System A Amino Acid Transporter Activity in Term Placenta Is Substrate Specific and Inversely Related to Amino Acid Concentration

Abstract

Using intact villous fragments from normal term placentas, the authors characterize the e fect 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—su ficient 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—su ficient 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.