An in vivo study of ovine placental transport of essential amino acids

Under normal physiological conditions, essential amino acids (EA) are trans ported from mother to fetus at different rates. The mechanisms underlying t hese differences include the expression of several amino acid transport sys tems in the placenta and the regulation of EA concentrations in maternal an d fetal plasma. To study the relation of EA transplacental flux to maternal plasma concentration, isotopes of EA were injected into the circulation of pregnant ewes. Measurements of concentration and molar enrichment in mater nal and fetal plasma and of umbilical plasma flow were used to calculate th e ratio of transplacental pulse flux to maternal concentration (clearance) for each EA. Five EA (Met, Phe, Leu, Ile, and Val) had relatively high and similar clearances and were followed, in order of decreasing clearance, by Trp, Thr, His, and Lys. The five high-clearance EA showed strong correlatio n (r(2) = 0.98) between the pulse flux and maternal concentration. The stud y suggests that five of the nine EA have similar affinity for a rate-limiti ng placental transport system that mediates rapid flux from mother to fetus , and that differences in transport rates within this group of EA are deter mined primarily by differences in maternal plasma concentration.