Long-term effects of triiodothyronine and thiouracil on myocardial beta-adrenergic receptor numbers and cyclic AMP concentration in rats.

Long-term (35 days) effects of thyroid hormone on the number of myocardial beta-adrenergic receptors and the c-AMP concentration were studied in the rat. The ventricles from triiodothyronine-administered rats (T3 rats) showed an increase in the number of beta-receptors compared to controls (54.3 +/- 3.1 vs 39.3 +/- 1.8 fmole of [125I]-iodohydroxybenzylpindolol (IHYP) binding sites/mg protein p less than 0.001) on the 35th day. Conversely, the ventricles from thiouracil-administered rats (TU rats) showed a decrease in the number of beta-receptors compared to controls (31.4 +/- 1.6 vs. 38.4 +/- 2.6 fmole/mg protein, p less than 0.05). The equilibrium dissociation constants (Kd) for the interaction of receptors with IHYP did not differ significantly (0.10 to 0.15 nM). The myocardial concentration of cyclic AMP was not significantly different (T3 rats, 1.09 +/- 0.09 nM/g wet weight tissue; T3 controls, 1.12 +/- 0.07; TU rats, 1.13 +/- 0.07; TU controls, 1.16 +/- 0.12) on the 35th day. On serial effects of triiodothyronine from the first to the 35th day, the number of beta-receptors of T3 rats increased significantly on the 24th and the 35th day, but the c-AMP concentration was not significantly different from that in control rats. These results demonstrated that thyroid hormone affects the number of myocardial beta-receptors in rats, and suggested a different mechanism of action of thyroid hormone on the myocardium from that of catecholamines.