Effects of KATP channel modulation on myocardial glycogen content, lactate, and amino acids in nonischemic and ischemic rat hearts

Cardioprotective effects of a new glutamic acid derivative in chronic alcohol intoxication

Alcohol abuse is a risk factor for heart damage and deterioration of its inotropic function. Currently, there is no pathogenetic pharmacological treatment for alcohol-induced myocardial injury. Therefore, the study of drugs with cardioprotective action is of current interest. Our earlier studies of stress-induced heart damage showed that a new derivative of glutamic acid - glufimet - protects the myocardium's inotropic function and limits lipid peroxidation. Additionally, we found that it increases the activity of antioxidant enzymes and improves mitochondrial respiration. The purpose of our study was to assess the effect of glufimet on the heart after chronic alcohol intoxication (CAI). The comparison drug was mildronate, which possesses cardioprotective properties and is used to treat alcohol withdrawal. We conducted our study using female Wistar rats (10 months old, 280-320 g). CAI was simulated by replacing drinking water with a 10% ethanol solution sweetened with sucrose (50 g/L) over a period of 24 weeks. The day after the animals stopped ethanol solution drinking, the control group was injected intraperitoneally (i.p.) with a saline solution once a day for 14 days, while the experimental groups received glufimet (28.7 mg/kg) and the drug of comparison mildronate (50 mg/kg), respectively. After that, we studied the heart contractility by measuring volume load, adrenergic reactivity, and maximum isometric load. Under CAI, the control group showed significantly lower growth in left ventricular pressure (LVP), myocardium contraction rate, and relaxation rate during functional tests. Higher concentrations of LPO products (malondialdehyde) and low activity of antioxidant enzymes (superoxide dismutase, glutathione peroxidase), indicating a disturbance in mitochondrial respiration compared to the control group, were registered. While being treated with glufimet and mildronate, the animals demonstrated higher growth rates of myocardial contraction, myocardial relaxation, and LVP, compared to the control group. Mitochondrial functioning and activity of the antioxidant enzymes increased in the same group as well.

Impact of hyperglycemia on myocardial ischemia-reperfusion susceptibility and ischemic preconditioning in hearts from rats with type 2 diabetes

Background

The mechanisms underlying increased mortality in patients with diabetes and admission hyperglycemia after an acute coronary syndrome may involve reduced capacity for cardioprotection. We investigated the impact of hyperglycemia on exogenously activated cardioprotection by ischemic preconditioning (IPC) in hearts from rats with type 2 diabetes mellitus (T2DM) that were endogenously cardioprotected by an inherent mechanism, and the involvement of myocardial glucose uptake (MGU) and myocardial O-linked β-N-acetylglucosamine (O-GlcNAc).

Methods and results

In isolated, perfused rat hearts subjected to ischemia–reperfusion, infarct size (IS) was overall larger during hyper- ([Glucose] = 22 mmol/L]) than normoglycemia ([Glucose] = 11 mmol/L]) (p < 0.001). IS was smaller in 12-week old Zucker diabetic fatty rats with recent onset T2DM (fa/fa) than in rats without T2DM (fa/+) (n = 8 in each group) both during hyperglycemia (p < 0.05) and normoglycemia (p < 0.05). IPC (2 × 5 min cycles) reduced IS during normo- (p < 0.01 for both groups) but not during hyperglycemia independently of the presence of T2DM. During hyperglycemia, an intensified IPC stimulus (4 × 5 min cycles) reduced IS only in hearts from animals with T2DM (p < 0.05). IPC increased MGU and O-GlcNAc levels during reperfusion in animals with and without T2DM at normoglycemia (MGU: p < 0.05, O-GlcNAc: p < 0.01 for both groups) but not during hyperglycemia. Intensified IPC at hyperglycemia increased MGU (p < 0.05) and O-GlcNAc levels (p < 0.05) only in hearts from animals with T2DM.

Conclusion

While the effect of IPC is reduced during hyperglycemia in rats without T2DM, endogenous cardioprotection in animals with T2DM is not influenced by hyperglycemia and the capacity for exogenous cardioprotection by IPC is preserved. MGU and O-GlcNAc levels are increased by exogenously induced cardioprotection by IPC but not by endogenous cardioprotection in animals with T2DM reflecting different underlying mechanisms by exogenous and endogenous cardioprotection.

