| Abstract: | The subtype of the beta-adrenergic receptor expressed in 3T3-L1 preadipocytes and adipocytes differentiated with dexamethasone and methylisobutylxanthine was determined by comparing the affinity of the receptors for epinephrine, norepinephrine, and beta-1 and beta-2 selective antagonist, 8-fold more avidly than adipocyte receptors. In contrast, adipocyte beta-receptors had a 10-fold higher affinity for epinephrine than for norepinephrine and complexed the beta-2 selective agonist zinterol with a 20-fold higher affinity than preadipocyte receptors. Hofstee plots and computer analyses of the binding data revealed that the populations of beta-1 receptors in preadipocytes and beta-2 receptors in adipocytes were nearly homogeneous. Preliminary characterizations of the beta-receptor phenotype in (nondifferentiating) 3T3-C2 cells treated with dexamethasone and methylisobutylxanthine and 3T3-422A adipocytes differentiated with insulin indicated that the expression of beta-2 receptors was not correlated with differentiation, but rather with exposure of the cells to dexamethasone and methylisobutylxanthine. The regulator of beta-receptor subtype was identified as the glucocorticoid analog, dexamethasone, by employing 3T3-L1 adipocytes which were stimulated to differentiate with methylisobutylxanthine and insulin. Detailed binding studies showed that under these conditions the adipocyte receptors retain beta-1 character. Subsequent treatment with 0.5 microM dexamethasone promoted the loss of beta-1 receptors, the appearance of beta-2 receptors, and a net 2- to 3-fold increase in the number of beta-receptors. Dexamethasone effected a complete switch from beta-1 to beta-2 subtype at concentrations as low as 2.5 nM while other steroids were ineffective below a concentration of 10 microM. |
