Effects of synthetic omega-conotoxin, a new type Ca2+ antagonist, on frog and mouse neuromuscular transmission.

A new type Ca2+ antagonist, synthetic omega-conotoxin (omega-CgTX) decreased the peak height of the endplate potential (EPP) in frog muscle but had no effect on the mouse neuromuscular junction. The reduction of endplate potential in frogs was due to a decrease in transmitter release, since the mean quantal content estimated by variance of EPPs (m) and from the peak heights of EPPs and miniature EPPs (m1) was reduced by omega-CgTX, but the postsynaptic sensitivity to ACh was unaltered. The decrease of mean quantal content caused by omega-CgTX was reversed by 4-aminopyridine, guanidine and Bay K 8644. Also, the effect of omega-CgTX was weakened in the presence of 10 mM Ca2+ or 12 mM Mg2+. Statistical analysis revealed that omega-CgTX decreased the number of quanta available (n) whereas the probability of release (p) remained unaffected.