Effect of K ions on 45Ca fluxes in myelinated nerve

The effects of extracellular K ions and depolarization on 45Ca fluxes were investigated in desheathed sciatic nerves of Rana pipiens and the results compared to Na-Ca countertransport models which postulate the exchange of three or more Na ions for one Ca ion. Changes in the extracellular K ion concentration ranging from 1 to 40 mM at a constant Na gradient did not affect Ca efflux significantly. In Na-Ca-free solutions maintained isotonic with sucrose, increasing K concentrations stimulated Ca efflux. Increasing K concentrations inhibit Ca influx in Na-free solutions. Although membrane potential differences was reduced by up to 40 m v during these procedures, in no instance did depolarization reduce the Ca efflux as predicted by the model and as reported for poisoned squid axon. The results suggest that K ions inhibit Ca influx and activate efflux similar to Na. Furthermore, a fraction of the "residual" Ca efflux observed in the absence of Na and Ca appears to be due to extracellular K ions. This study provides further evidence that mechanisms other than Na-Ca countertransport participate in Ca homeostasis in myelinated nerve.