Current aspects of lactate exchange: lactate/H+ transport in human skeletal muscle.

Skeletal muscle is capable of producing and releasing large amounts of lactate and at the same time taking up lactate and using it as a respiratory fuel. The release and uptake of lactate both involve transmembrane transport, which is mediated mainly by a membrane protein called the monocarboxylate transporter (MCT). MCTs mediate membrane transport with an obligatory 1:1 coupling between lactate and H+ fluxes, and is therefore of great importance for pH regulation, especially during intense muscle activity. The total lactate and H+ transport capacity is higher in membranes from oxidative fibers than in membranes from more glycolytic fibers. There are two isoforms of MCT present in skeletal muscle, MCT1 and MCT4. In human muscle samples, there is a positive correlation between the proportion of type I fibers and MCT1 density. In contrast, the MCT4 density in human muscle is independent of fiber type and displays a large interindividual variation. Although the two isoforms have identical transport kinetics (Km), they may have different roles in muscle. MCT1 and MCT4 respond differently to a high-intensity training session, which suggests that these two isoforms are regulated differently.