Post-rest contractions of amphibian cardiac muscle.

Post-rest potentiated contractions have been reported to be more dependent on Ca2+ stored in intracellular sites than on transmembrane Ca2+ influx. The phenomenon was examined using toad ventricular strips contracting isometrically and superfused with Ringer solution. Toad ventricular strips did not present post-rest potentiation, a fact that may be explained by the reduced volume of the sarcoplasmic reticulum in the amphibian myocardium. Potentiated post-rest contractions were obtained after calcium influx blockade with 10 microM verapamil or 2 mM Mn2+ and the slow reduction of extracellular Ca2+ concentration obtained by slowly exchanging the bath fluid was accompanied by a progressive decrease of both steady-state and post-rest contractions. These data confirm the observations made on other species regarding the post-rest potentiation phenomenon and support the suggestion of the dependence of post-rest potentiation on activator calcium liberated from intracellular stores.