Glucocorticoid Regulation of Myocardial β-Adrenergic Receptors*

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Abstract
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 β-adrenergic receptors, we characterized and quantified myocardial β-adrenergic receptors in adrenalectomized and glucocorticoid-replaced rats. Adrenalectomy was associated with an increase in myocardial β-adrenergic receptors, as measured by [3H]dihydroalprenolol binding (P < 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 β-adrenergic receptors (P < 0.01). The data indicate that glucocorticoids modulate myocardial β-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 β-adrenergic receptor site.