Actions of some cholinergic antagonists on fast-twitch and slow-twitch skeletal muscle of the cat.

1. The anticholinergic drug N-ethyl-2-pyrrolidylmethylcyclopentylphenyl glycollate (PMCG) has been studied for its effects on the contraction of fast-twitch (flexor hallucis longus, FHL) and slow-twitch (soleus) muscles in the cat.2. In both muscles lower doses (0.25 to 10 mg intra-arterially) potentiated and higher doses (10 mg and above) depressed twitches produced by indirect stimulation. Similar effects were obtained in directly stimulated muscles.3. The effects of the drug on muscles stimulated repetitively were dependent on both dose and frequency of stimulation. Doses which potentiated twitches also potentiated low frequency tetani (5-30 Hz for soleus; 10-50 Hz for FHL). There was a depression and non-maintenance of higher frequency tetani (80 Hz and above for soleus; 150 Hz and above for FHL).4. Both atropine and caramiphen had similar dose dependent potentiating and depressant actions. Hyoscine acted similarly in some cats; in others it had only a depressant action.5. It is suggested that the potentiating actions of PMCG, caramiphen, atropine and hyoscine are due to a direct musculotropic action. The depressant actions resulting from higher doses are due, in part, to a curare-like action and in part to a direct action on the muscle fibres.