Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “facilitated diffusion”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,657 records · Page 92Linked to original sources
PubMed1989

Transfers between free and combined oxygen flows in determining facilitated transport with membranes on the transport path.

The facilitated transport of a species brought about by a diffusible carrier is mediated by the sharing of the total flow between the flow of the free species (fd) and of the species-carrier compound (fc). The presence of membranes impermeable to the carrier on the transport path is accompanied by transfers between fd and fc in regions next to the membranes. These transfers are associated with differences between the current species-carrier concentration and the one that would exist at chemical equilibrium with the free species. These differences determine that the actual facilitated transport is smaller than the possible maximum. Analytic approximations to describe these differences are obtained; they are expressed in terms of physiochemical entities germane to the transport. This leads to a better conceptual understanding of the process. A relatively small system of algebraic equations for the numerical solution of the facilitated transport is formulated. A feature of this approach is that, instead of matching the values of functions at some points, the flows derived from integrals evaluated over some intervals are matched. The expressions for the physicochemical entities obtained contain the parameters explicitly; this permits analysis of the mechanisms involved in the dependence of the facilitated transport on the parameters of the system. It is shown, for instance, that the increase of the "on" chemical coefficient may result in either an increase or a decrease of the facilitated transport, depending on the current values of the parameters.

Biological Transport, Active↗
PubMed1996

Preparation of bank bone using defatting, freeze-drying and sterilisation with ethylene oxide gas. Part 1. Experimental evaluation of its efficacy and safety.

We devised a method of sterilising bone allografts which consists of defatting in chloroform and methanol, freeze-drying and sterilisation with ethylene oxide gas. The purpose of defatting and freeze-drying was to facilitate subsequent sterilisation by eliminating the barrier to diffusion of the gas into bone, to lower residual levels of ethylene oxide and its toxic by-products, to eliminate alloantigens and to make storage possible at room temperature. The efficacy and safety of the method were evaluated by testing the sterilisation of infected bone from 6 patients with active chronic osteomyelitis, the penetration of ethylene oxide into human femoral heads treated by this or by freeze-drying or freeze-thawing, and the desorption of ethylene oxide and its toxic by-products from pieces of bone treated by these methods. All the samples of infected bone tested negative for bacteria after treatment. The gas penetrated into the central area of the femoral heads in a few hours. Residual levels of ethylene oxide and its toxic by-products were much lower in the treated bone than in freeze-dried or freeze-thawed bone, and decreased quickly in flowing air. Prior defatting and freeze-drying facilitated penetration of ethylene oxide into bone during sterilisation and the desorption of ethylene oxide and its toxic by-products after sterilisation. Preparation under clean, but not sterile, conditions and storage at room temperature make bone banking more practical and efficient.

Bone Transplantation↗
PubMed1999

Comparison of scalar measures used in magnetic resonance diffusion tensor imaging.

The tensors derived from diffusion tensor imaging describe complex diffusion in tissues. However, it is difficult to compare tensors directly or to produce images that contain all of the information of the tensor. Therefore, it is convenient to produce scalar measures that extract desired aspects of the tensor. These measures map the three-dimensional eigenvalues of the diffusion tensor into scalar values. The measures impose an order on eigenvalue space. Many invariant scalar measures have been introduced in the literature. In the present manuscript, a general approach for producing invariant scalar measures is introduced. Because it is often difficult to determine in clinical practice which of the many measures is best to apply to a given situation, two formalisms are introduced for the presentation, definition, and comparison of measures applied to eigenvalues: (1) normalized eigenvalue space, and (2) parametric eigenvalue transformation plots. All of the anisotropy information contained in the three eigenvalues can be retained and displayed in a two-dimensional plot, the normalized eigenvalue plot. An example is given of how to determine the best measure to use for a given situation by superimposing isometric contour lines from various anisotropy measures on plots of actual measured eigenvalue data points. Parametric eigenvalue transformation plots allow comparison of how different measures impose order on normalized eigenvalue space to determine whether the measures are equivalent and how the measures differ. These formalisms facilitate the comparison of scalar invariant measures for diffusion tensor imaging. Normalized eigenvalue space allows presentation of eigenvalue anisotropy information.

