Density Of Transporter Sites Research Articles

Hole transport in polyfluorene-based sandwich-type devices: Quantitative analysis of the role of energetic disorder

The current density versus voltage $[J(V)]$ curves of hole-only sandwich-type devices containing a blue-emitting polyfluorene-based copolymer were measured for a wide range of temperatures and for several thicknesses of the active organic layer. We show that the $J(V)$ curves cannot be accurately described using a commonly used model within which the mobility depends only on the electric field, but that a consistent and quantitatively precise description of all curves can be obtained using the recently introduced extended Gaussian disorder model (EGDM). Within the EGDM, the mobility depends on the electric field and on the carrier concentration. Two physically interpretable parameters, viz. the width of the density of states, $\ensuremath{\sigma}$, and the density of transport sites, ${N}_{t}$, determine the shape of the curves. For the semiconductor studied, we find $\ensuremath{\sigma}=0.13\ifmmode\pm\else\textpm\fi{}0.01\text{ }\text{eV}$ and ${N}_{t}=(6\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{26}\text{ }{\text{m}}^{\ensuremath{-}3}$. Consistent with the EGDM, the logarithm of the mobility in the low carrier concentration and low-field limit is found to show a $1/{T}^{2}$ temperature dependence. It is shown that analyses which neglect the carrier-concentration dependence of the mobility yield an apparent $1/T$ temperature dependence, as reported for many different materials, and that the incorrectness of such an approach would readily follow from a study of the layer thickness dependence of the mobility.

Neurobiologie et pharmacothérapie de la phobie sociale

Resume Bien que Marks ait propose des 1970 une classification des phobies comprenant la phobie sociale, l’apparition de l’entite diagnostique phobie sociale est encore plus recente. Elle a lieu en 1980 avec le DSM III, 3e edition de la nomenclature de l’American Psychiatric Association, qui etablit des criteres operationnels pour son diagnostic. De ce fait, les recherches visant a identifier les facteurs genetiques, familiaux et temperamentaux, les correlats neurochimiques et neuroendocriniens ainsi que les structures cerebrales s’inscrivant dans la pathogenie de ce trouble en sont a leur debut. Neanmoins, elles ont permis d’apprecier l’intervention des influences genetiques, du milieu familial, et de traits temperamentaux, comme l’inhibition comportementale face a l’inconnu, dans le risque d’apparition d’une phobie sociale. Elles ont egalement contribue a mettre en evidence des perturbations du fonctionnement des systemes de neurotransmission monoaminergique dans la phobie sociale. Parallelement, les progres realises dans l’analyse des relations structure-fonction, et notamment l’apport recent de la neuro-imagerie, ont souligne l’importance des voies cortico-limbiques dans la physiopathologie de la phobie sociale, avec la participation de regions et structures cerebrales comme le cortex prefrontal, l’hippocampe, l’amygdale et le striatum. Ceci n’a rien de surprenant si l’on considere leur role essentiel dans la signification attribuee aux informations emanant de l’environnement, les processus emotionnels, mnesiques et de conditionnement contextuel, ainsi que dans la programmation des comportements dont on peut supposer l’alteration a la lumiere des symptomes caracteristiques de la maladie. La demarche pharmacotherapique aujourd’hui proposee repose sur l’efficacite demontree d’agents medicamenteux, au premier rang desquels figurent les inhibiteurs selectifs de la recapture de la serotonine dont l’emploi peut aujourd’hui prevaloir sur celui d’autres produits egalement actifs, comme les inhibiteurs de la monoamine oxydase non selectifs et irreversibles ou les benzodiazepines, de par la simplicite de leur utilisation et leur tres bonne tolerance.

Developmental changes in glucose transport, lipid composition, and fluidity of jejunal BBM.

Na(+)-dependent D-glucose uptake was studied in jejunal brush-border membrane (BBM) vesicles of chickens at 2 days and 1, 2, 5-6, and 12-14 wk of age. Both initial rates and accumulation ratios of the Na(+)-dependent D-glucose transport were significantly higher during the 1st wk than at other ages. To explain the age-related changes observed in the transport of D-glucose, the phlorizin-specific binding, Na+ permeability, lipid composition, and fluidity were studied. Transporter site density was quantified using 50 mumol/l phlorizin and found to be higher during the 1st wk. During the 2nd wk it decreased and then remained constant. Permeability of Na+, studied using 22Na+, showed that fluxes were similar during the first 6 wk, and a significant decrease was observed in the oldest group. Furthermore, membrane fluidity results showed a significant age-dependent decrease that correlated well with both the increased molar ratio of cholesterol to phospholipid and the decreased ratio of lipid to protein found during development. In conclusion, changes in the density of Na(+)-dependent D-glucose transporter as well as in lipid content and fluidity might be involved in the changes observed in D-glucose uptake during the posthatching development.

