A mammalian muscle with the pharmacological characteristics of slow tonic muscle.

The guinea-pig isolated cremaster muscle gave tetrodotoxin-resistant dose-related contractures with acetylcholine, carbachol and the depolarizing neuromuscular blocking agents. The dose-response curve to suxamethonium in tetrodotoxin 2 X 10(-7)M could be shifted to the right with tubocurarine 10(-6)M. KCl, 0.1M, produced slow sustained contractures of the muscle. With the cremaster nerve-muscle preparation tetanic contractions at 20 Hz were maintained over several minutes. Tetrodotoxin eliminated the twitch response to single shock nerve stimulation but not the sustained increase in tension produced by suxamethonium. The results suggest either that there is a component of slow tonic muscle in the guinea-pig cremaster or that the cremaster consists of a type of focally innervated muscle which has pharmacological responses qualitatively different from those of most focally innervated muscles so far described.