Swelling-activated K+ transport via two functionally distinct pathways in eel erythrocytes

Following osmotic swelling, erythrocytes from the European eel, Anguilla an guilla, underwent a regulatory volume decrease. This was prevented by replacement of Na+ with K+ in the suspending medium, consistent with a role for the (normally outward) electrochemical K+ gradient in the volume-regulatory response. The effect of cell swelling on K+ transport in these cells was investigated using Rb-86(+) as a tracer for K+. Osmotic swelling resulted in an increase in ouabain-insensitive K+ transport that was highest for cells in Cl- and Br- media but which was also significant in I- and NO3- media. Treatment of eel erythrocytes suspended in isotonic Cl- or Br- (but not I- or NO3-) media with the sulfhydryl reagent N-ethylmaleimide (NEM) resulted in a large increase in K+ transport. A quantitative comparison of the pharmacological properties of the ''Cl--dependent'' NEM-activated pathway with those of the ''Cl--independent'' pathway mediating swelling-activated K+ transport in cells in Cl--free (NO3- containing) media showed there to be significant differences between them. By contrast, the pharmacological properties of the Cl--independent swelling-activated K+ pathway were indistinguishable from those of the pathway responsible for the swelling-activated transport of taurine, the major organic osmolyte in these cells. A pharmacological analysis of ouabain-insensitive K+ transport in cells swollen in a hypotonic Cl--containing medium showed there to be two components, one with the characteristics of the NEM-activated system, the other showing the characteristics of the Cl--independent swelling-activated pathway. The data are consistent with the presence of two functionally distinct swelling-activated K+ transport mechanisms in eel erythrocytes: a KCl cotransporter that is activated under isotonic conditions by NEM and a Cl--independent, broad-specificity channel that accommodates a diverse range of organic and inorganic solutes.