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Passive Diffusion (passive + diffusion)
Selected AbstractsReaktions-Diffusions-Systeme für intrazellulären Transport und KontrolleANGEWANDTE CHEMIE, Issue 25 2010Siowling Soh Abstract Chemische Reaktionen können Zellen nur am Leben erhalten, wenn die beteiligten Verbindungen an den erforderlichen Stellen zeitlich präzise angeliefert werden. Die meisten Forschungen haben sich bislang auf aktive Transportmechanismen konzentriert, obwohl die passive Diffusion oft gleich schnell ist und weniger Energie erfordert. Um die Vorteile dieser Transportform zu nutzen, haben die Zellen ausgeklügelte Reaktions-Diffusions(RD)-Systeme entwickelt, die zahlreiche zelluläre Funktionen kontrollieren , von Chemotaxis und Zellteilung über Signalkaskaden und -oszillationen bis hin zur Zellbeweglichkeit. Diese nur scheinbar unterschiedlichen Systeme sind nach allgemeinen Prinzipien aufgebaut und haben viele Gemeinsamkeiten. Wiederkehrende Elemente sind nichtlineare Kinetik, Autokatalyse und Rückkopplungsschleifen. Um die Funktion dieser komplexen (bio)chemischen Systeme zu verstehen, muss man die Transportkinetik-Gleichungen analysieren oder die charakteristischen Zeiten der Teilprozesse zumindest qualitativ betrachten. Während wir Beispiele für zelluläre RD-Systeme vorstellen, versuchen wir daher auch, den Leser mit den theoretischen Grundlagen der RD-Phänomene vertraut zu machen. [source] Benzo[a]pyrene bioavailability from pristine soil and contaminated sediment assessed using two in vitro modelsENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 3 2007Luba Vasiluk Abstract A major route of exposure to hydrophobic organic contaminants (HOCs), such as benzo[a]pyrene (BaP), is ingestion. Matrix-bound HOCs may become bioavailable after mobilization by the gastrointestinal fluids followed by sorption to the intestinal epithelium. The purpose of this research was to measure the bioavailability of [14C]-BaP bound to pristine soils or field-contaminated sediment using an in vitro model of gastrointestinal digestion followed by sorption to human enterocytes (Caco-2 cells) or to a surrogate membrane, ethylene vinyl acetate (EVA) thin film. Although Caco-2 cells had a twofold higher lipid-normalized fugacity capacity than EVA, [14C]-BaP uptake by Caco-2 lipids and EVA thin film demonstrated a linear relationship within the range of BaP concentrations tested. These results suggest that EVA thin film is a good membrane surrogate for passive uptake of BaP. The in vitro system provided enough sensitivity to detect matrix effects on bioavailability; after 5 h, significantly lower concentrations of [14C]-BaP were sorbed into Caco-2 cells from soil containing a higher percentage of organic matter compared to soil with a lower percentage of organic matter. The [14C]-BaP desorption rate from Caco-2 lipids consistently was twofold higher than from EVA thin film for all matrices tested. The more rapid kinetics observed with Caco-2 cells probably were due to the greater surface area available for absorption/desorption in the cells. After 5 h, the uptake of BaP into Caco-2 lipid was similar in live and metabolically inert Caco-2 cells, suggesting that the primary route of BaP uptake is by passive diffusion. Moreover, the driving force for uptake is the fugacity gradient that exists between the gastrointestinal fluid and the membrane. [source] Fluorescence Recovery After PhotobleachingIMAGING & MICROSCOPY (ELECTRONIC), Issue 4 2006Gap Junction Communication in Tenocyte Monolayers Gap junctions (GJ) are ubiquitous channels that directly connect the cytoplasm of neighbouring cells, allowing electrical and chemical communication via rapid passive diffusion of ions, metabolites and small molecules up to 1 kDa. Each GJ channel is comprised of transmembrane proteins termed connexins, six of which form a hemi-channel or connexon that docks with a hemi-channel in the membrane of an adjacent cell. [source] Synthesis and evaluation of two uncharged 99mTc-labeled derivatives of thioflavin-T as potential tracer agents for fibrillar brain amyloidJOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 6 2009K. Serdons Abstract Thioflavin-T is a fluorescent dye for in vitro detection of fibrillar amyloid ,, a protein found in the brain of patients suffering from Alzheimer's disease. We synthesized and