A COMPARISON OF AFFINITY CONSTANTS FOR MUSCARINE‐SENSITIVE ACETYLCHOLINE RECEPTORS IN GUINEA‐PIG ATRIAL PACEMAKER CELLS AT 29°C AND IN ILEUM AT 29°C AND 37°C

Abstract

The affinity of 17 compounds for muscarine‐sensitive acetylcholine receptors in atrial pacemaker cells and ileum of the guinea‐pig has been measured at 29°C in Ringer‐Locke solution. Measurements were also made at 37°C with 7 of them. Some of the compounds had much higher affinity for the receptors in the ileum than for those in the atria. For the most selective compound, 4‐dipnenylacetoxy‐N‐methylpiperidine methiodide, the difference was approximately 20‐fold. The receptors in the atria are therefore different in structure from those in the ileum. The effects of temperature on affinity are not the same for all the compounds tested, indicating different enthalpies and entropies of adsorption and accounting for some of the difficulty experienced in predicting the affinity of new compounds.