Regulation of intestinal sugar transport.

The recent surge in knowledge of cellular and molecular mechanisms of intestinal sugar transport has fueled an enormous interest in adaptive mechanisms regulating sugar transport. We first review several functional considerations that help us interpret the different patterns of adaptation for different nutrients. We then distinguish nonspecific adaptive mechanisms leading to parallel changes in transport of different nutrients from specific adaptive mechanisms only affecting the transport of a single nutrient. Nonspecific adaptive mechanisms include changes in mucosal surface area and in the ratio of transporting to nontransporting cells; specific mechanisms include changes in site density of transporters and in affinity constants. We also enumerate the patterns of regulation and describe how sugar transport is affected by changes in diet, energy budgets, and environmental salinity as well as by intestinal resection, starvation, stress, and age. We relate the various signals linking these stimuli to adaptive mechanisms and make predictions about the nature of these signals. Finally, we describe the significance of the interactions among sugar, fluid, and electrolyte transport mechanisms and of the paracellular pathway to transepithelial transport of sugars. We close by drawing attention to promising directions for future research.

The modification of electrophotographic and mechanical properties of organic photoconductors by ultra-violet irradiation

Experiments have shown that UV treatment has a hardening effect on the surface of a drum-based organic photoconductor (OPC) and improves the retentivity. The dark decay and the photoinduced discharge rates were reduced linearly with the increase of time of UV irradiation. The dark decay rate and photoinduced discharge rates were reduced to 60% and more than 70% of the initial rate, respectively, while there was an increase of the activation energy of the charge transport layer. An initial increase in the residual potential was also observed from 20 to about 70 V. After 40 min of UV irradiation, the residual potential was seen to drop to 50 V again, and at the same time a slight increase of the hardness of the photoreceptor surface was detected from 13 to 16 in Vickers scale (Hv). The phenomena is most likely explained by a photochemical change which reduces the density of charge transport sites, builds up the number of deep traps and changes the molecular distribution. The decrease in charge transport site density and buildup of deep traps reduces the conductivity and hence the dark decay and photoinduced discharge rates. The fragmentation of charge transport molecules may result in a change of activation energy as well as the increase in the residue potential. Moreover, the redistribution of molecules leads to the change in molecular density and hardness.

Novel Radioligands for the Dopamine Transporter Demonstrate the Presence of Intrastriatal Nigral Grafts in the MPTP-Treated Monkey: Correlation with Improved Behavioral Function

This preliminary study demonstrates by in vitro autoradiography the value of novel radiolabeled cocaine analogs to characterize the neurochemical identity and measure the density and growth of fetal ventral mesencephalic cells transplanted to the caudate nucleus of the adult MPTP-treated monkey. Two cocaine derivatives were used, one with extremely high affinity for both dopamine and serotonin transporters and one which was selective for the dopamine transporter. The results suggest that the transplantation procedure was able to increase the density of transporter sites in most of the caudate nucleus, although the proportion of dopaminergic and serotonergic fibers may have been altered from normal.

Site density of mouse intestinal glucose transporters declines with age

To evaluate the effect of age on nutrient transport, the absorption rates of D-glucose, D-fructose, L-alanine, L-aspartate, L-leucine, L-lysine, L-proline, folic acid, and nicotinamide were determined in isolated jejunal tissues of young (6.7 mo old) and aged (23.7 and 27.0 mo old) mice (COBS:SFW). D-Glucose and D-fructose uptakes per milligram tissue were approximately 20-120% higher in the proximal jejunum and 15-50% higher in the distal jejunum of young mice. Amino acid and vitamin uptakes per milligram were also higher in young mice, but differences were not statistically significant. The number of Na(+)-D-glucose transporters per milligram tissue as estimated by specific phlorizin binding decreased with age. There was no age-related change in passive L-glucose permeability, in Kd of specific phlorizin binding, in transporter turnover rate, and in the molecular weight of the Na(+)-D-glucose transporter. Thus a reduction in D-glucose transporter site density fully accounts for the age-related decline in D-glucose transport rate per milligram small intestine.

