Muscarinic Receptor Binding in the Guinea Pig Urinary Bladder

Abstract

Muscarinic receptors in the guinea pig uinary bladder and ileal longitudinal muscle were studied by means of a receptor-binding technique. l-Quinuclidinyl [phenyl4-3H] benzilate ((.sbd.) 3H-QNB) was employed as ratio-ligand and the separation of bound from free (.sbd.) 3H-QNB was performed by microcentrifugation. Under conditions of equilibrium (.sbd.)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 noninteracting binding sites. The apparent Kd were 2.6 .cntdot. 10-10 M in the bladder and 1.2 .cntdot. 10-9 M in the ileum, whereas the Kd-values, estimated by extrapolation to an infinitely low receptor concentration, were 1.1 .cntdot. 10-10 M (bladder) and 3.1 .cntdot. 10-10 M (ileum). The binding of (.sbd.)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 noncholinergic drugs.