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. l-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±10−10 M in the bladder and 1.2±10−9 M in the ileum, whereas the KD-values, estimated by extrapolation to an infinitely low receptor concentration were 1.1±10−10 M (bladder) and 3.1±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.