Membrane transporters are considered essential for placental amino acid transfer, but the contribution of other factors, such as blood flow and metabolism, is poorly defined. In this study we combine experimental and modeling approaches to understand the determinants of [14C]phenylalanine transfer across the isolated perfused human placenta. Transfer of [14C]phenylalanine across the isolated perfused human placenta was determined at different maternal and fetal flow rates. Maternal flow rate was set at 10, 14, and 18 ml/min for 1 h each. At each maternal flow rate, fetal flow rates were set at 3, 6, and 9 ml/min for 20 min each. Appearance of [14C]phenylalanine was measured in the maternal and fetal venous exudates. Computational modeling of phenylalanine transfer was undertaken to allow comparison of the experimental data with predicted phenylalanine uptake and transfer under different initial assumptions. Placental uptake (mol/min) of [14C]phenylalanine increased with maternal, but not fetal, flow. Delivery (mol/min) of [14C]phenylalanine to the fetal circulation was not associated with fetal or maternal flow. The absence of a relationship between placental phenylalanine uptake and net flux of phenylalanine to the fetal circulation suggests that factors other than flow or transporter-mediated uptake are important determinants of phenylalanine transfer. These observations could be explained by tight regulation of free amino acid levels within the placenta or properties of the facilitated transporters mediating phenylalanine transport. We suggest that amino acid metabolism, primarily incorporation into protein, is controlling free amino acid levels and, thus, placental transfer.

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The electron resonance spectrum of the spin probe vanadyl acetylacetonate dissolved in the nematic mesophase of 4,4'-dimethoxyazoxybenzene has been studied as a function of temperature. The line shape is observed to deviate from a true lorentzian probably as a result of thermal fluctuations in the director orientation. An approximate expression for the true line shape in the presence of such fluctuations was, therefore, derived and employed to analyse the spectra to yield the line pofitions and, more importantly, their widths. A standard treatment of these line positions gives Pz, the component of the solute ordering matrix along the V-0 bond. The dominant spin relaxation process contributing to the linewidths comes from modulation of the g and vanadium hyperfine tensors by the random molecular reorientation ; theoretical expressions for the linewidth coefficients are obtained for vanadyl acetylacetonate in a liquid crystal with due allowance for the anisotropic environment and the large hyperfine interaction. The linewidth coefficients are found to depend