Characterization and localization of the muscarinic cholinergic receptor in human prostatic tissue

Abstract

Radioligand receptor binding and autoradiography were used to characterize and localize the muscarinic cholinergic receptor in human benign prostatic hyperplastic tissue. These methods have not been used previously to investigate the autonomic innervation of the human prostate. The binding of [3H]N-methylscopolamine ([3H]NMS), a muscarinic cholinergic antagonist, to homogenates of human prostate was saturable and of high affinity. The equilibrium dissociation constant, (Kd), for [3H]NMS binding to human prostate homogenates was 0.10 +/- 0.03 nM (mean +/- SEM). The values of the Kd's for [3H]NMS binding to prostates of man (0.10 nM), dog (0.20 nM), pig (0.11 nM), rat (0.07 nM) and rabbit (0.15 nM) were similar, suggesting homogeneity of muscarinic cholinergic receptors in varying species. The mean density, B(max), of muscarinic cholinergic receptors identified in the human prostate was 2.1 fmol./mg. prostate wet weight. The relative density of receptors in the human prostates were similar in the homogenates and slide-mounted tissue sections. The pharmacology of NMS binding sites on slide-mounted tissue sections was evaluated by competitive binding experiments using [3H]NMS and atropine. The IC50 corrected of atropine on slide-mounted tissue sections (0.42 nM) was similar to values obtained in prostate homogenates (1.16 nM). Autoradiography on slide-mounted tissue sections demonstrated that the muscarinic cholinergic receptors were localized to the epithelium of the prostate. The ratio of specific NMS binding in the epithelial and stromal components of the prostate, expressed as autoradiographic grains/unit area and autoradiographic grains/cell, was 71:1 and 33:1 respectively. Because prostatic secretion is dramatically enhanced by muscarinic cholinergic agonists, localization of muscarinic cholinergic receptors to the epithelium is consistent with the neuropharmacology of prostatic secretion. These studies have provided basic insight into the neuropharmacology of the prostate. Future studies will be necessary to characterize and localize other neurotransmitters in the human prostate in order to further enhance our understanding of prostatic function.