PERINATAL CHANGES IN THE COUPLING OF BETA1 AND BETA3 ADRENERGIC-RECEPTORS TO BROWN FAT ADENYLYL CYCLASE

The stimulation of adenylyl cyclase by catecholamines in neonatal brown adipose tissue (BAT) is markedly biphasic, suggesting the existence of receptors that have both high and low affinities for catecholamines. The identities of these receptors were examined by comparing responses in neonatal BAT membranes to those of Chinese hamster ovary cells which had been transfected to express the cloned rat beta1 and beta3 receptors. The results from these experiments indicate that high-affinity stimulation of adenylyl cyclase by catecholamines in BAT is mediated by beta1 receptors, as evidenced by the potencies of norepinephrine and isoproterenol at this receptor and the potent blockade of the receptor by alprenolol. The low-affinity catecholamine receptor appears to be the beta3 receptor, as indicated by the low potency of catecholamine agonists and the inability of low concentrations of alprenolol to block this activity. Furthermore, this receptor, like the cloned rat beta 3 receptor, was antagonized by (-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one (CGP 12177) and was stimulated by (R',R')-4-{2-[(2[(3-chlorophenyl)-2-hydroxyethyl] amino)propyl]phenyl}phenoxyacetic acid (BRL 37344). These results indicate that both beta1 and beta3 receptors couple to adenylyl cyclase in BAT and that activation of adenylyl cyclase in neonatal BAT is mediated primarily by beta3 receptors. Beta3 receptors were also clearly detected in weanling BAT with the beta3-selective agonist BRL 37344. However, when catecholamines were used to stimulate activity, the activation of adenylyl cyclase by beta1 receptors, which occurred at low concentrations of catecholamines, obscured the activation of adenylyl cyclase by beta3 receptors, which occurred only at high concentrations.