Crypt-villus site of glucose transporter induction by dietary carbohydrate in mouse intestine.

Intestinal brush-border glucose uptake rate is regulated by dietary carbohydrate level. However, this uptake response takes a day or more after a change in dietary carbohydrate level. Is this dietary signal perceived in the crypts, and is the glucose transporter activity of enterocytes irreversibly programmed there? If so, this time lag could reflect cell migration times along the crypt-villus axis, since glucose transport is not fully expressed until cells reach the midvillus. Alternatively, however, the time lag could arise from the induction process itself, if glucose transporter activity in mature villus enterocytes can be reversibly reprogrammed by dietary carbohydrate levels. Hence, we measured glucose-protectable phlorizin binding (as a measure of glucose transporter site density) in mouse enterocytes fractionated along the crypt-villus axis by the Weiser method, as a function of time after an abrupt switch in dietary carbohydrate level. For an increase or decrease in dietary carbohydrate, an increase or decrease in phlorizin binding site density first appeared in the crypts and marched over the course of several days to the villus tips. Hence, the signal for glucose transporter regulation is perceived in the crypts, and the observed lag in uptake is due largely to cell migration times.

Effect of diet on glucose transporter site density along the intestinal crypt-villus axis.

High-carbohydrate diets stimulate intestinal brush-border glucose uptake and increase the number of glucose-protectable phlorizin binding sites, but it has been unknown where along the crypt-villus axis these effects are expressed. We attacked this problem by three methods. First, by measuring phlorizin binding to isolated mouse enterocytes fractionated along the crypt-villus axis by the Weiser method, we identified a high-affinity binding site predominating from villus tip to midvillus and a site of possibly lower affinity predominating in the crypts. A high-carbohydrate diet increased by severalfold the density of the villus sites and probably also of the crypt sites, without changing their binding constants. Second, autoradiography revealed increased glucose-protectable phlorizin binding along the whole crypt-villus axis on a high-carbohydrate diet. Finally, a polyclonal antibody against the Na(+)-glucose cotransporter recognized a protein in the brush-border membrane of villus cells. Hence, substrate-dependent upregulation of intestinal glucose transport involves increased numbers of transporters along the crypt-villus axis.

Postnatal development of monosaccharide transport in pig intestine.

In vitro brush-border transport of three monosaccharides by pig intestine was studied as a function of postnatal age from immediately after birth before suckling to after weaning. Rates of transport normalized to tissue weight or surface area for glucose (Glc), galactose (Gal), and fructose (Fru) were highest at birth, with a steep decline after the onset of suckling, probably caused by any combination of three or more factors: reduced transporter site density, shifts in relative abundances of different monosaccharide transporters, and/or changes in activities of individual transporters. Whereas highest rates of Glc and Fru transport shifted from proximal to midintestine after weaning, Gal transport remained highest in proximal intestine. Postnatal increases in Km values for Gal, but not Glc, indicate there are multiple aldohexose transporters that undergo separate developmental trajectories. The presence of Fru transport in neonatal pigs may reflect a more advanced state of development than neonatal rats and rabbits, or may be an adaptation for early weaning. Changes in Fru-to-Glc and Gal-to-Glc transport ratios before weaning suggest transporter development is partly genetically hard-wired, apparently to prepare pigs for weaning. Curiously, Fru-to-Glc transport ratios were lower than those of rat and rabbit, but closely paralleled those of the carnivorous cat.

Rapid regulation of D-glucose transport in basolateral membrane of rat jejunum.

D-Glucose transport and D-glucose inhibitable [3H]cytochalasin B binding to jejunal basolateral membrane vesicles were measured to investigate the possible association between changes in transport activity seen in hyperglycemia and density of transporter sites. Comparison was made between hyperglycemic animals, noninfused rats, and a group infused with sorbitol. Vascular infusion of D-glucose produced a rapid increase in D-glucose transport followed by a delayed and smaller increase in [3H]cytochalasin B binding. The Vmax for glucose uptake was increased after only 30 min of glucose infusion and continued to rise up to 6 h. Comparison with noninfused and sorbitol-infused controls showed that 2 h of glucose infusion produced a 3.5-fold increase in the Vmax for D-glucose uptake while D-glucose-inhibitable binding of [3H]cytochalasin B was unaffected. Six hours of hyperglycemia resulted in the further stimulation of glucose transport (4.1-fold) and a significant 1.8-fold increase in cytochalasin B binding over that for noninfused animals. Vesicles prepared from animals 4 h after an in vivo injection of cycloheximide showed an 80% reduction in glucose transport with no significant change in the cytochalasin B binding density. These results suggest that D-glucose transport in the basolateral membrane is regulated by a combination of a modulation of carriers already in the membrane and subsequent changes in carrier site density.

