REGULATORY VOLUME DECREASE IN CULTURED ASTROCYTES .1. POTASSIUM-ACTIVATED AND CHLORIDE-ACTIVATED PERMEABILITY

Regulatory volume decrease (RVD) in detached cerebellar astrocytes in culture after acute exposure to hyposmolarity was characterized in thi s and the accompanying paper [H. Pasantes-Morales, R. A. Murray, R. Sa nches-Olea, and J. Moran. Am. J. Physiol. 266 (Cell Physiol. 35): C172 -C178, 1994]. RVD was independent of extracellular calcium, was accele rated at pH 8-9 and retarded at pH 6, and was reduced at temperatures < 18 degrees C. The cationic pathway activated by hyposmolarity was sp ecific for K+ and Rb+, since RVD was abolished and secondary swelling occurred when these ions replaced Na+. However, Li+, choline, tris(hyd roxymethyl)aminomethane, and glucosamine, all as Cl- salts, did not af fect RVD. The anion pathway was unselective, since RVD was inhibited w hen NaCl was replaced by anion K+ salts with a permeability rank of SC N- = I- > NO3- > Cl- > benzoate > acetate > > SO3- > gluconate. RVD wa s unaffected by bumetanide (50 mu M) and weakly inhibited by furosemid e (2 mM). Quinidine but not other K+ channel blockers inhibited RVD, a nd its effect was reversed by gramicidin. RVD was inhibited by 4,4'-di isothiocyanostilbene-2,2'-disulfonic acid and dipyridamole but not by diphenylamine-2-carboxylate or anthracene-9-carboxylate. These results suggest that diffusion possibly via channels rather than cotransporte rs is involved in the swelling-activated K+ and Cl- fluxes. Gramicidin did not change astrocyte volume in isosmotic conditions, but greatly accelerated RVD, suggesting that low Cl- permeability in isosmotic con ditions markedly increases by swelling, thus making K+ permeability th e rate-limiting step for RVD.