Intracellular potential changes following coronary occlusion in isolated perfused rat hearts

A model of myocardial ischemia for the simultaneous assessment of electrophysiological changes and arrhythmias in intact rabbits

A method of recording epicardial monophasic action potentials (MAPs) and ischemia-induced arrhythmias following coronary artery ligation in intact rabbits is described. It is expected that this model will have utility in analyzing drug effects and mechanisms of ischemic arrhythmogenesis. Rabbits were found to have two arrhythmic phases following coronary artery occlusion which correspond to phase Ia and Ib arrhythmias in other species. Epicardial MAPs recorded from ischemic tissue allowed electrophysiological effects to be correlated with these phases. Phase Ia arrhythmias occurred within the first 2 min of coronary artery occlusion and were associated with a reduction in the maximum upstroke velocity of MAPs and changes in MAP duration, including the occurrence of alternans in duration. Phase Ib arrhythmias occurred between 8 and 15 min after coronary artery occlusion. These arrhythmias were associated with a decrease in MAP duration and amplitude, alternans in MAP duration as well as conduction block. Coronary artery occlusion reliably induced arrhythmias in rabbits if the left branch of the coronary artery and the left anterior descending artery were occluded. There was a 95% incidence of premature ventricular contractions, 38% of ventricular tachycardia, and 48% of ventricular fibrillation (n = 21). The results of this study show that epicardial MAPs can be used to aid in the characterization of the electrophysiological mechanisms of ischemia-induced arrhythmias in vivo.

Glibenclamide, but not class III drugs, prevents ischaemic shortening of the refractory period in guinea-pig hearts

The effective refractory period was measured in paced (4 Hz) perfused guinea-pig hearts in vitro. The effective refractory period was linearly correlated with temperature of the perfusing solution: as temperature was reduced the effective refractory period was increased. Reduction of the coronary flow rate to 10% of control resulted in a marked reduction in the effective refractory period. UK-66,914, dofetilide, ibutilide and phentolamine caused a prolongation in the effective refractory period, but during ischaemia the effective refractory period was reduced by the same degree as in vehicle-treated hearts. Glibenclamide had no effect on the effective refractory period prior to ischemia but it abolished the ischaemia-induced shortening. These results suggest that the opening of KATP channels may be responsible for the ischaemia-induced shortening of the effective refractory period in perfused guinea-pig hearts and that the class III effects of UK-66,914, dofetilide and ibutilide are attenuated during ischaemia.

A protective effect of sulphinpyrazone against coronary occlusion-induced shortening of myocardial refractory periods in the rat

The hearts of anaesthetized, artificially ventilated rats were exposed, and the left coronary artery occluded. The diastolic threshold voltage for stimulation (DTV), the duration of the bipolar electrogram (DBE) and the functional refractory period (FRP) of the ischaemic area were measured at minute intervals for an hour after occlusion. Coronary occlusion caused a rise in DTV, a prolongation of the DBE and a biphasic change in the FRP, with an initial prolongation phase (1-4 min) followed by a decline to below pre-occlusion values (5-15 min). Episodes of ventricular tachyarrhythmia (VT) were most frequent during the period 5-15 min after the onset of occlusion of the coronary artery. This coincided with the period when FRP was minimal and the difference between DBE and FRP was maximal. Pretreatment of rats with sulphinpyrazone (2.5-40 mg kg-1) or indomethacin (5-20 mg kg-1) protected against the episodes of coronary occlusion-induced VT and against the associated decline in FRP of the ischaemic muscle. Sulphinpyrazone was more effective than indomethacin in this respect and a combination of the two drugs was approximately as effective as sulphinpyrazone alone. It was concluded that sulphinpyrazone protects rats against coronary occlusion-induced episodes of VT by reducing the risk of ventricular action potential re-entry. This effect is probably due to protection against the ischaemia-induced shortening of the myocardial FRP.

Actions of cardiotoxins from the southern Chinese cobra (Naja naja atra) on rat cardiac tissue

The cardiotoxic actions of southern Chinese cobra (Naja naja atra) venom have been investigated. Cardiotoxicity accounted for lethality of crude venom in anaesthetized rats and cardiotoxic polypeptides constituted the bulk of the venom when it was fractionated by CM-Sephadex chromatography. The cardiotoxic actions of these polypeptides were investigated in isolated cardiac tissue. Actions of cardiotoxins on rate, force and intracellular potentials occurred more readily in ventricular tissue and were reversible in all cardiac tissues. Bolus injections of cardiotoxins to perfused rat hearts produced profound changes in intracellular potentials, but such changes were reversed by 10 min after toxin wash-out. The changes included depolarization and loss of the fast phase (0) of the action potential. The loss of beating and contracture produced by cardiotoxins in isolated atria and ventricles showed such marked tachyphylaxis that toxicity was lost after 2-5 exposures. Both calcium and heparin ameliorated the cardiotoxic actions of cardiotoxins. Since the actions of cardiotoxin were partially selective for ventricular tissue, reversible, demonstrated tachyphylaxis and could be prevented, it is suggested that a specific molecular mechanism may be involved, rather than a non-specific action such as cell membrane destruction.