Facilitation of heart muscle contraction and its dependence on external calcium and sodium

1. The magnitude and time course of the facilitation of contraction following previous stimulation have been studied in strips of frog ventricle when the external concentrations of Na and Ca were varied. The maximum tension and the maximum rate of rise of tension have been used as indices of activation of the contractile element. Under the experimental conditions these two parameters changed similarly.2. At low stimulus frequencies (3-12 beats/min), and moderate concentrations of external Ca (2-5 mM), the facilitation of contraction with repetitive stimulation can be predicted by assuming each contraction to produce an amount of facilitation which sums linearly with that remaining from previous responses. Thus, the staircase can be constructed from the decay of facilitation following a single contraction.3. The decay of facilitation appears to consist of two components. The first has a half-time of about 3 sec; the second about 50 sec. The decay of the second component of facilitation is slowed and its magnitude increased by raising the external Ca from 2 to 5 mM.4. Reducing the external Na concentration increases both the initial strength of contraction and the amount of facilitation.5. When the ratio Ca/Na(2) is kept constant while changing the Ca and Na concentrations, the first contraction in a series remains constant, but facilitation is less at low concentrations of Ca and Na. Facilitation, therefore, does not depend solely on the amount of initial activation of the contractile system.6. The results are consistent with the hypothesis that facilitation depends on the amount of calcium retained in some cellular store.