The Role of Calcium in the Regulation of the Steady-State Levels of Sodium and Potassium in the HeLa Cell

Studies on HeLa cells in spinner culture at pH 7.0 and 37° have shown that [Na]_i decreased and [K]_i increased with increasing [Ca]_o. In Na-free (choline) medium [K]_i remained high whether or not Ca was present in the medium. [Na]_i and [K]_i approached a new steady state within 1 min after transfer to Ca-free medium and returned to the initial values within 15 min upon readdition of Ca. 40% of the cell Ca exchanged within 1 min followed by a slow exchange of the remaining Ca over several hours. [Ca]_i increased with decreasing [Na]_o but was independent of [K]_o. Equimolar Mg did not substitute for Ca in maintaining low [Na]_i and high [K]_i. Under steady-state conditions about 50% of the cell Na exchanged in accordance with a single rate constant. The initial Na influx was 270, 100, and 2.5 µM/liter of cell water/sec for 0, 0.10, and 1.0 mM [Ca]_o, respectively. When Na transport was inhibited with strophanthidin and [Na]_i and [K]_i allowed to reach a steady state, Na influx was more rapid for cells incubated in Ca-free medium than for cells incubated in medium containing 1.0 mM Ca. These results suggest that Ca competes with Na at the cell membrane and thus controls the passive diffusion of Na into the cell.