Does l-thyroxine administration alter cardiac size and myosin content in heterotopically transplanted rat hearts?
Thyroxine-induced cardiac hypertrophy appears to be mediated indirectly via changes in cardiac work, while myosin isoenzyme expression remains responsive to thyroid hormone levels regardless of workload.
Infrarenal heterotopic cardiac isografts maintain structural and functional integrity. We have used this transplantation model to further explore the mechanisms of thyroid hormone-induced cardiac hypertrophy. Thyroid hormone administration, 1-thyroxine (T4) 10 Mg/animal per d, led to a significant 30% increase in total heart weight and a 40% increase in the myosin content of the in situ heart when compared with control. In contrast, T4 treatment was without effect on the heart weight, protein content, rate of protein synthesis, or calculated myosin content of the heterotopic, nonworking heart. Heterotopic hearts demonstrated a significant decrease in the percentage of the VI myosin iso- enzyme from 95% to 61%. This shift occurred in euthyroid an- imals but was prevented by T4 treatment. These results suggest that thyroxine-induced cardiac hypertrophy is mediated indirectly via changes in cardiac work. Myosin isoenzyme expression can be altered by changes in work load but is still responsive to increased levels of thyroid hormone.
Klein et al. (Thu,) studied this question.
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