Contraction of the frog muscle in 4-aminopyridine by rapid cooling

In the 4-aminopyridine (4AP) Ringer in which the concentration of 4AP is above 0.005 mM, the frog muscle contracted repetitively by rapid cooling. This contraction was named AP-RCC. It took more than 20 min for 4AP to show the fall 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 400 mM 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 acetyl-choline release from the nerve terminal by the combined action of 4AP and depolarization during cooling.