Alanine umbilical uptake, disposal rate, and decarboxylation rate in the fetal lamb

Fetal plasma alanine disposal rate (DR) and decarboxylation rate were measured at 132 +/- 1 days gestation in nine fetal lambs infused with L-[1-14C]alanine via a brachial vein. In five experiments, L-[1-13C]alanine was added to the infusate. Using L-[1-14C]alanine, we found mean DR to be 15.5 +/- 1.8 mumol.min-1 . kg-1. DR was significantly correlated to both arterial plasma alanine and whole blood lactate concentrations. Placental uptake of fetal plasma alanine accounted for 19 +/- 4% of DR. Fetal and placental production of CO2 from the first carbon of alanine were 61 +/- 2 and 16 +/- 2% of DR, respectively, for a total uterine excretion of 77 +/- 3%. Net alanine flux from placenta to fetus was 5.2 +/- 0.5 mumol.min-1 x kg-1, which was less than fetal plasma alanine decarboxylation (9.4 +/- 1.2 mumol.min-1 x kg-1) plus fetal alanine accretion (2.4 mumol.min-1 x kg-1). Utilizing L-[1-13C]alanine, we found DR to be 14.2 +/- 0.8 mumol.min-1 x kg-1, not significantly different from the 14C data. We conclude that both the umbilical uptake of alanine from the placenta and fetal alanine synthesis contribute to fetal alanine supply and that oxidation is the main route of fetal plasma alanine disposal.