Glucocorticoid Regulation of Myocardial β-Adrenergic Receptors*

One action of glucocorticoids is to enhance the tropic effect of catecholamines on heart muscle. To test the hypothesis that this action of glucocorticoids is mediated by modulation of beta-adrenergic receptors, we characterized and quantified myocardial beta-adrenergic receptors in adrenalectomized and glucocorticoid-replaced rats. Adrenalectomy was associated with an increase in myocardial beta-adrenergic receptors, as measured by [3H]dihydroalprenolol binding (P less than 0.001). This increase occurred by 6 h, with no difference over time to 7 days. The administration of cortisol (80 mg/kg.day) to adrenalectomized rats prevented the increase in beta-adrenergic receptors (P less than 0.01). The data indicate that glucocorticoids modulate myocardial beta-adrenergic receptors. However, these results do not support the hypothesis that glucocorticoid enhancement of catecholamine action is mediated by these changes, suggesting that glucocorticoids exert this action at a level other than the beta-adrenergic receptor site.