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Pasantes Morales, H; Franco, R; Torres-Marquez, ME (2001)

EVIDENCE FOR TWO MECHANISMS OF AMINO ACID OSMOLYTE RELEASE FROM HIPPOCAMPAL SLICES

PFLUG ARCH EUR J PHY 442(5):791-800
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A 30% decrease in osmolarity stimulated H-3-taurine, H-3-GABA and glutamate (followed as H-3-D-aspartate) efflux from rat hippocampal Slices. H-3-taurine efflux was activated rapidly but inactivated slowly. It was decreased markedly by 100 muM 5 -nitro-(3-phenylpropylamino)benzoic acid (NPPB) and 600 muM niflumic acid and inhibited strongly by tyrphostins AG18, AG879 and AG 112 (25-100 muM), suggesting a tyrosine kinase-mediated mechanism. Hyposmolarity activated the mitogen-activated protein kinases (MAPK) extracellular-signal-related kinase-1/2 (ERK1/ERK2) and p38, but blockade of this reaction did not affect H-3-taurine efflux. Hyposmosis also activated phosphatidylinositide 3-kinase (PI3K) and its prevention by wortmannin (100 nM) essentially abolished H-3-taurine efflUX. H-3-taurine efflux was insensitive to the protein kinase C (PKC) blocker chelerythrine (2.5 muM) or to cytochalasin E (3 muM). The release of H-3-GABA and H-3-D-aspartate occurred by a different mechanism, characterized by rapid activation and inactivation, insensitivity to NPPB, niflumic acid, tyrphostins or wortmannin. H-3-GABA and H-3-D-aspartate efflux was not due to external [NaCl] decrease, cytosolic Ca2+ increase or depolarization, or to reverse operation of the carrier. This novel mechanism of amino acid release may be mediated by Ca2+-independent exocytosis and modulated by PKC and actin cytoskeleton disruption, as suggested by its inhibition by chelerythrine and potentiation by 100 nM phorbol-12-rayristate-13 acetate (PMA) and cytochalasin E. GABA and glutamate osmo-sensitive efflux may explain the hyposmolarity-elicited increase in amplitude of inhibitory and excitatory postsynaptic potentials in hippocampal slices as well as the hyperexcitability associated with hyponatraemia.