Picrotoxin Inhibits Contractions in Frog Heart Atrium

Effects of 5mM picrotoxin on action potentials and contractions of frog heart muscle were examined by simultaneous recording of electrical and mechanical activity using double mannitol-gap method. With short time exposure, picrotoxin did not affect the action potential appreciably, but reduced the twitch tension to about one third. On return to Ringer solution, the reduction in the tension was almost fully recovered within several minutes. With long time exposure, picrotoxin considerably reduced the depolarization at the plateau phase of the action potential, and the twitch tension was reduced to below one tenth. These effects could not be recovered within relatively short time. The results reveal, for the first time, the effect of picrotoxin to inhibit contraction in heart muscle, in contrast to its potentiation of contraction in skeleton muscle. The long time effects of picrotoxin could be ascribed to the inhibition of Ca current with picrotoxin, but the short time effect of picrotoxin to inhibit contraction should be caused by other mechanism.