Characterization of the α‐adrenoceptors in the female rabbit urethra

1 A radioligand binding technique was used to evaluate the proportions of α 1 ‐ and α 2 ‐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 α 1 ‐ and/or α 2 ‐adrenoceptors are located postjunctionally in the urethral smooth muscle. 2 Studies of the inhibition of [ 3 H]‐dihydroergocryptine binding by the selective α 1 ‐adrenoceptor antagonist prazosin or the selective α 2 ‐adrenoceptor antagonist rauwolscine revealed the α‐adrenoceptor population to consist of approximately 25% α 1 ‐adrenoceptors and 75% α 2 ‐adrenoceptors. These proportions were confirmed in saturation studies with [ 3 H]‐prazosin and [ 3 H]‐rauwolscine. The sum of α 1 ‐ and α 2 ‐adrenoceptors labelled by these selective α 1 ‐ and α 2 ‐adrenoceptor antagonists was about equal to the number labelled by the non‐selective α‐adrenoceptor antagonist [ 3 H]‐dihydroergocryptine. 3 Noradrenaline, as well as the selective α 1 ‐adrenoceptor agonist phenylephrine and the selective α 2 ‐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. 4 In addition to showing that both α 1 ‐ and α 2 ‐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.