Characterization of Muscarinic Cholinergic Receptor Binding in the Vas Deferens, Bladder, Prostate and Penis of the Rabbit

Abstract

Radioligand receptor binding techniques were used to characterize the muscarinic cholinergic receptor in the vas deferens, bladder, prostate and penis of the rabbit. This study represents the first comparative investigation of a neurotransmitter receptor in the genitourinary tract using radioligand receptor binding methods. A single high affinity muscarinic binding site was identified in the vas deferens (Kd = 0.16 nM), bladder (Kd = 0.15 nM) and prostate (Kd = 0.17 nM), using [3H]N-methylscopolamine ([3H]NMS), a muscarinic antagonist. Two high affinity [3H]NMS binding sites (Kd1 = 0.08 nM; Kd2 = 1.39 nM) were found in the penis. The pharmacology of the NMS binding sites in the vas deferens, bladder and prostate was characterized by competitive binding experiments with [3H]NMS and several unlabeled muscarinic and nonmuscarinic drugs. Noncholinergic drugs, as expected, were weak inhibitors of [3H]NMS binding. The values of the IC50''s for the muscarinic drugs atropine, pirenzepine and oxotremorine in the genitourinary tissues of the rabbit were similar to values reported in nongenitourinary tissues demonstrating homogeneity of muscarinic receptors. The mean Hill coefficients for the muscarinic antagonist (atropine 0.81 to 0.878) were significantly different than for the muscarinic agonist (oxotremorine 0.39 to 0.44) in all genitourinary tissues, a binding property of muscarinic receptors identified in other tissues. The requirements for the characterization of cholinergic muscarinic receptors were fulfilled for several genitourinary tissues of the rabbit. Radioligand receptor binding methods can now be applied to investigating the relationship between genitourinary dysfunction and alterations in the muscarinic cholinergic receptors.