Calcium-induced calcium increase in secretory vesicles of permeabilized rat neurohypophysial nerve terminals.
Digitonin-permeabilized isolated neurohypophysial nerve terminals are known to release their secretory vesicle content under calcium challenge. On this preparation, we monitored intra-organelle Ca2+ concentration using digital fluorescence microscopy of Fura-2. The superfusion of artificial intracellular solution containing 10 to 50 microM Ca2+ induced an intra-organelle [Ca2+] increase. Two major organelles are candidates for this increase: secretory vesicles and mitochondria. In an attempt to detect calcium changes in the vesicles, ruthenium red was used to impair mitochondrial calcium uptake. Part of the ruthenium red-insensitive intra-organelle [Ca2+] increase was abolished by raising sodium in the solution. Removing sodium boosted the intra-organelle [Ca2+] increase. These results taken together suggest the participation of Na/Ca exchange, known to exist in the membrane of these secretory vesicles. In addition to Na/Ca exchange, there would be at least another mechanism of vesicular calcium intake, as suggested by the partial inhibition of intra-organelle [Ca2+] increase obtained under acidic compartments: neutralization with NH4Cl. This mechanism remains to be defined. The main conclusion presented here, that an intravesicular [Ca2+] increase takes place at the rate of secretion, was predicted by the hypothesis that intravesicular Ca2+ changes would be involved in stimulus-secretion coupling.
Troadec
J D
JD
Laboratoire de Physiologie Cellulaire et Moléculaire (CNRS-UMR 6548), Nice, France.
Thirion
S
S
Laugier
J P
JP
Nicaise
G
G
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology*; D002451:Cell Compartmentation; D002463:Cell Membrane Permeability; D003594:Cytoplasmic Granules / Q000187:drug effects / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D008297:Male; D008856:Microscopy, Fluorescence; D008928:Mitochondria / Q000378:metabolism; D010904:Pituitary Gland, Posterior / Q000187:drug effects* / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000187:drug effects* / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology
D005456:Fluorescent Dyes; D012964:Sodium; D002118:Calcium; D016257:Fura-2