Abstract
The effects of norepinephrine on the activity of phosphorylase a, the concentrations of creatine-P and ATP, and contractile amplitude were determined in isolated perfused hearts from normal rats and rats given triiodothyronine (100 μg/day) for 3 days. The hearts were perfused with oxygenated salt solution at a constant rate of flow at either 30° or 37°. Hearts of both euthyroid and hyperthyroid rats had control activities of phosphorylase a equal to about 10 per cent of the total enzyme activity. The concentration of creatine-P was lower in hearts of triiodothyronine-treated rats than in those of euthyroid rats whether perfusions were done at 30° or at 37°. Graded doses of norepinephrine caused a greater increase of the percentage phosphorylase a in hearts from hyperthyroid rats than in those from euthyroid rats at both perfusion temperatures, but the contractile amplitude response was not potentiated. After norepinephrine, the concentration of creatine-P was decreased in hearts from both euthyroid and hyperthyroid rats when perfused at 37°, but not when perfused at 30°. However, at both temperatures, there was a correlation between the concentration of creatine-P and the activation of phosphorylase produced by norepinephrine. In addition, nonperfused hearts from triiodothyronine-treated rats, which have been found previously to have elevated activities of phosphorylase a, had the lowest concentrations of creatine-P of the conditions tested. These relationships are consistent with the hypothesis that thyroid hormone has no special effect on the cardiac adrenergic receptor, and that the greater activation of phosphorylase produced by norepinephrine in hearts of hyper-thyroid rats may be due in part to increased phosphorylase b kinase activity resulting from lower concentrations of creatine-P.
Published Version
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