Effects of Naja nivea venom on nerve, cardiac and skeletal muscle activity of the frog.

1. The effects of 1 mg/ml whole Naja nivea (Cape cobra) venom, and of three different toxins isolated from it, on spinal reflex excitability and cardiac and skeletal muscle activity of the frog (Xenopus laevis), were studied. Isolated spinal cords, sciatic nerve-gastrocnemius muscle units and isolated heart preparations were used.2. Synaptic efficiency and/or motoneurone excitability ceased totally and irreversibly after the application of whole venom within 18.5 minutes. The application of alpha toxin caused polysynaptic reactivation of the motoneurones after the monosynaptic response elicited by electrical stimulation, i.e. it had an excitatory effect, which was retained after washing off the toxin. The effect of beta toxin resembled that of alpha toxin, except that the effect was lost after the toxin was washed off. The delta toxin abolished all nervous reactivity irreversibly and was found to be 2.64 times more effective in this respect than whole venom.3. Responses of the gastrocnemius muscle to electrical stimulation of its motor nerve ceased 10 to 20 min after application of the venom. The muscle usually still responded slightly to direct stimulation for a few minutes. The effect was irreversible. The venom did not only block neuromuscular transmission, but also affected the muscle fibre itself. Nerve conductivity was relatively resistant to the venom; alpha and beta toxins produced similar results but were less effective.4. The application of whole venom (1 mg/ml) to the isolated heart resulted in complete spasm of the ventricle within 3 to 8 minutes. The atria continued contracting long after the ventricle ceased. When the venom was washed off, contractions of the atria were restored, while the ventricle remained irreversibly in a state of spasm. The alpha, beta and delta toxins weakened the contractions of the ventricle, but did not produce spasm. The effect on the ventricle was reversible. Contractions of the atria remained apparently unaltered.