Changes in volume densities and distribution of mitochondria in rat skeletal muscle after chronic hypoxia.

In order to test the hypothesis that subsarcolemmal aggregation of skeletal muscle mitochondria, as noted in some patients with mitochondrial myopathies, is caused by an impaired oxygen supply to mitochondria, we exposed two groups of rats to 8% O2 during 44 or 45 days. One group performed mild exercise, as did a normoxic control group. After hypoxia we investigated fibre type composition, fibre diameter and capillarity of soleus and extensor digitorum longus (e.d.l.) muscles. Mitochondrial volume densities were determined by morphometric analysis. In both e.d.l. and soleus muscle fibre type composition was unchanged and fibre diameter was reduced after hypoxia. Capillary network was unchanged, hence capillary density was increased in both muscles after hypoxia (P less than 0.05 in untrained soleus). Total mitochondrial volume density was increased in soleus muscle, which was due to an increase in the volume density of intermyofibrillar mitochondria (P less than 0.05 in the trained group), whereas the volume density of subsarcolemmal mitochondria decreased. In e.d.l. muscle the volume density of both intermyofibrillar and subsarcolemmal mitochondria decreased. We conclude that subsarcolemmal aggregation of mitochondria did not occur after chronic hypoxia. In soleus and e.d.l. muscle, different changes in the volume densities of mitochondria occurred. This may be related to the relative amounts of type I and type II fibres within these muscles.