The positive inotropic action of transmural electrical stimulation of the heart ventricles in the ontogeny of the frog Rana temporaria

Studies have been made of the effect of transmural electrical stimulation on twitch tension produced by atropinized ventricular preparations from tadpoles and adult frogs. In preparations from tadpoles at stage 42 and all the following stages, as well as in adult frogs, transmural electrical stimulation evoked positive inotropic responses which consisted of a slow propranolol-sensitive component or of a slow and fast components. It is highly probable that the slow component is induced by adrenergic transmitter. The fast propranolol-resistant component appears at stage 43. It may be prevented by bretilium being probably induced by a comediator which is released together with the adrenergic transmitter from the sympathetic nerve endings.