biologically evaluated two uncharged 99mTc-labeled derivatives of thioflavin-T. The precursors for labeling were synthesized by coupling an S,S, -bis-triphenylmethyl- N - tert -butoxycarbonyl bis-amino-bis-thiol tetradentate ligand via a propoxy spacer to 2-(4,-aminophenyl)-1,3-benzothiazole at the 6-position or the 2,-position. Deprotection and labeling with 99mTc were done via a one-pot procedure (15% yield) after which the labeled compound was isolated by high performance liquid chromatography (LC). LC in combination with mass spectrometry (MS) was used for identity confirmation of the labeled compounds. Results of electrophoresis and log,P determination supported the assumption that the radiolabeled compounds could cross the blood,brain barrier by passive diffusion. However, in normal mice both compounds showed a low brain uptake 2,min post injection. They were mainly excreted through the hepatobiliary system, with some accumulation in the stomach. Sixty minutes after intravenous injection, 37% of the 99mTc-activity in the blood corresponded to the original compound. In view of the low brain uptake, it is concluded that the studied 99mTc-labeled derivatives of thioflavin-T are not suitable as tracer agents for in vivo visualization of amyloid in brain. Copyright © 2009 John Wiley & Sons, Ltd. [source] Methoxypolyethylene glycol- block -polycaprolactone diblock copolymers reduce P-glycoprotein efflux in the absence of a membrane fluidization effect while stimulating P-glycoprotein ATPase activityJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2007Jason Zastre Abstract We have previously shown that amphiphilic diblock copolymers composed of methoxypolyethylene glycol- b -polycaprolactone (MePEG- b -PCL) increased the cellular accumulation and reduced the basolateral to apical flux of the P-glycoprotein substrate, rhodamine 123 (R-123) in caco-2 cells. The purpose of this study was to investigate membrane perturbation effects of MePEG- b -PCL diblock copolymers with erythrocyte membranes and caco-2 cells and the effect on P-gp ATPase activity. The diblock copolymer MePEG17 -b-PCL5 induced increasing erythrocyte hemolysis at concentrations which correlated with increasing accumulation of R-123 into caco-2 cells. However, no increase in cellular accumulation of R-123 by non-P-gp expressing cells was observed, suggesting that diblock did not enhance the transmembrane passive diffusion of R-123, but that the accumulation enhancement effect of the diblock in caco-2 cells was likely mediated primarily via P-gp inhibition. Fluorescence anisotropy measurements of membrane fluidity and P-gp ATPase activity demonstrated that MePEG17 - b -PCL5 decreased caco-2 membrane fluidity while stimulating ATPase activity approximately threefold at concentrations that maximally enhanced R-123 caco-2 accumulation. These results suggest that inhibition of P-gp efflux by MePEG17 - b -PCL5 does not appear to be related to increases in membrane fluidity or through inhibition in P-gp ATPase activities, which are two commonly reported cellular effects for P-gp inhibition mediated by surfactants. © 2006 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 864,875, 2007 [source] Combined strategies for enhancing the transdermal absorption of midazolam through human skinJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 9 2010Cristina Balaguer-Fernández Abstract Objectives, Midazolam administration by intravenous or intramuscular injection produces pain and stress. For this reason, alternative methods of administration have been proposed. The transdermal administration of midazolam could improve patient comfort, which is especially important for children in the pre-operative period. We aimed to assess the effect of iontophoresis and chemical percutaneous enhancers applied individually and together, to determine if a synergistic effect is achieved when both enhancement techniques are simultaneously employed. Methods, This work reports the characterization of the passive diffusion of midazolam hydrochloride through human skin in vitro and evaluates the effect of iontophoresis application and chemical percutaneous enhancers on said diffusion when employed both individually and in combination. Key findings, Percutaneous absorption assays demonstrated that the physical technique of iontophoresis, when applied alone, moderately