An experimental morphometric study of neutral lipid accumulation in skeletal muscles

The neutral lipid content of skeletal muscle fibers was determined by computing the lipid accumulation index (LAI) on transverse cryostat sections stained with Oil Red O. The LAI was defined as: (total area of neutral lipid droplets in a fiber) x 100/(total cross-sectional area of a fiber). The biceps, diaphragm, and soleus muscles were studied in 3 groups of guinea pigs: normal animals, animals fasted for 48 hours, and animals subjected to muscle denervation and then fasted for 48 hours. In normal animals, the highest mean LAI was found in the diaphragm (4.93) in comparison with lower values in the biceps (2.25) and soleus (2.09). After fasting, these values were markedly increased; there was also a concomitant increase in plasma free fatty acid (FFA) concentration. Prior denervation further increased the LAI in biceps and soleus but reduced it in the diaphragm. Type 2A fibers tended to show high lipid accumulation when the plasma FFA concentration was high. Type 2B fibers never accumulated much lipid under any circumstances. Type 1 fibers varied in their response in the different muscles.