blister and Ventricular-Fibrillation

Blebbistatin (BS) is a recently discovered inhibitor of the myosin II isoform and has been adopted as the mechanical uncoupler of choice for optical mapping, because previous studies suggest that BS has no significant cardiac electrophysiological effects in a number of species. The aim of this study was to determine whether BS affects cardiac electrophysiology in isolated New Zealand White rabbit hearts. Langendorff-perfused hearts (n=39) in constant-flow mode had left ventricular monophasic action potential duration (MAPD) measured at apical and basal regions during constant pacing (300 ms cycle length). Standard action potential duration restitution was obtained using the single extrastimulus method with measurement of the maximal restitution slope. Ventricular fibrillation threshold was measured as the minimal current inducing sustained ventricular fibrillation with burst pacing (30 stimuli, at 30 ms intervals). Optical action potentials were recorded using the voltage-sensitive dye di-4-ANEPPS. Measurements were taken at baseline and after 60 min perfusion with BS (5 μm). Blebbistatin significantly prolonged left ventricular apical (mean±SEM; from 129.9±2.9 to 170.7±4.1 ms, P<0.001, n=8) and basal MAPD (from 135.0±2.3 to 163.3±5.6 ms, P<0.001) and effective refractory period (from 141.3±4.8 to 175.6±3.7 ms, P<0.001) whilst increasing the maximal slope of restitution (apex, from 0.79±0.09 to 1.57±0.16, P<0.001; and base, from 0.71±0.06 to 1.44±0.24, P<0.001) and ventricular fibrillation threshold (from 5.3±1.1 to 17.0±2.9 mA, P<0.001). In other hearts, blebbistatin significantly prolonged optically recorded action potentials (from 136.5±6.3 to 173.0±7.9 ms, P<0.05, n=4). In control experiments, the increase of MAPD with blebbistatin was present whether the hearts were perfused in constant-pressure mode (n=5) or in unloaded conditions (n=5). These data show that blebbistatin significantly affects cardiac electrophysiology. Its use in optical mapping studies should be treated with caution.

Topics: Action Potentials; Animals; Electrophysiological Phenomena; Heart; Heart Ventricles; Heterocyclic Compounds, 4 or More Rings; In Vitro Techniques; Male; Myocardial Contraction; Perfusion; Rabbits; Ventricular Fibrillation

Myocardial ischemia results in metabolic changes, which collapse the mitochondrial network, that increase the vulnerability of the heart to ventricular fibrillation (VF). It has been demonstrated at the single cell level that uncoupling the mitochondria using carbonyl cyanide p-(tri-fluoromethoxy)phenyl-hydrazone (FCCP) at low concentrations can be cardioprotective. The aim of our study was to investigate the effect of FCCP on arrhythmogenesis during ischemia in the whole rabbit heart. We performed optical mapping of isolated rabbit hearts (n = 33) during control and 20 min of global ischemia and reperfusion, both with and without pretreatment with the mitochondrial uncoupler FCCP at 100, 50, or 30 nM. No hearts went into VF during ischemia under the control condition, with or without the electromechanical uncoupler blebbistatin. We found that pretreatment with 100 (n = 4) and 50 (n = 6) nM FCCP, with or without blebbistatin, leads to VF during ischemia in all hearts, whereas pretreatment with 30 nM of FCCP led to three out of eight hearts going into VF during ischemia. We demonstrated that 30 nM of FCCP significantly increased interventricular (but not intraventricular) action potential duration and conduction velocity heterogeneity in the heart during ischemia, thus providing the substrate for VF. We showed that wavebreaks during VF occurred between the right and left ventricular junction. We also demonstrated that no VF occurred in the heart pretreated with 10 μM glibenclamide, which is known to abolish interventricular heterogeneity. Our results indicate that low concentrations of FCCP, although cardioprotective at the single cell level, are arrhythmogenic at the whole heart level. This is due to the fact that FCCP induces different electrophysiological changes to the right and left ventricle, thus increasing interventricular heterogeneity and providing the substrate for VF.

Topics: Action Potentials; Animals; Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; Glyburide; Heterocyclic Compounds, 4 or More Rings; Mitochondria, Heart; Myocardial Ischemia; Proton Ionophores; Rabbits; Ventricular Fibrillation