Effects of palytoxin on mechanical and electrical activities of guinea pig papillary muscle.

Effects of palytoxin (PTX) on isolated papillary muscles of guinea pigs were studied in an attempt to elucidate the mechanical and electrical activities. Inotropic effects of PTX above 3 x 10(-9) g/ml; an early positive inotropic effect, slowly developing contracture accompanied by decline in phasic tension, appearance of aftercontractions and arrhythmias at high doses. The positive inotropic effect of PTX was enhanced in high Ca2+ medium but was not modified by propranolol. PTX induced a sustained depolarization and decrease in the amplitude, upstroke velocity and duration of action potential. During development of depolarization, arrhythmias occurred, which lasted for 5--10 min and reappeared 30--60 min after. Oscillatory afterpotential often appeared. Neither reserpine nor practolol prevented the PTX-induced arrhythmia while propranolol prevented it. Tetrodotoxin slowed the development of depolarization due to PTX and inhibited PTX-arrhythmias. In low Na+ medium, PTX exerted fewer effects on resting and action potentials and produced no arrhythmia. The results suggest that PTX-induced depolarization is responsible for the generation of contracture and arrhythmia and that the depolarization is due to the change in membrane Na permeability.