Abnormal repolarization of ventricular fibers in vicinity of subacute myocardial infarction in dogs

It has been reported that action potential duration (APD) in dog Purkinje fibers with myocardial infarction (MI) is prolonged. The authors found similar APD prolongation in ventricular (V) fibers from papillary muscles and V trabeculae in the immediate vicinity of subacute MI (4 to 7 day old) produced by slow occlusion of left circumflex artery (application of laminaria) in 15 dogs. AP were recorded at rate 0.5 c/sec in fibers superfused with Tyrode solution (K = 5.4 mM/L) from 'normal' fibers (NI) and vicinity of MI (I). The authors found no differences in resting membrane potential (RMP), AP amplitude and dV/dt max but phase 2 was significantly shorter and phase 3 significantly longer in I than NI. The ADP in I was 329 ± 46 and in NI 256 ± 32 msec (p < 0.001) and the repolarization slope 0.39 ± 0.11 in I, and 0.71 ± 0.15 V/sec in NI (p < 0.001). To understand the mechanism of slow repolarization and prolonged ADP in I fibers, the authors compared the I and NI fibers alter the following interventions: superfusion with high Na (218 mM), high K (16.2 mM), low K (1.4 and 0.7 mM) and isoproteronol (1 μg/ml). The qualitative effects of all interventions were similar in I and NI but the differences in APD and repolarization velocity between I and NI persisted after all interventions. The results do not elicit a specific cause of abnormal repolarization in I fibers but suggest a 'non specific' decrease in membrane conductance.