Microcalorimetric study of myocardial tissue metabolism in experimental hyperthyroidism in rats

Abstract

The metabolic consequences in myocardial tissue, specifically concerning the importance of aerobic metabolism, following the stimulatory actions of thyroid hormones were studied in an experimental model for hyperthyrodism in rats, treated with triiodothyronine (T3) for two weeks. The metabolic activity in myocardial tissue was measured by microcalorimetry and related to oxygen consumption and to the ATP content. In myocardial muscle tissue from rats given 10–25 μg T3 daily over two weeks, heat production was 1.53 ± 0.09 mWg−1, compared to 0.94 ± 0.05 mWg−1 in controls, p <0.001. Similarly, oxygen consumption was higher in T3-treated rats, 2.75 ± 0.16 nmol O2s−1g−1, compared to 1.55 ± 0.14 nmol O2s−1g−1 in the controls, p < 0.001. There was a significant correlation between heat production and oxygen consumption values when data from controls and T3-treated rats were put together, r = 0.66, p < 0.001. The ATP content in the myocardial tissue was significantly lower in T3-treated rats than in control rats, 2.82 ± 0.53 and 7.91 ± 0.44 μmolg−1 respectively, p < 0.001. The myocardial tissue ATP content was inversely correlated to myocardial heat production, r = −0.54, p < 0.01, and oxygen consumption, r = −0.50, p < 0.05. In conclusion, the present data demonstrate the close relationship between oxygen consumption and metabolic activity in rat myocardial tissue in the euthyroid and hyperthyroid states. Aerobic metabolism was calculated to account for about 80% of the myocardial heat production in the euthyroid state and for about 87% in the hyperthyroid state. In addition, T3 treatment was associated with a significant reduction of the ATP content in the myocardial tissue studied, suggesting that myocardial ATP generation may be insufficient to meet the increased demand on heart performance in hyperthyroidism.