Geographutoxin II, a novel peptide inhibitor of Na channels of skeletal muscles and autonomic nerves.

Geographutoxin II (GTX II, 3 X 10(-9) to 10(-7) M) from a cone shell Conus geographus inhibited twitch responses of the isolated mouse diaphragm to direct stimulation in a dose-dependent manner. The contraction of the diaphragm induced by grayanotoxin I or veratridine was abolished by GTX II (3 X 10(-7) M), whereas the contractile response to KCI or caffeine was not affected. GTX II induced similar effects on isolated bullfrog sartorius muscles, but required higher concentrations (6 X 10(-7) to 3 X 10(-6) M). GTX II (greater than 10(-6) M) inhibited or abolished the action potential evoked in sartorius muscles markedly. In the isolated guinea pig vas deferens and ileum, GTX II caused a dose-dependent inhibition of the twitch responses to indirect nerve stimulation at concentrations of 3 X 10(-8) to 10(-6) M and 10(-7) to 10(-6) M, respectively. But the toxin had no effect on the dose contractile-response curves for norepinephrine, carbamylcholine or KCI in the vas deferens and for carbamylcholine or histamine in the ileum. GTX II (5 X 10(-8) to 10(-6) M) decreased norepinephrine release induced by veratridine from the vas deferens in a dose-dependent manner. These results suggest that GTX II blocks the voltage-sensitive Na channels in the cell membrane of skeletal muscles and autonomic nerves and these may play an important role in the mechanism of inhibitory effects of GTX II on contractile responses of these tissues to electrical stimulation.