The dual face of glutamate: from a neurotoxin to a potential survival factor-metabolic implications in health and disease

Glutamate is the major excitatory neurotransmitter in the central nervous system. Beyond this function, glutamate also plays a key role in intermediary metabolism in all organs and tissues, linking carbohydrate and amino acid metabolism via the tricarboxylic acid cycle. Under both physiological and pathological conditions, we have recently found that the ability of glutamate to fuel cell metabolism selectively relies on the activity of two main transporters: the sodium-calcium exchanger (NCX) and the sodium-dependent excitatory amino-acid transporters (EAATs). In ischemic settings, when glutamate is administered at the onset of the reoxygenation phase, the coordinate activity of EAAT and NCX allows glutamate to improve cell viability by stimulating ATP production. So far, this phenomenon has been observed in both cardiac and neuronal models. In this review, we focus on the most recent findings exploring the unusual activity of glutamate as a potential survival factor in different settings.

Essential role of the Na+-Ca2+ exchanger (NCX) in glutamate-enhanced cell survival in cardiac cells exposed to hypoxia/reoxygenation

Myocardial ischemia culminates in ATP production impairment, ionic derangement and cell death. The provision of metabolic substrates during reperfusion significantly increases heart tolerance to ischemia by improving mitochondrial performance. Under normoxia, glutamate contributes to myocardial energy balance as substrate for anaplerotic reactions, and we demonstrated that the Na⁺/Ca²⁺ exchanger1 (NCX1) provides functional support for both glutamate uptake and use for ATP synthesis. Here we investigated the role of NCX1 in the potential of glutamate to improve energy metabolism and survival of cardiac cells subjected to hypoxia/reoxygenation (H/R). Specifically, in H9c2-NCX1 myoblasts, ATP levels, mitochondrial activities and cell survival were significantly compromised after H/R challenge. Glutamate supplementation at the onset of the reoxygenation phase significantly promoted viability, improved mitochondrial functions and normalized the H/R-induced increase of NCX1 reverse-mode activity. The benefits of glutamate were strikingly lost in H9c2-WT (lacking NCX1 expression), or in H9c2-NCX1 and rat cardiomyocytes treated with either NCX or Excitatory Amino Acid Transporters (EAATs) blockers, suggesting that a functional interplay between these transporters is critically required for glutamate-induced protection. Collectively, these results revealed for the first time the key role of NCX1 for the beneficial effects of glutamate against H/R-induced cell injury.

Comparative proteomic analysis of the ATP-sensitive K+ channel complex in different tissue types

ATP-sensitive K(+) (K(ATP)) channels are expressed ubiquitously, but have diverse roles in various organs and cells. Their diversity can partly be explained by distinct tissue-specific compositions of four copies of the pore-forming inward rectifier potassium channel subunits (Kir6.1 and/or Kir6.2) and four regulatory sulfonylurea receptor subunits (SUR1 and/or SUR2). Channel function and/or subcellular localization also can be modified by the proteins with which they transiently or permanently interact to generate even more diversity. We performed a quantitative proteomic analysis of K(ATP) channel complexes in the heart, endothelium, insulin-secreting min6 cells (pancreatic β-cell like), and the hypothalamus to identify proteins with which they interact in different tissues. Glycolysis is an overrepresented pathway in identified proteins of the heart, min6 cells, and the endothelium. Proteins with other energy metabolic functions were identified in the hypothalamic samples. These data suggest that the metabolo-electrical coupling conferred by K(ATP) channels is conferred partly by proteins with which they interact. A large number of identified cytoskeletal and trafficking proteins suggests endocytic recycling may help control K(ATP) channel surface density and/or subcellular localization. Overall, our data demonstrate that K(ATP) channels in different tissues may assemble with proteins having common functions, but that tissue-specific complex organization also occurs.

Is preconditioning by oxytocin administration mediated by iNOS and/or mitochondrial K(ATP) channel activation in the in vivo anesthetized rabbit heart?

AIMS

Oxytocin (OXT) pretreatment protects the heart during ischemia-reperfusion injury by activating ATP-dependent potassium (K(ATP)) channels. The aim of the current study was to elucidate the roles of nitric oxide synthaseNOS and myocardial biochemistry in the cardioprotective effects of OXT and ischemic preconditioning (IPC).

MAIN METHODS

Male New Zealand White anesthetized rabbits (13 groups) were subjected to 30 min of occlusion of the left coronary artery and 120 min of reperfusion with or without IPC.

