The ability of muscarinic receptor antagonists to compete with (−)-[3H]quinuclidinyl benzilate ([3H]QNB) binding was compared with their ability to block carbachol-mediated stimulation of particulate guanylate cyclase activity in rat colonocytes. The binding of [3H]QNB to membranes was inhibited by antagonists with the following rank order of potencies (inhibitory constants, nM): atropine (2.5) ≈ 4- diphenylacetoxy -N- methylpiperidine iodide (4- DAMP ) [4.6] ⪢ pirenzepine (121) > methoctramine (385) . 4-DAMP ( IC 50 = ∼ 10 nM ) was also more potent in blocking carbachol-induced stimulation of guanylate cyclase activity than either pirenzepine ( IC 50 = ∼ 700 nM ) or methoctramine ( IC 50 = ∼ 1500 nM ).
