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

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Abstract
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 fmol of [125I]-iodohydroxybenzylpindolol (IHYP) binding sites/mg protein P < 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 fmol/mg protein, P < 0.05). The equilibrium Kd for the interaction of receptors with IHYP did not differ significantly (0.10-0.15 nM). The myocardial concentration of cyclic AMP was not significantly different (T3 rats, 1.09 .+-. 0.09 nM/g wet wt 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 1st to the 35th day, the number of .beta.-receptors of T3 rats increased significantly on the 24th and the 35th day, but the cAMP concentration was not significantly different from that in control rats. The 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.