Determination of the coupling ratio for Na+-H+ exchange in renal microvillus membrane vesicles

We evaluated the H⁺:Na⁺ coupling ratio of the Na⁺-H⁺ exchanger present in microvillus membrane vesicles isolated from the rabbit renal cortex. Our approach was to impose transmembrane Na⁺ and H⁺ gradients of varying magnitude and then to measure the net flux of Na⁺ over the subsequent 5-s period. The Na⁺-H⁺ exchanger was observed to be at equilibrium (i.e. no significant net Na⁺ flux) whenever Na⁺]_i/Na⁺]_o was equal to H⁺]_i/H⁺]_o. Moreover, under all conditions the magnitude and direction of net Na⁺ flux was independent of changes in the transmembrane electrical potential difference. These results are consistent with a value of 1.0 for the coupling ratio of Na⁺-H⁺ exchange in renal microvillus membrane vesicles.