MICROCALORIMETRIC STUDY OF MYOCARDIAL TISSUE METABOLISM IN EXPERIMENTAL HYPERTHYROIDISM IN RATS

The metabolic consequences in myocardial tissue, specifically concerni ng the importance of aerobic metabolism, following the stimulatory act ions of thyroid hormones were studied in an experimental model for hyp erthyroidism in rats, treated with triiodothyronine (T3) for two weeks . The metabolic activity in myocardial tissue was measured by microcal orimetry and related to oxygen consumption and to the ATP content. In myocardial muscle tissue from rats given 10-25 mu g T3 daily over two weeks, heat production was 1.53+/-0.09 mW g(-1), compared to 0.94+/-0. 05 mW g(-1) in controls, p < 0.001. Similarly, oxygen consumption was higher in T3-treated rats, 2.75+/-0.16 nmol O(2)s(-1) g(-1), compared to 1.55+/-0.14 nmol O-2 s(-1) g(-1) in the controls, p < 0.001. There was a significant correlation between heat production and oxygen consu mption values when data from controls and T3-treated rats were put tog ether, r = 0.66, p < 0.001. The ATP content in the myocardial tissue w as significantly lower in T3-treated rats than in control rats, 2.82+/ -0.53 and 7.91+/-44 mu mol g(-1) respectively, p < 0.001. The myocardi al tissue ATP content was inversely correlated to myocardial heat prod uction, 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 t issue 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. I n addition, T3 treatment was associated with a significant reduction o f the ATP content in the myocardial tissue studied, suggesting that my ocardial ATP generation may be insufficient to meet the increased dema nd on heart performance in hyperthyroidism.