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

I joined the department of medicine at Yale as an assistant professor in 1977 and had a laboratory in the department of physiology. My initial studies and grants were for the characterization of Na+-glucose cotransport in renal brush border membrane vesicles, a research interest and experimental model system that I had brought from my research training with Bert Sacktor at the Gerontology Research Center of the National Institutes of Health in Baltimore (1,2). In a previous issue of this journal, I described the fortuitous manner in which my work with Bert on renal brush border glucose transport came about (3). I was fortunate enough to receive funding for a postdoctoral fellow on my initial grant and one of the applicants for the advertised position was Jim Kinsella. Jim joined my laboratory in 1978 after receiving his Ph.D. in pharmacology from the State University of New York Upstate Medical Center at Syracuse. Jim’s thesis work with Charles Ross had involved the elegant use of both brush border and basolateral membrane vesicles to provide the first description of the pathways for organic cation transport in this preparation (4,5). In retrospect, I am not sure why Jim came to work with such a junior mentor who scarcely had more research experience than he had. Perhaps the outstanding environment for renal research due to the presence of more senior investigators such as Gerhard Giebisch, Emile Boulpaep, and Fred Wright in the department of physiology at Yale was the real attraction. In any event, Jim planned to study the kinetics of the Na+-glucose cotransporter as an extension of work that I had done that showed potential-dependent translocation of the empty substrate binding site (6). Prior to his arrival in the laboratory, Jim submitted what proved to be a successful application for an individual National Institutes of Health postdoctoral research fellowship to study this subject. This saved my grant-funded fellowship position, which then was used to support a renal fellow named Jeff Blomstedt, who went on to describe the anion exchange process mediating KINSELLA, JAMES L., AND PETER S. ARONSON. Properties of the Na+-H+ exchanger in renal microvillus membrane vesicles. Am. J. Physiol. 238 (Renal Fluid Electrolyte Physiol. 7): F461–F469, 1980.—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 SO4 2 . 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. We conclude that isolated renal microvillus membranes contain a tightly coupled Na+-H+ exchanger that may play an important role in proximal tubular acidification.