Ovine Placental Transport of Essential Aminoacids

Abstract 334 Poster Session I, Saturday, 5/1 (poster 136)

To investigate placental transport and umbilical uptakes of essential aminoacids across the ovine placenta, L-[1-¹³C]-leucine, L-[1-¹³C]-valine, L-[1-¹³C]-isoleucine, L-[1-¹³C]-phenylalanine, L-[methyl-d3]-methionine, L-[1-¹³C]-lysine, L-[1-¹³C]-threonine, L-[ring 2-¹³C]-histidine and L-[indole-d5]-tryptophan were infused as a bolus into the maternal circulation into 7 pregnant sheep at 122±1.3 days gestation. Aminoacid concentrations and stable isotopic enrichments were measured in maternal arterial, uterine venous, fetal arterial and umbilical venous plasma at time zero and for 120 minutes after the bolus. The umbilical plasma flow was measured and the respective tracer and tracee concentrations and fluxes were calculated.

The net umbilical tracee uptakes calculated using tracer fluxes and enrichments expressed as µmol·min^- 1(Kg fetus)^-1 were: leucine (3.8 µmol·Kg^-1·min^-1), isoleucine (3.3 µmol·Kg^-1·min^-1), phenylalanine (1.5 µmol·Kg^-1·min^-1), lysine (0.7 µmol·Kg^-1·min^-1), valine (7.5 µmol·Kg^-1·min^-1), methionine (0.5 µmol·Kg^- 1·min^-1), threonine (1.6 µmol·Kg^-1·min^-1), histidine (0.3 µmol·Kg^-1·min^-1). These data are in agreement with previous estimates of uptakes based on nontracer methodology except for lysine and valine. Tryptophan umbilical uptake was measured for the first time: 0.8 µmol·Kg^-1·min^-1.

The essential aminoacids were grouped into four groups according to the time to peak in the umbilical vein: 1, leucine, phenylalanine, isoleucine (4.6±0.4 min); 2, lysine, methionine, tryptophan, valine (7.4±0.7 min); 3, histidine (14.2±1.5 min); 4, threonine (22.6±2.5 min). Significant differences were found among these groups (p<0.0001 in group 1 vs 2 and both 1 and 2 vs either histidine or threonine; histidine vs threonine, p<0.05) but not within the same group. These different in vivo placental transport rates reflect the combined effect of affinity to the same transport system shared by a group of aminoacids and of different transport systems utilized.