SPONTANEOUS ACETYLCHOLINE-RELEASE IN MAMMALIAN NEUROMUSCULAR-JUNCTIONS

Spontaneous secretion of the neurotransmitter acetylcholine in mammali an neuromuscular synapsis depends on the Ca2+ content of nerve termina ls. The Ca2+ electrochemical gradient favors the entry of this cation. We investigated the possible involvement of three voltage-dependent C a2+ channels (VDCC) (L-, N-, and P/Q-types) on spontaneous transmitter release at the rat neuromuscular junction. Miniature end-plate potent ial (MEPP) frequency was clearly reduced by 5 mu M nifedipine, a block er of the L-type VDCC, and to a lesser extent by the N-type VDCC block er, omega-conotoxin GVIA (omega-CgTx, 5 mu M). On the other hand, nife dipine and omega-CgTx had no effect on K+-induced transmitter secretio n. omega-Agatoxin IVA (100 nM), a P/Q-type VDCC blocker, prevents acet ylcholine release induced by K+ depolarization but failed to affect ME PP frequency in basal conditions. These results suggest that in the ma mmalian neuromuscular junction Ca2+ enters nerve terminals through at least three different channels, two of them (L- and N-types) mainly re lated to spontaneous acetylcholine release and the other (P/Q-type) mo stly involved in depolarization-induced neurotransmitter release. Ca2-binding molecule-related spontaneous release apparently binds Ca2+ ve ry rapidly and would probably be located very close to Ca2+ channels, since the fast Ca2+ chelator (BAPTA-AM) significantly reduced MEPP fre quency, whereas EGTA-AM, exhibiting slower kinetics, had a lower effec t. The increase in MEPP frequency induced by exposing the preparation to hypertonic solutions was affected by neither external Ca2+ concentr ation nor L-, N-, and P/Q-type VDCC blockers, indicating that extracel lular 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 p romoted by ''bulk'' intracellular Ca2+ concentration increases.