KEY FINDINGS

IPC (1 cycle), OXT (0.03 μg/kg, i.p.) or IPC + OXT yield significant infarct size reductions (21.8±1.5%, 20.5±1.2% and 19.4±1.4%, respectively, versus 38.9±3.5% in the S-CONT group; P<0.01) and antiarrhythmic effects, including VF (0%, 0% and 0%, versus 50% in S-CONT group; P<0.05) sustained VT (13%, 13% and 13%, versus 100% in S-CONT group; P<0.005) and other arrhythmias (25%, 13% and 25%, versus 100% in S-CONT group; P<0.005, P<0.01 and P<0.005, respectively). Atosiban (ATO, a selective OXT receptor antagonist), 5-HD and L-NAME (a nonspecific NOS inhibitor) abolished the beneficial effects of IPC and OXT, suggesting that the benefits are achieved via selective activation of OXT receptors, mitochondrial K(ATP) channels and NO. An iNOS inhibitor (1400 W) blocked the beneficial effects of IPC but not OXT. The IPC, OXT, IPC + OXT and 1400 W + OXT interventions significantly preserved ATP levels in the heart.

SIGNIFICANCE

This study demonstrates similarities between acute OXT pretreatment and IPC in terms of infarct size reduction, antiarrhythmic activity, and metabolic status.

Metformin improves cardiac function in a nondiabetic rat model of post-MI heart failure

Metformin is the first choice drug for the treatment of patients with diabetes, but its use is debated in patients with advanced cardiorenal disease. Epidemiological data suggest that metformin may reduce cardiac events, in patients both with and without heart failure. Experimental evidence suggests that metformin reduces cardiac ischemia-reperfusion injury. It is unknown whether metformin improves cardiac function (remodeling) in a long-term post-MI remodeling model. We therefore studied male, nondiabetic, Sprague-Dawley rats that were subjected to either myocardial infarction (MI) or sham operation. Animals were randomly allocated to treatment with normal water or metformin-containing water (250 mg·kg(-1)·day(-1)). At baseline, 6 wk, and 12 wk, metabolic parameters were analyzed and oral glucose tolerance tests (OGTT) were performed. Echocardiography and hemodynamic parameters were assessed 12 wk after MI. In the MI model, infarct size was significantly smaller after 12-wk metformin treatment (29.6 ± 3.2 vs. 38.0 ± 2.2%, P < 0.05). Moreover, metformin resulted in less left ventricular dilatation (6.0 ± 0.4 vs. 7.6 ± 0.6 mm, P < 0.05) and preservation of left ventricular ejection fraction (65.8 ± 3.7% vs. 48.6 ± 5.6%, P < 0.05) compared with MI control. The improved cardiac function was associated with decreased atrial natriuretic peptide mRNA levels in the metformin-treated group (50% reduction compared with MI, P < 0.05). Insulin resistance did not occur during cardiac remodeling (as indicated by normal OGTT) and fasting glucose levels and the pattern of the OGTT were not affected by metformin. Molecular analyses suggested that altered AMP kinase phosphorylation status and low insulin levels mediate the salutary effects of metformin. Altogether our results indicate that metformin may have potential to attenuate heart failure development after myocardial infarction, in the absence of diabetes and independent of systemic glucose levels.

Comparison of two sulfonylureas with high and low myocardial K(ATP) channel affinity on myocardial infarct size and metabolism in a rat model of type 2 diabetes

AIMS/HYPOTHESIS

Sulfonylureas (SUs) may impair outcome in patients with acute coronary syndrome. Most experimental studies of the myocardial effects of SU treatment are performed in non-diabetic models. We compared the effect of two widely used SUs, glibenclamide (gb) and gliclazide (gc), with high and low myocardial K(ATP) channel affinity, respectively, at therapeutic concentrations on infarct size, left ventricular (LV) function and myocardial glycogen, lactate and alanine content before and after ischaemia/reperfusion (I/R).

METHODS

Non-diabetic Wistar and diabetic Goto-Kakizaki rat hearts were investigated in a Langendorff preparation. Gb (0.1 μmol/l) and gc (1.0 μmol/l) were administrated throughout the study. Infarct size was evaluated after 120 min of reperfusion. Myocardial metabolite content was measured before and after ischaemia.

