Characterization of the α‐adrenoceptors in the female rabbit urethra

Open Access
Publisher Website
Google Scholar
Abstract
A radioligand binding technique was used to evaluate the proportions of α1‐ and α2‐adrenoceptors in crude membrane preparations obtained from the female rabbit bladder base and urethra. In addition, urethral rings were studied in vitro in an attempt to determine if α1‐ and/or α2‐adrenoceptors are located postjunctionally in the urethral smooth muscle. Studies of the inhibition of [3H]‐dihydroergocryptine binding by the selective α1‐adrenoceptor antagonist prazosin or the selective α2‐adrenoceptor antagonist rauwolscine revealed the α‐adrenoceptor population to consist of approximately 25% α1‐adrenoceptors and 75% α2‐adrenoceptors. These proportions were confirmed in saturation studies with [3H]‐prazosin and [3H]‐rauwolscine. The sum of α1‐ and α2‐adrenoceptors labelled by these selective α1‐ and α2‐adrenoceptor antagonists was about equal to the number labelled by the non‐selective α‐adrenoceptor antagonist [3H]‐dihydroergocryptine. Noradrenaline, as well as the selective α1‐adrenoceptor agonist phenylephrine and the selective α2‐adrenoceptor agonist clonidine, induced contractions of urethral ring preparations. Prazosin blocked contractions induced by phenylephrine to a greater extent than contractions induced by clonidine. The opposite was true for the inhibitory effect of rauwolscine. In addition to showing that both α1‐ and α2‐adrenoceptor binding sites exist in membrane preparations of the rabbit bladder base and urethra, the results reveal the presence of both adrenoceptor subtypes postjunctionally in the rabbit urethra; and both mediate contraction of the smooth muscle.
Keywords