NEUTRAL AMINO-ACID-UPTAKE BY THE MICROVILLOUS PLASMA-MEMBRANE OF THE HUMAN PLACENTA IS INVERSELY RELATED TO FETAL SIZE AT BIRTH IN NORMAL-PREGNANCY

Understanding the physiological regulation of fetal growth is importan t, as normal variations in size at birth relate to differences in neon atal and adult health. Although fetal growth directly reflects net pla cental transfer, little is known about how normal fetal growth relates to the transfer capabilities of the placental epithelium, the syncyti otrophoblast. The Na+-dependent and Na+-independent uptakes of methyla minoisobutyric acid (MeAIB) by vesicles prepared from the syncytiotrop hoblast microvillous plasma membrane give measurements of system A neu tral amino acid transporter activity and diffusive permeability, respe ctively. In 62 normal pregnancies, we related vesicle MeAIB uptakes to neonatal anthropometry. Smaller babies with a lower abdominal circumf erence had higher placental system A activity per mg membrane protein (P = 0.004); activity rose from 0.020 to 0.043 nmol/30 sec/mg protein as abdominal circumference fell from 34.6 cm or more to 32.0 cm or les s. Within the normal range of fetal and placental size, this may refle ct a tendency toward compensatory up-regulation of the placental syste m A transporter in smaller babies. Babies with a lower abdominal circu mference also had higher Na+-independent MeAIB uptakes (P = 0.0005); t his could reflect important compositional changes in the microvillous plasma membrane, leading in vivo to increased back-diffusion of amino acids out of the syncytiotrophoblast.