Determination of beta-adrenoceptor subtypes in human pulmonary artery and thoracic aorta by radioligand binding

The beta-adrenoceptors of human pulmonary artery (PA) and thoracic aorta (TA) were studied by the use of a high specific activity radioligand [125I]pindolol (Pin). To identify the subtypes of beta-adrenoceptors in the 2 blood vessels, the competitive inhibition curves of [125I]Pin by beta 1-antagonist atenolol and beta 2-agonist salbutamol were analyzed using a computer program LIGAND of a mathematical model of the ligand-binding system. The Bmax (15.3 +/- 1.2 fmol/mg protein) and the dissociation constant Kd (44 +/- 4 pmol.L-1) for PA were similar to those for TA (Bmax, 12.8 +/- 1.2 fmol/mg protein, Kd, 45 +/- 4 pmol.L-1). Competitive inhibition analysis showed that the beta 1- and beta 2-adrenoceptor subtypes coexisted in human PA and TA, with beta 1 adrenoceptor dominant. The ratios of beta 1:beta 2 in PA and TA were 2.9:1.0 and 2.1:1.0, respectively.