Skeletal muscle metabolism: effect of age, obesity, thyroid and nutritional status

To investigate the potential role of skeletal muscle metabolism in the determination of whole body energy balance, the rate of oxygen consumption was measured in soleus muscles isolated from mice of differing age, sex, strain (thin and obese), thyroid and nutritional status. As expected, T3-induced hyperthyroidism resulted in an increase in the intrinsic rate of energy utilization by the muscles; qualitatively similar changes were noted in both younger animals and mice induced to increase their energy intake by sucrose-overfeeding. In contrast, soleus muscle oxygen consumption was significantly reduced in genetically obese NZO mice. Parallel changes in the activity of the Na-K pump were observed in all groups of animals, with a good correlation (r greater than 0.6, P less than 0.01) noted between these two independently measured aspects of muscle metabolism. The results suggest that the intrinsic metabolic rate of the skeletal muscle mass plays an important role in the control of overall energy balance, with changes in this rate being potentially partly responsible for the altered energetic efficiency of genetic obesity and adaptive thermogenesis models. In addition, measurement of the activity of the Na-K-pump may provide a convenient marker for such observed changes in energy utilization.