increased midazolam hydrochloride permeation flux through human skin, producing a similar effect to that obtained with R -(+)-limonene chemical enhancer. Among the strategies assayed, it was observed that Azone produced the most pronounced enhancement effect when applied separately. The combination of pre-treatment with Azone and iontophoresis exhibited a higher capacity for enhancing the transdermal flux of midazolam through human skin than Azone alone. Conclusions, In conclusion, when applied individually, Azone exhibited the greatest enhancement effect on the transdermal diffusion of midazolam of the various strategies assayed. The combination of Azone and iontophoresis produce the highest transdermal steady-state flux of midazolam but no synergic effect was achieved when the two enhancement strategies were applied in combination, showing that although selecting the best conditions for iontophoresis application, it is less effective for augmenting the transdermal delivery of midazolam than the chemical enhancer Azone. [source] Prediction of human pharmacokinetics , renal metabolic and excretion clearanceJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 11 2007Urban Fagerholm The kidneys have the capability to both excrete and metabolise drugs. An understanding of mechanisms that determine these processes is required for the prediction of pharmacokinetics, exposures, doses and interactions of candidate drugs. This is particularly important for compounds predicted to have low or negligible non-renal clearance (CL). Clinically significant interactions in drug transport occur mostly in the kidneys. The main objective was to evaluate methods for prediction of excretion and metabolic renal CL (CLR) in humans. CLR is difficult to predict because of the involvement of bi-directional passive and active tubular transport, differences in uptake capacity, pH and residence time on luminal and blood sides of tubular cells, and limited knowledge about regional tubular residence time, permeability (Pe) and metabolic capacity. Allometry provides poor predictions of excretion CLR because of species differences in unbound fraction, urine pH and active transport. The correlation between fraction excreted unchanged in urine (fe) in humans and animals is also poor, except for compounds with high passive Pe (extensive/complete tubular reabsorption; zero/negligible fe) and/or high non-renal CL. Physiologically based in-vitro/in-vivo methods could potentially be useful for predicting CLR. Filtration could easily be predicted. Prediction of tubular secretion CL requires an in-vitro transport model and establishment of an in-vitro/in-vivo relationship, and does not appear to have been attempted. The relationship between passive Pe and tubular fraction reabsorbed (freabs) for compounds with and without apparent secretion has recently been established and useful equations and limits for prediction were developed. The suggestion that reabsorption has a lipophilicity cut-off does not seem to hold. Instead, compounds with passive Pe that is less than or equal to that of atenolol are expected to have negligible passive freabs. Compounds with passive Pe that is equal to or higher than that of carbamazepine are expected to have complete freabs. For compounds with intermediate Pe the relationship is irregular and freabs is difficult to predict. Tubular cells are comparably impermeable (for passive diffusion), and show regional differences in enzymatic and transporter activities. This limits the usefulness of microsome data and makes microsome-based predictions of metabolic CLR questionable. Renal concentrations and activities of CYP450s are comparably low, suggesting that CYP450 substrates have negligible metabolic CLR. The metabolic CLR of high-Pe UDP-glucuronyltransferase substrates could contribute to the total CL. [source] Design, synthesis and properties of novel iron(III)-specific fluorescent probesJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 4 2004Wei Luo ABSTRACT Bidentate chelators such as hydroxypyridinones and hydroxypyranones are highly iron selective. The synthesis of two novel fluorescent probes N -[2-(3-hydroxy-2-methyl-4-oxopyridin-1(4H)-yl)ethyl]-2-(7-methoxy-2-oxo-2H -chromen-4-yl)acetamide (CP600) and N -[(3-hydroxy-6-methyl-4-oxo-4H -pyran-2-yl)methyl]-2-(7-methoxy-2-oxo-2H -chromen-4-yl)acetamide (CP610) is reported. The method involves coupling the bidentate ligands, 3-hydroxypyridin-4-one and 3-hydroxypyran-4-one, with the well-characterised fluorescent probe methoxycoumarin. Fluorescence emission of both probes at 380 nm is readily quenched by Fe3+. The fluorescence was quenched to a greater extent by Fe3+ than by Mn2+, Co2+, Zn2+, Ca2+, Mg2+, Na+ and K+ and to approximately the same extent as Cu2+. Comparison of the fluorescence-quenching ability by a range of metal ions on CP600 and CP610 and the hexadentate chelator, calcein, under in-vitro conditions, demonstrated advantages of the two novel fluorescent probes with respect to both iron(III) sensitivity and selectivity. Chelation of iron(III) by CP600 and CP610 leads to the formation of a complex with a metal-to-ligand ratio of 1:3. Fluorescence is quenched on formation of such complexes. These probes possess a molecular weight less than 400 and thus they are predicted to permeate biological membranes by passive diffusion, and have potential for reporting intracellular organelle labile iron levels. [source] Three-dimensional MRI of cerebral projections in rat brain in vivo after intracortical injection of MnCl2NMR IN BIOMEDICINE, Issue 5 2003Peter R. Allegrini Abstract In this study we investigated the potential of in vivo MRI detection of axonal Mn2+ transport for tracing neuronal projections originating in the sensorimotor cortex in healthy and lesioned rat brains. Special attention was given to the potential of visualizing neuronal sprouting of central nervous system across the midline. After injecting unchelated MnCl2 into the forelimb area of sensorimotor cortex of 18 healthy and 10 lesioned rats corticofugal projections could be traced through the internal capsule to the cerebral peduncle and the pyramidal decussation. Although the neuronal tract was visible as early as 6,h after MnCl2 injection, best contrast was achieved after 24,48,h. Beside the cortico-spinal tract, the cortico-thalamic fibres were also visualized by anterograde Mn2+ transport. Cortico-striatal fibres were partially masked by the very high signal near the MnCl2 injection site but could be discerned as well. Slight, diffuse signal enhancement of cortical tissue contralateral to the MnCl2 injection site in healthy rat brains suggests interhemispheric connections or passive diffusion of Mn2+. However, enhanced fibre tract contrast connecting both hemispheres was visible 16 weeks after onset of focal photothrombotic cortical injury. In conclusion our study has shown that we were able to visualize reproducibly the main descending corticofugal projections and interhemispheric connections by non-invasive MRI after localized injection of MnCl2. The appearence of interhemispheric Mn2+ -enhanced fibres after photothrombotic focal injury indicates that the method may bear potential to follow non-invasively gross plastic changes of connectivity in the brain after injury. Copyright © 2001 John Wiley & Sons, Ltd. [source] CO2 -concentrating mechanisms in Egeria densa, a submersed aquatic plantPHYSIOLOGIA PLANTARUM, Issue 4 2002María V. Lara Egeria densa is an aquatic higher plant which has developed different mechanisms to deal with photosynthesis under conditions of low CO2 availability. On the one hand it shows leaf pH-polarity, which has been proposed to be used for bicarbonate utilization. In this way, at high light intensities and low dissolved carbon concentration, this species generates a low pH at the adaxial leaf surface. This acidification shifts the equilibrium HCO3,/CO2 towards CO2, which enters the cell by passive diffusion. By this means, E. densa increases the concentration of CO2 available for photosynthesis inside the cells, when this gas is limiting. On the other hand, under stress conditions resulting from high temperature and high light intensities, it shows a biochemical adaptation with the induction of a C4 -like mechanism but without Kranz anatomy. Transfer from low to high temperature and light conditions induces increased levels of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) and NADP-malic enzyme (NADP-ME, EC 1.1.1.40), both key enzymes participating in the Hatch-Slack cycle in plants with C4 metabolism. Moreover, one PEPC isoform, whose synthesis is induced by high temperature and light, is phosphorylated in the light, and changes in