Placenta-fetal “alanine-lactate cycle” in the human during late gestation

Summary The hypothesis of an “alanine-lactate cycle”, existing between the human fetus and placenta during late gestation, has been examined. During 77 elective cesarean sections, plasma lactate, glucose, alanine, glutamic acid, and aspartic acid concentrations have been determined in both umbilical and uterine blood vessels, and, simultaneously, the placental activities of the enzymes involved in the expected metabolic pathways have been also measured (pyruvate kinase, lactate dehydrogenase, glutamate pyruvate transaminase, and glutamate oxaloacetate transaminase). The movements of the nutrient substrates, estimated from the arteriovenous differences, indicated a placental production of alanine and a placental utilization of lactate, glutamate, and aspartate. Moreover, a significant correlation between umbilical venous alanine and umbilical arterial lactate was observed. The placental activities of the enzymes were fully compatible with the nutrient fluxes. Such an “alanine-lactate cycle” could be helpful to the fetus for at least two reasons: 1) to receive amine nitrogen for the synthesis of amino acids and nucleotides, and 2) to lower the reduction potential in fetal tissues.