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E M Wright

Publications and source records attributed to E M Wright.

At least 19 recordsLinked to original sources
PubMed2003

Physical factors limiting the spectral extent and band gap dependence of supercontinuum generation.

Based on simulations of femtosecond pulse propagation in water we elucidate the physical factors that limit the attainable spectral extent of supercontinuum generation in bulk media. In contrast to the standard scenario, where arrest of self-focusing collapse by multiphoton absorption and plasma defocusing are viewed as imposing a limit to supercontinuum broadening, we show that linear chromatic dispersion also plays a major role. This insight provides an intuitive explanation of the observed band gap dependence of the attainable supercontinuum generation in condensed media.

Journal Article↗
PubMed2002

Presteady-state and steady-state kinetics and turnover rate of the mouse gamma-aminobutyric acid transporter (mGAT3).

We expressed mouse gamma-aminobutyric acid (GABA) transporter (mGAT3) in Xenopus laevis oocytes and examined its steady-state and presteady-state kinetics and turnover rate by using tracer flux and electrophysiological methods. In oocytes expressing mGAT3, GABA evoked a Na+-dependent and Cl(-)-facilitated inward current. The dependence on Na+ was absolute, whereas that for Cl(-) was not. At a membrane potential of -50 mV, the half-maximal concentrations for Na+, Cl(-), and GABA were 14 mM, 5 mM, and 3 microM. The Hill coefficient for GABA activation and Cl(-) enhancement of the inward current was 1, and that for Na+ activation was > or =2. The GABA-evoked inward current was directly proportional to GABA influx (2.2 +/- 0.1 charges/GABA) into cells, indicating that under these conditions, there is tight ion/GABA coupling in the transport cycle. In response to step changes in the membrane voltage and in the absence of GABA, mGAT3 exhibited presteady-state current transients (charge movements). The charge-voltage (Q-V) relation was fitted with a single Boltzmann function. The voltage at half-maximal charge (V(0.5)) was +25 mV, and the effective valence of the moveable charge (zdelta) was 1.6. In contrast to the ON transients, which relaxed with a time constant of < or =30 msec, the OFF transients had a time constant of 1.1 sec. Reduction in external Na+ ([Na+]o) and Cl(-) ([Cl(-)]o) concentrations shifted the Q-V relationship to negative membrane potentials. At zero [Na+]o (106 mM Cl(-)), no mGAT3-mediated transients were observed, and at zero [Cl(-)]o (100 mM Na+), the charge movements decreased to approximately 30% of the maximal charge (Q(max)). GABA led to the elimination of charge movements. The half-maximal concentrations for Na+ activation, Cl(-) enhancement, and GABA elimination of the charge movements were 48 mM, 19 mM, and 5 mM, respectively. Q(max) and I(max) obtained in the same cells yielded the mGAT3 turnover rate, 1.7 sec(-1) at -50 mV. The low turnover rate of mGAT3 may be due to the slow return of the empty transporter from the internal to the external membrane surface.

Animals↗
PubMed2002

Interference of a thermal tonks gas on a ring.

A nonzero temperature generalization of the Fermi-Bose mapping theorem is used to study the exact quantum statistical dynamics of a one-dimensional gas of impenetrable bosons on a ring. We investigate the interference produced when an initially trapped gas localized on one side of the ring is released, split via an optical-dipole grating, and recombined on the other side of the ring. Nonzero temperature is shown not to be a limitation to obtaining high visibility fringes.

Journal Article↗
PubMed2002

Crossover from one to three dimensions for a gas of hard-core bosons.

We develop a variational theory of the crossover from the one-dimensional (1D) regime to the 3D regime for ultracold Bose gases in thin waveguides. Within the 1D regime we map out the parameter space for fermionization, which may span the full 1D regime for suitable transverse confinement.

Journal Article↗
PubMed2002

Functional expression of the Vibrio parahaemolyticus Na+/galactose (vSGLT) cotransporter in Xenopus laevis oocytes.

We have successfully expressed a bacterial cotransporter in a functional form in the Xenopus laevis oocyte expression system. The goals were to compare the kinetics and selectivity of the cotransporter expressed in oocytes with those obtained in bacteria and in proteoliposomes, and to determine if it is possible to measure the electrical properties of the bacterial cotransporter expressed in oocytes. The Vibrio parahaemolyticus Na+/galactose cotransporter (vSGLT) expressed in oocytes has functional properties that are similar to those expressed in bacteria and those of the purified cotransporter reconstituted into liposomes. vSGLT is a Na+-dependent transporter that is saturable with Na+ (K(0.5)=17 mM) and D-galactose (K(0.5)=237 microM) and is sensitive to both D-fucose and phlorizin. In addition, vSGLT in oocytes shows a sugar specificity in the order of D-galactose >D-fucose > D-glucose, distinguishing it from the animal members of the Na+/glucose cotransporter family. The level of transport by vSGLT in oocytes is lower overall (V(max) approximately 10 pmol/oocyte/hour) compared to other plant and animal cotransporters (V(max) approximately 1000 pmol/oocyte/hour). The low level of expression does not permit us to carry out electrophysiological studies of the bacterial cotransporter. This study shows the potential and unique advantages of utilizing a eukaryotic oocyte expression system to study bacterial cotransporters.

