Fiber Type-Specific Localization of Monocarboxylate Transporters MCT1 and MCT4 in Rat Skeletal Muscle

For many years it was thought that lactic acid traverses plasma membranes by diffusion, however, it has been shown in recent years that lactic acid and other monocarboxylates are transported through these membranes together with H+ by monocarboxylate transporters (MCT5). Of these transporters, rat skeletal muscle has been found to contain MCT1 and MCT4. It is thought that MCT1 transports lactic acid into the skeletal muscle from outside the skeletal muscle cells, while MCT4 is involved in the extrusion of lactic acid out of the muscle cells. It has been reported that the concentration of MCT1 within the skeletal muscle is highest in muscle fibers with superior oxidative glycolytic capacity, whereas MCT4 concentrations are highest in fibers with greater anaerobic glycolytic capacity. However, the relation between MCT1 and MCT4 localization and muscle fiber type has not been clarified from a morphological viewpoint. The present study applied morphological methods to examine the relation between fiber type and localization of MCT1 and MCT4 in Wistar rat skeletal muscle. After the animals were perfusion-fixed in 4% paraformaldehyde, the extensor digitorum longus muscle (EDL) and the soleus muscle (SOL) were dissected out and MCT1 and MCT4 localization were immunohistochemically determined in serial sections. In addition, adjacent sections were immunohistochemically stained with parvalbumin to identify muscle fiber types. Results showed clearly that MCT1 was present on the plasma membranes of all type I fibers, whereas MCT4 was localized on the plasma membranes of all type Ilb fibers. Both MCT1 and MCT4 were found on nearly all intermediate type Ila fibers. The authors consider that these relationships between muscle fiber type and MCT1 or MCT4 localization reflect the differences in glycolytic metabolism that have been reported between the different muscle fiber types.