Effect of fetal growth restriction on system A amino acid transporter activity in the maternal facing plasma membrane of rat syncytiotrophoblast.

To determine whether reduced maternofetal transfer of neutral amino acids in growth-restricted fetal rats is due to decreased system A transporter activity, we measured Na(+)-dependent MeAIB uptake by membrane vesicles from placentas of fetuses growth-restricted due to uterine artery ligation and control placentas (sham ligation). Na(+)-dependent uptake of methylamino-isobutyric acid (MeAIB) was linear over 15-60 s in vesicles from both ligated and sham-ligated sides of the uterus. Na(+)-dependent uptake of MeAIB at 30 s did not differ in paired measurements on vesicles from ligated and sham-ligated horns, 0.063 +/- 0.004 versus 0.056 +/- 0.005 nmol/mg of vesicle protein. The kinetics of Na(+)-dependent MeAIB uptake were similar in paired measurements on vesicles from ligated and sham-ligated horns, with overall K(m) = 4.4 +/- 0.5 mM and Vmax = 0.93 +/- 0.08 nmol/mg vesicle protein per 30 s. Uptake of tracer was inhibited 85-95% by known substrates for the system A amino acid transporter (alanine > or = serine > MeAIB > glycine = proline). We conclude that the system A transporter is present in the maternal facing plasma membrane of rat syncytiotrophoblast, but that the activity of this system, per mg of vesicle protein, is unaffected in fetal growth restriction induced by a decrease in maternal placental blood flow in late pregnancy.