RESULTS

Infarct size was smaller in diabetic hearts than in non-diabetic hearts (0.33 ± 0.03 vs 0.51 ± 0.05, p < 0.05). Gb increased infarct size (0.54 ± 0.04 vs 0.33 ± 0.03, p < 0.05) and reduced post-ischaemic LV developed pressure (60 ± 3 vs 76 ± 3 mmHg, p < 0.05) and coronary flow (4.9 ± 0.5 vs 7.1 ± 0.4 ml min(-1) g(-1), p < 0.05) in gb-treated diabetic rats compared with untreated diabetic rats. On comparing gb-treated diabetic rats with untreated diabetic rats, glycogen content was reduced before (9.1 ± 0.6 vs 13.6 ± 1.0 nmol/mg wet weight, p < 0.01) and after ischaemia (0.9 ± 0.2 vs 1.8 ± 0.2 nmol/mg wet weight, p < 0.05), and lactate (4.8 ± 0.4 vs 3.2 ± 0.3 nmol/mg wet weight, p < 0.01) and alanine (1.38 ± 0.12 vs 0.96 ± 0.09 nmol/mg wet weight, p < 0.05) contents were increased during reperfusion. Gc-treatment of diabetic and non-diabetic rats did not affect any of the measured variables.

CONCLUSIONS/INTERPRETATIONS

Gb, but not gc, exacerbates I/R injury and deteriorates LV function in diabetic hearts. These effects of gb on diabetic hearts may be due to detrimental effects on myocardial carbohydrate metabolism.

Concept paper: antihyperglycemic therapy and the diabetic heart--do we really know enough?

The purpose of this article is to provide reasons to start looking more critically at the existing glucose-lowering therapies in diabetes, from the point of their effect on cardiac metabolism. The presented arguments begin with the description of major differences between metabolism in myocardium and the skeletal muscle and of examples of myocardial metabolic inflexibility observed in heart failure and Type 2 diabetes. It is proposed that the metabolic inflexibility of diabetic myocardium should be taken into consideration as a factor to explain causes of unexpected cardiovascular mortality observed in the recently published outcome studies such as Action to Control Cardiovascular Risk in Diabetes (ACCORD) and Veterans Affairs Diabetes Feasibility Trial. The same reasoning was applied to challenge the "legacy effect" of the UK Prospective Diabetes Study and Steno-2 trials. A striking paucity of data on the effects of antihyperglycemic therapies on cardiac metabolism is brought to attention in spite of the fact that the technology to study human cardiac metabolism in vivo is available. It is hoped that increased focus on research in this area could contribute to improved cardiovascular safety monitoring of various antihyperglycemic regimens and thereby enhance our ability to save more lives of patients with Type 2 diabetes.

Retention of Nanoparticles-Labeled Bone Marrow Mononuclear Cells in the Isolated Ex Vivo Perfused Heart After Myocardial Infarction in Animal Model

Cell therapy of myocardial infarction (MI) is under clinical investigation, yet little is known about its underlying mechanism of function. Our aims were to induce a sub-lethal myocardial infarction in a rabbit, to evaluate the abilities of labeled bone marrow mononuclear cells to migrate from the vessel bed into extracellular space of the myocardium, and to evaluate the short-term distribution of cells in the damaged left ventricle. Sub-lethal myocardial infarction was induced in rabbits by ligation of the left coronary vessel branch ( in vivo). The Langendorff heart perfusion model ( ex vivo) was used in the next phase. The hearts subjected to MI induction were divided into 3 groups (P1–P3), and hearts without MI formed a control group (C). Nanoparticles-labeled bone marrow mononuclear cells were injected into coronary arteries via the aorta. Perfusion after application lasted 2 minutes in the P1 group, 10 minutes in the P2 and C groups, and 25 minutes in the P3 group. The myocardium of the left ventricle was examined histologically, and the numbers of labeled cells in vessels, myocardium, and combined were determined. The numbers of detected cells in the P1 and C groups were significantly lower than in the P2 and P3 groups. In the P2 and P3 groups, the numbers of cells found distally from the ligation were significantly higher than proximally from the ligation site. Bone marrow mononuclear cells labeled with iron oxide nanoparticles proved the ability to migrate in the myocardium interstitium with significantly higher affinity for the tissue damaged by infarction.

Diazoxide increases liver and kidney HSP25 and HSP70 after shock and stroke

BACKGROUND

The compound, diazoxide (DZ), is known to induce preconditioning through its effect as a mitochondrial K(ATP) channel opener and succinate dehydrogenase inhibitor. Our team tested the hypothesis that pharmacological induction of ischemic preconditioning with DZ can offer cytoprotection and preserve vital tissues after hemorrhagic shock and stroke.

