Pharmacological characterization of α1‐adrenoceptor subtypes in the human prostate: functional and binding studies

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Abstract
Objective To characterize the α1,‐adrenoceptor subtypes of the human benignly enlarged prostate using functional and binding studies.Material and methods Strips of prostatic tissue taken from nine patients with benign prostatic hypertrophy who were undergoing open prostatectomy were used in the styudy.Results The strips isolated from five prostates produced a large contraction in response to noradrenaline and phenylphrine but not to clonidine. The contractile response induced by noradrenaline was competitively antagonized by representative α1‐adrenoceptor antagonists (prazosin, WB4101, 5‐methylurapidil and HV723), the dissociation constants (pKb) being <.5. Pre‐treatment with chloroethyclonidine was without effect on the contractile response to noradrenaline. In saturation experiments with five prostates. [3H]‐prazosin bound to the prostate membranes with two distinct affinities (pKd‐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 [3H]‐prazosin; the resulting high and low pK1 values for each of the antagonists were consistent with the two pKD values obtained for [3H]‐prazosin in the saturation experiments. 5‐Methylurapidil and HV723 displaced the [3H]‐prazosin binding monophasically with an affinity (pKI) close to 8.5.These results suggest the presence of at least two distinct α‐adrenoceptor subtypes (presumably an αIC subtype with a high affinity for prazosin and WB4101, and a putative αIL 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.