Neutral Amino Acid Uptake by Microvillous and Basal Plasma Membrane Vesicles from Appropriate- and Small-for-Gestational Age Human Pregnancies

Abstract

Fetal growth depends upon nutrient availabilty in the maternal circulation as well as appropriate transfer across the placenta via known transport mechanisms in the maternal- and fetal-facing plasma membranes of the syncytiotrophoblast. In this study, microvillous (maternal-facing) and basal (fetal-facing) plasma membrane vesicles were isolated from the placentas of appropriate-for-gestational age (AGA) and small-for-gestational age (SGA) human pregnancies and the uptakes of tritiated leucine, alanine, and glycine were determined. There was no significant difference in the uptakes of either alanine or leucine by the microvillous and basal vesicles from AGA and SGA pregnancies. In contrast, glycine uptake by both microvillous and basal membrane vesicles from SGA pregnancies was significantly less than vesicles from AGA pregnancies. Glycine is delivered to the umbilical circulation in amounts only minimally in excess of its accretion into protein resulting in a narrow margin of safety for fetal growth. The relative decrease in glycine uptake by SGA vesicles may reflect compromised in vivo availability with resultant depletion of marginal reserves and impaired fetal growth.

Key Words:

• Placenta
• Fetal growth
• Alanine
• Leucine
• Glycine