Effects of denervation and reinnervation on the responses of kitten muscles to acetylcholine and suxamethonium.

1. The development of the high sensitivity of the adult neuromuscular junction to the blocking and depolarizing action of suxamethonium was studied in kittens of different ages.2. It was found that the sensitivity to the depolarizing effects of suxamethonium increases simultaneously with that to the blocking effects of the drug during the 3rd-6th week of life. The increase of sensitivity to the blocking effects of suxamethonium during development can therefore be accounted for by the depolarizing effect of the drug.3. When the nerve is crushed during the first week of the animal's life and then left to regenerate, the development of the high sensitivity of the neuromuscular junction to suxamethonium and acetylcholine is impaired. Thus during development the nerve is essential for the induction of the high sensitivity of the neuromuscular junction to acetylcholine as well as to other depolarizing drugs.4. The blocking and depolarizing effects of suxamethonium on reinnervated muscles of cats at different ages were compared. The depolarizing action of suxamethonium on reinnervated end-plates is always smaller, but its blocking effect is not. In young animals the blocking effect of suxamethonium on reinnervated muscles is smaller, whereas reinnervated muscles of adult animals are extremely sensitive to the blocking effects of the drug.5. 24-36 hr after denervation cat muscles were more sensitive to the blocking effects of suxamethonium but the depolarizing effect of the drug on the end-plate region was smaller. It thus appears that the nerve not only induces but also maintains the high sensitivity of the end-plate region to depolarizing drugs.