Basolateral Na⁺-H⁺ antiporter: Mechanisms of electroneutral and conductive ion transport

The basolateral Na-H antiporter of the turtle colon exhibits both conductive and electroneutral Na⁺ transport (Post and Dawson. 1992. American Journal of Physiology. 262:C1089-C1094). To explore the mechanism of antiporter-mediated current flow, we compared the conditions necessary to evoke conduction and exchange, and determined the kinetics of activation for both processes. Outward (cell to extracellular fluid) but not inward (extracellular fluid to cell) Na⁺ or Li⁺ gradients promoted antiporter-mediated Na⁺ or Li⁺ currents, whereas an outwardly directed proton gradient drove inward Na⁺ or Li⁺ currents. Proton gradient-driven, "counterflow" current is strong evidence for an exchange stoichiometry of > 1 Na⁺ or Li⁺ per proton. Consistent with this notion, outward Na⁺ and Li⁺ currents generated by outward Na⁺ or Li⁺ gradients displayed sigmoidal activation kinetics. Antiporter-mediated proton currents were never observed, suggesting that only a single proton was transported per turnover of the antiporter. In contrast to Na⁺ conduction, Na⁺ exchange was driven by either outwardly or inwardly directed Na⁺, Li⁺, or H⁺ gradients, and the activation of Na⁺/Na⁺ exchange was consistent with Michaelis-Menten kinetics (K1/2 = 5 mM). Raising the extracellular fluid Na⁺ or Li⁺ concentration, but not extracellular fluid proton concentration, inhibited antiporter-mediated conduction and activated Na⁺ exchange. These results are consistent with a model for the Na-H antiporter in which the binding of Na⁺ or Li⁺ to a high-affinity site gives rise to one-for-one cation exchange, but the binding of Na⁺ or Li⁺ ions to other, lower-affinity sites can give rise to a nonunity, cation exchange stoichiometry and, hence, the net translocation of charge. The relative proportion of conductive and nonconductive events is determined by the magnitude and orientation of the substrate gradient and by the serosal concentration of Na⁺ or Li⁺.