Distribution of the lactate/H+ transporter isoforms MCT1 and MCT4 in humanskeletal muscle

The profiles of the lactate/H+ transporter isoforms [monocarboxylate transp orter isoforms (MCT)] MCT1 and MCT4 (formerly MCT3 of Price, N. T., V. N. J ackson, and A. P. Halestrap. Biochem. J. 329: 321-328, 1998) were studied i n the soleus, triceps brachii, and vastus lateralis muscles of six male sub jects. The fiber-type compositions of the muscles were evaluated from the o ccurrence of the myosin heavy chain isoforms, and the fibers were classifie d as type I, IIA, or IIX. The total content of MCT1 and MCT4 was determined in muscle homogenates by Western blotting, and MCT1 and MCT4 were visualiz ed an cross-sectional muscle sections by immunofluorescence microscopy. The Western blotting revealed a positive, linear relationship between the MCT1 content and the occurrence of type I fibers in the muscle, but no signific ant relation was found between MCT4 content and fiber type. Moreover, the i nterindividual variation in MCT4 content was much larger than the interindi vidual variation in MCT1 content in homogenate samples. The immunofluoresce nce microscopy showed that within a given muscle section, the MCT4 isoform was clearly more abundant in type II fibers than in type I fibers, whereas only minor differences existed in the occurrence of the MCT1 isoform betwee n type I and II fibers. Together the present results indicate that the cont ent of MCT1 in a muscle varies between different muscles, whereas fiber-typ e differences in MCT1 content are minor within a given muscle section. In c ontrast, the content of MCT4 is clearly fiber-type specific but apparently quite similar in various muscles.