L-arginine transport at the fetal side of human placenta: Effect of aspirin in pregnancy

L-Arginine transport by the fetal side of human placenta was investigated through the characterization of L-[H-3]arginine uptake in isolated perfused cotyledon. Competitive inhibition experiments suggest the presence of at least two transport systems: a Na+-independent, pH-insensitive system inhibitable by cationic amino acids, similar to system y(+), and a Na+-dependent system which recognizes both cationic and neutral amino acids only in the presence of Na+, i.e. a B-o,B-+-like system. The kinetic analysis of L-arginine uptake in the presence of Na+ revealed that the process is mediated by saturable components: a high-affinity system (K-m = 167 +/- 18.0 mu M; V-max = 0.174 +/- 0.012 mu mol min(-1)) and a low-affinity carrier (K-m = 980 +/- 112 mu M; V-max = 1.60 +/- 0.12 mu mol min(-1)). In the absence of Na+, L-arginine uptake was fitted by one model with a Michaelis-Menten constant of 200 +/- 24.8 mu M. These results suggest that the high-affinity component corresponds to the Na+-independent system y(+), whilst the low-affinity system may represent the activity of the Na+-dependent B-o,B-+ transporter. Kinetic studies in placentae taken from aspirin-treated pregnancies showed that L-arginine is transported with a significantly higher affinity (K-m = 42.5 +/- 5.7 mu M), but with a lower capacity (V-max = 0.064 +/- 0.003 mu mol min(-1)) than in the non-treated group. The latter finding suggests that aspirin would facilitate the uptake of the NO precursor only at very low arginine concentrations.