RESUMEN
The effect of thyroid hormones (thyroxine and triiodothyronine) on catecholamine receptors in isolated rat fat cells was investigated. Binding of (3H)isoproterenol and (3H)norepinephrine were increased by thyroid hormones. (3H)isoproterenol binding was more enhanced than (3H)norepinephrine binding. Triiodothyronine had a more potent effect than thyroxine. (3H)isoproterenol was used to estimate the number or affinity of beta-adrenergic receptors in rat fat cells treated with thyroid hormones. The binding sites for (3H)isoproterenol were the same in untreated and with triiodothyronine treated fat cells. The equilibrium dissociation constants (KD) for the interaction of receptors with (3H)isoproterenol were altered in thyroid hormone treated cells. There was a significant difference between the untreated and triiodothyronine treated fat cells in the affinity of beta-adrenergic receptor binding sites for (3H)isoproterenol. Thyroid hormone could alter negative cooperative site-to-site interaction among the binding sites for (3H)isoproterenol.
Asunto(s)
Tejido Adiposo/efectos de los fármacos , Receptores Adrenérgicos beta/efectos de los fármacos , Receptores Adrenérgicos/efectos de los fármacos , Hormonas Tiroideas/farmacología , Animales , Sitios de Unión , Catecolaminas/metabolismo , Técnicas In Vitro , Lipólisis/efectos de los fármacos , Masculino , Ratas , Receptores Adrenérgicos beta/metabolismoRESUMEN
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.
Asunto(s)
Tejido Adiposo/metabolismo , Isoproterenol/metabolismo , Receptores Adrenérgicos beta/metabolismo , Receptores Adrenérgicos/metabolismo , Agonistas Adrenérgicos beta/metabolismo , Animales , Unión Competitiva , Catecolaminas/metabolismo , Células Cultivadas , Humanos , Cinética , Masculino , Ratas , Relación Estructura-ActividadRESUMEN
The role of thyroid hormones on lipolysis in human subcutaneous adipose tissue was investigated. Incubation of subcutaneous fat pads with thyroxine (0.1--10 000 nM) augmented the subsequent isoproterenol stimulation of lipolysis, measured by glycerol release. The basal lipolysis could not by stimulated by thyroxine. The theophylline- and dibutyryl-cyclic AMP stimulated lipolysis also could not be increased by thyroxine at these concentrations. In separate studies, the effect of thyroxine (0.01 pM--1 microM) and triiodothyronine (0.01 pM--1 microM) on cyclic AMP accumulation was examined. No effect of thyroid hormones on cyclic AMP accumulation was seen in non-isoproterenol stimulated tissue. Fat pads stimulated by isoproterenol and then treated with thyroid hormones showed marked increases in accumulation of cyclic AMP as compared to control tissue in the presence of isoproterenol alone.