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

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Abstract
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.