Figure 1.

Illustration of the essential elements of the recently introduced intracellular lactate shuttle (red) in comparison to the more well-known malate-aspartate and glycerol-phosphate NAD⁺/NADH shuttles (blue). LDH, lactate dehydrogenase; MCT, monocarboxylate transporter; ETC, electron transport chain; Shuttles, malate-aspartate and glycerol-phosphate NAD⁺/NADH shuttles. The H⁺ ions for pyruvate and lactate are inserted to emphasize that the MCT symports a proton; the same MCT carrier can transport both pyruvate and lactate. Note that the mitochondrial LDH may actually be in the intermembrane space of the mitochondria and on the outer surface of the inner membrane. Note also that operation of the intracellular lactate shuttle delivers both reducing equivalents and substrate for oxidation to mitochondria. The intracellular lactate shuttle explains HLa production and accumulation under aerobic exercise conditions.