Characterization of the alpha-adrenoceptors in the female rabbit urethra.

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