Expression of MCT1 and MCT4 in a patient with mitochondrial myopathy.

Mitochondrial myopathies (MM) are characterized by alterations in oxidative phosphorylation. The resultant increase in glycolytic flux produces a variable lactic acidosis. Intracellular acidification can induce both metabolic and, in the case of skeletal muscle, contractile dysfunction. Skeletal muscle lactate transporters have recently been identified which include both monocarboxylate transporter 1 (MCT1) and 4 (MCT4). Lactate import into oxidative skeletal muscle appears to be catalyzed by MCT1, whereas its extrusion from glycolytic fibers may be mediated by MCT4. We describe the expression of these isoforms in a patient with MM as compared to controls (n = 5). MCT4 content was 86% (>3 SD) higher in the patient with MM, whereas MCT1 content was less markedly elevated (47%), as compared to controls. These findings support previous work suggesting that the major role of MCT4 is to defend intracellular pH by extruding lactate and H(+) to the interstitium. The role of MCT1 in MM is less clear.