[Analysis of the relationship between the rate and force of myocardial contractions in dogs, rats and frogs].

Under maximally physiological conditions of arterial blood supply to isolated hearts of dogs, rats and frogs it was found that frequency potentiation of contractility was most distinct in the ventricles of frogs, less distinct in the left ventricle of dogs, and absent in rats. It was demonstrated that in connection with the rhythmic activity the myocardial cells accumulate quantities of calcium proportional to the degree of frequency potentiation. Amounts of calcium were determined that are necessary for a full activation of myocardial contractility proteins, and the amount of calcium entering the myocardial cells of a frog per one systole was calculated. A close approximation of both the values indicates that the activation of a contraction occurs at the expense of the entrance of extracellular calcium during the depolarization of a sarcolemoma.