Augmentation of Myocardial Oxygen Consumption in Hyperthyroid Cats

To clarify the effects of hyperthyroidism on myocardial oxygen consumption (VO 2 ), a polarographic method was employed to compare the VO 2 of isolated papillary muscles from 13 normal euthyroid cats with that of 11 hyperthyroid cats. Basal VO 2 was greater in the hyperthyroid group (3.03±0.20 vs. 2.36±0.19 SE filter-ing dry wrr 1 · hour 1 , P < 0.05). In muscles studied under afterloaded isotonic conditions, hyperthyroidism shifted the forcevelocity curve upward and to the right, with an increase in both extent and velocity of shortening at equivalent loads. These changes in myocardial behavior in hyperthyroidism were associated with an increase in myocardial VO 2 . Isometrically contracting muscles from hyperthyroid animals demonstrated significant increases in both developed tension (6.3±0.7 vs. 4.7±0.4 g/mm 2 , P < 0.05) and rate of tension development (32.6±3.5 vs. 19.4±1.5 g/mm 2 · second- 1 , P < 0.01), as compared to the euthyroid group. Myocardial VO 2 , expressed per g/mm 2 isometric developed tension, was significantly greater in the hyperthyroid group (0.641±0.09 vs. 0.42±0.04 milliter · mg dry wt- 1 · beat-, P < 0.02). Thus, experimental hyperthyroidism augments myocardial VO 2 whether measured in resting or contracting cardiac muscle. This increase can be attributed, at least in part, to $ie altered contractile function of the heart in hyperthyroidism.