Isomyosin and thyroid hormone levels in pressure-overloaded weanling and adult rat hearts

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Abstract
The relationship was examined between ventricular isomyosin composition and plasma thyroxine (T4) 5 wk after partial constriction of the abdominal aorta in weanling (21 day) and adult (8 wk) rats. Cardiac enlargement in weanling aorta-constricted animals was associated with a significant (P < 0.001) decrease in %V1 isomyosin in both left (32%) and right ventricles (25%) with a corresponding increase in the %V3 isomyosin and a reduction in plasma T4 levels. The ratio of V1/T4 was similar in weanling control (17.8 .+-. 0.8) and aorta-constricted (18.0 .+-. 1.4) rats. In adult aorta-constricted animals, there was a significant (P < 0.001) reduction in the %V1 (16%) isomyosin in the left ventricle and a smaller decrease in the right ventricular V1 (8%) with no change in plasma T4 levels. There was also a significant difference in V1/T4 between control (16.1 .+-. 0.4) and aorta-constricted (13.9 .+-. 0.7) adult rats in contrast to the maintenance of the V1/T4 in weanling aorta-constricted animals. Both increased workload and changes in thyroxine levels contribute to the isomyosin redistribution seen in weanling rats subjected to a pressure overload, whereas, in adult hypertrophied hearts, alterations of the ventricular isomyosin composition appear to be due solely to the increased pressure overload.