Absence of significant sodium-hydrogen exchange by rabbit erythrocyte sodium-lithium countertransporter

Red blood cells of several mammalian species (e.g., human, rabbit, bovine) possess a countertransport system for Li and Na. We have used rabbit red blood cells to determine the extent of sodium-proton exchange by this countertransporter. Ouabain-insensitive 22Na efflux into phosphate-buffered K-acetate media of varying Na concentrations was determined at extracellular pH 7.5 and 6.9. To maintain the intracellular pH at approximately 7.2, most of the acetate in the pH 6.9 medium was replaced by gluconate, a nonpenetrating anion. Extracellular Na strongly (greater than 10-fold) stimulated the 22Na efflux in both high- and low-pH media, but the K1/2 for this stimulation was higher at pH 6.9. This is consistent with a competitive binding of H to the outward-facing transport site. However, the lower pH itself, in a Na-free medium, stimulated the 22Na efflux only very slightly. The 22Na efflux stimulated by lowering the extracellular pH to 6.9 was less than 1% of the stimulation produced by 100 mM extracellular Na. It is concluded that, although H appears to bind to the substrate site of the Na-Li exchanger, there is no significant H transport in the physiological pH range.