Relationship between the effects of Goniopora toxin on action potential and on contractile force in guinea-pig papillary muscle.

Effects of Goniopora toxin (GPT) on cardiac action potential and on contractile force were investigated in isolated guinea-pig papillary muscle. GPT produced a positive inotropic effect by increasing contractile force and prolonging the relaxation time. The time-to-peak force was little affected. GPT prolonged the action potential duration but did not affect the resting membrane potential nor the amplitude of the action potential. Thus there was a correlation between the positive inotropic effect and prolongation of the action potential duration. Tetrodotoxin or a reduction in extracellular sodium concentration attenuated both the positive inotropic effect and the prolonged duration of the action potential induced by GPT. Lanthanum or a reduction in extracellular calcium concentration also inhibited the increased contraction but did not shorten the prolonged durations of contraction and action potential. Verapamil attenuated the positive inotropic effect by reducing both the contractile force and the duration of contraction, but did not shorten the action potential duration. These results show that the positive inotropic effect of GPT depends on the increase in both sodium and calcium influxes while the prolonging effect on the action potential probably depends only on an increase in sodium influx. Hence, it is concluded that the prolongation of the action potential due to the increased sodium permeability is an essential process for the appearance of the positive inotropic effect of GPT.