Abstract

The interrelations between the inotropic effects of altering the external calcium concentration and of altering frequency of stimulation were studied. Calcium influenced the positive inotropic portion of the frequency-force curve, affecting its slope and the frequency range over which it extends, but had no influence on the negative inotropic portion of the frequency-force curve. Calcium also influenced the magnitude and rate of decay of poststimulation potentiation. Frequency in turn influenced the positive inotropic effects of calcium, affecting the speed of action of calcium on cardiac contractility and the calcium concentration able to increase contractility maximally. All of these results are consistent with known effects of frequency of stimulation and of the external calcium concentration on calcium uptake. These results support the hypothesis that the positive, but not the negative, inotropic effect of frequency is calcium-dependent. Reduction of the sodium concentration produced effects similar to those produced by an increase in calcium concentration.