Alkaline pH and internal calcium increase Na+ and K+ effluxes in LK sheep red blood cells in Cl--free solutions

We examined the effects of pH, internal ionized Ca (Ca-i(2+)), cellular ATP, external divalent cations and quinine on Cl-independent ouabain-resistant K+ efflux in volume-clamped sheep red blood cells (SRBCs) of normal high (HK) and low (LK) intracellular K+ phenotypes. In LK SRBCs the K+ efflux was higher at pH 9.0 (350%) than at pi-Is 7.4 and 6.5, and was inhibited by external divalent cations, quinine, and cellular ATP depletion. The above findings suggest that the increased K+ efflux at alkaline pH is due to the opening of ion channels or specific transporters in the cell membrane. In addition, K+ efflux was activated (100%) when Ca-i(2+) was increased (+A23187, +Ca-o(2+)) into the mu M range. However, in comparison to human red blood cells, the Ca-i(2+)-induced increase in K+ efflux in LK SRBCs was fourfold smaller and insensitive to quinine and charybdotoxin. The Na+ efflux was also higher at pH 9.0 than at pH 7.4, and activated (about 40%) by increasing Ca-i(2+). In contrast, in HK SRBCs the K+ efflux at pH 9.0 was neither inhibited by quinine nor activated by Ca-i(2+). These studies suggest the presence in LK SRBCs, of at least two pathways for Cl-independent K+ and Na+ transport, of which one is unmasked by alkalinization, and the other by a rise in Ca-i(2+).