What factors determine placental glucose transfer kinetics?

Introduction: transfer of glucose across the human placenta is directly proportional to maternal glucose concentrations even when these are well above the physiological range. This study investigates the relationship between maternal and fetal glucose concentrations and transfer across the placenta.Methods: transfer of d-glucose, 3H-3-o-methyl-d-glucose (3H-3MG) and 14C-l-glucose across the isolated perfused human placental cotyledon was determined for maternal and fetal arterial d-glucose concentrations between 0 and 20 mmol/l.Results: clearance of 3H-3MG or 14C-l-glucose was not affected by maternal or fetal d-glucose concentrations in either circulation.Discussion: based on the arterial glucose concentrations and the reported KM for GLUT1, the transfer of d-glucose and 3H-3MG would be expected to show signs of saturation as d-glucose concentrations increased but this did not occur. One explanation for this is that incomplete mixing of maternal blood and the rate of diffusion across unstirred layers may lower the effective concentration of glucose at the microvillous membrane and subsequently at the basal membrane. Uncertainties about the affinity of GLUT1 for glucose, both outside and inside the cell, may also contribute to the difference between the predicted and observed kinetics.Conclusion: these factors may therefore help explain why the observed and predicted kinetics differ and they emphasise the importance of understanding the function of transport proteins in their physiological context. The development of a computational model of glucose transfer may improve our understanding of how the determinants of placental glucose transfer interact and function as a syste

Extended Gari-Krumpelmann model fits to nucleon electromagnetic form factors

Nucleon electromagnetic form factor data (including recent data) is fitted with models that respect the confinement and asymptotic freedom properties of QCD. Gari-Krumpelmann (GK) type models, which include the major vector meson pole contributions and at high momentum transfer conform to the predictions of perturbative QCD, are combined with Hohler-Pietarinen (HP) models, which also include the width of the rho meson and the addition of higher mass vector meson exchanges, but do not evolve into the explicit form of PQCD at high momentum transfer. Different parameterizations of the GK model's hadronic form factors, the effect of including the width of the rho meson and the addition of the next (in mass) isospin 1 vector meson are considered. The quality of fit and the consistency of the parameters select three of the combined HP/GK type models. Projections are made to the higher momentum transfers which are relevant to electron-deuteron experiments. The projections vary little for the preferred models, removing much of the ambiguity in electron-nucleus scattering predictions.Comment: 18pp, 7 figures, using RevTeX with BoxedEPS macros; 1 new figure, minor textual changes; email correspondence to [email protected]

Octet-Baryon Form Factors in the Diquark Model

We present an alternative parameterization of the quark-diquark model of baryons which particularly takes care of the most recent proton electric form-factor data from the E136 experiment at SLAC. In addition to electromagnetic form factors of the nucleon, for which good agreement with data is achieved, we discuss the weak axial vector form factor of the nucleon as well as electromagnetic form factors of $\Lambda$ and $\Sigma$ hyperons. Technical advance in calculating the pertinent analytic expressions within perturbative quantum chromodynamics is gained by formulating the wave function of the quark-diquark system in a covariant way. Finally, we also comment on the influence of Sudakov corrections within the scope of the diquark model.Comment: 16 pages, WU-B 93-07, latex, uuencoded postscript files of 7 figures appended at the end of the latex fil

Effect of recent R_p and R_n measurements on extended Gari-Krumpelmann model fits to nucleon electromagnetic form factors

The Gari-Krumpelmann (GK) models of nucleon electromagnetic form factors, in which the rho, omega, and phi vector meson pole contributions evolve at high momentum transfer to conform to the predictions of perturbative QCD (pQCD), was recently extended to include the width of the rho meson by substituting the result of dispersion relations for the pole and the addition of rho' (1450) isovector vector meson pole. This extended model was shown to produce a good overall fit to all the available nucleon electromagnetic form factor (emff) data. Since then new polarization data shows that the electric to magnetic ratios R_p and R_n obtained are not consistent with the older G_{Ep} and G_{En} data in their range of momentum transfer. The model is further extended to include the omega' (1419) isoscalar vector meson pole. It is found that while this GKex cannot simultaneously fit the new R_p and the old G_{En} data, it can fit the new R_p and R_n well simultaneously. An excellent fit to all the remaining data is obtained when the inconsistent G_{Ep} and G_{En} is omitted. The model predictions are shown up to momentum transfer squared, Q^2, of 8 GeV^2/c^2.Comment: 14 pages, 8 figures, using RevTeX4; email correspondence to [email protected] ; minor typos corrected, figures added, conclusions extende

Chronology of Dune Development in the White River Badlands, Northern Great Plains, USA

Aeolian dune field chronologies provide important information on drought history on the Great Plains. The White River Badlands (WRB) dunes are located approximately 60 km north of the Nebraska Sand Hills (NSH), in the western section of the northern Great Plains. Clifftop dunes, sand sheets, and stabilized northwest-southeast trending parabolic dunes are found on upland mesas and buttes, locally called tables. The result of this study is a dune stabilization history determined from samples collected from stratigraphic exposures and dune crests. Thirty-seven OSL ages, from this and previous investigations, show three periods of dune activity: 1) ∼21,000 years ago to 12,000 years ago (a), 2) ∼9 to 6 ka, and 3) post-700 a. Stratigraphic exposures and low-relief dune forms preserve evidence of late Pleistocene and middle Holocene dune development, while high-relief dune crests preserve evidence of late Holocene dune development. Results of 12 OSL ages from the most recent dune activation event indicate that Medieval Climate Anomaly (MCA) droughts and Little Ice Age (LIA) droughts caused dune reactivation on the tables. Dune reactivation was accompanied by other drought-driven geomorphological responses in the WRB, including fluvial incision of the prairie and formation of sod tables. Regional significance of the MCA and LIA droughts is supported by similarities in the aeolian chronologies of the NSH at 700–600 a and some western Great Plains dune fields at 420–210 a. Aerial photographs of the WRB show little activity during the Dust Bowl droughts of the 1930s