Direct labelling of ? 2-adrenoceptors

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Abstract
A radioligand that selectively labels β 2-adrenoceptors, 3H-ICI 118,551 (3H-ICI), is introduced. Experiments were performed on guinea-pig tissues. The binding characteristics of 3H-ICI on lung membrane particles are compared with the blocking characteristics of ICI 118,551 against the tracheo-relaxant effects of (−)-noradrenaline, (−)-adrenaline and (±)-fenoterol. Binding to both β 1- and β 2-adrenoceptors were also performed with 3H-(−)-bupranolol on lung and ventricular myocardium. The binding inhibition characteristics of unlabelled ICI 118,551 on ventricle were compared with its characteristics as antagonist of the positive chronotropic effects of (−)-noradrenaline, (−)-adrenaline and (±)-fenoterol in spontaneously beating right atria. ICI 118,551 blocked more the relaxant effects of (±)-fenoterol and (−)-adrenaline than those of (−)-noradrenaline on trachea. The positive chronotropic effects of (±)-fenoterol on sinoatrial node were blocked more than those of both (−)-adrenaline and (−)-noradrenaline. A non-linear regression analysis of blocking data with ICI 118,551 according to the model of Lemoine and Kaumann (1983) revelas that both β 1- and β 2-adrenoceptors contribute to the tracheo-relaxant and positive chronotropic effects of agonists. The estimated equilibrium dissociation constants pKB (-log K B=pK B; mol/l) were 7.1 and 9.6 for β 1- and β 2-adrenoceptors, respectively. Tracheal β 2-adrenoceptors contribute 99%, 97% and 7%, sinoatrial β 2-adrenoceptors contribute 76%, 3% and 0% to the fractional stimuli induced by (±)-fenoterol, (−)-adrenaline and (−)-noradrenaline, respectively. 3H-ICI associated to β 2-adrenoceptors of lung membranes with a k on of 0.52 l·nmol−1·min−1 and dissociated with a k off of 0.19 min−1. 3H-ICI bound to lung β 2-adrenoceptors with an equilibrium dissociation constant pKL* of 9.2. Unlabelled ICI 118,551, (−)-bupranolol, (+)-bupranolol, (−)-adrenaline, (−)-noradrenaline and (±)-fenoterol competed with 3H-ICI for lung β 2-adrenoceptors with pK L-values of 9.0, 9.4, 8.1, 5.9, 4.9 and 6.4, respectively. 3H-(−)-bupranolol associated to β-adrenoceptors of lung membranes with a k on 1.2 l·nmol−1·min−1 and dissociated with a k off of 0.26 min−1. 3H-(−)-bupranolol bound to lung β 2-adrenoceptors and to heart β 1-adrenoceptors with a pK L of 9.6 and a pK L of 8.8, respectively. Lung β 2- and β 1-adrenoceptors comprised 3/4 and 1/4 of the β-adrenoceptor population, as estimated independently with 3H-ICI and 3H-(−)-bupranolol; 1/5 of ventricular β-adrenoceptors was β 2, 4/5 β 1. The binding characteristics including stereoselectivity show that 3H-(−)-ICI 118,551 is useful to label nearly exclusively β 2-adrenoceptors in a system containing both β 1- and β 2-adrenoceptors. The affinity for β 2-adrenoceptors of competing ligands can be determined straightforwardly without interference of β 1-adrenoceptors. The low affinity for lung β 2-adrenoceptors but high tracheorelaxant potency of agonists suggest the existence of a large β 1-adrenoceptor reserve.

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