Effects of exercise on protein synthesis and myosin heavy chain gene expression in hypothyroid rats.

Hypothyroidism suppresses muscle growth and alters myosin heavy chain (MHC) gene expression. To study the role of thyroid hormones in exercise-induced muscle growth and protein synthesis, we measured skeletal and cardiac muscle protein synthesis and MHC gene expression in hypothyroid rats allowed to exercise voluntarily. Female Sprague-Dawley rats (200-210 g) were separated into four groups for 28 days of treatment: control, hypothyroid (TX), hypothyroid plus running-wheel exercise (TX+Ex), and hypothyroid plus 25% overfed (TX+OF). Fractional protein synthesis rates (% incorporation/day) were measured using [3H]phenylalanine incorporation 10 min postinjection. The heart weight-to-body weight ratios of the TX and the TX+OF groups showed marked cardiac atrophy over the 28-day period (2.76 +/- 0.12 and 2.50 +/- 0.22 vs. 3.37 +/- 0.18 mg/g, respectively; P < 0.01). However, the TX+Ex group prevented heart, gastrocnemius, and soleus muscle atrophy over the same time period. Heart, gastrocnemius, and soleus muscles had markedly suppressed protein synthesis rates in the TX and TX+OF groups vs. the euthyroid controls (mean fall -72%; P < 0.01, analysis of variance). However, exercise increased protein synthesis rate by 50% (P < 0.05) compared with TX alone in all three muscle groups. Exercise did not modify hypothyroid-induced alterations of cardiac myosin isoform expression. Exercise-mediated effects on skeletal and cardiac muscle growth but not cardiac MHC gene expression appear to be independent of thyroid hormones.