Relationship of fetal alanine uptake and placental alanine metabolism to maternal plasma alanine concentration

Uterine and umbilical uptakes of alanine (Ala) were measured in 10 ewes bef ore (control) and during intravenous infusion of Ala, which increased mater nal arterial Ala concentration from 115 +/- 14 to 629 +/- 78 mu M (P < 0.00 1). In 8 of these ewes, placental Ala fluxes were traced by constant intrav enous infusion of L-[3,3,3-H-2(3)]Ala in the mother and L-[1-C-13]Ala in th e fetus. Rates are reported as micromoles per minute per kilogram fetus. Al a infusion increased uterine uptake (2.5 +/- 0.6 to 15.6 +/- 3.1, P < 0.001 ), umbilical uptake (3.1 +/- 0.5 to 6.9 +/- 0.8, P < 0.001), and net uterop lacental utilization (-0.7 +/- 0.8 to 8.6 +/- 2.7, P < 0.01) of Ala. Contro l Ala flux to fetus from mother (R-f,R-m) was much less than the Ala flux t o fetus from placenta (R-f,R-p) (0.17 +/- 0.04 vs. 5.0 +/- 0.6). Two additi onal studies utilizing L-[U-C-13]Ala as the maternal tracer confirmed the s mall relative contribution of R-f,R-m to R-f,R-p. During maternal Ala infus ion, R-f,R-m increased significantly (P < 0.02) but remained a small fracti on of R-f,R-p (0.71 +/- 0.2 vs. 7.3 +/- 1.3). We conclude that maternal Ala entering the placenta is metabolized and exchanged for placental Ala, so t hat most of the Ala delivered to the fetus is produced within the placenta. An increase in maternal Ala concentration increases placental Ala utilizat ion and the fetal uptake of both maternal and placental Ala.