An investigation of the mechanism involved in the cholinergic action of meptazinol

In concentrations above 20 μM, (±)‐meptazinol produced a contraction of the guinea‐pig isolated ileum and this effect was antagonized by atropine (0.01 to 0.3 μM) in a manner which was not competitive. Cooling the preparation to 15 °C blocked the contractile action of meptazinol and of dimethylphenylpiperazinium (DMPP) but did not affect the action of carbachol. Twitch responses of the rat phrenic nerve‐diaphragm preparation induced by indirect electrical stimulation in the presence of naloxone (20 nM) were potentiated by meptazinol (1 to 40 μM) which also reversed a partial blockade of the twitch induced by tubocurarine. Neither of these effects was seen in tissues which had been pretreated with the cholinesterase inhibitor BW284C51 (0.2 μM) though tetraethylammonium iodide (40 μM) was still able to enhance the responses to stimulation. In the presence of naloxone (20 nM) electrically induced responses of the rat isolated rectum were abolished by cinchocaine (10 μM), partially blocked by atropine (0.1 to 0.4 μM) and potentiated by meptazinol (1 to 30 μM). The latter action was not seen when meptazinol was administered in the presence of BW284C51. It is concluded that the cholinergic action of meptazinol in these tissues is due to an indirect effect, probably involving inhibition of cholinesterase and that no evidence was seen of any ability to increase the release of acetylcholine itself.