In vivo characteristics of placental amino acid transport and metabolism in ovine pregnancy - A review

The placental transport of amino acids which is nutritionally important is the net entry rate into the fetal circulation (the umbilical uptake). This entry rate is a function of transport across cell membranes, the effect of competition among amino acids for transport, particularly across the fetal surface of the trophoblast, and their metabolism and interconversion within the placenta. The result of these different interactive fluxes is that the relationship between maternal concentration and fetal supply of an amino acid differs for each amino acid. For some amino acids there are relatively large bidirectional fluxes at both the fetal and maternal surfaces of the placenta. These fluxes can be measured in vivo utilizing stable isotope methodology. There is an important interorgan exchange of amino acids between the placenta and fetal liver. This exchange is, at least in part, a function of the absence of gluconcogenesis in the fetal liver, Both glutamate and serine, which are released from the fetal liver, are taken up by the placenta from the fetal circulation and metabolized within the placenta. (C) 2002 IFPA and Elsevier Science Ltd.