Development of supersensitivity in denervated slow muscle of the frog. Its dependence on the nerve stump length.

Sensitivity to carbachol has been studied in paired normal and denervated gastrocnemius slow muscle fibers of the frog. Dose response curves showed that after 25 days the ED50 was ten times less in denervated muscle than in the normal. The denervated muscle showed a three-fold increase in affinity constant to d-tubocurarine. In studying the development of supersensitivity of paired muscles denervated near or far from the neuromuscular junction it was found that muscles with a long axon stump started to increase sensitivity three days later than muscles with a short stump. We suggested that in slow muscle fibers, denervation supersensitivity is not only due to a spread in cholinergic receptor area but also to a change in the receptor properties. As the onset of the supersensitivity depends on the length of the nerve stump, it is also suggested that it is regulated by presynaptic factors which would be transported along the axon at a rate of a few centimeters per day.