Contraction of the Frog Muscle in 4-Aminopyridine by Rapid Cooling

Abstract

In the 4-aminopyridine (4AP) Ringer in which the concentration of 4AP is above 0.005mM, the frog muscle contracted repetitively by rapid cooling.This contraction was named AP-RCC.It took more than 20min for 4AP to show the full effect on the AP-RCC.During the AP-RCC action potentials of the muscle membrane which synchronized with the repetitive contractions were observed.Tetrodotoxin abolished the AP-RCC.In muscle pretreated with 400mM glycerol, no AP-RCC was obtained.During the AP-RCC no action potential of the nerve was seen. d-Tubocurarine abolished both the AP-RCC and the accompanying action potential of the muscle.Mn²⁺ and, to a lesser extent, Mg²⁺, inhibited both the AP-RCC and the twitch by indirect stimulation.The repetitive nerve stimulation which lasted long enough to exhaust acetylcholine at the nerve terminal abolished the AP-RCC. 4AP restored the contraction suppressed by dantrolene sodium.The AP-RCC was masked by caffeine rapid cooling contracture when the concentration of caffeine was high.
From these results, the AP-RCC is considered to be generated by acetylcholine release from the nerve terminal by the combined action of 4AP and depolarization during cooling.