Amino Acid Uptake by Liver in Pregnant and Lactating Rats

Abstract

Two new sites of active amino acid metabolism develop during the reproductive cycle of mammals, the fetoplacental unit and the mammary gland. Amino acid requirements are fulfilled by different means before and after parturition; thus, pregnancy is associated with a nitrogen sparing effect which is mediated by a low ureagenic flux (1), whereas lactation is characterized by a marked hyperphagia (2). It has been claimed that a low substrate availability and/or a low liver capacity to take up amino acids should be the major point(s) responsible for the low ureagenic flux during pregnancy (1) and that the amino acid excess during lactation should be counterbalanced by a high liver uptake (2). It is well stablished now that the gestational hypoalaninemia which develops at mid-pregnancy in the rat is related to an increase in alanine turnover and alanine metabolism in liver (3,4), but the role of this organ in amino acid homeostasis before parturition and during lactation has been scarcely studied. The next lines are addressed to discuss this specific point, especially the availability and net uptake of amino acids by the liver in these physiological situations, as well as the mechanisms by which the observed changes are being mediated.