BINDING AND FUNCTIONAL-PROPERTIES OF ENDOTHELIN RECEPTOR SUBTYPES IN THE HUMAN PROSTATE

The objective of the present study was to characterize the binding and functional properties of endothelin (ET) receptor subtypes in the hum an prostate. Human prostatic tissue was obtained from male subjects un dergoing radical prostatectomy for low-volume prostate cancer. The opt imal assay conditions for characterizing human prostatic ET-1 binding sites on slide-mounted tissue sections were defined. Maximal specific I-125-ET-1 binding was achieved after a 10-min preincubation, a 120-mi n incubation, and a washing procedure that consisted of a brief rinse and a 1-min wash. The mean equilibrium dissociation constant (K-d) and density (B-max) of ET-1 binding sites determined from six saturation studies were 0.72 +/- 0.13 nM and 40.4 +/- 6.9 fmol/mg of wet weight, respectively. The mean Hill coefficient was 0.99 +/- 0.01, indicating that I-125-ET-1 identifies a single population of binding sites. The p harmacology of I-125-ET-1 binding sites was characterized using compet itive binding experiments. The competition plots for ET-1 were best fi t by a one-binding site model, whereas the plots for sarafotoxin 6C (S 6C) and BQ123 were consistently best fit by a two-site model. The mean K-i value of ET-1 was 0.34 +/- 0.12 nM. The mean K-i values for the h igh and low affinity S6C binding sites were 0.50 +/- 0.09 nM and 0.84 +/- 0.28 mu M, respectively. The mean K-i values for the high and low affinity BQ123 binding sites were 5.51 +/- 1.05 nM and 24.9 +/- 6.5 mu M, respectively. The ratio of ET(A) to ET(B) binding sites was approx imately 2:1. The ET receptor subtype mediating prostatic smooth muscle tension was investigated using agonist-antagonist competition studies . ET-1, a nonselective ET agonist, elicited a potent contraction of pr ostate smooth muscle. The pA(2) of BQ123 for inhibiting ET-1-mediated contraction was 6.84. S6C, a selective ET(B) agonist, also elicited a potent contraction of prostate smooth muscle. BQ123 at concentrations between 0.1 and 10 mu M did not shift the S6C dose-response curve. The se functional studies suggest that both ET(A) and ET(B) receptors medi ate the tension of prostate smooth muscle. Endogenous ETs may be invol ved in the pathophysiology of bladder outlet obstruction in men with b enign prostatic hyperplasia. If this is the case, then ET antagonists may represent effective treatment for benign prostatic hyperplasia.