The effect of calcium and beat interval on the contractile force and refractoriness of the isolated rat atrium in the absence and presence of nifedipine: a possible mechanism for the negative staircase phenomenon.

An increase in the calcium concentration of the Ringer solution increased the contractile force and reduced the effective refractory period of the isolated rat left atrium paced at a frequency of 60 per min. An increase in the frequency of stimulation decreased the contractile force (negative staircase) and reduced the effective refractory period; these effects being most pronounced at the highest calcium levels. A calcium-antagonistic drug, nifedipine, decreased the contractile force and the effective refractory period. The effect of an increased frequency of stimulation on the contractile force at different calcium levels in the absence and presence of nifedipine paralleled its effect on the effective refractory period. This observation suggests that both these phenomena are caused by a decreased quantity of inflowing calcium ions during the period of exciation, and thus might explain the absence of positive staircase in the rat myocardium.