Thiol-dependent passive K/Cl transport in sheep red cells: I. Dependence on chloride and external ions.

Treatment with 2 mM N-ethylmaleimide (NEM) caused a marked increase in K+ permeability of low K+ but not of high K+ sheep red cells suspended in isosmotic Cl- media with 10(-4) M ouabain. The Na+ permeability was unaltered. Kinetic analysis by K+ efflux and K+ or Rb+ influx measurements suggests that NEM primarily increased the bidirectional fluxes of K+ and Rb+, since (a) no significant change in the apparent external affinities of these ions was found, and (b) below unity, the observed flux ratios were close to those calculated from the Ussing relationship. Replacement of Cl- by NO3 abolished the NEM-stimulated and reduced the basal K+ flux rates. Similarly, 10(-3) M furosemide inhibited Cl- -dependent K+ fluxes in both control and NEM-treated LK red cells. Exposure of LK cells to hyposmotic but not to hyperosmotic salt solutions increased the basal Cl- dependent K+ flux twofold as reported by Dunham and Ellory (J. Physiol. (London) 318:511-530, 1981) but did not affect its fractional stimulation by NEM. The action of NEM is interpreted as a stimulation of a temperature-dependent and Cl- -requiring K+ transport pathway genetically preserved in adult LK but turned off in HK sheep red cells. In addition, common to both LK and HK sheep red cells was a basal K+ flux that operated in the presence of either Cl- or NO3-.