Positive inotropic effect of cycloprotobuxine-A on isolated guinea pig myocardium.

Cycloprotobuxine-A (CPB-A) 0.1-100 mumoles/L produced positive inotropic effects in left and right atria in a concentration-dependent manner. CPB-A 30 mumoles/L enhanced post-rest contraction, augmented the response of left atrium to increase in stimulating frequency, and increased the developing tension evoked by paired pulse stimulation. By taking simultaneous recordings of action potentials and contractile force of papillary muscles, it was found that CPB-A 30 mumoles/L increased the contractile force and prolonged the action potential duration at 50% of depolarization. It is concluded that the positive inotropic effect of CPB-A on myocardium may be associated with an increase in transsarcolemmal influx of calcium as well as an augmentation of the amount of calcium released from intracellular stores.