Anisotropy↗
PubMed2003

Mechanical forces impeding exocytotic surfactant release revealed by optical tweezers.

The release of surfactant from alveolar type II cells is essential to lower the surface tension in the lung and to facilitate inspiration. However, the factors controlling dispersal and diffusion of this hydrophobic material are still poorly understood. Here we report that release of surfactant from the fused vesicle, termed lamellar body (LB), resisted mechanical forces applied by optical tweezers: At constant trapping force, the probability to expand LB contents, i.e., to "pull" surfactant into the extracellular fluid, increased with time after LB fusion with the plasma membrane, consistent with slow fusion pore expansion in these cells. Elevations of the cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) had a similar effect. Inasmuch as surfactant did not disintegrate in the extracellular space, this method permitted for the first time the determination of elastic and recoil properties of the macromolecular complex, yielding a spring constant of approximately 12.5 pN/ micro m. This is the first functional evidence that release of hydrophobic material is mechanically impeded and occurs in an "all-or-none" fashion. This mode of release is most probably the result of cohesive forces of surfactant, combined with adhesive forces and/or retaining forces exerted by a constrictive fusion pore acting as a regulated mechanical barrier, withstanding forces up to 160 pN. In independent experiments equiaxial strain was exerted on cells without optical tweezers. Strain facilitated surfactant release from preexisting fused vesicles, consistent with the view of mechanical impediments during the release process, which can be overcome by cell strain.

Animals↗
PubMed2006

How fatty acids of different chain length enter and leave cells by free diffusion.

Opposing views exist as to how unesterified fatty acids (FA) enter and leave cells. It is commonly believed that for short- and medium-chain FA free diffusion suffices whereas it is questioned whether proteins are required to facilitate transport of long-chain fatty acid (LCFA). Furthermore, it is unclear whether these proteins facilitate binding to the plasma membrane, trans-membrane movement, dissociation into the cytosol and/or transport in the cytosol. In this mini-review we approach the controversy from a different point of view by focusing on the membrane permeability constant (P) of FA with different chain length. We compare experimentally derived values of the P of short and medium-chain FA with values of apparent permeability coefficients for LCFA calculated from their dissociation rate constant (k(off)), flip-flop rate constant (k(flip)) and partition coefficient (Kp) in phospholipid bilayers. It was found that Overton's rule is valid as long as k(flip)<<k(off). With increasing chain length, the permeability increases according to increasing Kp and reaches a maximum for LCFA with chain length of 18 carbons or longer. For fast flip-flop (e.g. k(flip)=15s(-1)), the apparent permeability constant for palmitic acid is very high (P(app)=1.61 cm/s). Even for a slow flip-flop rate constant (e.g. k(flip)=0.3s(-1)), the permeability constant of LCFA is still several orders of magnitude larger than the P of water and other small non-electrolytes. Since polyunsaturated FA have basically the same physico-chemical properties as LCFA, they have similar membrane permeabilities. The implications for theories involving proteins to facilitate uptake of FA are discussed.

Animals↗
PubMed2004

Mathematical modelling of therapies targeted at bacterial quorum sensing.

