Fig. 1. The processes involved in the lactate shuttle hypothesis (Brooks, 1986). The pathway proposes that (1) glucose enters the cell, where it is sequentially broken down to pyruvate (2). Pyruvate enters the mitochondrion, allowing respiration to continue in the tricarboxylic acid (TCA) cycle (3). Lactate is subsequently formed via the lactate dehydrogenase (LDH) reaction (4) and is then exported from the cytosolic compartment via monocarboxylate transporter (MCT) transport (5), where it is redistributed to a variety of functional sites. Note the suggested presence of mitochondrial lactate dehydrogenase (mLDH) (6), which forms the construct of the intracellular shuttle system (7) (see text for description).