An ionophoretic model for the study of calcium-calcium exchange

A Pressman cell was used to study the Br-X537A-mediated translocation of ⁴⁵Ca from one aqueous phase into another across an immiscible organic phase. In this system, an increase in the concentration of ⁴⁰Ca in one chamber (mimicking the extracellular compartment) resulted in an increased efflux of ⁴⁵Ca added in trace amount to the other chamber (simulating the cytosolic compartment). Such a Ca-Ca exchange process was also observed when the concentration of ionophore was increased, and could be blocked by the organic Ca-antagonist suloctidil. However, no Ca-Ca exchange occurred when the downhill rate of ⁴⁰Ca translocation was increased by use of a pH gradient. This ionophoretic model also suggests that a low intracellular concentration of Ca tends to maintain Ca influx at a low rate, and that the asymetrical distribution of Ca facilitates rather than impeeds the ionophoretic extrusion of intracellular Ca against its chemical gradient.