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Comparison of the Effect of a Protein-Free and Restricted High Protein—Low Carbohydrate Diet on Ventricular Myosin ATPase Activity and Isomyosin Profile in Young Rats: Evidence That Protein-Depleted Animals Are Euthyroid¹

Veterans Administration Medical Center and Department of Medicine, University of Arkansas for Medical Sciences, Little Rock, AR 72205

Young rats fed a protein-deficient diet have elevated total triiodothyronine (T₃) levels in spite of decreased skeletal muscle protein turnover. Interpretation of the thyroid status of these animals is complicated by increased T₃ binding by serum proteins. Free T₃ levels ranging from normal to low and decreased resting oxygen consumption have been reported. To investigate the thyroid status of animals fed a protein-free diet, ventricular myosin ATPase activity and isomyosin profile have been used as an index of thyroid hormone activity. The effect of the protein-free diet has been compared to a restricted high protein-low carbohydrate diet, which causes clear evidence of decreased thyroid hormone effect. After 28 d, calcium-activated myosin ATPase activity was 1.50 ± 0.05 µmol P₁/(mg protein · min) in animals fed the basal diet, 1.16 ± 0.03 µmol P₁/ (mg protein · min) in animals fed the restricted high protein-low carbohydrate diet and 1.48 ± 0.04 µmol P₁/(mg protein · min) in animals fed the protein-free diet. In addition, a shift in isomyosin content with the appearance of V₂ and V₃ isomyosins occurred with the restricted high protein-low carbohydrate diet but not the protein-free diet. The failure of the protein-depleted rats to decrease myosin ATPase activity and alter isomyosin content suggests that they are euthyroid.

KEY WORDS: • protein deficiency • thyroid status • ventricular isomyosin • ventricular myosin ATPase activity

¹ This work was supported by Veterans Administration Research funds, Project No. 430-66-8721-0002, and by the National Institutes of Health, Grant No. 2-R01-AM20718-08.

Manuscript received 3 April 1987. Revision accepted 31 August 1987.