PHARMACOLOGICAL CHARACTERIZATION OF ALPHA(1)-ADRENOCEPTOR SUBTYPES IN THE HUMAN PROSTATE - FUNCTIONAL AND BINDING-STUDIES

Objective To characterize the alpha(1)-adrenoceptor subtypes of the human benignly enlarged prostate using functional and binding studies. Materials and methods Strips of prostatic tissue taken from nine patients with benign prostatic hypertrophy who were undergoing open prostatectomy were used in the study. Results The strips isolated from five prostates produced a large contraction in response to noradrenaline and phenylephrine but not to clonidine. The contractile response induced by noradrenaline was competitively antagonized by representative alpha(1)-adrenoceptor antagonists (prazosin, WB4101, 5-methylurapidil and HV723), the dissociation constants (pK(B)) being <8.5. Pre-treatment with chloroethylclonidine was without effect on the contractile response to noradrenaline. In saturation experiments with five prostates, [H-3]-prazosin bound to the prostate membranes with two distinct affinities (pK(D) = 9.95+/-0.07 and 8.71+/-0.04, Bmax = 151+/-8 and 138+/-3 fmol/mg protein, respectively). Unlabelled prazosin and WB4101 biphasically displaced the binding of 200 pM [H-3]-prazosin; the resulting high and low pK(I) values for each of the antagonists were consistent with the two pK(D) values obtained for [H-3]-prazosin in the saturation experiments. 5-Methylurapidil and HV723 displaced the [H-3]-prazosin binding monophasically with an affinity (pK(I)) close to 8.5. Conclusions These results suggest the presence of at least two distinct alpha(1)-adrenoceptor subtypes (presumably an alpha(1C) subtype with a high affinity for prazosin and WB4101, and a putative alpha(IL) subtype with a low affinity for the antagonists) in the human prostate, in which the latter subtype may be predominantly involved in the contractile response to noradrenaline.