Bacteria commonly use diffusible signal molecules to synchronise their behaviour by facilitating population dependent co-ordination. This cell-to-cell signalling mechanism is known as quorum sensing (QS) and provides a way of ensuring that certain genes are 'switched on' only when a certain signal concentration (typically corresponding to a large population density) has been reached. In this paper we focus on the QS system of the human pathogen Pseudomonas aeruginosa, which employs a complex hierarchy of QS signalling systems, which regulate the formation of multiple exoproducts, swarming and biofilm differentiation. In P. aeruginosa, the signal molecules are N-acylated homoserine lactones (AHLs; e.g., N-(3-oxododecanoyl)-homoserine lactone [3-oxo-C12-HSL]), which bind to transcriptional regulator proteins (LasR in the case of 3-oxo-C12-HSL) to activate the expression of target genes including lasI, which codes for the 3-oxo-C12-HSL synthase. Since the virulence of P. aeruginosa is controlled by QS, agents (QSBs) designed to block this cell-to-cell communication have potential as novel antibacterials. By drawing on existing models for the reaction kinetics of this system, we model a growing population subject to treatment with two kinds of QSB, together with a conventional antibiotic. The first kind of QSB is assumed to act by diffusing through the cell membrane and then destabilising/sequestering LasR, while the second kind remains outside the cell and degrades the AHL signal molecule itself. Numerical and mathematical analysis of the resulting systems of ordinary differential equations reveals in particular that, while a sufficiently high dose of QSB is, in all cases considered, able to reduce the AHL concentration (and hence virulence) to a negligible level, the qualitative response to treatment is sensitive to parameter values.

4-Butyrolactone↗
PubMed1992

Axotomized frog sciatic nerve releases diffusible neurite-promoting factors.

Using the bullfrog (Rana catesbeiana) dorsal root ganglia (DRG) and its sciatic nerve (ScN) as a model system, we have previously described neuronal and non-neuronal molecular changes associated with the early regenerative response of DRG neurons to axotomy. Since diffusible molecular factors, released by axotomized ScN, might function to stimulate axon regrowth, we have assayed the ability of ScN-conditioned bath to promote in vitro neurite outgrowth from PC-12 cells. Diffusible ScN proteins were collected by incubating segments of normal or axotomized ScN in a small volume of RPMI media for 4 h (nerve bath). The nerve baths, supplemented with serum, were then added to PC-12 cell cultures to assay for the presence of neurite growth factors released by ScN. Results showed that nerve baths, collected from sham-operated or axotomized ScN, could not induce the differentiation of PC-12 into neurite-bearing cells. Therefore, in all subsequent neurite growth assay experiments, an exogenous source of nerve growth factor (NGF) (50 ng/ml) was added to the nerve baths or unconditioned media to generate and maintain PC-12 neuritic structure. We found that nerve baths, collected from previously axotomized (at least 3 days post-injury) nerve, contained diffusible factors which enhanced PC-12 neurite growth, relative to unconditioned media. No neurite growth factors were observed to be released by sham-operated ScN or 1-day post-axotomized ScN. Further experiments were conducted to identify the diffusible neurite growth factors released from axotomized ScN. We showed that the release (if any) of endogenous diffusible NGF or laminin from axotomized nerve could not have accounted for the facilitation of neurite growth. Analysis of radiolabelled ScN proteins by two-dimensional polyacrylamide gel could not have accounted for the facilitation of neurite growth. Analysis of radiolabelled ScN proteins by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) showed that the relative abundance of two diffusible proteins (M(r) approximately 35 and 70 kDa) in the nerve bath was directly correlated with the ability of the nerve bath to facilitate PC-12 neurite growth.

Animals↗
PubMed1977

Pharmacokinetics of ampicillin and methoxymethyl ester of hetacillin in dogs.

Concentrations of ampicillin and the methoxymethyl ester of hetacillin were measured in plasma, prostatic fluid, and spinal fluid of dogs receiving ampicillin or the hetacillin ester by continuous intravenous infusion. The ratios of ampicillin in prostatic fluid relative to plasma levels were higher after dosing with the hetacillin ester. Pharmacokinetic parameter values were consistent with urinary excretion characteristics. Ready diffusion of the ester across biological membranes may facilitate eradication of pathogenic organisms in prostatic and spinal fluids.

Ampicillin↗
PubMed1977

Role of carbonate in acid neutralization of aluminum hydroxide gel.

