Barbieri A, Santagostino-Barbone M G, Zonta F, Lucchelli A
Institute of Pharmacology, School of Pharmacy, University of Pavia, Italy.
Naunyn Schmiedebergs Arch Pharmacol. 1998 Jun;357(6):654-61. doi: 10.1007/pl00005221.
The alpha-adrenoceptors that mediate contractions in strips of splenic artery from the pig were characterized by the use of selective agonists and subtype-selective antagonists. Noradrenaline, the alpha1-selective agonist phenylephrine and the alpha1-/alpha2-agonist oxymetazoline caused the preparations to contract with potency (pD2) values of 6.94, 6.14 and 7.27, respectively. Compared to noradrenaline, phenylephrine and oxymetazoline induced 93% and 78% of noradrenaline maximum effect. Conversely, the two alpha2-selective agonists clonidine and B-HT 920 induced only 31% and 13% of noradrenaline maximum effect. B-HT 920 only marginally contracted the tissue even when it was precontracted with phenylephrine. The alpha2-selective antagonist yohimbine antagonized oxymetazoline- and phenyleprine-induced contractions with affinity (pA2) values (6.80 and 6.74, respectively) consistent with alpha1-adrenoceptor interaction. This suggests that the pig splenic artery possesses only functional alpha1-adrenoceptors. The alpha1-adrenoceptor antagonists of varying subtype selectivities like WB-4101, 5-methylurapidil, benoxathian and BMY 7378 competitively antagonized phenylephrine-induced contractions with affinity values of 9.46, 8.26, 9.06 and 6.91, respectively. These values correlated highly with published affinity values for functional alpha1A-adrenoceptors (r=0.92) and alpha1a-clones (r=0.94) and less well with affinity values for functional alpha1B-adrenoceptors (r=0.84) and alpha1b-clones (r=0.87). Conversely, correlation with functional alpha1D-adrenoceptors (r=0.26) and alpha1d-clones (r=0.33) was poor. In addition the alpha1D-selective antagonist BMY 7378 had a low affinity value compared to that reported for alpha1D-adrenoceptors. Therefore, based on correlation studies, the plot that resembled the line of equal values most closely was that for the alpha1A-subtype. The alpha1A-selective antagonist RS-17053 antagonized phenylephrine-induced contractions in an apparently non-competitive manner and gave an apparent pA2 value of 7.06 which is similar to the "low" affinity values reported in other alpha1A-containing tissues. Exposure to the irreversible alpha1B/D-antagonist chloroethylclonidine slightly decreased maximum response to phenylephrine without significantly affecting its potency value, indicating that the phenylephrine response is substantially chloroethylclonidine-insensitive. It is concluded that splenic artery strips from the pig contract in response to phenylephrine through activation of alpha1-adrenoceptors which display the pharmacological profile of the alpha1A-subtype for which the recently reported alpha1A-selective antagonist RS-17053 shows low affinity. Evidence for contribution of the alpha1B-subtype in the overall contractile response is elusive while no evidence was obtained for the involvement of the alpha1D-subtype. The contribution of functional alpha2-adrenoceptors to the contractile response was ruled out.
通过使用选择性激动剂和亚型选择性拮抗剂，对介导猪脾动脉条收缩的α-肾上腺素能受体进行了表征。去甲肾上腺素、α1选择性激动剂苯肾上腺素和α1/α2激动剂羟甲唑啉使制剂收缩，效价（pD2）值分别为6.94、6.14和7.27。与去甲肾上腺素相比，苯肾上腺素和羟甲唑啉分别诱导出去甲肾上腺素最大效应的93%和78%。相反，两种α2选择性激动剂可乐定和B-HT 920仅诱导出去甲肾上腺素最大效应的31%和13%。即使在用苯肾上腺素预收缩后，B-HT 920对组织的收缩作用也很微弱。α2选择性拮抗剂育亨宾拮抗羟甲唑啉和苯肾上腺素诱导的收缩，其亲和力（pA2）值（分别为6.80和6.74）与α1肾上腺素能受体相互作用一致。这表明猪脾动脉仅具有功能性α1肾上腺素能受体。不同亚型选择性的α1肾上腺素能受体拮抗剂，如WB-4101、5-甲基尿嘧啶、苯恶噻嗪和BMY 7378，竞争性拮抗苯肾上腺素诱导的收缩，其亲和力值分别为9.46、8.26、9.06和6.91。这些值与已发表的功能性α1A肾上腺素能受体（r=0.92）和α1a克隆（r=0.94）的亲和力值高度相关，与功能性α1B肾上腺素能受体（r=0.84）和α1b克隆（r=0.87）的亲和力值相关性稍低。相反，与功能性α1D肾上腺素能受体（r=0.26）和α1d克隆（r=0.33）的相关性很差。此外，与报道的α1D肾上腺素能受体相比，α1D选择性拮抗剂BMY 7378的亲和力值较低。因此，基于相关性研究，最接近等值线的图是α1A亚型的图。α1A选择性拮抗剂RS-17053以明显的非竞争性方式拮抗苯肾上腺素诱导的收缩，表观pA2值为7.06，这与其他含α1A组织中报道的“低”亲和力值相似。暴露于不可逆性α1B/D拮抗剂氯乙可乐定后，对苯肾上腺素的最大反应略有降低，但对其效价无明显影响，表明苯肾上腺素反应对氯乙可乐定基本不敏感。结论是，猪脾动脉条对苯肾上腺素的收缩反应是通过激活α1肾上腺素能受体介导的，该受体表现出α1A亚型的药理学特征，最近报道的α1A选择性拮抗剂RS-17053对其显示出低亲和力。α1B亚型对总体收缩反应的贡献证据难以捉摸，而未获得α1D亚型参与的证据。功能性α2肾上腺素能受体对收缩反应的贡献被排除。
