title: Effects of exercise on protein synthesis and myosin heavy chain gene expression in hypothyroid rats.
abstract: 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.
authors.affiliation: Department of Medicine, North Shore University Hospital-Cornell University Medical College, Manhasset, New York 10030.
authors.forename: H L
authors.initials: HL
authors.lastname: Katzeff
authors.forename: K M
authors.initials: KM
authors.lastname: Ojamaa
authors.forename: I
authors.initials: I
authors.lastname: Klein
mesh_terms: D000818:Animals; D004435:Eating; D005260:Female; D015870:Gene Expression; D007037:Hypothyroidism / Q000235:genetics* / Q000473:pathology / Q000503:physiopathology*; D008285:Major Histocompatibility Complex / Q000235:genetics; D009043:Motor Activity / Q000502:physiology*; D009132:Muscles / Q000473:pathology; D009206:Myocardium / Q000473:pathology; D009218:Myosins / Q000235:genetics*; D009929:Organ Size; D014176:Protein Biosynthesis; D051381:Rats
chemical_list: D009218:Myosins