Transport of amino acids by placenta

Abstract

Placental transport has been studied in vivo in the guinea pig and in vitro with guinea pig and human placenta. Following the infusion of the nonmetabolized amino acid, AIB-14C, into pregnant guinea pigs for 2 to 4 hours, the fetal plasma concentration of the amino acid exceeded that in the maternal plasma. The concentration of the amino acid in maternal placenta (decidua) exceeded that in the maternal plasma. The concentration in the fetal placenta was higher than in both the maternal and fetal plasma, suggesting that transport may involve at least two steps, concentrative uptake into the placenta and release into fetal plasma. Perfusion of guinea pig placenta in situ, following removal of the fetus, demonstrated the primary role of the placenta in establishing a concentration gradient toward the fetus. It was also demonstrated that the amino acid was transferred across the placenta in both directions, apparently more rapidly toward the fetus than in the reverse direction. In vitro experiments with guinea pig and human placenta revealed concentrative uptake of the amino acid. This was decreased by metabolic poisons. The isolated villus of human placenta concentrated the amino acid more effectively than did placental slices. Insulin increased uptake into human placenta but not into guinea pig placenta. Growth hormone, lactogenic hormone, estradiol, estrone sulfate, and oxytocin produced no measurable response.