Anion-selectivity of the swelling-activated osmolyte channel in eel erythrocytes

Osmotic swelling of fish erythrocytes activates a broad-specificity permeation pathway that mediates the volume-regulatory efflux of taurine and other intracellular osmolytes. This pathway is blocked by inhibitors of the erythrocyte band 3 anion exchanger, raising the possibility that band 3 is involved in the volume-regulatory response. In this study of eel erythrocytes, a quantitative comparison of the pharmacology of swelling-activated taurine transport with that of band S-mediated SO₄^2- transport showed there to be significant differences between them. N-ethylmaleimide and quinine were effective inhibitors of swelling-activated taurine transport but caused little, if any, inhibition of band 3. Conversely, DIDS was a more potent inhibitor of band 3-mediated SO₄^2- flux than of swelling-activated taurine transport. In cells in isotonic medium, pretreated then coincubated with 0.1 mM DIDS, the band 3-mediated transport of SO₄^2- and Cl^- was reduced to a low level. Exposure of these cells to a hypotonic medium containing 0.1 mM DIDS was followed by the activation of a Cl^- permeation pathway showing the same inhibitor sensitivity as swelling-activated taurine transport. The data are consistent with swelling-activated transport of taurine and Cl^- being via a common pathway. A comparison of the swelling-activated transport rates for taurine and Cl^- with those for several other solutes was consistent with the hypothesis that this pathway is an anion-selective channel, similar to those that mediate the volume-regulatory efflux of Cl^- and organic osmolytes from mammalian cells.