The measurement of intracellular sodium activity and its relationship to the action of calcium ions upon the low-sodium contracture in frog atrial trabeculae.

If the Na concentration of the solution bathing a frog atrial trabecula is reduced a contracture rapidly develops and then relaxes spontaneously. If the Ca concentration of the low-Na solution is raised during this relaxation a further contracture may develop. The amplitude of this Ca-addition contracture depends in a complex manner upon [Na]o. It also shows a time dependence which is similar to the rate of spontaneous relaxation and the time course of the fall in aiNa as measured by ion-sensitive micro-electrodes. These observations are discussed in terms of transmembrane ion movements, the processes occurring during spontaneous relaxation and a possible compartmentalization of both intracellular Ca and Na ions. A relationship between [Na]i and [Na]o is derived from the contracture data that compares well with direct measurements made using ion-sensitive micro-electrodes.