Reentrant Ventricular Arrhythmias in the Late Myocardial Infarction Period. 7. Effect of Verapamil and D-600 and the Role of the "slow Channel"

Reentrant ventricular arrhythmias (RVA) were analyzed in dogs 3--7 days after ligation of the anterior descending coronary artery using averaged "composite" recordings of electrical activity of reentrant pathways (RP) from the epicardial surface of the infarction zone (IZ). Verapamil (V) and D-600 (D) (0.2--0.5 mg/kg i.v.) resulted in slight-to-moderate improvement of conduction in RP with abolition of spontaneous RVA and RVA initiated by premature depolarizations. The effect of V was not blocked by pretreatment with propranolol (0.5 mg/kg i.v.). Using a standard microelectrode technique and strips of epicardial muscle from the IZ, D (0.5--1 X 10(-6) g/ml) slightly improved the upstroke velocity and membrane responses of depressed ischemic cells. In contrast, tetrodotoxin (5 X 10(-7) g/ml) further depressed or abolished action potentials of ischemic cells. We conclude: 1) the moderate antiarrhythmic effect of V and D on RVA is the result of improved conduction in RP; 2) this action is partly explained by improvement of a depressed sodium channel and is not related to catecholamine release; 3) slow-response action potentials play no significant role in the genesis of ischemia-related RVA, which probably results from depression of the fast response.