Lactate transporters (MCT proteins) in heart and skeletal muscles

Lactate traverses the cell membranes of many tissues, including the heart a nd skeletal muscle via a facilitated monocarboxylate transport system that functions as a proton symport and is stereoselective for L-lactate. In the past few years, seven monocarboxylate transporters have been cloned. Monoca rboxylate transporters are ubiquitously distributed among many tissues, and the transcripts of several monocarboxylate transporters are present within many of the same tissues. This complicates the identification of their met abolic function. There is also evidence that that there is some species spe cificity, with differences in MCT tissue distributions in hamsters, rats, a nd humans. MCT1 and MCT3-M/MCT4 are present in rat and human muscles, and M CT1 expression is highly correlated with the oxidative capacity of skeletal muscles and with their capacity to take up lactate from the circulation, M CT1 is also present in heart and is located on the plasma membrane (in subd omains), T-tubules, and in caveolae. With training, MCT1 is increased in ra t and human muscle, and in rat hearts, resulting in an increased uptake of lactate from the buffers perfused through these tissues and an increase in lactate efflux out of purified vesicles. In humans, the training-induced in creases in MCT1 are associated with an increased lactate efflux out of musc le. MCT3-M/MCT4 is not correlated with the muscles' oxidative capacities bu t is equally abundant In Type IIa and IIb muscles, whereas it is markedly l ower in slow-twitch (Type I) muscles. Clearly, we are at the threshold of a new era in understanding the regulation of lactate movement into and out o f skeletal muscle and cardiac cells.