Unusual α-adrenoceptor subtype in canine saphenous vein: comparison to mesenteric vein

摘要

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 α_2-adrenoceptor antagonist to inhibit competitively contractions to these α_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 [~3H]-rauwolscine binding sites with a high and a low affinity binding site (K_i of 1.49 ± 0.65 and 94.7 ± 51 μM, n = 6, respectively). 3 Pretreatment with 100 μM chloroethylclonidine (CEC) for 15 min abolished [~3H]-prazosin binding in microsomes from both veins and reduced binding (B_(max)) of [~3H]-rauwolscine in microsomes by 55.1±0.8% (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 [~3H]-rauwolscine binding site. In both the DSV and the DMV, CEC (100 μM) 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 WB 4101 or 5-methylurapidil (5-MU) suggested the presence of α_(1D)-adrenoceptors in the DSV and α_(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 α_(1D) and α_(1A)-receptors present in the two veins. Studies of radioligand binding interactions of these two antagonists with [~3H]-prazosin, were consistent with the presence of some α_(1D)-receptors in DSV and α_(1A)-receptors in DMV. These findings raise questions about the selectivity of CEC in differentiating α_1-adrenoceptor subtypes. 5 B-HT 920 caused contractions in the DSV smaller than those to the α_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 [~3H]-rauwolscine binding was not significantly affected by CEC pretreatment. 6 These results suggest the presence of unusual α-adrenoceptors in the DSV. In addition to α_2-adrenoceptors, receptors recognizing rauwolscine as well as prazosin, WB 4101, phenylephrine and methoxamine and susceptible to inactivation by CEC are present. They appear to be, in part, unusual α_(1D)-adrenoceptors.