Responses of the toad isolated rectus abdominis muscle to cumulative doses of carbachol were recorded in the absence or presence of varying concentrations of cimetidine or ranitidine. The corresponding cumulative log concentration-response curves for carbachol were then plotted for each antagonist studied. Cimetidine (1 mmol/l) produced a straight and parallel 1.8-fold shift of the carbachol curve to the right of the corresponding control curve with no significant change in the maximal response. Cimetidine, at doses of 2.5 mmol/l and 5 mmol/l, and ranitidine, at doses of 0.1 mmol/l, 0.25 mmol/l and 0.5 mmol/l, produced a concentration-dependent and non-parallel shift of the carbachol curve to the right of the corresponding control curve accompanied by a marked decline of maximal responses. The results provide further evidence that ion-channel block may be involved in the neuromuscular blockade produced by cimetidine or ranitidine, the latter being more potent in this respect. Competitive antagonism may also be partly responsible for cimetidine-induced neuromuscular blockade, especially at lower drug concentrations.
1. Cimetidine and ranitidine can inhibit various cholinergic sites, which can account for some of their clinically documented adverse effects; ranitidine can also inhibit adrenergic transmission, closely resembling the action of guanethidine. The effects of cimetidine on adrenergic transmission in the rat isolated anococcygeus muscle (Acm) were therefore investigated. 2. The contractile (motor) responses of the Acm to electrical field stimulation (EFS; 20-30V, 10 s, 1 ms pulse width, every 2 min) at varying frequencies (Hz: 5, 10, 20) and to 3 microM noradrenaline (NA) were inhibited in a concentration-dependent manner by cimetidine (mM: 1, 2, 4, 8). Inhibition of the EFS-induced responses was inversely related to the stimulation frequency. 3. Cimetidine (nM: 2, 4, 8) produced a concentration-dependent and non-parallel shift of the NA cumulative log concentration-response curves (CRCs; curves 2, 3, 4) to the right of the control curve (curve 1); at the highest concentration (8 mM) used, cimetidine produced a 4.3-fold shift of curve 4 accompanied by a decline of 9.4 +/- 1.5% in the maximal response to NA (compared to essentially no change in maximal responses for the corresponding CRCs in the NA control series). Cimetidine therefore inhibited the postjunctional alpha-adrenoceptor sites. 4. The contractile responses of the Acm to EFS (i.e. prejunctionally mediated responses) were more sensitive to inhibition by cimetidine than the NA-induced (postjunctionally mediated) responses: 8 mM cimetidine inhibited the responses to EFS by about 97%, whereas the responses to NA were inhibited by only 41 +/- 5%.(ABSTRACT TRUNCATED AT 250 WORDS)
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.