Mitochondrial distribution in relation to changes in muscle metabolism in rat soleus

Rats were subjected to exercise programs of either brief running once weekly or endurance running daily on a laddermill for 6 weeks. Both groups of exercised rats showed significant hypertrophy of the soleus. This muscle was analyzed morphometrically to determine if these running programs induced significant differences in the distribution of interfibrillar mitochondria. Interfibrillar mitochondrial volume density in the endurance-trained and control groups was highest near the fiber border and decreased with distance toward the fiber center, whereas in the weekly-run rats interfibrillar mitochondrial volume density was not correlated with distance within the fiber. Absolute interfibrillar mitochondrial volume (Vmi) was approximately 50% higher in endurance-trained rats than in controls at all locations sampled, but Vmi was nearly 80% higher in the weekly-run animals only in the fiber center. Thus the distribution of mitochondria was significantly different in weekly-run versus endurance-trained animals which may reflect differences in substrate source or high-energy phosphate flux in these fibers.