Effects of adrenalectomy on muscle fibre growth and fibre-type composition in obese-hyperglycaemic (ob/ob) and lean mice.

Adrenalectomy of obese mice increases the weights of their skeletal muscles towards normal. We have investigated whether this increased growth was specific for the fast phasic muscles and white, glycolytic fibres most affected by obesity and whether the proportions of the different fibre types were restored to normal. Male obese hyperglycaemic mice (ob/ob) and lean mice were adrenalectomized at 6 weeks of age and examined 6 weeks later. In obese mice body weight gain and blood glucose were markedly decreased, although still higher than in lean mice. The weight of the heart was decreased to normal in obese mice and unaffected in lean mice. In lean mice the weights of the biceps brachii, gastrocnemius and soleus muscles increased by similar small amounts after adrenalectomy. In obese mice the fast gastrocnemius and biceps brachii recovered most in weight, the soleus somewhat less, but all remained smaller than in lean adrenalectomized mice. The increased growth of the biceps brachii in obese mice after adrenalectomy was non-specific with a similar increase in size of all fibre types, including the fast red fibres which were not affected by obesity. Fibre type proportions were not normalized by adrenalectomy of obese mice but were significantly changed in adrenalectomized lean mice. We conclude that it is possible only partially to normalize muscle weight and fibre type composition in obese mice by adrenalectomy. We suppose that its effects are manifest because of other defects such as insulin resistance and that these are not all reversed by adrenalectomy.