Correlation of the effect of Ca⁺² on Na⁺ and K⁺ permeability and membrane potential of Ehrlich ascites tumor cells

The effect of Ca⁺² on the transport and intracellular distribution of Na⁺ and K⁺ in Ehrlich ascites tumor cells was investigated in an effort to establish the mechanism of Ca⁺²‐induced hyperpolarization of the cell membrane. Inclusion of Ca⁺² (2mM) in the incubation medium leads to reduced cytoplasmic concentrations of Na⁺, K⁺ and Cl⁻ in steady state cells. In cells inhibited by ouabain, Ca⁺² causes a 41% decrease in the rate of net K⁺ loss, but is without effect on the rate of net Na⁺ accumulation. Net K⁺ flux is reduced by 50%, while net Na⁺ flux is unchanged in the transport‐inhibited cells. The membrane potential of cells in Ca⁺²‐free medium (‐13.9 ± 0.8 mV) is unaffected by the addition of ouabain. However, the potential of cells in Ca⁺²‐containing medium (‐23.3 ± 1.2 mV) declines in one hour after the addition of ouabain to values comparable to those of control cells (‐15.2 ± 0.7 mV). The results of these experiment are consistent with the postulation that Ca⁺² exerts two effects on Na⁺ and K⁺ transport. First, Ca⁺² reduces the membrane permeability to K⁺ by 25%. Second, Ca⁺² alters the coupling of the Na/K active transport mechanism leding to an electrogenic hyperpolarization of the membrane.