The relative potencies of some agonists at M2 muscarinic receptors in guinea‐pig ileum, atria and bronchi

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Abstract
The effects of some agonists on isolated preparations of guinea-pig ileum, atria and bronchial muscle were compared with those of carbachol. The concentrations producing comparable responses were used to estimate the equipotent molar ratio relative to carbachol. Arecaidine propargyl ester was 4-5 times as active as carbachol on the ileum but > 10 times as active as carbachol on atrial rate or atrial force; this compound had a 2- to 3-fold selectivity for receptors in atria. Ethoxyethyltrimethylammonium iodide was 1/4-1/3 as active as carbachol on ileum but only 1/10 as active as carbachol on atrial rate or atrial force, and showed a 3-4 fold selectivity for receptors in ileum. The other compounds tested, which included acetylcholine, methacholine, n-pentyltrimethylammonium iodide and bethanechol showed less selectivity. There were no obvious differences between effects on atrial rate and effects on atrial force, though with esters it was often difficult to obtain effects on atrial rate in the absence of an inhibitor of cholinesterase. Activity on bronchial muscle was generally similar to activity on ileum.