained anoxia. This effect apparently depends on opening of K_ATP but is independent of ATP preservation.     

Role of mitochondrial and sarcolemmal K(ATP) channels in ischemic preconditioning of the canine heart

We tested whether mitochondrial or sarcolemmal ATP-sensitive K(+) (K(ATP)) channels play a key role in ischemic preconditioning (IP) in canine hearts. In open-chest beagle dogs, the left anterior descending artery was occluded four times for 5 min each with 5-min intervals of reperfusion (IP), occluded for 90 min, and reperfused for 6 h. IP as well as cromakalim and nicorandil (nonspecific K(ATP) channel openers) markedly limited infarct size (6.3 +/- 1.2, 8.9 +/- 1.9, and 7.2 +/- 1.6%, respectively) compared with the control group (40.9 +/- 4.1%). A selective mitochondrial K(ATP) channel blocker, 5-hydroxydecanoate, partially blunted the limitation of infarct size in the animals subjected to IP and those treated with cromakalim and nicorandil (21.6 +/- 3.8, 25.1 +/- 4.6, and 19.8 +/- 5.2%, respectively). A nonspecific K(ATP) channel blocker, glibenclamide, completely abolished the effect of IP (38.5 +/- 6.2%). Intracoronary or intravenous administration of a mitochondria-selective K(ATP) channel opener, diazoxide, at >100 micromol/l could only partially decrease infarct size (19.5 +/- 4.3 and 20.1 +/- 4.4%, respectively). In conclusion, mitochondrial and sarcolemmal K(ATP) channels independently play an important role in the limitation of infarct size by IP in the canine heart.     

Preconditioning by hypoventilation increases ventricular arrhythmia threshold in Wistar rats

Hypoventilation, as one of ventilatory disorders, decreases the electrical stability of the heart similarly as ischemia. If preconditioning by short cycles of ischemia has a cardioprotective effect against harmful influences of a prolonged ischemic period, then preconditioning by hypoventilation (HPC) can also have a similar effect. Anesthetized rats (ketamine 100 mg/kg + xylasine 15 mg/kg i.m., open chest experiments) were subjected to 20 min of hypoventilation followed by 20 min of reoxygenation (control group). The preconditioning (PC) was induced by one (1PC), two (2PC) or three (3PC) cycles of 5-min hypoventilation followed by 5-min reoxygenation. The electrical stability of the heart was measured by a ventricular arrhythmia threshold (VAT) tested by electrical stimulation of the right ventricle. Twenty-minute hypoventilation significantly decreased the VAT in the control and 1PC groups (p<0.05) and non-significantly in 2PC vs. the initial values. Reoxygenation reversed the VAT values to the initial level only in the control group. In 3PC, the VAT was increased from 2.32+/-0.69 mA to 4.25+/-1.31 mA. during hypoventilation (p<0.001) and to 4.37+/-1.99 mA during reoxygenation (p<0.001). It is concluded that cardioprotection against the hypoventilation/ reoxygenation-induced decrease of VAT proved to be effective only after three cycles of HPC.     

Cardioprotection by multiple preconditioning cycles does not require mitochondrial K(ATP) channels in pigs

To test whether cardioprotection induced by ischemic preconditioning depends on the opening of mitochondrial ATP-sensitive K(+) (K(ATP)) channels, the effect of channel blockade was studied in barbital-anesthetized open-chest pigs subjected to 30 min of complete occlusion of the left anterior descending coronary artery and 3 h of reflow. Preconditioning was elicited by two cycles of 5-min occlusion plus 10-min reperfusion before the 30-min occlusion period. 5-Hydroxydecanoate (5 mg/kg iv) was injected 15 min before preconditioning or pharmacological preconditioning induced by diazoxide (3.5 mg/kg, 1 ml/min iv). Infarct size (percentage of the area at risk) after 30 min of ischemia was 35.1 +/- 9.9% (n = 7). Preconditioning markedly limited myocardial infarct size (2.7 +/- 1.6%, n = 7), and 5-hydroxydecanoate did not abolish protection (2.4 +/- 0.9%, n = 8). Diazoxide infusion also significantly limited infarct size (14.6 +/- 7.4%, n = 7), and 5-hydroxydecanoate blocked this effect (30.8 +/- 8.0%, n = 7). Thus the opening of mitochondrial K(ATP) channels is cardioprotective in pigs, but these data do not support the hypothesis that opening of mitochondrial K(ATP) channels is required for the endogenous protection afforded by preconditioning.     

Role of protein phosphatases in hypoxic preconditioning

To find a protein kinase C (PKC)-independent preconditioning mechanism, hypoxic preconditioning (HP; i.e., 10-min anoxia and 10-min reoxygenation) was applied to isolated rat hearts before 60-min global ischemia. HP led to improved recovery of developed pressure and reduced end-diastolic pressure in the left ventricle during reperfusion. Protection was unaffected by the PKC inhibitor bisindolylmaleimide (BIM; 1 micromol/l). It was abolished by the inhibitor of protein phosphatases 1 and 2A cantharidin (20 or 5 micromol/l) and partially enhanced by the inhibitor of protein phosphatase 2A okadaic acid (5 nmol/l). In adult rat cardiomyocytes treated with BIM and exposed to 60-min simulated ischemia (anoxia, extracellular pH 6.4), HP led to attenuation of anoxic Na(+)/Ca(2+) overload and of hypercontracture, which developed on reoxygenation. This protection was prevented by treatment with cantharidin but not with okadaic acid. In conclusion, HP exerts PKC-independent protection on ischemic-reperfused rat hearts and cardiomyocytes. Protein phosphatase 1 seems a mediator of this protective mechanism.     

