Effect of dextromethorphan on guinea pig ileal contractility in vitro: comparison with levomethorphan, loperamide and codeine.

Dextromethorphan (DM) was tested for its effect on the contractility of the guinea pig ileum in vitro. Comparisons were made with levomethorphan, levorphanol, codeine and loperamide. DM and codeine inhibited the contractions of the electrically stimulated ileum, with IC50 values of 15 and 8 microM, respectively. The inhibitory effect of DM, in contrast to codeine, was not blocked by the opiate antagonist naloxone. Pretreatment (8, 50 and 100 microM) with DM and its I-isomer levomethorphan reduced in a dose-dependent fashion both phasic and tonic contractions produced by maximally effective concentrations of carbachol and 80 mM KCl. Pretreatment (8, 50 and 100 microM) with codeine, dextrorphan or levorphanol, unlike DM, did not reduce carbachol or KCl-induced contractions. The concentration-response curves to calcium in K+ depolarized ileum were shifted to the right in a parallel manner suggesting competitive antagonism for DM (pA2 5.3), levomethorphan (pA2 5.3) and loperamide (pA2 6.6). Codeine and levorphanol (8, 50 and 100 microM) did not antagonize calcium-induced contractions. In two assays of calmodulin-dependent processes DM was inactive (IC50 greater than 1 mM), whereas trifluoperazine and calmidazolium (standard calmodulin antagonists) were active in the micromolar range. In summary, these data suggest that DM, through a nonopiate mechanism, antagonizes gut contractility possibly by "stabilizing" neuronal and/or muscle cell membranes.