VOLUME-SENSITIVE KC1 COTRANSPORT AND TAURINE FLUXES IN HORSE RED-BLOOD-CELLS

Potassium (using Rb-86+ as a tracer), amino acid and taurine fluxes we re measured in horse red blood cells (RBCs). No volume-sensitive compo nent of alanine and glycine transport was observed, and although volum e-sensitive taurine fluxes were observed in most animals, their absolu te magnitudes were small. K+ fluxes, however, were shown to be particu larly volume sensitive; they were stimulated by cell swelling and inhi bited by cell shrinkage. Sizeable fluxes were present at normal cell v olumes. The volume-sensitive K+ flux was Cl- dependent and was abolish ed by Cl- replacement with methylsulphate. The Cl--dependent K+ fluxes in horse red blood cells were stimulated by lowering in external pH t o 6.9 and by treatment with the sulphydryl-reacting agent, N-ethylmale imide. They were inhibited by the potent K+-Cl- co-transport inhibitor , DIOA, ([(dihydroindenyl)oxy]alkanoic acid) but were insensitive to t he Na+-K+-Cl- co-transport inhibitors, frusemide and bumetanide. A Cl- channel inhibitor, 5-nitro-2-(phenylpropyl-amino)-benzoate (NPPB), pr oduced partial inhibition. These results suggest that regulatory volum e decrease in horse red blood cells is achieved predominantly by volum e-sensitive K+ efflux mediated via a K+-Cl- co-transport system with s imilar properties to those observed in the red blood cells of other sp ecies. The significance of these findings and their rheological conseq uences are discussed.