Inability of muscles in the obese mouse (ob/ob) to respond to changes in body weight and activity.

The fat-free carcass weight of the obese mouse (ob/ob) is generally less than that of wild-type siblings. The aim of this investigation was to examine the effect of obesity on muscle weights and histochemistry and to determine whether any effects could be eliminated when the obesity was largely prevented or reduced by limiting food intake. For 5 muscles examined the weights were significantly greater (except for biceps brachii) in the wild-type than in obese mice. Although there was a significant correlation between muscle weight (except for soleus) and body weight in the wild-type mice, no such correlation held for the obese mice. No remarkable differences between groups of mice were found in the histochemistry of the biceps brachii and soleus muscles except that fibre sizes were generally smaller in the obese mice. It is concluded that the skeletal muscles of obese mice cannot respond to the increased activity associated with prevented or reduced obesity.