Characterization of beta-adrenergic receptors by (3H) isoproterenol in adipocytes of humans and rats.
The specific binding of (3H) isoproterenol to isolated fat cells of human and rat was characterized. Binding of (3H) isoproterenol to isolated fat cells of rat was saturable with 420 pmol of (3H) isoproterenol bound/100 mg of lipid. Half-maximal saturation occurred at 5 microM providing an estimate of the equilibrium dissociation constant, KD, for the interaction of (3H) isoproterenol with its binding sites. Kinetic analysis of (3H) isoproterenol binding provided a value of 2.01 x 10(4) min-1. M-1 for the forward bimolecular rate constant, k1. Dissociation of (3H) isoproterenol was a first order reaction with a rate constant, k2, of 0.62 x 10(-1) min-1. The ratio k2/k1 = 3.07 microM provides an independent measurement of the KD for the interaction of (3H) isoproterenol with its binding sites which is in agreement with the values obtained by steady state analysis (3 to 5 microM). The apparent equilibrium dissociation constant, KD, for the interaction of (3H) isoproterenol with its receptor in human fat cells obtained by steady state analysis was 1 to 0.9 microM. Scatchard- and Hill-analysis suggest the possibility of different negatively cooperative interactions among the binding sites in human and rat. beta-Adrenergic agonists competed for the binding sites. The order of potency was isoproterenol greater than epinephrine greater than norepinephrine. Compounds such as DOPA, dopamine and (m-Hydroxyphenyl)2-methyl-aminoethanol which are structurally related to catecholamines had little or no affinity for (3H) isoproterenol binding sites.
D000273:Adipose Tissue / Q000378:metabolism*; D000318:Adrenergic beta-Agonists / Q000378:metabolism; D000818:Animals; D001667:Binding, Competitive; D002395:Catecholamines / Q000378:metabolism; D002478:Cells, Cultured; D006801:Humans; D007545:Isoproterenol / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D051381:Rats; D011941:Receptors, Adrenergic / Q000378:metabolism*; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D013329:Structure-Activity Relationship
D000318:Adrenergic beta-Agonists; D002395:Catecholamines; D011941:Receptors, Adrenergic; D011943:Receptors, Adrenergic, beta; D007545:Isoproterenol