Thyroid Hormone Regulation of β-Adrenergic Receptor Number in Aging Rats*

To evaluate the capacity of the myocardium in aged rats to respond to hyperthyroidism, we quantified myocardial beta-adrenergic receptors in female Fischer 344 rats of 3, 12, and 24 months of age. In T3-treated rats (500 microgram T3/kg . day for 3 days), myocardial beta-adrenergic receptors, as measured by [3H]dihydroalprenolol binding, were significantly increased (P less than 0.01) over controls in 3-, 12-, and 24-month-old animals. The data demonstrate that senescent rats retain the capacity to increase myocardial beta-adrenergic receptors in response to exogenous hyperthyroidism. In the myocardium, the mechanism of decreased catecholamine responsiveness in aging appears to be at other than the beta-adrenergic receptor site, since receptor density is unaltered with age, as is receptor modulation in response to hyperthyroidism.