Sudden cardiac death resulting from ventricular fibrillation (VF) remains a major cause of mortality. The purpose of this study was to investigate the roles of loss of oxidative phosphorylation and activation of the mitochondrial ATP-sensitive K+ channel and permeability transition pore in VF development during myocardial ischemia by using mitochondrial uncoupling agents (carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol) and channel blockers (5-hydroxydecanoate and cyclosporine A) at concentrations that have been demonstrated to block the intended targets selectively. Isolated rat hearts (n = 8 per group) were perfused with 0.3 μM carbonyl cyanide m-chlorophenylhydrazone, 100 μM 2,4-dinitrophenol, 0.2 μM cyclosporine A, 100 μM 5-hydroxydecanoate, or vehicle solution and regional ischemia induced after 10 minutes. Carbonyl cyanide m-chlorophenylhydrazone and 2,4 dinitrophenol caused profound QT shortening and triggered VF in 100% of hearts before ischemia. During ischemia, neither cyclosporine A (88%) nor 5-hydroxydecanoate (100%) reduced VF incidence compared with control (100% VF). In separate hearts, carbonyl cyanide m-chlorophenylhydrazone decreased tissue ATP content, and glibenclamide or glimepiride delayed the QT shortening and onset of VF triggered by carbonyl cyanide m-chlorophenylhydrazone. In conclusion, mitochondrial uncoupling agents trigger VF, likely as a result of ATP depletion with subsequent activation of sarcolemmal ATP-sensitive K+ currents. The mechanism of VF in ischemia does not involve activation of the mitochondrial ATP-sensitive K+ channel or permeability transition pore.
An isolated heart method that has been proposed to aid in ascertaining the involvement of L-type calcium channel blockade in the mechanism of action of novel antiarrhythmic drugs involves increasing the calcium concentration in the perfusion buffer. The purpose of this study was to determine the validity of this method using an established L-type calcium channel blocker, verapamil. Isolated rat hearts were perfused with normal calcium (1.4 mM) Krebs solution containing drug vehicle only, a normal calcium solution containing verapamil (300 nM), or a high calcium (2.8 mM) solution containing verapamil. The occurrence of ventricular fibrillation during a subsequent period of regional myocardial ischemia was monitored. The incidence of ventricular fibrillation was significantly reduced from 80% in controls to 20% by perfusion with verapamil in normal calcium Krebs solution (P < 0.05). Perfusion with the high calcium solution increased the incidence of ventricular fibrillation in the presence of verapamil to 40% (P > 0.05 versus controls). We conclude that the antiarrhythmic effect of verapamil in isolated hearts can be attenuated by increasing the calcium content of the perfusion solution, but a twofold increase in the calcium concentration failed to fully restore susceptibility to ventricular fibrillation to that observed in verapamil-free controls.
The vast majority of studies employing the isolated perfused rat heart model to study ischemic arrhythmias have used male rats only. The objective of this study was to determine the variation in susceptibility to ischemia‐induced ventricular fibrillation (VF) in isolated female rat hearts with the changing endogenous reproductive hormonal environment associated with the estrous cycle.Hearts were isolated from female rats under anesthesia and perfused with 3 mM K+ Krebs solution. Experiments were grouped according to estrous cycle stage that was determined by prior vaginal lavage (n=9–13 per group). A group of male rats was used as the control. Regional ischemia was induced by coronary ligation and maintained for 30 min. The incidence of VF was determined from the ECG.The incidence of VF in male hearts was 100%, while the incidence of VF in female hearts was also high but varied with stage of the estrous cycle (proestrus 89%, diestrus 78%, metestrus 100%, estrus 69%; p>0.05). There was no difference between groups in time to onset of VF or ischemic zone size (range 40–44% of total ventricular weight).In conclusion, the susceptibility of isolated female rat hearts to ischemic VF is comparable to that of male rat hearts. However, the variation in VF susceptibility in female rat hearts associated with the different stages of the estrous cycle may affect the statistical power of a study.Supported by a WVSOM intramural grant.
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