Placental amino acid uptake. V. Relationship to placental maturation in the rat.

The functional maturation of the placenta during the latter portion of pregnancy is almost certainly essential to fetal growth but its mechanism is largely unknown. To determine the role of changes in intrinsic cellular transport in this process we measured the activity of transport systems for AIB between day 14 and term, a period of known marked increase in in vivo AIB transfer in the rat. In vitro incubation demonstrated that the labyrinthine tissue possessed two transport systems for cellular AIB uptake. Their maximum velocities remained essentially constant from day 16 to term and the intracellular concentration achieved during incubation actually decreased with gestational and the intracellular concentration achieved during incubation actually decreased with gestational age. In vitro tissue preincubation increased cellular uptake of AIB and this response also decreased with maturation. Thus changes in intrinsic transport mechanisms do not at all parallel the very large maturational increase in in utero amino acid transfer. Changes in intrauterine factors such as blood flow, the hormonal millieu, or fetal utilization and the resultant placental-fetal concentration gradients are much more likely to account for the increase in transfer than are alterations in cellular transport mechanisms.