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Muscarinic receptor binding in the guinea pig urinary bladder.

The muscarinic receptors in the guinea pig urinary bladder and the longitudinal muscle of the guinea pig ileum were studied by means of a receptor-binding technique. 1-Quinuclidinyl [phenyl 4-3H]benzilate ((-)3H-QNB) was employed as radio-ligand and the separation of bound from free (-) 3H-QNB was performed by microcentrifugation. Under conditions of equilibrium (-)3H-QNB was specifically bound with high affinity to a limited number of sites, 0.32 and 1.62 pmol/mg protein in the bladder and ileum, respectively. The binding appeared to represent a single population of non-interacting binding sites. The apparent dissociation constants were 2.6 x 10(-10) M in the bladder and 1.2 x 10(-9) M in the ileum, whereas the KD-values, estimated by extrapolation to an infinitely low receptor concentration were 1.1 x 10(-10) M (bladder) and 3.1 x 10(-10) M (ileum). The binding of (-)3H-QNB appears to represent an interaction with muscarinic receptors, as it was effectively inhibited by muscarinic antagonists and agonists, but not by a variety of non-cholinergic drugs.