In microinjected Myxicola giant axons with elevated [Na]i, Na efflux was sensitive to Cao under some conditions. In Li seawater, sensitivity to Cao was high whereas in Na seawater, sensitivity to Cao was observed only upon elevation of [Ca]o above the normal value. In choline seawater, the sensitivity of Na efflux to Cao was less than that observed in Li seawater whereas Mg seawater failed to support any detectable Cao-sensitive Na efflux. Addition of Na to Li seawater was inhibitory to Cao-sensitive Na efflux, the extent of inhibition increasing with rising values of [Na]o. The presence of 20 mM K in Li seawater resulted in about a threefold increase in the Cao-activated Na efflux. Experiments in which the membrane potential, Vm, was varied or held constant when [K]o was changed showed that the augmentation of Ca-activated Na efflux by Ko was not due to changes in Vm but resulted from a direct action of K on activation by Ca. The same experimental conditions that favored a large component of Cao-activated Na efflux also caused a large increase in Ca influx. Measurements of Ca influx in the presence of 20 mM K and comparison with values of Ca-activated Na efflux suggest that the Na:Ca coupling ratio may be altered by increasing external [K]o. Overall, the results suggest that the Cao-activated Na efflux in Myxicola giant axons requires the presence of an external monovalent cation and that the order of effectiveness at a total monovalent cation concentration of 430 mM is K + Li greater than Li greater than Choline greater than Na.

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