Amino acid uptake by the human placenta is known to occur via several transport mechanisms. However, regulation by extracellular factors has received relatively little attention. A recent report by this laboratory characterized the uptake of alpha-aminoisobutyric acid (AIB) stimulated by insulin in the cultured human placental trophoblast. The current study evaluated the effect of insulin-like growth factor-1 (IGF-1) on AIB uptake in cultured human placental trophoblasts. Na(+)-dependent AIB uptake was significantly stimulated by IGF-I in a time-dependent manner, as early as 30 min after hormone exposure. The maximum effect was at 2-4 hr of continuous exposure to IGF-I and the stimulation was dependent upon IGF-1 concentration approaching maximal stimulation at 50 ng.ml-1. AIB uptake was inhibited by increasing concentrations of alpha-(methylamino)isobutyric acid (MeAIB). Approximately 75% of basal (unstimulated) Na(+)-dependent AIB uptake was inhibited by MeAIB. The IGF-1-stimulated increment above basal AIB uptake was completely inhibited by MeAIB. IGF-1 increased the maximum uptake velocity but not Km. Using equimolar concentrations, stimulation was greater with IGF-1 than with IGF-2. Stimulation by IGF-1, but not insulin, was inhibited by anti-IGF-1 receptor antibody, indicating mediation via the IGF-1 receptor. H7, a nonspecific inhibitor of serine-threonine kinase, inhibited IGF-1-dependent stimulation of AIB uptake. In addition, calphostin C (a specific inhibitor of protein kinase C), but not H89 (a specific inhibitor of protein kinase A), inhibited the IGF-1 action. This study further characterizes regulated amino acid uptake by the human placental trophoblasts and demonstrates that the Na(+)-dependent component of AIB uptake is stimulated by physiologic concentrations of IGF-1.

Insulin responsiveness in the human placenta is controversial. This study evaluated insulin stimulation of alpha-aminoisobutyric acid (AIB) uptake in cultured human placental trophoblasts. Both Na(+)-dependent and -independent components of AIB uptake were present in cultured trophoblasts. Na(+)-dependent AIB uptake was significantly stimulated by insulin in a time-dependent manner, as early as 2 h, with a maximum at 12 h of continuous exposure to hormone. Insulin treatment for 4 h increased both the initial uptake rate and the final intracellular concentration. Stimulation was dependent on insulin concentration, with significant stimulation beginning at 10(-9) M. Insulin treatment increased maximum velocity but not the Michaelis constant. Approximately 75% of basal (unstimulated) AIB uptake was inhibited by 10 mM alpha-methylaminoisobutyric acid (MeAIB). The insulin-stimulated increment above basal AIB uptake was completely inhibited by 10 mM MeAIB. Cycloheximide treatment significantly reduced basal and stimulated AIB uptake, although a significant response to insulin persisted. Na(+)-dependent AIB uptake was also stimulated by glucagon, dexamethasone, and 8-bromoadenosine 3',5'-cyclic monophosphate, but not by vasopressin. This study further characterizes amino acid uptake by the human placenta and demonstrates that the Na(+)-dependent component of AIB uptake by the cultured trophoblasts is stimulated by physiological concentrations of insulin.

Human placental uptake and maternal-to-fetal (M-to-F) net transfer of taurine were evaluated in purified microvillous membrane vesicles (MMV) and the isolated perfused cotyledon. Taurine uptake by MMV was specifically stimulated by an inward Na+ gradient [maximum velocity (Vmax), 24.5 +/- 0.6 pmol.mg-1.30 s-1; Michaelis constant (Km), 6.2 +/- 0.7 microM], with uptake stoichiometry showing approximately 2 Na+/taurine molecule. In the presence or absence of Na+, Cl- did not stimulate uptake. Na(+)-stimulated uptake was enhanced by an outward K+ gradient. beta-Alanine and hypotaurine competitively inhibited uptake of taurine in MMV. Two-way uptake inhibition studies showed no interaction between taurine and non-beta-amino acids. When MMV were preloaded with taurine there was enhancement of Na(+)-stimulated uptake. In the perfused placentas, saturable M-to-F net transfer of taurine was not observed, despite saturation of tissue uptake from the maternal circulation. During 3 h of perfusion, no fetal-to-maternal (F-to-M) gradient formed for taurine; yet, a 2:1 gradient simultaneously occurred for histidine. This study demonstrates that taurine uptake by the microvillous membrane of the human placenta is highly specific, of high affinity, and Na+ coupled.