Elevated intracellular Ca²⁺ affects Li_i-Na_o countertransport in human red blood cells

Changes in cytoplasmic Ca²⁺ concentration and in Li_i-Na_o countertransport activity have been shown to be associated with essential hypertension. Elevated intracellular free [Ca²⁺], as well as abnormalities of Ca²⁺ binding and transport have been reported in cells from different tissues of hypertensive laboratory animals and essential hypertensive patients. Similarly, enhanced rates of Li_i-Na_o countertransport and the modified pattern of the temperature dependence of this activity in red blood cells from essential hypertensive patients have been previously demonstrated. The aim of the present study was to investigate possible interaction between changes in intracellular free [Ca²⁺] and the Li_i-Na_o exchange in human red blood cells. The ionophore ionomycin was used to allow Ca²⁺ incorporation into the cells in a dose-dependent manner. The elevation of intracellular [Ca²⁺], in turn, resulted in enhanced Li⁺ efflux from the cells. At 3 μM, ionomycin selectively and significantly enhanced the Li_i-Na_o countertransport but not Li⁺ leakage from the cells. EGTA totally abolished the effect of ionomycin, indicating that the effect is directly related to Ca²⁺. As low as 0.4 μM Ca²⁺ caused a statistically significant effect. The maximal effect of Ca²⁺ on the Li_i-Na_o countertransport was achieved around the external pH range of 6.8-7.5. In contrast, the leakage of Li⁺ was significantly enhanced by Ca²⁺ at a pH of 7.4 and above. Ca²⁺ did not affect the K_m of the Li_i-Na_o countertransport for Li⁺. Amiloride, which inhibits Na⁺ H⁺ exchange, inhibited only 10% of the Ca²⁺-enhanced countertransport. It is concluded that Ca²⁺ may play a role in the regulation of Li_i-Na_o countertransport in erythrocytes.