MATERIALS AND METHODS

Sprague-Dawley male rats received an intraperitoneal injection of sterile saline or 5 mg/kg DZ in saline 24 h prior to 1 h of hemorrhagic shock, by approximately 40% total blood loss volume (Shock Study), or a permanent unilateral common carotid ligation just before shock (Stroke + Shock Study). While remaining under isoflurane anesthesia, animals then received 81 mL/kg intravenous sterile saline over the next 45 min for recovery and survived for another 24 h.

RESULTS

When DZ was administered 24 h prior to shock, it significantly reduced hyperglycemia, which in vehicle-treated animals persisted after resuscitation. DZ also attenuated hyperlactatemia during the 1-h shock period. With more severe trauma from combined stroke and shock, DZ also decreased hyperlactatemia and hyperglycemia levels but the reduction was only significant for hyperglycemia. The expression levels of heat shock proteins 25 (HSP25) and 70 (HSP70) were used as biomarkers for response of the kidney and liver to DZ and combined stroke and shock. Compared to vehicle-treated animals, DZ-treated rats subjected to shock and stroke exhibited increased HSP25 and HSP70 in kidney and liver tissue.

CONCLUSIONS

DZ-attenuated physiological indicators of metabolic stress following shock or combined shock and stroke and enhanced the up-regulation of cytoprotective heat shock protein expression.

Pretreatment with sarafotoxin 6c prior to coronary occlusion protects against infarction and arrhythmias via cardiomyocyte mitochondrial K(ATP) channel activation in the intact rabbit heart during ischemia/reperfusion

BACKGROUND

Endothelial ET(B) receptor activation by exogenously administered sarafotoxin 6c(a snake venom peptide with a sequence homology to ET-1 prior to ischemia activates release of nitric oxide(NO) and previous studies have shown that NO facilitates mitochondrial K(ATP) activation in cardiac cells and cardioprotection.

OBJECTIVES AND METHODS

The aim of this investigation was to test whether the administration of sarafotoxin 6c(a selective ET(B) receptor agonist) has cardioprotective and antiarrhythmic effects against ischemia and reperfusion injury in a well-standardized model of reperfusion arrhythmias in anesthetized adult male rabbits (n = 53) subjected to 30 min occlusion of the left coronary artery followed by 120 min of reperfusion.

RESULTS

Pretreatment with sarafotoxin 6c (0.24 nmol/kg, i.v.) prior to the period of coronary occlusion offers significant infarct size reduction (19.1 +/- 2.0% versus 39.7 +/- 3.7% in the saline control group; P < 0.01) and antiarrhythmic effects. Sarafotoxin 6c treatment significantly attenuated the incidence of life-threatening arrhythmias like sustained VT (13 versus 100% in the saline control group; P < 0.005) and other arrhythmias (25 versus 100% in the saline control group; P < 0.005), and increased the number of surviving animals without arrhythmias. Pretreatment with 5-HD but not HMR 1883 abolished the beneficial effects of sarafotoxin 6c on reperfusion induced arrhythmias and cardioprotection suggesting that benefits have been achieved via the selective activation of cardiomyocyte mitochondrial K(ATP) channels. Sarafotoxin 6c evoked NO release and selective activation of mitoK(ATP) channels in cardiomyocytes contributes to cardioprotection and antiarrhythmic activity during ischemia-reperfusion in the anesthetized rabbit.

CONCLUSIONS

We conclude that the selective activation of ET(B) receptors by sarafotoxin 6c prior to coronary occlusion contributes to cardioprotective and antiarrhythmic properties.

Pharmacological preconditioning by levosimendan is mediated by inducible nitric oxide synthase and mitochondrial KATP channel activation in the in vivo anesthetized rabbit heart model

BACKGROUND

Provocation of fatal cardiac arrhythmias has limited the use of inotropic agents as heart failure therapy. Levosimendan (LEV) is a new inodilator, whose mechanism of action includes calcium sensitization of contractile proteins and the opening of ATP-dependent potassium channels.

OBJECTIVES AND METHODS

The aim of this investigation was to test whether the administration of LEV has cardioprotective and antiarrhythmic effects against ischemia and reperfusion injury in a manner similar to ischemic preconditioning (IPC) in a well-standardized model of reperfusion arrhythmias in anesthetized adult male rabbits (n=122) subjected to 30 min occlusion of the left coronary artery followed by 120 min of reperfusion.

