Neurochemical evidence that [Ca2+]o antagonizes the effect of neomycin on acetylcholine release from mouse hemidiaphragm preparation: an attempt to assess the margin to safety.

Although the neuromuscular junction is the most thoroughly studied synapse of any type and has become the model of our understanding of synaptic transmission, some questions remain unanswered; e.g. there has been no direct assessment of the size of margin of safety. In this study the [Ca2+]o-dependent, quantally released acetylcholine measured by a neurochemical method, and the contraction of the mouse hemidiaphragm in response to phrenic nerve stimulation were recorded, and the effect of neomycin was studied. It was found that a much higher concentration of neomycin was needed to depress contractions, than to reduce acetylcholine release to the same extent, and that there was an inverse correlation between [Ca2+]o and the inhibitory effect of neomycin on acetylcholine release. Ninety percent of the release of acetylcholine had to be reduced by neomycin before any reduction in muscle responses could be seen. This indicates that the margin of safety is about 10. In conclusion, at the neuromuscular junction any reduction in ACh release, whatever the mechanism, first produces a reduction in the margin of safety. The nondepolarizing neuromuscular blocking agents block primarily the nicotinic receptors located on the postjunctional site. Many receptors have to be blocked before a reduction of muscle response is observed. This is probably the reason why unexpected clinical problems (e.g. recurarization) have been described when a patients has been treated with antibiotics, even though the dose of muscle relaxant injected was relatively low.