Determination Of Transport Rates Research Articles

No signature of selection on the C-terminal region of glucose transporter 2 with the evolution of avian nectarivory

BackgroundFlying birds, especially those that hover, need to meet high energetic demands. Birds that meet this demand through nectarivory face the added challenges of maintaining homeostasis in the face of spikes in blood sugar associated with nectar meals, as well as transporting that sugar to energetically demanding tissues. Nectarivory has evolved many times in birds and we hypothesized that the challenges of this dietary strategy would exert selective pressure on key aspects of metabolic physiology. Specifically, we hypothesized we would find convergent or parallel amino acid substitutions among different nectarivorous lineages in a protein important to sensing, regulating, and transporting glucose, glucose transporter 2 (GLUT2).MethodsGenetic sequences for GLUT2 were obtained from ten pairs of nectarivorous and non-nectarivorous sister taxa. We performed PCR amplification of the intracellular C-terminal domain of GLUT2 and adjacent protein domains due to the role of this region in determination of transport rate, substrate specificity and glucosensing.ResultsOur findings have ruled out the C-terminal regulatory region of GLUT2 as a target for selection by sugar-rich diet among avian nectarivores, though selection among hummingbirds, the oldest avian nectarivores, cannot be discounted.ConclusionOur results indicate future studies should examine down-stream targets of GLUT2-mediated glucosensing and insulin secretion, such as insulin receptors and their targets, as potential sites of selection by nectarivory in birds.

Determination of transport rates in the Yellow River–Bohai Sea mixing zone via natural geochemical tracers

In light of the current problems facing the Yellow River and surrounding areas (e.g., periods of zero river discharge, increasing nitrate concentrations of the Bohai Sea), we examined the coastal mixing dynamics around the mouth of the Yellow River. Naturally occurring radium isotopes ( 223Ra, 224Ra, 226Ra, and 228Ra) and other geochemical tracers (Ba, Si, and salinity) were employed to determine river plume transport scales and rates. Barium and radium exhibit elevated concentrations within the salinity gradient where they are desorbed from particles via ion-exchange. Once they are added to the system, they decrease offshore from dilution with lower concentration Bohai Sea water, and in the case of 224Ra and 223Ra, by radioactive decay. Using radium “ages” to assess the dissolved material transport scales and rates proved to be a useful tool in this environment. The ages based on the 224Ra/ 228Ra activity ratio increased gradually until salinities reached ∼25 when they rapidly increased due to decreased mixing at higher salinities. Integrated net transport rates through the salinity front ranged from 1.4 to 1.6 cm/s and did not vary significantly with river discharge. Thus, tidal mixing appears to dominate in this system, at least over the range of discharges investigated (80–600 m 3/s). Determining the temporal scale of flow across the coastal zone in this region is a valuable first step toward examining whether the Yellow River is contributing to the increasing inorganic nitrogen concentrations in the central Bohai Sea.

An in vitro method for studying the iontophoretic enhancement of drug transport through skin

An electrodiffusion cell and measurement system has been developed to study the factors governing the transport of ionized drugs through skin under the influence of an electric field. The system allows the determination of transport rates in vitro under conditions of either constant current or constant voltage across the skin. Both accelerating and retarding voltages may be applied. To test the method, the iontophoretic transport of a negatively charged bone resorption agent, etidronate disodium (ethanehydroxydiphosphonate, EHDP) was measured across excised human skin. An enhancement factor of 50-70 over passive diffusion was obtained using a constant current density of 140 μA-cm −2, and a factor of approximately 100 was obtained with a constant applied voltage of -0.5 V. At the lowest power levels tested (14 μA-cm −2 or -0.25 V) the enhancements were in reasonable agreement with the predictions of a constant field model for electrodiffusion; however, at higher power levels a phenomenological parameter, the drug transference number in the membrane, appeared to be more useful. Neither the constant current nor the constant voltage mode was found to be a completely satisfactory way of controlling drug delivery in this model. A current-voltage regimen in which both parameters are varied may be required in order to achieve consistent results.