THE MEASUREMENT OF INTRACELLULAR SODIUM ACTIVITY AND ITS RELATIONSHIP TO THE ACTION OF CALCIUM IONS UPON THE LOW-SODIUM CONTRACTURE IN FROG ATRIAL TRABECULAE

¹ Department of Physiology, The University, Leicester LE1 7RH

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 aⁱ_Na 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.