Competitive and non‐competitive antagonism exhibited by ‘selective’ antagonists at atrial and ileal muscarinic receptor subtypes

1 The affinity of a number of ‘selective’ agonists and antagonists has been assessed at atrial or ileal muscarinic receptors by use of in vitro functional analysis. 2 The most selective compound for ileal muscarinic receptors was silabenzhexol (approx. 50 fold), and to a lesser extent benzhexol (approx. 5 fold). Conversely, the most selective compound for the atrial muscarinic receptors was AF‐DX 116 (approx. 6 fold). 3 The novel M1‐receptor antagonist, telenzepine and other antagonists such as propantheline and isopropamide did not distinguish between atrial and ileal receptors. 4 Dicyclomine, adiphenine, hexahydroadiphenine and oxyphenonium exhibited competitive antagonism at atrial receptors but non‐competitive antagonism at ileal receptors. No conclusions could, therefore, be drawn with regard to their selectivity. 5 The agonists, arecaidine propargyl ester (APE), ethoxyethyltriethylammonium (EOE) and carbachol, exhibited some selectivity in potency but little difference in affinity. 6 It is concluded that the study supports the existence of ileal and atrial muscarinic receptor subtypes. However, the use of dicyclomine and related compounds in receptor classification is limited.