An analysis of the mode of action of carbachol on the chick biventer cervicis nerve-muscle preparation

Abstract

The mechanism of contracture evoked by carbachol in the isolated chick biventer cervicis nerve-muscle preparation was studied. At concentrations lower than 11 μM, carbachol progressively induced contracture at a rate much slower than did acetylcholine. A spontaneous increase of the response to carbachol, but not to acetylcholine, was observed 2–4 hr after isolation of the muscle. By contrast, no change of the response occured in the denervated muscle. Anticholinesterase treatment shifted the dose-response curve for carbachol markedly to the left as far as the contracture attained after 4–6 min incubation was concerned. The shift was much less marked for the dose-response curve plotted against the initial rate of response defined as the contracture obtained after 1 min incubation. No enhancement of response was detected by inhibition of acetylcholinesterase in the denervated muscle. β-Bungarotoxin which blocked neuromuscular transmission within 30 min, caused a transient enhancement of the response to carbachol. After 2–3 hr treatment, however, the sponteneous increase of response to carbachol was counteracted by the toxin. No potentiation of the response by anticholinesterase agents was observed after toxin treatment. When added in the presence of physostigmine or echothiophate, β-bungarotoxin reduced the response to carbachol to the control level in 2 hr. The response to carbachol in the muscle treated with α-bungarotoxin and washed subsequently was also not potentiated by anticholinesterase treatment. Repetitive nerve stimulation in the presence of hemicholinium-3 caused a neuromuscular blockade but did not appreciably antagonize the response to carbachol more than that to acetylcholine either in the absence or presence of physostigmine. When calcium ion in the medium was decreased from 2.7 to 0.54 mM, the response to nerve stimulation was nearly completely inhibited, but the response to carbachol was not affected and could still be potentiated by anticholinesterase. It is concluded that carbachol has both direct and indirect effects on the chick biventer cervicis muscle. At a low concentration, particularly in the presence of an anticholinesterase and if sufficient time of incubation is allowed, the indirect effect caused by release of acetylcholine may become more prominent than the direct action on the post-synaptic receptor. Both the mechanism and the store of acetylcholine for this indirect action appear to be different from those for the nerve impulse.