RESULTS

Pretreatment with either 1 cycle of IPC, LEV (0.1 micromol/kg, i.v.), or IPC+LEV prior to the period of coronary occlusion offers significant infarct size reduction (21.6+/-1.6%, 22.1+/-2.2%, and 21.4+/-1.4%, respectively vs 38.7+/-3.6% in saline control group; P<0.01) and antiarrhythmic effects. IPC, LEV and IPC+LEV treatment significantly attenuated the incidence of life-threatening arrhythmias like sustained VT (13%, 13% and 13%, respectively vs 100% in saline control group; P<0.005) and other arrhythmias (25%, 25% and 13%, respectively vs 100% in saline control group; P<0.005), and increased the number of surviving animals without arrhythmias. Pretreatment with 5-HD, N(omega)-nitro-L-arginine methyl ester (L-NAME, a nonspecific NOS inhibitor) and the specific iNOS inhibitor 1400 W [N-(-3-(aminomethyl)benzyl) acetamidine] abolished the beneficial effects of IPC, and LEV on reperfusion induced arrhythmias and cardioprotection suggesting that benefits have been achieved via both the selective activation of cardiomyocyte mitochondrial K(ATP) channels and NO. One cycle of IPC and LEV pretreatment significantly preserved the level of ATP in the 30 min ischemic heart and 120 min reperfused heart.

CONCLUSIONS

The present study demonstrates similarities between acute LEV treatment and IPC of the rabbit myocardium in terms of survival, cardioprotection, antiarrhythmic activity, and metabolic status.

Similarities between ischemic preconditioning and 17beta-estradiol mediated cardiomyocyte KATP channel activation leading to cardioprotective and antiarrhythmic effects during ischemia/reperfusion in the intact rabbit heart

The aims of our present work were to assess whether treatment with either ischemic preconditioning (IPC) or 17beta-estradiol or both combined produce proarrhythmic or antiarrhythmic effects, and whether opening of the sarcolemmal or mitochondrial KATP channels is relatable to this effect; to assess biochemically the effects of IPC and/or 17beta-estradiol on oxidant stress and antioxidant defenses in the myocardium; to examine the effects of nitric oxide (NO) synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME) pretreatment in rabbits treated with either IPC or 17beta-estradiol (because 17beta-estradiol evoked NO release has been implicated in KATP activation and IPC); and examine the effects of ischemic preconditioning and 17beta-estradiol on myocardial energy metabolism during ischemia and reperfusion in a well-standardized model of reperfusion arrhythmias in anesthetized adult male New Zealand White rabbits (n = 124) subjected to 30 minutes occlusion of the left coronary artery followed by 120 minutes of reperfusion. Pretreatment with either 17beta-estradiol (10 microg/kg, i.v.) or one cycle of ischemic preconditioning prior to the period of coronary occlusion offers significant infarct size reduction (18.6 +/- 2.2% and 19.4 +/- 1.9%, respectively versus 40.1 +/- 3.9% in saline control and 39.2 +/- 3.2% in vehicle control groups; P < 0.01) and antiarrhythmic effects. Both 17beta-estradiol and ischemic preconditioning treatment significantly attenuated the incidence of life-threatening arrhythmias like sustained VT (13% and 13%, respectively versus 100% in saline control and 100% in vehicle control groups; P < 0.001) and other arrhythmias (25% and 25%, respectively versus 100% in saline control and 100% in vehicle control groups; P < 0.001), and were quite effective in increasing the number of animals that survived without developing any arrhythmia during ischemia and reperfusion. 5-hydroxydecanoate(5-HD; 5 mg/kg, i.v.) alone offered no cardioprotective and antiarrhythmic activities. Pretreatment with 5-HD but not HMR 1883 (3 mg/kg, i.v.) abolished the beneficial effects of 17beta-estradiol and ischemia preconditioning on reperfusion-induced arrhythmias and cardioprotection suggesting that such effects have been achieved via the selective activation of cardiomyocyte mitochondrial KATP channels rather than sarcolemmal KATP channels. The reduced reperfusion arrhythmic incidence and durations induced by estrogen was not significantly altered by ICI 182 720 (2.5 mg/kg, i.v.). The lack of effect of ICI 182 720 on antiarrhythmic and infarct-limiting effects of 17beta-estradiol and ischemic preconditioning suggest that these favorable effects are rapid, direct, and non-genomic effects. This study demonstrates similarities between 17beta-estradiol and ischemic preconditioning of the rabbit myocardium in terms of cardioprotection, antiarrhythmic, and metabolic activities. Ischemic preconditioning and 17beta-estradiol appear to share a final common effector; the mitochondrial KATP channel.