The Lactate/Pyruvate Shuttle in Spermatozoa: Operation in Vitro

The operation of a shuttle for the transfer of reducing equivalents in the special mitochondria present in the middle piece of spermatozoa (sperm-type mitochondria, STM) was studied in reconstituted systems in vitro with mouse, rat, and rabbit STM. The redox couple lactate/ pyruvate and the lactate dehydrogenase isozyme C4 are involved in the shuttle. It is active with rat and rabbit STM, while it does not work with mouse STM, probably because the influx of lactate into the mouse organelles is relatively poor. Ratios of consumption of pyruvate/lactate by STM were 21.6, 1.28, and 1.6 for mouse, rat, and rabbit organelles, respectively. The shuttle is inhibited by 0.6 mM mersalyl, a blocker of lactate transport. The operation of the shuttle would oxidize cytosolic NADH produced during aerobic glycolysis (or fructolysis) in spermatozoa of those species having an efficient lactate carrier in mitochondria.