kinetic and regulatory properties are correlated with changes in the phosphorylation state of this enzyme. In the present review, we describe these two processes in this submersed angiosperm that appear to help it perform photosynthesis under conditions of extreme temperatures and high light intensities. [source] Involvement of an influx transporter in the blood,brain barrier transport of naloxoneBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 4 2010Toyofumi Suzuki Abstract Naloxone, a potent and specific opioid antagonist, has been shown in previous studies to have an influx clearance across the rat blood,brain barrier (BBB) two times greater than the efflux clearance. The purpose of the present study was to characterize the influx transport of naloxone across the rat BBB using the brain uptake index (BUI) method. The initial uptake rate of [3H]naloxone exhibited saturability in a concentration-dependent manner (concentration range 0.5,µM to 15,mM) in the presence of unlabeled naloxone. These results indicate that both passive diffusion and a carrier-mediated transport mechanism are operating. The in vivo kinetic parameters were estimated as follows: the Michaelis constant, Kt, was 2.99±0.71,mM; the maximum uptake rate, Jmax, was 0.477±0.083,µmol/min/g brain; and the nonsaturable first-order rate constant, Kd, was 0.160±0.044,ml/min/g brain. The uptake of [3H]naloxone by the rat brain increased as the pH of the injected solution was increased from 5.5 to 8.5 and was strongly inhibited by cationic H1 -antagonists such as pyrilamine and diphenhydramine and cationic drugs such as lidocaine and propranolol. In contrast, the BBB transport of [3H]naloxone was not affected by any typical substrates for organic cation transport systems such as tetraethylammonium, ergothioneine or L -carnitine or substrates for organic anion transport systems such as p -aminohippuric acid, benzylpenicillin or pravastatin. The present results suggest that a pH-dependent and saturable influx transport system that is a selective transporter for cationic H1 -antagonists is involved in the BBB transport of naloxone in the rat. Copyright © 2010 John Wiley & Sons, Ltd. [source] Efficacy of peritoneal dialysis of tolbutamide in rats under conditions of the plasma unbound fraction being increasedBIOPHARMACEUTICS AND DRUG DISPOSITION, Issue 1 2009Takashi Makita Abstract Peritoneal dialysis of a highly protein-bound compound, tolbutamide, was examined in rats to clarify whether the efficacy of the peritoneal dialysis of such compounds increases proportionally as their unbound fractions increase. As expected, it was shown that the tolbutamide concentration of the peritoneal dialysate rose as the unbound fraction of tolbutamide increased. However, the efficacy of peritoneal dialysis of tolbutamide was proportionally elevated only when the unbound fraction was slightly increased by sulfamethoxazole treatment. When the unbound fraction of tolbutamide was increased 7.8 times by sulfadimethoxine treatment, the dialysis efficacy was increased to only 58% of that expected. This discrepancy between the observed and expected values regarding dialysis efficacy was more marked when experiments were performed in rats with experimentally induced acute renal failure. Pharmacokinetic analysis indicated that the intrinsic dialysis clearance of tolbutamide decreased when its unbound fraction was greatly increased. These findings suggest that peritoneal dialysis may be mediated not only by passive diffusion, but also by concentration-dependent processes. The efficacy of the peritoneal dialysis of therapeutic compounds may be overestimated if the estimation is based only on their unbound fraction measured under control conditions. Copyright © 2009 John Wiley & Sons, Ltd. [source] MECHANISMS MEDIATING PRESSURE NATRIURESIS: WHAT WE KNOW and WHAT WE NEED TO FIND OUTCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 5-6 2005Roger G Evans SUMMARY 1.,It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2.,Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in ,leaky' tubular elements. 3.,Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4.,The key unresolved question is, how is increased renal artery pressure ,sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5.,Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF. [source] | ||||||||||||||||