Zn2+ stimulates spontaneous transmitter release at mouse neuromuscular junctions

1. Experiments were carried out to examine the effect of Zn2+ on the rate of spontaneous release of transmitter at the neuromuscular junction of the mouse diaphragm muscle, in the presence and absence of external Ca2+. Miniature endplate potentials (m.e.p.ps) were measured in vitro. 2. Zn2+ markedly elevated the frequency of m.e.p.ps without affecting the resting membrane potential of muscle fibres. This effect was time- and concentration-dependent but was independent of the presence of external Ca2+. In a Ca2+-free bathing solution, Zn2+ frequently produced twitching in several fibers. The twitching dislodged the microelectrode. Replacement of the 10 mM NaCl in the Ca2+-free solution with equimolar KCl overcame this difficulty. The experiments summarized below were done in the Ca2+-free bathing solution which contained 10 mM KCl instead of 10 mM NaCl. 3. The effect of Zn2+ was transient and required a latent period of many minutes. Low temperature (24 degrees C) increased the length of this latent period and reduced the maximum effect of Zn2+. 4. Zn2+ increased the frequency of m.e.p.ps in K+-free (replaced with NaCl) solution. The effect appeared with shorter latency in this solution compared to the standard Krebs-Ringer solution. 5. The effect of Zn2+ was partially antagonized by dantrolene sodium or by neomycin. Both agents also reduced the effect of external Ca2+ on m.e.p.ps in depolarizing solution. 6. Cd2+ and 2,3-bisphosphoglycerate also elevated the frequency of m.e.p.ps in a manner independent of external Ca2+, but the latter compound was much less potent than Cd2+. 7. These experiments provide evidence for a role of intracellularly stored Ca24 in the release of transmitter at the motor nerve terminal. The release of Ca24 from the storage site may be coupled with the metabolism of phosphatidylinositol.