EFFECT OF THYROXINE AND THIOURACIL ON THE MG++-ACTIVATED ATPASE ON THE RAT MYOCARDIUM

Mechanochemistry of cardiac muscle. V. Influence of thyroid state on energy utilization

The possibility that alterations in the rate or efficiency of energy utilization could be involved in the control of cellular oxygen consumption by thyroid hormone was examined in right ventricular papillary muscles isolated from normal euthyroid cats and cats with experimentally induced hyperthyroidism and hypothyroidism. Energy production in the muscles was inhibited and isolated from the process of energy utilization by exposure to iodoacetic acid and nitrogen. After resting or performing variable amounts of contractile element work under isometric conditions, muscles were frozen, and the total amount of chemical energy ( approximately P = creatine phosphate + ATP) used was determined. The resting rate of energy utilization in muscles from euthyroid animals was 0.78+/-0.07 mumoles/g per min of approximately P. This rate was elevated in muscles from hyperthyroid cats to 1.00+/-0.09 mumoles/g per min and decreased in muscles from hypothyroid cats to 0.23+/-0.14 mumoles/g per min. Isometrically contracting muscles from cats with hypothyroidism utilized only 64% as much energy as muscles from euthyroid cats while performing 81% as much contractile element work at a moderately decreased level of contractile state. Muscles from hyperthyroid cats utilized an average of 41% more energy than did muscles from euthyroid cats while contracting an identical number of times and performing an equal amount of contractile element work at a slightly increased level of contractile state. These results suggest that thyroid hormone directly influences the rate of cellular energy utilization. Furthermore, the increase in energy utilization in muscles from hyperthyroid cats could not be attributed entirely to observed alterations in contractile behavior, which indicates that excess thyroid hormone may decrease the efficiency of the conversion of cellular energy to work. However, the opposite effect, an increased efficiency of energy utilization, was not observed in muscles from hypothyroid cats. Thus, it is concluded that the calorigenic effects of thyroid hormone may be explained, at least in part, by alterations in the process of energy utilization.

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.

Anaerobic performance and metabolism of the hyperthyroid heart

Anaerobically periused hearts from rats with experimentally induced hyperthyroidism exhibited accelerated deterioration of pacemaker activity and ventricular performance. The diminished anaerobic performance of hyperthyroid hearts was associated with decreased adenosine triphosphate (ATP) levels and a reduced rate of anaerobic glycolysis as reflected in decreased lactic acid production during 30 min of anoxic perfusion.Studies on whole heart homogenates demonstrated inhibition at the phosphofructokinase (PFK) step of the glycolytic pathway. Such inhibition was not demonstrated in the hyperthyroid heart cytosol. It is postulated that an inhibitor of PFK which resides dominantly in the particulate fraction is probably responsible for the diminished anaerobic glycolysis and performance of the hyperthyroid heart.

Cardiac failure in the dog as a consequence of exogenous hyperthyroidism

Thirty dogs were made hyperthyroid by feeding them 1 g/kg USP thyroid powder or injecting 1.2 mg/kg of 1-thyroxine/day. Seven of the 30 dogs used had surgically induced mild valvular lesions of the right heart to determine whether preexistent organic disease was a requisite to the induction of failure in hyperthyroidism.

At 2 to 27 months the animals were subjected to cardiac catheterization for measurement of cardiac work and metabolism in vivo. The animals were then killed and the levels of the high energy phosphate compounds creatine phosphate (CP) and the adenine nucleotides (ATP+ADP) in the heart were measured as an index of the net efficiency of the processes of oxidative phosphorylation.

By hemodynamic or histopathologic criteria, 13 dogs showed signs of failure, only 3 of which had preexistent valvular lesions. The failure was not accompanied by a decreased efficiency of oxidative phosphorylation. Net levels of CP and ATP + ADP in the myocardium were normal. Preliminary studies with sarcosomes isolated from 5 animals also revealed normal P/O ratios in 3 dogs with and in 2 without signs of cardiac failure. Liver mitochondria isolated from these same 5 animals all demonstrated decreased P/O ratios. Decreased myocardial extractions of both glucose and pyruvate occurred with failure, suggesting some degree of myocardial hypoxia with increased intracellular glycolysis. Areas of relative hypoxia may exist in a hypertrophied myocardium of the hyperthyroid animal.