Effect of functional overload on substrate oxidation capacity of skeletal muscle.

We determined the effects of surgically induced functional overload (O) on rodent fast-twitch plantaris (P) and slow-twitch soleus (S) skeletal muscle substrate oxidative capacity. Compared with normal control muscles of weight-matched rats, bilateral overload produced 68 and 23% increases in the wet weight of OP and OS muscles, respectively (p less than 0.05). Total protein concentrations of the O muscles remained unchanged relative to controls. The enzymatic capacity to oxidize pyruvate, palmitate, and alpha-glycerophosphate was unchanged in OP muscles relative to controls. Certain ketone oxidative enzyme markers were increased in the whole as well as in the inner "red" and outer "white" regions of OP muscle. Citrate synthase activity (a marker for tricarboxylic acid cycle oxidative capacity) was decreased in the whole and in the red region but unchanged in the white region of OP muscles. In contrast, the above measurements were significantly decreased in the OS muscles compared with controls (p less than 0.05). These findings suggest that there is both an expansion and a change in composition of the mitochondrial pool of enlarged P muscle. The effects on OS muscle, however, suggest the possibility that the oxidative capacity is not altered parallel with the enlargement, although fibre-typing (fast-twitch to slow-twitch) changes and altered mitochondrial composition could also contribute in part to the change.