Hyposmolarity evokes norepinephrine efflux from synaptosomes by a depolarization- and Ca2+ -dependent exocytotic mechanism.
Osmolarity reduction (20%) elicited 3H-norepinephrine (NE) efflux from rat cortical synaptosomes. The hyposmotic NE release resulted from the following events: (i) a Na+-dependent and La3+-, Gd3+- and ruthenium red-sensitive depolarization; (ii) a cytosolic Ca2+ ([Ca2+]i) rise with contributions from external Ca2+ influx and internal Ca2+ release, probably through the mitochondrial Na+-Ca2+ exchanger; and (iii) activation of a [Ca2+]i-evoked, tetanus toxin (TeTX)-sensitive, PKC-modulated NE efflux mechanism. This sequence was established from results showing a drop in the hyposmotic [Ca2+]i rise by preventing depolarization with La3+, and by the inhibitory effects of Ca2+-free medium (EGTA; 50%), CGP37157 (the mitochondrial Na+-Ca2+ exchanger blocker; 48%), EGTA + CGP37157 or by EGTA-AM (> 95% in both cases). In close correspondence with these effects, NE efflux was 92% decreased by Na+ omission, 75% by La3+, 47% by EGTA, 50% by CGP37157, 90% by EGTA + CGP37157 and 88% by EGTA-AM. PKC influenced the intracellular Ca2+ release and, mainly through this action, modulated NE efflux. TeTX suppressed NE efflux. The K+-stimulated NE release, studied in parallel, was unaffected by Na+ omission, or by La3+, Gd3+ or ruthenium red. It was fully dependent on external Ca2+, insensitive to CGP37157 and abolished by TeTX. These results suggest that the hyposmotic events, although different from the K+-evoked depolarization and [Ca2+]i rise mechanisms, are able to trigger a depolarization-dependent, Ca2+-dependent and TeTX-sensitive mechanism for neurotransmitter release.
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Department of Biophysics, Institute of Cell Physiology, National University of Mexico, UNAM, Apartado Postal 70-253, 04510 México D.F., Mexico City, Mexico.
Pasantes-Morales
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D000818:Animals; D001464:Barium / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002227:Carbazoles / Q000494:pharmacology; D002540:Cerebral Cortex / Q000166:cytology*; D002998:Clonazepam / Q000031:analogs & derivatives / Q000494:pharmacology; D004347:Drug Interactions; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D007211:Indoles / Q000494:pharmacology; D007811:Lanthanum / Q000494:pharmacology; D009638:Norepinephrine / Q000378:metabolism*; D009994:Osmolar Concentration; D010703:Phorbol Esters / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011493:Protein Kinase C / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D012430:Ruthenium Red / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger / Q000494:pharmacology; D013050:Spectrometry, Fluorescence / Q000379:methods; D013574:Synaptosomes / Q000187:drug effects / Q000502:physiology*; D013744:Tetanus Toxin / Q000494:pharmacology; D013841:Thiazepines / Q000494:pharmacology; D013997:Time Factors; D014316:Tritium / Q000378:metabolism; D020960:omega-Conotoxins / Q000494:pharmacology
D002121:Calcium Channel Blockers; D002227:Carbazoles; D004791:Enzyme Inhibitors; D007211:Indoles; D010703:Phorbol Esters; D019831:Sodium-Calcium Exchanger; D013744:Tetanus Toxin; D013841:Thiazepines; D020960:omega-Conotoxins; D002104:Cadmium; D014316:Tritium; D012430:Ruthenium Red; C081021:Go 6976; C075851:omega-conotoxin-MVIIC; D001464:Barium; D004533:Egtazic Acid; D002998:Clonazepam; D011189:Potassium Chloride; D007811:Lanthanum; C079032:CGP 37157; C032974:12-O-retinoylphorbol-13-acetate; C046770:EGTA acetoxymethyl ester; D012964:Sodium; D011493:Protein Kinase C; D002118:Calcium; D009638:Norepinephrine