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Ligands at β2-, β3-, and the Low-Affinity State of β1-Adrenoceptors Block the α1-Adrenoceptor-Mediated Constriction in Human Pulmonary and Rat Mesenteric Arteries

We examined whether the β2-adrenoceptor agonists fenoterol and salbutamol, the β3-adrenoceptor agonists CL 316243 and ZD 2079, and the agonists of the low-affinity state of β-adrenoceptors, cyanopindolol and CGP 12177 block α1-adrenoceptors in that concentration range in which they relax the human pulmonary and rat mesenteric arteries preconstricted with phenylephrine 10 μM and 1 μM, respectively. For quantification of vasodilatation pEC25 values and for the antagonism toward α1-adrenoceptors, pA2 values were determined. We found that in the rat mesenteric artery, (1) the pEC25 values of the β-adrenoceptor ligands resemble their respective pA2 values (difference ≤ 0.9 log units), and (2) the order of potencies is the same for both parameters, ie, cyanopindolol ∼ fenoterol > CGP 12177 > salbutamol > ZD 2079 > CL 316243. In the human pulmonary artery, (1) the pEC25 values are slightly lower (by 0.6-1.3 log units) than their respective pA2 values, and (2) the rank order of potencies is the same for both parameters. In conclusion, the present study suggests that ligands of β2-adrenoceptors and of non-β1-non-β2-adrenoceptors relax rat and human vessels preconstricted with phenylephrine or norepinephrine mainly through their α1-adrenolytic effects. Hence, for the investigation of the role of β-adrenoceptors in vessels, the constrictor agent should be chosen with great caution.