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To investigate the mechanism of thyroid hormone-induced cardiac hypertrophy, we have studied the in vivo changes in cardiac size and total myocardial content of both membrane and cytoskeletal enzymes in the rat after the administration of excess thyroid hormone. In response to 50 micrograms T4/day, there is a significant increase in heart rate and heart work associated with an increase in total heart size and protein content. Measurements of the specific activity of Na,K-ATPase and p-nitrophenol phosphatase demonstrate a small but significant increase in specific activity, while the specific activity of myosin ATPase is unchanged. To further probe the mechanism for T4-mediated hypertrophy we studied the in vivo effects of beta-adrenergic blockade on rat heart size. When animals were treated with both T4 and propranolol (10 mg/animal.day) cardiac hypertrophy was prevented. Propranolol alone at this dose did not affect heart rate, heart weight, or serum levels of T4 and T3. The present data suggest that 1) the hypertrophic response of the myocardium to excess thyroid hormone involves cytoplasmic as well as membrane proteins, 2) the increase in total myocardial protein, which can be blocked by propranolol, is indirectly mediated by increases in cardiac work rather than a direct effect of thyroid hormone.
Irwin Klein (Fri,) studied this question.