Effects of diltiazem on the electrophysiological properties of cat ventricular muscle fibers during experimentally induced right ventricular systolic hypertension.

The electrophysiological effects of diltiazem were studied in right ventricular muscle fibers from normal cats and cats with experimentally induced right ventricular systolic hypertension (RVSH). Two types of action potential (AP) abnormalities were observed in preparations from cats with RVSH: Type I cells, found in most areas of the right ventricular free wall, demonstrated reduced maximum diastolic potential (MDP) (-64.4 mV) and Vmax (89.6 V/s) while Type II cells showed a "slow response" AP configuration (MDP, -48.8 mV; AP amplitude, 48.9 mV; AP duration at 50% repolarization, 47.5 ms; AP duration at 90% repolarization, 90.2 ms; Vmax, 13.1 V/s) and were often monitored near the tricuspid valve. Diltiazem (2.2 X 10(-7) and 2.2 X 10(-6) M) had no effect on MDP of normal, Type I, or Type II cells. Diltiazem at 2.2 X 10(-6) M significantly reduced AP amplitude and Vmax of both Type I and normal cells. In contrast, even at 2.2 X 10(-7) M, diltiazem significantly reduced AP amplitude and Vmax of the Type II cells. Diltiazem, 2.2 X 10(-6) M, would often abolish AP of Type II cells, while Type I cells were more sensitive to tetrodotoxin. AP duration of normal cells was unaffected by diltiazem while that of Type I and II cells was significantly shortened.