Isovolumetric regulation mechanisms in cultured cerebellar granule neurons

Cultured cerebellar granule neurons exposed to gradual reductions in osmolarity (− 1.8 mOsm/min) maintained constant volume up to − 50% external osmolarity (π o ), showing the occurrence of isovolumetric regulation (IVR). Amino acids, Cl − , and K + contributed at different phases of IVR, with early efflux threshold for [ 3 H]taurine, d ‐[ 3 H]aspartate (as marker for glutamate) of π o − 2% and − 19%, respectively, and more delayed thresholds of − 30% for [ 3 H]glycine and − 25% and − 29%, respectively, for Cl − ( 125 I) and K + ( 86 Rb). Taurine seems preferentially involved in IVR, showing the lowest threshold, the highest efflux rate (five‐fold over other amino acids) and the largest cell content decrease. Taurine and Cl − efflux were abolished by niflumic acid and 86 Rb by 15 m m Ba 2+ . Niflumic acid essentially prevented IVR in all ranges of π o . Cl − ‐free medium impaired IVR when π o decreased to − 24% and Ba 2+ blocked it only at a late phase of − 30% π o . These results indicate that in cerebellar granule neurons: (i) IVR is an active process of volume regulation accomplished by efflux of intracellular osmolytes; (ii) the volume regulation operating at small changes of π o is fully accounted for by mechanisms sensitive to niflumic acid, with contributions of both Cl − and amino acids, particularly taurine; (iii) Cl − contribution to IVR is delayed with respect to other niflumic acid‐sensitive osmolyte fluxes (osmolarity threshold of − 25% π o ); and (iv), K + fluxes do not contribute to IVR until a late phase (< − 30% π o ).