Effect of higenamine on action potential of ventricular myocardial cells

Action potentials of isolated ventricular myocardial fibers of pigs and the electrophysiological actions of dl-demethylcoclaurine (DMC) or higenamine on the porcine myocardial cells were studied by glass microelectrodes. The action potentials of porcine ventricular myocardial fibers were similar to those of other mammalian hearts. Amplitude of the action potential was 115 +/- 9 mV, duration of action potential 242 +/- 30 ms, resting membrane potential -85 +/- 5 mV and the maximum rise of depolarization of action potential 151 +/- 27 V/s. After perfusion with normal Tyrode's solution containing DMC 5 micrograms/ml, the amplitude of action potential was increased and the duration of action potential was prolonged, the amplitude and duration of the plateau of action potential were increased and the slope of phase 2 of action potential was reduced (p less than 0.05). The resting potential, the slope of phase 3 and the maximum depolarization rate of phase 0 of action potential did not change. In Tyrode's solution containing Mn++ 3 mM, these electrophysiological actions of DMC disappeared. DMC may abolish the conduction block induced by K+-rich solution. In Tyrode's solution containing K+ 32 mM, the upstroke of action potential showed 2 phases under influence of DMC. The second phase disappeared when Mn++ was added. All findings indicate that DMC can be considered as an activator of the slow channel. The electrophysiological mechanism and clinical significance of DMC were discussed.