The role of carbonate in the acid neutralization of a carbonate-containing aluminum hydroxide gel was studied. The pH-stat neutralization reaction was comprised of three phases: an initial, immediately reacting phase; a slow, zero-order phase; and a terminal, fast zero-order phase. The evolution of carbon dioxide and the appearance of aluminum ions were monitored during neutralization. Acid neutralization is believed to occur predominantly at the aluminum-carbonate bond during the slow phase of neutralization. The reaction of structural carbonate disrupts the gel structure and increases the porosity and surface area of the gel. The diffusion of acid into the gel structure is facilitated and accelerates the neutralization rate, as seen in the final phase of the pH-stat titrigrams. The behavior of carbonate is probably responsible for the rapid and complete neutralization of acid generally observed with carbonate-containing aluminum hydroxide gel.

Acids↗
PubMed1977

Penis-retractor muscle of Aplysia: excitatory motor neurons.

Cobalt axonal iontophoresis and intracellular recordings were used to identify a cluster of several motor neurons innervating the penis-retractor muscle of Aplysia. Intracellularly recorded motor neuron action potentials elicited direct, one-for-one, constant latency excitatory junctional potentials (ejps) in individual muscle fibers. The axons of motor neurons could be recorded extracellularly in the penis-retractor nerve and stimulation of the nerve backfired the motor neurons. Perfusion of the ganglion, the muscle, or both with solutions of either increased Mg++/decreased Ca++ or increased Ca++ sea water indicated that the presumed motor neuron impaled was not a sensory cell and that interneurons were not intercalated in the pathway. Innervation of muscle fibers was found to be functionally polyneuronal and diffuse. The ejps were found to undergo marked facilitation with repetitive motor-neuron stimulation. The motor neurons were isolated in a distinct cluster in the right pedal ganglion. Their electrical activity was characterized by spontaneous irregular action potentials and a moderate input of postsynaptic potentials.

Animals↗
PubMed1998

Attenuation of gossypol cytotoxicity by cyclic AMP in a rat liver cell line.

The effect of pretreatment of a rat liver cell line (Clone 9) with 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), an agent previously shown to increase gap junctional intercellular communication (GJIC), was used to determine if increased GJIC would attenuate the cytotoxic effects of gossypol acetic acid (GAA). Pretreatment with 8-Br-cAMP increased the inhibitory concentration 50% (IC50) for gossypol from 4.1 to 6.1 microM in Clone 9 cells. GJIC was suppressed by about 57% within 5 min when Clone 9 cells were treated with 1.0 microM GAA alone, whereas cells treated with 3.0 or 10 microM GAA were completely uncoupled within the same time frame. In contrast, GJIC was maintained near control levels in Clone 9 cells preincubated with 1.0 mM 8-Br-cAMP for 10 min prior to 1.0 microM GAA exposure. 8-Br-cAMP partially restored GJIC in cells treated with 3.0 microM GAA but was unable to protect cells exposed to 10 microM GAA. The effects of GAA and 8-Br-cAMP pretreatment on connexin43 (Cx43) protein expression was analyzed with Western blots. GAA treatment at concentrations of 1 and 3 microM caused a time- and dose-dependent increase in phosphorylation of Cx43 over a 20-min period, whereas 10 microM GAA caused a time-dependent degradation of Cx43 over the same interval. Pretreatment of cells for 10 min with 8-Br-cAMP completely reversed the effect of 1 microM GAA and partially blocked the effect of 3 microM GAA on Cx43 phosphorylation and suppressed Cx43 degradation at 10 microM. Additional fluorescence endpoints associated with cellular homeostasis mechanisms were also monitored to evaluate cytotoxicity and potential protective effects of 8-Br-cAMP pretreatment in cells exposed to GAA, including generation of reactive oxygen species (ROS), cytoplasmic acidification, glutathione (GSH) content, intracellular calcium levels ([Ca2+]i), and mitochondrial and plasma membrane potential. The adverse effects of GAA on the production of ROS, cytoplasmic acidification, GSH content, and [Ca2+]i were also attenuated. The primary protective effect of 8-Br-cAMP was observed at or below the IC50 of GAA, with greatest protective effects detected on early endpoints affected by GAA exposure. These studies suggest that the protective effect of 8-Br-cAMP in GAA-treated cells results from enhanced GJIC, which facilitates cellular homeostasis by providing cell-cell diffusion of essential metabolites, ions, and regulatory and informational molecules.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed2001

[Ultrasound mammography and magnetic resonance mammography as adjunctive methods in mammography screening].

