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Analysis of cardiac muscarinic receptors recognized selectively by nonquaternary but not by quaternary ligands.

Three muscarinic receptor antagonists, [3H]quinuclidinyl benzilate ([3H]QNB), N-[3H]methylscopolamine ([3H]NMS and N-[methyl-3H]QNB ([3H]NMeQNB), each bind to an apparently homogeneous population of receptors on intact chick heart cells. [3H]QNB binds to approximately 9500 sites/cells, whereas [3H]NMS and [3H]NMeQNB bind to approximately 5000 sites/cell. Atropine and scopolamine compete with all three radioligands with a single, high affinity. Their quaternary analogs N-methylatropine and NMS and the quaternary agonist carbachol also show a single affinity for [3H]NMS and [3H]NMeQNB binding sites, but have biphasic competition curves for [3H]QNB sites with low "apparent" affinity for a subpopulation of sites. When 10 nM or greater propylbenzilylcholine mustard is used to alkylate receptors virtually all [3H]NMS binding is abolished, whereas [3H]QNB still labels a significant fraction of the binding sites seen in control cells. The sites with low apparent affinity for quaternary ligands are shown to have characteristics of muscarinic receptors, but do not appear necessary for muscarinic receptor-mediated phosphoinositide hydrolysis. We suggest that a subpopulation of nonfunctional muscarinic receptors are sequestered within the membrane or otherwise inaccessible to hydrophilic or charged ligands.