The effect of thyroid hormones on the aerobic development of locomotor and cardiac muscles in the barnacle goose

The role of thyroid hormones in the development of the locomotor and cardiac muscles of the barnacle goose (Branta leucopsis) was investigated. From 2 weeks of age, goslings were treated with thyroxine, triiodothyronine, or methimazole (a thyroid inhibitor). Birds were killed at 6 weeks (n = 5) or 9 weeks (n = 4) and various locomotor and cardiac muscle masses recorded and tissue samples taken for analysis of citrate synthase activity. The effects of thyroxine and triiodothyronine were not significantly different from each other, except in the case of the iliofibularis at 9 weeks. The mass-specific citrate synthase activity of the iliofibularis, semimembranosus muscles and liver were significantly increased by thyroid hormone treatment. Cardiac muscle showed a significant hypertrophy at 9 weeks of age following treatment with thyroxine. Hypothyroidism induced by treatment with methimazole exhibited the greatest effect on the pectoralis muscle, reducing citrate synthase activity by 33%. Mass-specific citrate synthase activity of the pectoralis and pectoralis mass (% body wt.) were found to be highly correlated (r2 = 0.74) at 6 weeks of age. It is suggested that thyroid hormones may be involved in controlling the tissue-specific timing of the maturation of locomotor and cardiac muscles.