1 We investigated the nature of the adrenoceptors in the dog saphenous vein (DSV) and dog mesenteric vein (DMV) to determine the nature of the unexpected interactions of phenylephrine and methoxamine with rauwolscine in the DSV, i.e. the ability of the putative alpha(2)-adrenoceptor antagonist ro inhibit competitively contractions to these alpha(1)-agonists. Radioligand binding studies were performed in parallel with contractility studies. 2 Functionally, in the DSV, phenylephrine and methoxamine-induced contractions were antagonized by rauwolscine with Schild slopes of -0.52 and -0.46, respectively and apparent pA(2) values of 8.5 and 9.2, respectively. Such antagonism was not observed in the DMV. In the DSV, prazosin competes for [H-3]-rauwolscine binding sites with a high and a low affinity binding site (K-i of 1.49+/-0.65 and 94.7+/-51 mu M, n = 6, respectively). 3 Pretreatment with 100 mu M chloroethylclonidine (CEC) for 15 min abolished [H-3]-prazosin binding in microsomes from both veins and reduced binding (B-max) of [H-3]-rauwolscine in microsomes by 55.1+/-0.80% (n=3) in the DSV but did not affect the B-max in the DMV. CEC pretreatment in the venular rings denuded of endothelium caused persistent contraction in the DSV but not in the DMV. In the DSV, CEC appeared to interact with a single [H-3]-rauwolscine binding site. In both the DSV and the DMV, CEC (100 mu M) caused a significant shift in the EC(50) values for phenylephrine and methoxamine. Maximum responses in the DMV were significantly attenuated while those in the DSV were unaffected when total tension was considered. 4 Studies of the functional interactions of the DSV and the DMV with WE 4101 or 5-methylurapidil (5-MU) suggested the presence of alpha(1D)-adrenoceptors in the DSV and alpha(1A)-adrenoceptors in the DMV. The receptors inactivated by CEC in the DMV and DSV may represent some or all of the receptors with properties of alpha(1D) and alpha(1A)-receptors present in the two veins. Studies of radioligand binding interactions of these two antagonists with [H-3]-prazosin, were consistent with the presence of some alpha(1D)-receptors in DSV and alpha(1A)-receptors in DMV. These findings raise questions about the selectivity of CEC in differentiating alpha(1)-adrenoceptor subtypes. 5 B-HT 920 caused contractions in the DSV smaller than those to the alpha(1)-agonists but the maximum was not affected by CEC pretreatment. The EC(50) values were shifted to the left after CEC. In radioligand binding studies, B-HT 920 competition for [H-3]-rauwolscine binding was not significantly affected by CEC pretreatment. 6 These results suggest the presence of unusual alpha-adrenoceptors in the DSV. In addition to alpha(2)-adrenoceptors, receptors recognizing rauwolscine as well as prazosin, WE 4101. phenylephrine and methoxamine and susceptible to inactivation by CEC are present. They appear to be, in part, unusual alpha(1D)-adrenoceptors.
