TISSUE-SPECIFIC EFFECTS OF HYPOTHYROIDISM ON POSTNATAL MUSCLE DEVELOPMENT IN THE BARNACLE GOOSE

The hypothesis that tissue-specific levels of thyroid hormones may be required for normal locomotor muscle development was investigated in t he barnacle goose Branta leucopsis. Hypothyroidism was induced in gosl ings by treatment with methimazole from either 3 days or 2 weeks of ag e, and birds were killed at 7 weeks of age. The masses of the pectoral is, iliofibularis, semimembranosus and cardiac ventricle muscles were measured, and samples from these tissues were analysed for the mass-sp ecific activity of the mitochondrial enzyme citrate synthase (CS), An ultrastructural electron micrograph analysis of the pectoralis was als o carried out. No significant differences were found between the two h ypothyroid groups except for the effect on the relative mass of the il iofibularis muscle. Developmental responses to hypothyroidism were fou nd to be tissue-specific. Hypothyroidism resulted in a significantly l ower relative cardiac ventricle mass (by 17%) and CS activity of the l eg muscles (by 34%), while absolute leg muscle mass was not affected. The relative mass of the pectoralis was significantly lower (by 57%) i n hypothyroid birds and showed a significant, uniformly lower CS activ ity (by 60-83%) as a result of a lower mitochondrial fractional volume , Haematocrit and capillary-to-fibre ratio in the pectoralis were also significantly lower in hypothyroid birds, and skeletal growth and plu mage development were affected.