Animals↗
PubMed2002

A ligand-dependent conformational change of the Na+/galactose cotransporter of Vibrio parahaemolyticus, monitored by tryptophan fluorescence.

Purification and reconstitution of the active Vibrio parahaemolyticus Na+/galactose transporter (vSGLT) enables us to do protein chemistry studies on a representative member of this class of membrane transporters. By measuring intrinsic tryptophan (Trp) fluorescence, conformational changes on the binding of substrates could be investigated. Trp fluorescence increased by 6% on the addition of saturating levels of both Na+ and D-galactose, with a K0.5 for D-galactose of 0.6 mM. No change was seen on the addition of Na+ alone or by adding D-galactose in the presence of K+. The Trp fluorescence could be quenched by acrylamide, but not by Cs+or I?. In the presence of Na+ or K+ alone, of Na+ or K+ and D-galactose, of Na+ and L-glucose, or in the absence of ligands, the fluorescence quenches by acrylamide were similar. This indicated that the tryptophan exposure to acrylamide was unchanged in the presence or absence of ligands. No shifts in lem maximum were observed. To find the Trp responsible for the change in fluorescence, Trp 448 in transmembrane helix 11 in the putative sugar-binding pocket was mutated. It was found that W448F showed a similar change in Trp fluorescence upon the addition of D-galactose in the presence of Na+. We conclude that the Trp fluorescence properties of the purified and reconstituted Na+/galactose cotransporter are selectively changed by the transported substrates Na+ and D-galactose, but it is not the Trp (W448) in the sugar translocation pathway that is involved.

Calcium-Binding Proteins↗
PubMed2002

Inhibition of gap junction hemichannels by chloride channel blockers.

Electrophysiological methods were used to assess the effect of chloride-channel blockers on the macroscopic and microscopic currents of mouse connexin50 (Cx50) and rat connexin46 (Cx46) hemichannels expressed in Xenopus laevis oocytes. Oocytes were voltage-clamped at -50 mV and hemichannel currents (ICx50 or ICx46) were activated by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 5 mM to 10 microM. Ion-replacement experiments suggested that ICx50 is carried primarily (>95%) by monovalent cations (PK : PNa : PCl = 1.0 : 0.74 : 0.05). ICx50 was inhibited by 18beta-glycyrrhetinic acid (apparent Ki, 2 microM), gadolinium (3 microM), flufenamic acid (3 microM), niflumic acid (11 microM), NPPB (15 microM), diphenyl-2-carboxylate (26 microM), and octanol (177 microM). With the exception of octanol, niflumic acid, and diphenyl-2-carboxylate, the above agents also inhibited ICx46. Anthracene-9-carboxylate, furosemide, DIDS, SITS, IAA-94, and tamoxifen had no inhibitory effect on either ICx50 or ICx46. The kinetics of ICx50 inhibition were not altered at widely different [Ca2+]o (10-500 microM), suggesting that drug-hemichannel interaction does not involve the Ca2+ binding site. In excised membrane patches, application of flufenamic acid or octanol to the extracellular surface of Cx50 hemichannels reduced single channel-open probability without altering the single-channel conductance, but application to the cytoplasmic surface had no effect on the channels. We conclude that some chloride-channel blockers inhibit lens-connexin hemichannels by acting on a site accessible only from the extracellular space, and that drug-hemichannel interaction involves a high-affinity site other than the Ca2+ binding site.

Animals↗
PubMed2001

Generation of electromagnetic pulses from plasma channels induced by femtosecond light strings.

We present a model that elucidates the physics underlying the generation of an electromagnetic pulse from a femtosecond laser induced plasma channel. The radiation pressure force from the laser pulse spatially separates the ionized electrons from the heavier ions and the induced dipole moment subsequently oscillates at the plasma frequency and radiates an electromagnetic pulse.

Journal Article↗
PubMed2001

Residue 457 controls sugar binding and transport in the Na(+)/glucose cotransporter.

