REGULATORY VOLUME DECREASE AND ASSOCIATED OSMOLYTE FLUXES IN CEREBELLAR GRANULE NEURONS ARE CALCIUM-INDEPENDENT

To investigate a possible role for Ca as a transduction signal for reg ulatory volume decrease (RVD), the effects of external Ca removal, Ca channel blockers (Cd, Co, La, Gd, verapamil, diltiazem, dihydropyridin es) and inhibitors of endoplasmic reticulum Ca release (dantrolene, ry anodine, TMB-8) were examined on RM) and on the swelling-activated eff lux of two main osmolytes: Cl (traced by I-125) and [H-3]taurine, Omis sion of Ca plus EGTA did not affect RVD or osmolyte release but when B APTA was the chelator, RVD decreased 20%, I-125 fluxes were unaffected and taurine stimulated efflux decreased (20%) white the basal efflux slightly increased (<10%). Verapamil, diltiazem, Co, Cd, La and Gd did not affect RVD or osmolyte fluxes. Nimodipine and nitrendipine (25-50 mu M) markedly decreased RVD and osmolyte fluxes (>90%) through a mec hanism independent of extracellular Ca. Swelling elicited an increase in cytosolic Ca measured by fura-2, which was notably variable ranging 50-350 nM. However, RVD and osmolyte fluxes were not affected by the blockers of endogenous Ca release dantrolene, ryanodine and TMB-8 or b y the permeable Ca chelator BAPTA-AM, even when the cytosolic Ca incre ase was abolished by the chelator. These results indicate that 1) RVD and osmolyte fluxes are independent of extracellular Ca 2) RVD, osmoly te release and cytosolic Ca raise are only coincident events. Conseque ntly, Ca is unlikely to be a transducing signal for RVD in neurons. (C ) 1997 Wiley-Liss, Inc.