Exogenous nanomolar zinc ion (Zn2+) as a negative modulator of neuromuscular transmission via presynaptic mechanism in mouse diaphragm.

Zinc (Zn2+) is the second most abundant trace element after iron, with most of it is stored in skeletal muscles. Although a large part of Zn2+ is tightly bound to metalloproteins, the small portion of free Zn2+ can participate in nerve signaling. Here we examined the effects of Zn2+ at nanomolar concentrations on neuromuscular transmission in the diaphragm, the main respiratory muscle. Zn2+ reduced spontaneous neurotransmitter release at both lowered and physiological external Ca2+ levels. Additionally, Zn2+ effectively decreased the probability of neurotransmitter release upon single nerve stimulation under lowered external Ca2+, and inhibited Ca2+-independent sucrose-induced exocytosis. At physiological external Ca2+ concentration, Zn2+ decreased neurotransmitter release during low-frequency stimulation. The reduction became increased during short trains of moderate-to-high frequency stimuli. Furthermore, Zn2+ diminished both neurotransmitter release and the participation of dye-labeled synaptic vesicles in exocytosis during prolonged nerve firing at moderate frequency. Zn2+ aggravated muscle fatigue and impaired contraction recovery upon nerve stimulation. This was linked to a reduction in peak inspiratory flow in mice, an indicator of diaphragm function, after injection of low-dose Zn2+. Our data suggest that at nanomolar concentrations, Zn2+ is a negative modulator of neuromuscular function.