Characterization of bicarbonate-dependent potassium uptake in cultured corneal endothelial cells

Bovine corneal endothelial (BCE) cells in culture demonstrated ⁸⁶Rb⁺ uptake which was mostly ouabain-sensitive with some (15 to 50%) ouabain-insensitive uptake that was dependent on the presence of bicarbonate in the incubation medium. Bovine smooth muscle (SM) cells demonstrated ouabain-sensitive ⁸⁶Rb⁺ uptake but the ouabain-insensitive ⁸⁶Rb⁺ uptake was not bicarbonate-dependent. Although omission of bicarbonate from the incubation buffer resulted in some reduction in the pH, this change was not responsible for the reduction in the ouabain-insensitive ⁸⁶Rb⁺ uptake. Furthermore, the removal of bicarbonate decreased the ⁸⁶Rb⁺ influx but not its efflux. This ouabain-insensitive and bicarbonate-dependent ⁸⁶Rb⁺ influx in BCE cells proceeded at a linear rate for at least 60 min and increased as a function of bicarbonate concentration such that almost maximal uptake was observed at a concentration of about 10 to 15 mM. Saturation of the bicarbonate-dependent ⁸⁶Rb⁺ pump in BCE cells occurred at a concentration of 2 mM Rb⁺ in the incubation buffer, similar to the previously observed value for the Na⁺, K⁺-ATPase. Competition experiments with both unlabeled Rb⁺ and K⁺ demonstrated that likewise in the Na⁺, K⁺-ATPase the ⁸⁶Rb⁺ influx represented physiological influx of K⁺. Furthermore, the energy requirements of the bicarbonate-dependent ⁸⁶Rb⁺ uptake were similar to those of the ⁸⁶Rb⁺ uptake via the Na⁺, K⁺-ATPase. The results described in this work demonstrated a novel bicarbonate-dependent K⁺ pump in addition to the Na⁺, K⁺-ATPase pump. This novel pump made a significant contribution to total K⁺ influx in BCE cells and may contribute to the fluid pump activity in addition to the well established contribution of the Na⁺, K⁺-ATPase pump.