Spontaneous acetylcholine release in mammalian neuromuscular junctions.
Spontaneous secretion of the neurotransmitter acetylcholine in mammalian neuromuscular synapsis depends on the Ca2+ content of nerve terminals. The Ca2+ electrochemical gradient favors the entry of this cation. We investigated the possible involvement of three voltage-dependent Ca2+ channels (VDCC) (L-, N-, and P/Q-types) on spontaneous transmitter, release at the rat neuromuscular junction. Miniature end-plate potential (MEPP) frequency was clearly reduced by 5 microM nifedipine, a blocker of the L-type VDCC, and to a lesser extent by the N-type VDCC blocker, omega-conotoxin GVIA (omega-CgTx, 5 microM). On the other hand, nifedipine and omega-CgTx had no effect on K(+)-induced transmitter secretion. omega-Agatoxin IVA (100 nM), a P/Q-type VDCC blocker, prevents acetylcholine release induced by K+ depolarization but failed to affect MEPP frequency in basal conditions. These results suggest that in the mammalian neuromuscular junction Ca2+ enters nerve terminals through at least three different channels, two of them (L- and N-types) mainly related to spontaneous acetylcholine release and the other (P/Q-type) mostly involved in depolarization-induced neurotransmitter release. Ca(2+)-binding molecule-related spontaneous release apparently binds Ca2+ very rapidly and would probably be located very close to Ca2+ channels, since the fast Ca2+ chelator (BAPTA-AM) significantly reduced MEPP frequency, whereas EGTA-AM, exhibiting slower kinetics, had a lower effect. The increase in MEPP frequency induced by exposing the preparation to hypertonic solutions was affected by neither external Ca2+ concentration nor L-, N-, and P/Q-type VDCC blockers, indicating that extracellular Ca2+ is not necessary to produce hyperosmotic neurosecretion. On the other hand, MEPP frequency was diminished by BAPTA-AM and EGTA-AM to the same extent, supporting the view that hypertonic response is promoted by "bulk" intracellular Ca2+ concentration increases.
Losavio
A
A
Instituto de Investigaciones Médicas A. Lanari, Universidad de Buenos Aires, Argentina.
Muchnik
S
S
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology*; D020864:Calcium Channels, N-Type; D003964:Diaphragm; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007700:Kinetics; D009469:Neuromuscular Junction / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D010455:Peptides / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D013111:Spider Venoms / Q000494:pharmacology; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA
D000109:Acetylcholine; D002118:Calcium; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D020864:Calcium Channels, N-Type; D004533:Egtazic Acid; D009543:Nifedipine; D010455:Peptides; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; C046770:EGTA acetoxymethyl ester