Band 3 modulation and hypotonic-stimulated taurine efflux in skate erythrocytes

Previous studies have shown that exposure of skate erythrocytes to hypotonic medium or isosmotic medium with permeant solutes such as ethylene glycol or ammonium chloride increases cell volume. Initial swelling is followed by a regulatory volume decrease accomplished by efflux of cell solutes, including the beta-amino acid taurine. Taurine efflux, as well as the cell volume recovery, is inhibited by stilbene disulfonates and other anion exchange inhibitors, suggesting involvement of the erythrocyte anion exchanger band 3. In the present study we show that binding of the stilbene dihydro 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (H2DIDS) to intact cells doubles after exposure to one-half hypotonic medium. Binding also increases after exposure of erythrocytes to medium of one-third hypotonic medium and ethylene glycol medium, but DIDS binding is not significantly elevated in ammonium chloride medium. To investigate possible mechanisms for modulation of band 3, cells were labeled with 32PO4 and levels of protein phosphorylation determined. Phosphorylation of three proteins of molecular masses of 99, 65, and 32 kDa increases with hypotonic stress. Because the anion exchanger migrates with molecular mass near 100 kDa, the anion exchanger was immunoprecipitated. Hypotonic stress induces a rapid and persistent increase in the 32PO4 phosphorylation of the exchanger. This modification may be responsible for the increased DIDS binding and could be related to stimulated taurine efflux.