Ultrasonography and magnetic resonance (MR) tomography are helpful tools to evaluate unclear lesions found at screening mammography. Ultrasound is particularly useful to prove the presence of a cyst or to further examine unclear, asymmetrical densities. With MR mammography, carcinomas can be found even when x-ray mammography or ultrasonography are limited due to diffuse, benign, proliferative changes. Ultrasound guidance has greatly facilitated core needle biopsy for suspicious lesions. Additionally, approved devices for MR-guided biopsy are commercially available. As a primary screening tool, ultrasound or MR mammography may be used only in women who are positive for BRCA-1 or BRCA-2 mutations or who are otherwise at high risk for breast cancer.

Adult↗
PubMed1987

Intracellular chromium reduction.

Two steps are involved in the uptake of Cr(VI): (1) the diffusion of the anion CrO4(2-) through a facilitated transport system, presumably the non-specific anion carrier and (2) the intracellular reduction of Cr(VI) to Cr(III). The intracellular reduction of Cr(VI), keeping the cytoplasmic concentration of Cr(VI) low, facilitates accumulation of chromate from extracellular medium into the cell. In the present paper, a direct demonstration of intracellular chromium reduction is provided by means of electron paramagnetic (spin) resonance (EPR) spectroscopy. Incubation of metabolically active rat thymocytes with chromate originates a signal which can be attributed to a paramagnetic species of chromium, Cr(V) or Cr(III). The EPR signal is originated by intracellular reduction of chromium since: (1) it is observed only when cells are incubated with chromate, (2) it is present even after extensive washings of the cells in a chromium-free medium; (3) it is abolished when cells are incubated with drugs able to reduce the glutathione pool, i.e., diethylmaleate or phorone; and (4) it is abolished when cells are incubated in the presence of a specific inhibitor of the anion carrier, 4-acetamido-4'-isothiocyanatostilbene-2-2'-disulfonic acid.

Animals↗
PubMed2005

Interaction of glucose transporter 1 with anion exchanger 1 in vitro.

The facilitative glucose transporter 1 (GLUT1) mediates the passive diffusion of d-glucose across the cell membrane, providing the energy resource in glycolysis in the erythrocytes. Anion exchanger 1 (band 3) is another important membrane protein that mediates rapid exchange of CO(2) through Cl(-)/HCO(3)(-) exchange across the erythrocyte membrane. For verifying the presumption over a decade that GLUT1 and band 3 in the erythrocyte would be interacting with each other, we cloned and expressed both the cytoplasmic domains of GLUT1 and band 3 in Escherichia coli, and tested their binding ability. By coimmunoprecipitation we found that among the tested N-terminal, C-terminal, and loop fraction of GLUT1, only the C-terminal of GLUT1 can interact with cytoplasmic domain of band 3. The interaction was further verified by coimmunoprecipitation and pull-down assay using both proteins as bait and target. These results showed that GLUT1 and band 3 form a protein complex that can regulate the activities of the proteins within it.

Anion Exchange Protein 1, Erythrocyte↗
PubMed2006

Release modulating hydrophilic matrix systems of losartan potassium: optimization of formulation using statistical experimental design.

