Functional and direct binding studies using subtype selective muscarinic receptor antagonists

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Abstract
1 Muscarinic receptor antagonists were examined in direct binding studies on guinea-pig cardiac and cortical muscarinic receptors. Pirenzepine, dicyclomine and hexahydroadiphenine were shown to be selective ligands for the putative M1-muscarinic receptor. 2 Functional affinity estimates of the muscarinic ligands studied was determined from their ability to inhibit carbachol-stimulated inositol phosphate (IP) accumulation in guinea-pig cortical slices. 3 The affinity estimates for the inhibition of muscarinic agonist-stimulated IP accumulation were better correlated with affinity estimates obtained from binding studies on the M1 than the M2 muscarinic receptor. 4 These data provide additional evidence, both from direct binding and functional studies, for the presence of M1 and M2 muscarinic receptor subtypes.