alpha-Latrotoxin releases calcium in frog motor nerve terminals.
alpha-Latrotoxin (alpha-LTX) is a neurotoxin that accelerates spontaneous exocytosis independently of extracellular Ca(2+). Although alpha-LTX increases spontaneous transmitter release at synapses, the mechanism is unknown. We tested the hypothesis that alpha-LTX causes transmitter release by mobilizing intracellular Ca(2+) in frog motor nerve terminals. Transmitter release was measured electrophysiologically and with the vesicle marker FM1-43; presynaptic ion concentration dynamics were measured with fluorescent ion-imaging techniques. We report that alpha-LTX increases transmitter release after release of a physiologically relevant concentration of intracellular Ca(2+). Neither the blockade of Ca(2+) release nor the depletion of Ca(2+) from endoplasmic reticulum affected Ca(2+) signals produced by alpha-LTX. The Ca(2+) source is likely to be mitochondria, because the effects on Ca(2+) mobilization of CCCP (which depletes mitochondrial Ca(2+)) and of alpha-LTX are mutually occlusive. The release of mitochondrial Ca(2+) is partially attributable to an increase in intracellular Na(+), suggesting that the mitochondrial Na(+)/Ca(2+) exchanger is activated. Effects of alpha-LTX were not blocked when Ca(2+) increases were reduced greatly in saline lacking both Na(+) and Ca(2+) and by application of intracellular Ca(2+) chelators. Therefore, although increases in intracellular Ca(2+) may facilitate the effects of alpha-LTX on transmitter release, these increases do not appear to be necessary. The results show that investigations of Ca(2+)-independent alpha-LTX mechanisms or uses of alpha-LTX to probe exocytosis mechanisms would be complicated by the release of intracellular Ca(2+), which itself can trigger exocytosis.
Tsang
C W
CW
Department of Physiology, University of Toronto, Toronto, Canada M5S 1A8.
Elrick
D B
DB
Charlton
M P
MP
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004963:Estrenes / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology; D005456:Fluorescent Dyes; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D008928:Mitochondria / Q000187:drug effects / Q000378:metabolism; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000378:metabolism; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D011726:Pyridinium Compounds; D011760:Pyrrolidinones / Q000494:pharmacology; D000644:Quaternary Ammonium Compounds; D011894:Rana pipiens; D012964:Sodium / Q000378:metabolism; D026941:Sodium Channel Blockers; D013111:Spider Venoms / Q000494:pharmacology*; D013779:Tetrodotoxin / Q000494:pharmacology; D010738:Type C Phospholipases / Q000037:antagonists & inhibitors; D014475:Uncoupling Agents / Q000494:pharmacology
D002614:Chelating Agents; D004963:Estrenes; C073804:FM1 43; D005456:Fluorescent Dyes; D010726:Phosphodiesterase Inhibitors; D011726:Pyridinium Compounds; D011760:Pyrrolidinones; D000644:Quaternary Ammonium Compounds; D026941:Sodium Channel Blockers; D013111:Spider Venoms; D014475:Uncoupling Agents; C060229:1-(6-((3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D013779:Tetrodotoxin; D004533:Egtazic Acid; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone; C020763:alpha-latrotoxin; D012964:Sodium; D010738:Type C Phospholipases; D002118:Calcium