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+]0 was equal to [H+]i/[H+]0. 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.