MODE OF ANTAGONISM OF METHOCTRAMINE, AF-DX-116 AND HEXAHYDROSILADIFENIDOL IN GUINEA-PIG LEFT ATRIUM AND ILEUM - COMPARISON OF SCHILD AND RESULTANT ANALYSIS

1 Methoctramine, AF-DX 116 and hexahydrosiladifenidol (HHSiD) are the muscarinic antagonists most widely used to study muscarinic receptor s ubtypes. 2 The present study was undertaken to examine the mode of ant agonism of these compounds in guinea-pig left atrium and ileum by comp arison of the Schild and resultant analysis. With this method the effe ct of various concentrations of the test antagonist on the antagonism produced by specific concentrations of a reference antagonist was meas ured and the equilibrium dissociation constant of the test antagonist- receptor complex estimated. Atropine was used for comparative purposes and scopolamine as the reference antagonist. 3 At the cardiac level t he affinity values obtained by Schild and resultant analysis for metho ctramine and AF-DX 116, as for atropine, are very similar: these resul ts indicate that the two cardio-selective antagonists and the non-sele ctive antagonist, atropine, bind at a common site with the reference a ntagonist scopolamine. The resultant plot for the ileo-selective HHSiD has a slope considerably less than unity: this finding might indicate that this antagonist binds to a site different from that of scopolami ne and it should be considered like an allosteric antagonist. 4 At the ileal level the affinity values obtained by Schild and resultant anal ysis are identical for the ileo-selective antagonist HHSiD as for atro pine but not for methoctramine and AF-DX 116. This indicates a mutual binding site with scopolamine for HHSiD and atropine but not for the t wo cardio-selective antagonists. However, it is worth emphasizing that the difference between affinity values obtained by Schild and resulta nt analysis is seen when relatively high concentrations are required: a dual mode of interaction (both competitive and allosteric) could be invoked.