The Na(+)/glucose cotransporter (SGLT1) is highly selective for its natural substrates, d-glucose and d-galactose. We have investigated the structural basis of this sugar selectivity on the human isoform of SGLT1, single site mutants of hSGLT1, and the pig SGLT3 isoform, expressed in Xenopus oocytes using electrophysiological methods and the effects of cysteine-specific reagents. Kinetics of transport of glucose analogues, each modified at one position of the pyranose ring, were determined for each transporter. Correlation of kinetics with amino acid sequences indicates that residue Gln-457 sequentially interacts with O1 of the pyranose in the binding site, and with O5 in the translocation pathway. Furthermore, correlation of the selectivity characteristics of the SGLT isoforms (SGLT1 transports both glucose and galactose, but SGLT2 and SGLT3 transport only glucose) with amino acid sequence differences, suggests that residue 460 (threonine in SGLT1, and serine in SGLT2 and SGLT3) are involved in hydrogen bonding to O4 of the pyranose. In addition, the results show that substrate specificity of binding is not correlated to substrate specificity of transport, suggesting there are at least two steps in the sugar translocation process.

Animals↗
PubMed2001

Evidence for the involvement of Ala 166 in coupling Na(+) to sugar transport through the human Na(+)/glucose cotransporter.

We mutated residue 166, located in the putative Na(+) transport pathway between transmembrane segments 4 and 5 of human Na(+)/glucose cotransporter (hSGLT1), from alanine to cysteine (A166C). A166C was expressed in Xenopus laevis oocytes, and electrophysiological methods were used to assay function. The affinity for Na(+) was unchanged compared to that of hSGLT1, whereas the sugar affinity was reduced and sugar specificity was altered. There was a reduction in the turnover rate of the transporter, and in contrast to that of hSGLT1, the turnover rate depended on the sugar molecule. Exposure of A166C to MTSEA and MTSET, but not MTSES, abolished sugar transport. Accessibility of A166C to alkylating reagents was independent of protein conformation, indicating that the residue is always accessible from the extracellular surface. Sugar and phlorizin did not protect the residue from being alkylated, suggesting that residue 166 is not located in the sugar pathway. MTSEA, MTSET, and MTSES all changed the pre-steady-state kinetics of A166C, independent of pH, and sugars altered these kinetics. The inability of MTSEA-labeled A166C to transport sugar was reversed (with no major change in Na(+) and sugar affinity) if the positive charge on MTSEA was neutralized by increasing the external pH to 9.0. These studies suggest that the residue at position 166 is involved in the interaction between the Na(+) and sugar transport pathways.

Alanine↗
PubMed2001

Polarization dynamics of femtosecond pulses propagating in air.

Polarization dynamics of femtosecond light pulses propagating in air is studied by computer simulation. A rich variety of dynamics is found that depends on the initial polarization state and power of the pulse. Effects of polarization on the plasma and supercontinuum generation are also discussed.

Journal Article↗
PubMed2001

Mapping the urea channel through the rabbit Na(+)-glucose cotransporter SGLT1.

1. The rabbit Na(+)-glucose cotransporter rbSGLT1 and its carboxy-terminal part, C5, which contains transmembrane helices 10-14 of SGLT1 and functions as a low affinity glucose uniporter, were expressed as individual proteins in Xenopus oocytes. Transport of 55 microM urea, ethylene glycol, mannitol and alpha-methyl-D-glucopyranoside (alphaMDG) by control oocytes and by oocytes expressing SGLT1 and C5 was studied by uptake measurements of the 14C-labelled substrates. 2. There was a 5- to 6-fold increase in urea transport mediated by C5, compared with control oocytes. Similar to SGLT1, the C5-urea uptake was cation independent, linear in time and with increasing urea concentration, and blocked with the same sensitivity by the inhibitor phloretin (K(i) approximately 1 mM). Like SGLT1 in choline buffer, the C5-mediated uptake was insensitive to phlorizin. 3. Mannitol was transported by C5 but not by SGLT1 or control oocytes. 4. The activation energy (E(a)) for urea transport through C5 was low (5 +/- 3 kcal mol(-1)) compared with that of non-injected oocytes (16 +/- 0.5 kcal mol(-1)) and comparable with the E(a) of passive urea or water transport through intact SGLT1. 5. The urea influx through C5 increased in the presence of alphaMDG, but not in the presence of the same concentration of mannitol. 6. We conclude that the five carboxy-terminal transmembrane helices of SGLT1 form a channel for the permeation of small molecules such as urea and water.

Animals↗
PubMed2001

Measurement of one-particle correlations and momentum distributions for trapped 1D gases.

Van Hove's theory of scattering of probe particles by a macroscopic target is generalized so as to relate the differential cross section for atomic ejection via stimulated Raman transitions to one-particle momentum-time correlations and momentum distributions of 1D trapped gases. This method is well suited to probing the longitudinal momentum distributions of 1D gases in situ, and examples are given for bosonic and fermionic atoms.

