Alpha-Adrenoceptors and Muscarinic Receptors in the Isolated Human Prostate

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Abstract
Prostatic specimens of adenomatous tissue (PA) were obtained from the lateral lobe of the prostate in patients with benign prostatic hyperplasia (BPH). Non-hyperplastic specimens of the outer prostatic tissue (PC) were taken from the dorsal part of the prostate in patients undergoing cystourethrectomy. Effects of .alpha.-adrenoceptor and muscarinic receptor active drugs were studied. Noradrenaline (NA) and the .alpha.1-adrenoceptor agonist phenylephrine induced concentration-dependent contractions in PC and PA preparations. The .alpha.2-adrenoceptor agonist clonidine was without effect on PC but contracted PA preparations; it was less potent and had less intrinsic activity than phenylephrine. In PC and PA strips, the .alpha.1-adrenoceptor antagonist prazosin was more effective than the .alpha.2-adrenoceptor antagonist rauwolscine to inhibit NA-induced contractions. Prazosin, but not rauwolscine, inhibited electrically induced contractions in PC strips. The muscarinic receptor agonists acetylcholine and carbachol were without effect in PC and PA preparations. In both PC and PA preparations, clonidine decreased and rauwolscine increased the electrically elicited 3H-efflux after pre-loading of the tissues with 3H-noradrenaline. Carbachol and scopolamine were without consistent effects. In radioligand receptor binding experiments, the occurrence of .alpha.1- and .alpha.2-adrenoceptor binding sites was demonstrated; the ratio between .alpha.1- and .alpha.2-adrenoceptor binding sites was 3/2 in PC, but 2/3 in PA tissue. By autoradiography, muscarinic receptors were found to be localized exclusively to the glandular epithelium, consistent with the lack of contractile effects of muscarinic receptor active drugs on PC and PA preparations. Our results thus suggest that the main .alpha.-adrenoceptor function in human prostatic smooth muscle is that of the .alpha.1-type and that muscarinic receptors in the prostate are involved in processes other than control of smooth muscle contraction.