Post M A, Dawson D C
Department of Physiology, University of Michigan Medical School, Ann Arbor 48109-0622.
J Gen Physiol. 1994 May;103(5):895-916. doi: 10.1085/jgp.103.5.895.
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+.
龟结肠基底外侧的钠-氢反向转运体表现出传导性和电中性的钠离子转运（波斯特和道森，1992年，《美国生理学杂志》，262卷：C1089 - C1094页）。为了探究反向转运体介导的电流流动机制，我们比较了引发传导和交换所需的条件，并确定了这两个过程的激活动力学。外向（从细胞到细胞外液）而非内向（从细胞外液到细胞）的钠离子或锂离子梯度促进了反向转运体介导的钠离子或锂离子电流，而外向的质子梯度驱动内向的钠离子或锂离子电流。质子梯度驱动的“逆流”电流是每质子交换化学计量比大于1个钠离子或锂离子的有力证据。与此概念一致，由外向钠离子或锂离子梯度产生的外向钠离子和锂离子电流呈现出S形激活动力学。从未观察到反向转运体介导的质子电流，这表明反向转运体每一次周转仅转运一个质子。与钠离子传导不同，钠离子交换由外向或内向的钠离子、锂离子或质子梯度驱动，并且钠离子/钠离子交换的激活符合米氏动力学（半最大激活浓度K1/2 = 5 mM）。提高细胞外液中的钠离子或锂离子浓度，但不提高细胞外液质子浓度，会抑制反向转运体介导的传导并激活钠离子交换。这些结果与钠-氢反向转运体的模型一致，在该模型中，钠离子或锂离子与高亲和力位点的结合导致一对一的阳离子交换，但钠离子或锂离子与其他低亲和力位点的结合可导致非单一的阳离子交换化学计量比，从而导致电荷的净转运。传导性和非传导性事件的相对比例由底物梯度的大小和方向以及钠离子或锂离子的浆膜浓度决定。
