Presynaptic inhibitory effect of geographutoxin II, a new peptide toxin from Conus geographus venom, in the guinea-pig vas deferens.

Geographutoxin II (GTX II), a new peptide toxin, isolated from Conus geographus venom, caused a dose-dependent inhibition of the contractile response of the guinea-pig vas deferens to transmural stimulation at concentrations of 3 X 10(-8) to 10(-6) M. The dose-response curves for noradrenaline, acetylcholine and KCl was not affected by GTX II (2 X 10(-7) to 10(-6) M). The release of noradrenaline induced by veratridine was strongly inhibited by GTX II. These results suggest that the GTX II-induced inhibition of the vas deferens may be a presynaptic effect causing a decrease of transmitter release.