LEUCINE TRANSPORT FROM MOTHER TO FETUS IN RAT - ROLE OF THE VISCERAL YOLK-SAC

It is known that intracellular proteolysis of exogenous protein in the visceral yolk sac (VYS) is a significant source of leucine for the fe tus from days 8.5 through 17.5 post-conception in the rat. In contrast , little attention has been paid to the potential of the VYS to transp ort maternal free amino acids to the fetus. We investigated this quest ion using the exteriorized 17.5-day rat fetus, enclosed within its amn ion and VYS, but with its placenta attached to the uterus and still fu nctional. We first demonstrated that [H-3]leucine in medium bathing th e exteriorized fetus is transported by the WS to the fetus and then ex changed with the leucine pool in the maternal circulation. This result indicates that the preparation remains viable for the duration of the experiment. We next injected a trace amount of [H-3] leucine intraven ously into the 17.5-day pregnant rat and found that the concentration of radioactivity in the plasma of the exteriorized fetus was not much lower than that in plasma of non-exteriorized fetuses in the same litt er (8-15 fetuses per litter). We interpret these results to indicate t hat the VYS transports free leucine to the fetus but that this process makes a minor contribution to the total transfer of free leucine into the 17.5-day rat fetus. (C) 1998 Elsevier Science Inc.