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

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Abstract
Palytoxin (PTX) [isolated from Palythoa fuberculosa] is one of the most potent marine toxins. Effects of PTX on isolated papillary muscles of guinea pigs were studied in an attempt to elucidate the mechanical and electrical activities. Among the inotropic effects of PTX above 3 .times. 10-9 g/ml were an early positive inotropic effect, slowly developing contracture accompanied by decline in phasic tensions, 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 later. Oscillatory afterpotential often appeared. Neither reserpine nor practolol prevented the PTX-induced arrhythmia but 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. PTX-induced depolarization evidently is responsible for the generation of contracture and arrhythmia and that the depolarization is due to the change in membrane Na permeability.