Vol. 275, Issue 6, E942-E950, December 1998
Relationship of fetal alanine uptake and placental alanine
metabolism to maternal plasma alanine concentration
Michelle
Timmerman1,
Misoo
Chung2,
Randall B.
Wilkening2,
Paul V.
Fennessey2,
Frederick C.
Battaglia2, and
Giacomo
Meschia2
1 Department of Obstetrics and
Gynecology, Erasmus University, Rotterdam, The Netherlands 3000 DR; and 2 Division of
Perinatal Medicine, Departments of Pediatrics, Pharmacology and
Physiology, University of Colorado Health Sciences Center, Denver,
Colorado 80262
Uterine and umbilical uptakes of alanine (Ala)
were measured in 10 ewes before (control) and during intravenous
infusion of Ala, which increased maternal arterial Ala concentration
from 115 ± 14 to 629 ± 78 µM
(P < 0.001). In 8 of these
ewes, placental Ala fluxes were traced by constant intravenous infusion
of
L-[3,3,3-2H3]Ala
in the mother and
L-[1-13C]Ala
in the fetus. Rates are reported as micromoles per minute per kilogram
fetus. Ala 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 uteroplacental utilization (
0.7 ± 0.8 to 8.6 ± 2.7, P < 0.01) of Ala. Control
Ala flux to fetus from mother
(Rf,m) was much
less than the Ala flux to fetus from placenta
(Rf,p) (0.17 ± 0.04 vs. 5.0 ± 0.6). Two additional studies utilizing
L-[U-13C]Ala
as the maternal tracer confirmed the small relative contribution of
Rf,m to
Rf,p. During
maternal Ala infusion,
Rf,m increased significantly (P < 0.02) but
remained a small fraction of
Rf,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 that most
of the Ala delivered to the fetus is produced within the placenta. An
increase in maternal Ala concentration increases placental Ala
utilization and the fetal uptake of both maternal and placental Ala.
alanine turnover; amino acids; umbilical uptake; alanine
fluxes
