Subsarcolemmal mitochondria and capillarization of soleus muscle fibers in young rats subjected to an endurance training. A morphometric study of semithin sections.

Rats, 6 weeks old, were subjected to a program of endurance running for 3, 6 and 12 weeks. 0.5 to 0.8 mum thick sections of Epon embedded soleus muscles were studied with morphometric methods. In cross-sections the area occupied by subsarcolemmal mitochondria was independent of the age, but was 53% higher after 12 weeks of training. The mean depth of the zones with subsarcolemmal mitochondria increased only 15% to about 0.9 mum. Thus, the subsarcolemmal mitochondria showed a pronounced spreading at the muscle fiber surface in trained muscles.-The number of capillaries per fiber decreased slightly in controls and increased not significantly in trained muscles. It is concluded that the subsarcolemmal mitochondria supply the energy for the active transport of metabolites through the sarcolemma in oxidative muscle fibers, and that they are the limiting factor for endurance performance of the soleus muscle fibers because the changes in the capillarization were only small. It is suggested that the subsarcolemmal and the interfibrillar mitochondria have different functions and may therefore represent different types of mitochondria which can be distinguished by their morphology as well as by their biochemical properties.