The aim of the present research work was to systemically device a model of factors that would yield an optimized sustained release dosage form of an anti-hypertensive agent, losartan potassium, using response surface methodology by employing a 3-factor, 3-level Box-Behnken statistical design. Independent variables studied were the amount of the release retardant polymers - HPMC K15M (X(1)), HPMC K100M (X(2)) and sodium carboxymethyl cellulose (X(3)). The dependent variables were the burst release in 15 min (Y(1)), cumulative percentage release of drug after 60 min (Y(2)) and hardness (Y(3)) of the tablets with constraints on the Y(2)=31-35%. Statistical validity of the polynomials was established. In vitro release and swelling studies were carried out for the optimized formulation and the data were fitted to kinetic equations. The polynomial mathematical relationship obtained Y(2)=32.91-2.30X(1)-5.69X(2)-0.97X(3)-0.41X(1)X(2)+0.21X(1)X(3)-0.92X(1)(2)-1.89X(2)(2) (r(2)=0.9944) explained the main and quadratic effects, and the interactions of factors influencing the drug release from matrix tablets. The adjusted (0.9842) and predicted values (0.9893) of r(2) for Y(2) were in close agreement. Validation of the optimization study indicated high degree of prognostic ability of response surface methodology. Tablets showed an initial burst release preceding a more gradual sustained release phase following a non-fickian diffusion process. The Box-Behnken experimental design facilitated the formulation and optimization of sustained release hydrophilic matrix systems of losartan potassium.

Angiotensin II Type 1 Receptor Blockers↗
PubMed2001

Plasma apolipoprotein(a) co-deposits with fibrin in inflammatory arthritic joints.

Extravascular coagulation and diminished fibrinolysis are processes that contribute to the pathology of both inflammatory arthritis and atherosclerosis. We hypothesized that, given its homology with plasminogen, apolipoprotein (apo) (a), the distinctive glycoprotein of the atherogenic lipoprotein (Lp) (a), may be equally implicated in inflammatory arthritis. We detected the presence of apo(a) as part of Lp(a) in human arthritic synovial fluid. The abundance of apo(a) in synovial fluid rose in proportion to plasma apo(a) levels and was higher in inflammatory arthritides than in osteoarthritis. In addition, apo(a) immunoreactive material, but not apo(a) transcripts, was detected in inflammatory arthritic synovial tissues. These data indicated that synovial fluid apo(a) originates from circulating Lp(a) and that diffusion of Lp(a) through synovial tissue is facilitated in inflammatory types of arthritis. In synovial tissues, apo(a) co-localized with fibrin. These observations could be reproduced in a model of antigen-induced arthritis, using transgenic mice expressing human Lp(a). Although in this mouse model the presence of apo(a) did not change the severity of arthritis, the co-localization of apo(a) with fibrin in synovial tissue suggests that, in humans, apo(a) may modulate locally the fibrinolytic activity and may thus contribute to the persistence of intra-articular fibrin in inflammatory arthritis.

Animals↗
PubMed1999

Mandatory practice self-appraisal: moving towards outcomes based continuing education.

Most organized physician continuing educational activities are undertaken without assessing their effects on long-term changes in physician practice patterns, patient outcomes or return on investment. Practice audit and practice self-appraisal are two activities that can be used to achieve these objectives. It is recommended that these be promoted from an educational perspective. A series of principles, including the mandatory application of practice audit and practice self-appraisal are proposed to guide the process. The identification of issues relating to learning, diffusion of information and behavioural change required to facilitate this change are briefly discussed.

Education, Medical, Continuing↗
PubMed2003

Enhancement of glucose transport in rat thymocytes by different radical sources.

This study demonstrates that oxidative stress induced in rat thymocytes by the hydrophilic 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH), the lipophilic cumene hydroperoxide (CumOOH) and the freely diffusible H2O2 is associated with an activation of facilitative glucose transport rate, mediated by GLUT1, the major transporter in this cell type. We compared the effects of the three tested radical sources on the kinetic transport parameters, showing that the transport rate enhancement in the treated cells can be ascribed to an increase in the Vmax value, apart from the site of generation of the oxidative stress. The enhancement of glucose transport by the three oxidants in thymocytes was significantly attenuated both by protein tyrosine kinase inhibitors as genistein and tyrphostin A23 and by U73122, a phospholipase C inhibitor. Genistein and U73122 reversed also the cited increase of Vmax values. It is concluded that the stimulation of glucose transport in response to different oxidants is mediated, at least in part, through reactive oxygen species (ROS)-induced stimulation of protein tyrosine kinase and phospholipase C pathways.

Animals↗