Efflux of osmolyte amino acids during isovolumic regulation in hippocampalslices

The efflux of potassium (K+) and amino acids from hippocampal slices was me asured after sudden exposure to 10% (270 mOsm), 25% (225 mOsm) or 50% (150 mOsm) hyposmotic solutions or after gradual decrease (-2.5 mOsm/min) in ext ernal osmolarity, In slices suddenly exposed to 50% hyposmotic solutions, s welling was followed by partial (74%) cell volume recovery, suggesting regu latory volume decrease (RVD), With gradual hyposmotic changes, no increase in cell water content was observed even when the solution at the end of the experiment was 50% hyposmotic, showing the occurrence of isovolumic regula tion (IVR), The gradual decrease in osmolarity elicited the efflux of H-3-t aurine with a threshold at -5 mOsm and D-[H-3]aspartate (as marker for glut amate) and at -20 mOsm for (3)[H]GABA. The efflux rate of [H-3]taurine was always notably higher than those of [H-3]GABA and D-[H-3]aspartate, with a maximal increase over the isosmotic efflux of about -/-fold for taurine and 3- and 2-fold for [H-3]GABA and D-[H-3]aspartate, respectively. The amino acid content in slices exposed to 50% hyposmotic solutions (abrupt change) during 20 min decreased by 50.6% and 62.6% (gradual change). Taurine and gl utamate showed the largest decrease. An enhancement in Rb-86 efflux and a c orresponding decrease in K+ tissue content was seen in association with RVD but not with IVR. These results demonstrate the contribution of amino acid s to IVR and indicate their involvement in this mechanism of cell volume co ntrol. (C) 2000 Wiley-Liss, Inc.