Functional and direct binding studies using subtype selective muscarinic receptor antagonists

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.