Journal Article↗
PubMed2001

Isotonic transport by the Na+-glucose cotransporter SGLT1 from humans and rabbit.

1. In order to study its role in steady state water transport, the Na+-glucose cotransporter (SGLT1) was expressed in Xenopus laevis oocytes; both the human and the rabbit clones were tested. The transport activity was monitored as a clamp current and the flux of water followed optically as the change in oocyte volume. 2. SGLT1 has two modes of water transport. First, it acts as a molecular water pump: for each 2 Na+ and 1 sugar molecule 264 water molecules were cotransported in the human SGLT1 (hSGLT1), 424 for the rabbit SGLT1 (rSGLT1). Second, it acts as a water channel. 3. The cotransport of water was tightly coupled to the sugar-induced clamp current. Instantaneous changes in clamp current induced by changes in clamp voltage were accompanied by instantaneous changes in the rate of water transport. 4. The cotransported solution was predicted to be hypertonic, and an osmotic gradient built up across the oocyte membrane with continued transport; this resulted in an additional osmotic influx of water. After 5-10 min a steady state was achieved in which the total influx was predicted to be isotonic with the intracellular solution. 5. With the given expression levels, the steady state water transport was divided about equally between cotransport, osmosis across the SGLT1 and osmosis across the native oocyte membrane. 6. Coexpression of AQP1 with the SGLT1 increased the water permeability more than 10-fold and steady state isotonic transport was achieved after less than 2 s of sugar activation. One-third of the water was cotransported, and the remainder was osmotically driven through the AQP1. 7. The data suggest that SGLT1 has three roles in isotonic water transport: it cotransports water directly, it supplies a passive pathway for osmotic water transport, and it generates an osmotic driving force that can be employed by other pathways, for example aquaporins.

Animals↗
PubMed2001

Risk factors for sporadic cases of Escherichia coli O157 infection: the importance of contact with animal excreta.

To determine environmental risk factors for sporadic E. coli O157 infection in Scotland we undertook a prospective, matched case-control study between 1 October 1996 and 31 March 1999. One hundred and eighty-three cases and 545 matched controls were recruited. Contact with animal faeces (OR = 3.65; 95% CI 1.81, 7.34: P < 0.0005) and likely contact with animal faeces (OR = 4.8; 95% CI 2.42, 9.48; P < 0.0005) emerged as strong risk factors for infection. Certain exposures (mainly food-related) were inversely associated with infection i.e. were statistically protective. Most striking was the consumption of bottled water (OR = 0.28; 95% CI 0.15, 0.52; P < 0.0005). Transmission of E. coli O157 does not occur simply through contaminated food. Members of the public need to be aware of the potential for acquiring E. coli O157 through contamination of the environment with animal faeces so that they may take measures to mitigate their risk.

Adolescent↗
PubMed2001

Common mechanisms of inhibition for the Na+/glucose (hSGLT1) and Na+/Cl-/GABA (hGAT1) cotransporters.

1. Electrophysiological methods were used to investigate the interaction of inhibitors with the human Na(+)/glucose (hSGLT1) and Na(+)/Cl(-)/GABA (hGAT1) cotransporters. Inhibitor constants were estimated from both inhibition of substrate-dependent current and inhibitor-induced changes in cotransporter conformation. 2. The competitive, non-transported inhibitors are substrate derivatives with inhibition constants from 200 nM (phlorizin) to 17 mM (esculin) for hSGLT1, and 300 nM (SKF89976A) to 10 mM (baclofen) for hGAT1. At least for hSGLT1, values determined using either method were proportional over 5-orders of magnitude. 3. Correlation of inhibition to structure of the inhibitors resulted in a pharmacophore for glycoside binding to hSGLT1: the aglycone is coplanar with the pyranose ring, and binds to a hydrophobic/aromatic surface of at least 7x12A. Important hydrogen bond interactions occur at five positions bordering this surface. 4. In both hSGLT1 and hGAT1 the data suggests that there is a large, hydrophobic inhibitor binding site approximately 8A from the substrate binding site. This suggests an architectural similarity between hSGLT1 and hGAT1. There is also structural similarity between non-competitive and competitive inhibitors, e.g., phloretin is the aglycone of phlorizin (hSGLT1) and nortriptyline resembles SKF89976A without nipecotic acid (hGAT1). 5. Our studies establish that measurement of the effect of inhibitors on presteady state currents is a valid non-radioactive method for the determination of inhibitor binding constants. Furthermore, analysis of the presteady state currents provide novel insights into partial reactions of the transport cycle and mode of action of the inhibitors.

Animals↗