System A amino acid transporter activity in term placenta is substrate specific and inversely related to amino acid concentration.

Using intact villous fragments from normal term placentas, the authors characterize the effect of reduced amino acid availability on amino acid uptake via the system A amino acid transporter. Villous fragments deprived of amino acids demonstrate increased system A activity compared with those incubated in an amino acid-sufficient medium (P < .05). Similarly, placental villous fragments exposed to media containing only amino acids not specifically transported by system A have a significant increase in system A activity compared with villous fragments incubated in an amino acid-sufficient medium containing only substrates of system A (P < .05). There is a significant trend for increasing system A activity as the concentrations of the system A amino acid substrates are decreased (P < .01). Collectively, these data indicate that normal placentas can increase system A amino acid transporter activity in a substrate-specific and dose-dependent manner as a means to ensure optimal fetal growth in the presence of amino acid limitation.