Depolarization, exocytosis and amino acid release evoked by hyposmolarity from cortical synaptosomes.
External osmolarity reduction (20%) led to labelled glutamate, GABA and taurine release from rat brain cortical synaptosomes. A Cl--independent, Na+-dependent, La3+-sensitive and tetrodotoxin (TTX) reduced depolarization of synaptosomes occurred upon hyposmolarity, suggestive of Na+ entry through nonselective cation channels. This depolarization, together with cytosolic Ca2+ ([Ca2+]I) increase, resulted in exocytosis, monitored by FM1-43. The release fraction resulting from these phenomena was estimated, by its decrease, by La3+, EGTA-AM and tetanus toxin (TeTX), as 34-44% for glutamate, 21-29% for GABA and 18-22% for taurine. Protein kinase C (PKC) activation by phorbol-12-myristate-13-acetate (PMA) increased the hyposmolarity-elicited exocytosis and this activation increased glutamate (80%), GABA (51%) and taurine (42%) hyposmotic efflux. Inhibition by chelerythrine reduced glutamate, GABA and taurine efflux by 64%, 50% and 24%, respectively. The Na+-dependence of amino acid release (glutamate 63%, GABA 46% and taurine 29%) may result from both, prevention of the depolarization-exocytosis efflux, and blockade of the carrier reversal operation. Carrier blockade by dl-threo-beta-benzyloxy aspartate (TBOA) and NO-711 resulted in 37% and 28% reduction of glutamate and GABA release, respectively. Contribution of the osmolyte leak pathway to amino acid release, estimated by the influence of Cl- (NPPB) and tyrosine kinase (AG18) blocker, was up to 55% for taurine, but only 10-18% for GABA, with apparently no contribution for glutamate. The predominant osmolyte-type mechanism of taurine release suggest its function in volume control in nerve endings, while glutamate and GABA respond to events concurrent with hyposmolarity by a neurotransmitter-like release mechanism. The hyposmolarity-induced amino acid efflux from nerve endings may have consequences for neuronal excitability during hyponatremia.
Department of Biophysics, Institute of Cell Physiology, National University of Mexico, Mexico.
Karina
K
Tuz
Claudia
C
Peña-Segura
Rodrigo
R
Franco
Herminia
H
Pasantes-Morales
D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D018698:Glutamic Acid / Q000378:metabolism; D007038:Hypotonic Solutions / Q000494:pharmacology; D008297:Male; D009994:Osmolar Concentration; D051381:Rats; D017208:Rats, Wistar; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013654:Taurine / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D000596:Amino Acids; D007038:Hypotonic Solutions; D013654:Taurine; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid