Properties of the Na+-H+ exchanger in renal microvillus membrane vesicles

Abstract

Transport of Na+ and H+ was evaluated in brush border membrane vesicles isolated from the rabbit renal cortex. Na+ transport was assayed by a rapid filtration technique; H+ transport was monitored with 5,5-dimethoxazolidine-2,4-dione by flow dialysis. Uphill Na+ uptake was induced by imposition of an in > out H+ gradient, and uphill H+ efflux by imposition of an out > in Na+ gradient, consistent with the action of a Na+-H+ exchanger. The uptake of Na+ was electroneutral either in the presence or absence of a H+ gradient, indicating a fixed 1:1 stoichiometry for the exchange process. Na+ transport was saturable and inhibited by Li+ and NH4+ but not by K+, Rb+, Cs+, or choline. Uphill H+ efflux was induced by imposition of an out > in Li+ gradient. Neither the uptake of Na+ nor H+ efflux were influenced by out > in gradients of Cl- compared to gradients of SCN- or SO42. If transport systems mediating Na+-Cl- co-transport and/or Cl--OH- exchange are present in the microvillus membrane, their respective rates must be slow compared to the rate of Na+-H+ exchange. Transport of Na+ was inhibited by harmaline and amiloride, but not by acetazolamide, furosemide, or 4-acetamido-4''-isothiocyanostilbene-2,2''-disulfonate. Isolated renal microvillus membranes apparently contain a tightly coupled Na+-H+ exchanger that may play an important role in proximal tubular acidification.