Salvia miltiorrhiza attenuates the changes in contraction and intracellular calcium induced by anoxia and reoxygenation in rat cardiomyocytes

The aim of the present study is to investigate the effect of Salvia miltiorrhiza (SM) on contraction and the intracellular calcium of isolated ventricular myocytes during normoxia or anoxia and reoxygenation using a video tracking system and spectrofluorometry. Cardiac ventricular myocytes were isolated enzymatically by collagenase and exposed to 5 min of anoxia followed by 10 min of reoxygenation. SM (1-9 g/L) depressed both contraction and the [Ca(2+)](i) transient in a dose-dependent manner. SM did not affect the diastolic calcium level and the sarcolemmal Ca(2+) channel of myocytes but decreased the caffeine-induced calcium release. During anoxia, the +/-dL/dtmax, amplitudes of contraction (dL) of cell contraction and [Ca(2+)](i) transients were decreased, while the diastolic calcium level was increased. None of the parameters returned to the pre-anoxia level during reoxygenaton. However, SM (3 g/L) did attenuate the changes in cell contraction and intracellular calcium induced by anoxia and reoxygenation. It is concluded that SM has different effects on normoxic and anoxic cardiomyocytes. The SM-induced reduction of changes in contraction and intracellular calcium induced by anoxia/reoxygenation indicates that SM may be beneficial for cardiac tissue in recovery of mechanical function and intracellular calcium homeostasis.     

Delayed cardioprotection by isoflurane: role of K(ATP) channels

Isoflurane mimics the cardioprotective effect of acute ischemic preconditioning with an acute memory phase. We determined whether isoflurane can induce delayed cardioprotection, the involvement of ATP-sensitive potassium (K(ATP)) channels, and cellular location of the channels. Neonatal New Zealand White rabbits at 7-10 days of age (n = 5-16/group) were exposed to 1% isoflurane-100% oxygen for 2 h. Hearts exposed 2 h to 100% oxygen served as untreated controls. Twenty-four hours later resistance to myocardial ischemia was determined using an isolated perfused heart model. Isoflurane significantly reduced infarct size/area at risk (means +/- SD) by 50% (10 +/- 5%) versus untreated controls (20 +/- 6%). Isoflurane increased recovery of preischemic left ventricular developed pressure by 28% (69 +/- 4%) versus untreated controls (54 +/- 6%). The mitochondrial K(ATP) channel blocker 5-hydroxydecanoate (5-HD) completely (55 +/- 3%) and the sarcolemmal K(ATP) channel blocker HMR 1098 partially (62 +/- 3%) attenuated the cardioprotective effects of isoflurane. The combination of 5-HD and HMR-1098 completely abolished the cardioprotective effect of isoflurane (56 +/- 5%). We conclude that both mitochondrial and sarcolemmal K(ATP) channels contribute to isoflurane-induced delayed cardioprotection.     

Kappa 阿片受体的抗缺血性心脏保护作用--信息机制

有证据表明,心脏细胞产生强腓肽和强腓肽类多肽,它们是kappa阿片受体(κ-0R)的激动剂。κ-0R是心脏一种优势的阿片受体,其激活可改变在体和离体心脏的功能。在正常和病理情况下,内源性κ-阿片肽可能通过自分泌或旁分泌的方式调节心脏功能。心肌缺血是导致心脏功能紊乱的一个常见原因,主要表现为心肌功能减弱,心律失常及心肌梗塞等。心肌缺血时,交感神经发放增强,从而增加作功负荷及氧消耗量;而这又使缺血引发的状况更为恶化。机体抵抗缺血引发心肌损害／心律失常的保护机制之一是抑制β-肾上腺素受体(β—AR)的兴奋。κ-0R确实能抑制β-AR的激动。这种抑制主要是由于GS蛋白受到抑制,也在较小程度上由于信息通路的腺苷酸环化酶的抑制。因为该种酶能通过对百日咳毒素敏感的G蛋白转导β—AR的激动。另一保护心肌对抗缺血性损害的机制是预处理。预处理是指预先受到缺血等损伤使心脏对随后更严重的损伤产生较强的耐受能力。这种保护作用可以在预处理后即时产生,也可延至预处理后1—3天。在采用缺血或其产生的后果之一——代谢抑制作为预处理而致的心脏保护中,κ-OR参与媒介预处理的作用。用κ—OR的特异性激动剂U50488H激活κ—OR(U50488H药理性预处理,UP)可激活蛋白激酶C(PKC),开放ATY敏感的钾通道(KATP channels)及增加热休克蛋白(HSP)的产生。阻断PKC的作用,关闭KATP通道或抑制HSP的合成,均可消除UP的心脏保护作用。这些发现表明,PKC、KATP通道和HSP在UP的心脏保护中均具重要作用。此外,UP也能减低缺血造成心肌损害的因素之一,即Ca^2 的超负荷。这个事实表明UP发挥心脏保护作用至少部分地是通过减低Ca^2 的超负荷。最有趣的是,以阻断剂阻塞KATP通道,在消除UP的延迟性心脏保护作用的同时也降低了UP对Ca^2 超负荷的抑制作用。这个事实揭示了KATP通道开放所致的心脏保护作用至少部分地可能是由于防止或减低了Ca^2 的超负荷。