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 [I-125] pindolol (Pin). To identify the subtypes of beta-adrenoceptors in the 2 blood vessels, the competitive inhibition curves of [I-125]Pin by beta1-antagonist atenolol and beta2-agonist salbutamol were analyzed using a computer program LIGAND of a mathematical model of the ligand-binding system. The B(max) (15.3 +/- 1.2 fmol/mg protein) and the dissociation constant K(d) (44 +/- 4 pmol -L-1) for PA were similar to those for TA (B(max), 12.8 +/- 1.2 fmol/mg protein, K(d), 45 +/- 4 pmol . L-1). Competitive inhibition analysis showed that the beta1-and beta2-adrenoceptor subtypes coexisted in human PA and TA, with beta1 adrenoceptor dominant. The ratios of beta1 : beta2 in PA and TA were 2.9:1.0 and 2.1:1.0, respectively.