A method for measuring apical glucose transporter site density in intact intestinal mucosa by means of phlorizin binding.

Phlorizin binding has been widely used to estimate the site density of glucose transporters on intestinal and renal brush-border vesicles. Glucose transport measurements in the intact intestinal mucosa show that changes in transport rate postulated to arise from changes in site density occur under many physiological and pathological conditions. Exploring the basis of these regulatory phenomena would be facilitated by comparing changes in transport rate and site density measured in the same preparation. Hence we developed methods for measuring phlorizin binding in everted sleeves of intact mouse intestine. Specific binding of phlorizin to glucose carriers reached an asymptotic value within 120 sec, while nonspecific binding continued to rise thereafter. Hence we used 120-sec incubations. The rate of dissociation of specifically bound phlorizin was accelerated by Na+-free solutions and even more by 50 mM glucose, while the rate of dissociation of nonspecifically bound phlorizin was independent of these solution changes. Hence we chose a 20-sec rinse in Ringer + 50 mM mannitol, because it washes out 30-40% of the nonspecifically bound phlorizin but virtually none of the specifically bound phlorizin. Ligand-binding analysis of specific binding against phlorizin concentration suggested two classes of binding sites, of which the one with stronger affinity for phlorizin probably has the higher capacity for glucose transport in mouse jejunum. The calculated affinity and capacity of this component are independent of whether one estimates the specific component of total binding by adding glucose or by removing Na+.

Use of phlorizin binding to demonstrate induction of intestinal glucose transporters.

We used specific binding of phlorizin to the intact intestinal mucosa in order to measure glucose transport site density in intestines of mice fed a high-carbohydrate or no-carbohydrate diet. Nonspecific binding varied with intestinal position but showed only modest dependence on diet. Specific binding to glucose transporters was 1.9 times greater in jejunum of high-carbohydrate mice than of no-carbohydrate mice; this ratio was the same as the ratio for Vmax values of active D-glucose uptake between the two diet groups. The gradient in specific binding of phlorizin along the intestine paralleled the gradient in Vmax of glucose transport. These results directly demonstrate that the increase in intestinal glucose transport caused by a high-carbohydrate diet is due to induction of glucose transporters. They also indicate that the normal positional gradient in glucose transport along the intestine arises from a gradient in transporters, induced by the normal gradient in luminal glucose concentration.

Electrogenic proton transport in the plasma membrane of Neurospora

A variety of experiments involving direct measurement of membrane potentials with microelectrodes have led to the notion that the generalized chemiosmotic hypothesis (4) can account for the major transport processes occurring in the plasma membrane of Neurospora; and that fungal plasma membranes, despite their apparent lack of redox elements, are useful models for many transport events which take place in energyconserving membranes. The diagram in Fig. 1 represents the current working hypothesis for transport by Neurospora. The measured resting membrane potential lies in the range -160 to -240 mV (cell interior negative) and is sustained largely by ejection of protons coupled to the splitting of ATP (8).' The membrane potential then serves as the main energy distributor for transport, and-together with a small inward chemical gradient for protons-drives the uptake of a variety of different substances. Direct evidence exists that glucose and its analogues are handled in this manner (9), and indirect evidence that amino acids, phosphate, and potassium are as well.2 From the measured membrane resistance (ca. l0,000Q cm2 under standard conditions; ref. 7) and potential, the ion flux driven by the proton pump can be estimated at 200 pmol/cm2 s. If the density of transport sites on the membrane were roughly the same as that of F,-ATPase sites on the mitochondrial inner membrane (3,000/im2, ref. 1), the flux would yield a turnover rate of 200/site * s, for a two-charge process. Major evidence in support of Fig. 1 comes from a survey of the speed and extent of depolarizations caused by certain ions, metabolic inhibitors, and antibiotics. Hydrogen ions, almost alone among the common inorganic cations and anions, have a steep depolarizing effect: 30-40 mV for each unit decrease of pH,, at least in the range pH 5 to pH 3 (6). Cyanide, at concentrations just sufficient to block cytochrome oxidase,