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

Following osmotic swelling, erythrocytes from the European eel, Anguilla anguilla, 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 ⁸⁶Rb⁺ 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.