EVIDENCE FOR A DIFFERENT RESPONSE OF RED AND WHITE SKELETAL MUSCLE OF THE RAT IN DIFFERENT THYROID STATES

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Abstract
The influence of thyroid hormone depletion and experimental hyperthyroidism on red and white skeletal muscle of the rat during periods of 2, 4 and 8 wk were studied. Body weight, muscle weight, mitochondrial protein content and specific activities of the mitochondrial enzymes .alpha.-glycerophosphate dehydrogenase (EC 1.1.99.5) (.alpha.-GPD) and succinate dehydrogenase (EC 1.3.99.1) (SDH) were used as parameters. The largest differences in body weight gain and muscle weight gain (both red and white muscle) in the hypothyroid rats were seen after 8 wk of T4 treatment. In the hyperthyroid rats the weight of the red muscle and the ratio of the red muscle weight to the body weight increased, whereas the white muscle weight and the ratio of the white muscle weight to the body weight decreased relative to the control animals. In hypothyroid rats the mitochondrial protein content was lowered in both red and white muscle, the specific .alpha.-GPD activity only in the latter. No changes in specific SDH activity were observed in either type of muscle. The hyperthyroid rats showed an increase in the mitochondrial protein content and the specific .alpha.-GPD and SDH activity in the red muscle, whereas no significant changes were observed in the white muscle. The effect of the thyroid state apparently differs in red and white muscle. An explanation for a possibly greater sensitivity of red than of white muscle to thyroid hormones was discussed.