A. Losavio et S. Muchnik, SPONTANEOUS ACETYLCHOLINE-RELEASE IN MAMMALIAN NEUROMUSCULAR-JUNCTIONS, American journal of physiology. Cell physiology, 42(6), 1997, pp. 1835-1841
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.