Chloride dependence of frusemide- and phloretin-sensitive passive sodium and potassium fluxes in human red cells.

1. In outdated human red blood cells, there was a Ko-dependent, ouabain-insensitive efflux of Na that was inhibited by frusemide and phloretin. 2. It was reduced or absent (a) in media containing Ca or Mg, (b) in fresh cells. 3. In media with Cl partly replaced by NO3, Na efflux, and its sensitivity to Ko and frusemide, were reduced. 4. The saturable component of ouabain-insensitive K influx was dependent on Cl and inhibited by frusemide and by phloretin. 5. Bromide also supported a frusemide-sensitive, ouabain-insensitive K influx but acetate and SO4 (like NO3) did not.