to the editor: We thank Dr. Shennan (5) for his comments and agree that plasma glutamate and its transporters play a role in regulating glutamate availability in lactating mammary epithelial cells. In our paper, we did not rule out this possibility, but rather suggested a potentially important role for extracellular l-leucine, a branched-chain amino acid (BCAA), in glutamate synthesis by these cells. A recent study involving dietary supplementation with BCAA to lactating sows supported this hypothesis (4). Specifically, concentrations of free and peptide-bound glutamate in the milk of sows that received BCAA supplementation increased significantly, as well as milk production (4). Because dietary BCAAs enter the blood circulation in proportion to their amounts in the diet and almost all glutamate in the diet is catabolized by the small intestine during the first pass, dietary l-leucine seems to be more readily available than dietary glutamate for transport into peripheral tissues with a favorable concentration gradient (3). Compared with l-leucine, the concentration of l-glutamate in plasma is much lower and more tightly regulated (1, 2). Thus, l-glutamate transport into mammary epithelial cells appears to be a costly antigradient process for the mammary tissue. Indeed, we also consider that future studies should focus on elucidating 1) the relative contribution of arterial plasma l-glutamate and l-leucine transamination to glutamate provision in the lactating mammary gland epithelial cells and 2) the elusive mechanism by which free l-glutamate and other amino acids cross the apical membrane of secretory cells to appear in milk.
H. Uneyama is a principal researcher at a food company producing amino acids.
T.M. and A.S.G. drafted manuscript; G.W. and H.U. edited and revised manuscript; H.U. approved final version of manuscript.
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