Vecuronium suppresses transmission at the rat phrenic neuromuscular junction by inhibiting presynaptic L-type calcium channels

Background and objectivesThe non-depolarizing muscle relaxant vecuronium inhibits contraction by competitive inhibition of postsynaptic acetylcholine receptors (AchRs), which decreases the number of quanta released per impulse in response to 50 Hz stimulation. The specific role of calcium influx through L-type calcium channels is the promotion of endocytosis and vesicle recycling during high-frequency stimulation. Vecuronium also induces four pulse tetanic fade, a proxy measure of decreased quanta release. We examined whether vecuronium suppresses neuromuscular transmission during high-frequency stimulation by inhibiting presynaptic L-type calcium channels.MethodsFifty male Sprague-Dawley rats were divided into five treatment groups: unstimulated control group, α-bungarotoxin (BTX) group, nifedipine group, vecuronium group, and nifedipine plus vecuronium group. Rat phrenic nerve-diaphragm neuromuscular juctions were stimulated at 50 Hz and field excitatory post-synaptic potentials (fEPSPs) were recorded. Expression levels of the presynaptic Ca2+-binding, protein synaptotagmin 1, and the presynaptic plasma membrane protein, syntaxin 1, were measured by Western blots.ResultsThe fEPSPs evoked by 50 Hz stimulus trains were decreased by vecuronium, nifedipine, and by vecuronium plus nifedipine. Nifedipine, an L-type calcium channel blocker, reduced the expression of synaptogamin and syntaxin and blocked the suppressive effect of vecuronium, suggesting that both agents inhibit presynaptic L-type calcium channels.ConclusionsVecuronium which blocked L-type calcium channels may suppress activity of the α3β2 nAChR subunit, which exists in the presynaptic membrane and enhances quantal release. This α3β2 nAChR-mediated positive feedback effect may be facilitated by L-type Ca2+ channel activity under high-frequency stimulation. Vecuronium may disrupt this positive feedback cycle, leading to suppression of fEPSPs. Vercuronium may reduce neuromuscular transmission through presynaptic and postsynaptic mechanisms.

► Reveal a new L-type calcium regulation process on Ach release. ► Electrophysiological and multi-chemical are used. ► Experimental results included use of four different configurations of neurotransmitter. ► Reveal regulation of L-type calcium blockers on synaptotagmin and syntaxin protein level.