The use of different agonists in antagonist affinity constant estimations.

1 The affinities of 4 muscarinic antagonists were estimated on intact pieces of guinea-pig ileum using the agonists carbachol and pentyl trimethylammonium both in separate experiments and in the same experiment. 2 The apparent affinities were slightly but consistently higher when estimated from the responses produced by pentyl trimethylammonium than when estimated from the responses produced by carbachol. 3 This difference was greatly reduced or abolished if totally denervated logitudinal muscle strips were used rather than intact pieces of ileum. It is therefore suggested that the difference is due to the presence of receptors in the ganglionic layer. 4 To explain the difference in apparent affinity of the antagonists these receptors can not be identical to the muscarinic receptors on the smooth muscle. 5 In addition they can not be nicotinic ganglionic receptors as the difference did not appear to be affected by the presence or absence of hexamethonium.