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PubMed1980

On facilitated oxygen diffusion in muscle tissues.

The role of myoglobin in facilitated diffusion of oxygen in muscle in examined in a tissue model that utilizes a central supplying capillary and a tissue cylinder concentric with the central capillary, and that includes the nonlinear characteristics of the oxygen-hemoglobin dissociation reaction. In contrast to previous work, this model exhibits the effect of blood flow and a realistic, though ideal, tissue-capillary geometry. Solutions of the model equations are obtained by a singular-perturbation technique, and numerical results are discussed for model parameters of physiologic interest. In contrast to the findings of Murray, Rubinow, Taylor, and others, fractional order perturbation terms obtained for the "boundary-layer" regions near the supplying capillaries are quite significant in the overall interpretation of the modeling results. Some closed solutions are found for special cases, and these are contrasted with the full singular-perturbation solution. Interpretations are given for parameters of physiologic interest.

Animals↗
PubMed1997

Evaluation of myoglobin function in the presence of axial diffusion.

Facilitation of oxygen transport by myoglobin has been assessed by many researchers. Yet, the models used in these studies often assume that radial diffusion is the primary transport mechanism in tissue. Axial diffusion is typically neglected. In this study, oxygen transport by myoglobin facilitation is added to a proven cardiac tissue model which contains axial diffusion in the tissue and capillary regions, the Radially-Averaged, Axially-Distributed (RAAD) model. Previous research has shown that the axial diffusion in the capillary and tissue regions becomes coupled, causing a reduction in the pO2 at the capillary inlet. The objective is to determine if this coupling effect increases the facilitation of oxygen transport by myoglobin. The RAAD model consists of non-interacting cylinders of tissue (Krogh cylinders), with each perfused by a central capillary. Derivation of the equations describing the RAAD model yields a stiff, fourth-order, non-linear, ODE, BVP. The equation set is solved numerically. Parameters for myoglobin concentration and diffusion coefficient are chosen to maximize myoglobin facilitation. The effect of myoglobin is assessed by observing changes in the pO2 profiles for the model with and without myoglobin. Also, the RAAD model is compared to experimental pO2 data to determine if the inclusion of myoglobin improves the model prediction. The computer simulations show that myoglobin does facilitate diffusion, but only to a small extent. The changes in the capillary pO2 profiles for the model with and without myoglobin are not significant, pO2 reductions are 0.8% at the inlet and 2% at the outlet. The model prediction is not substantially improved with the addition of myoglobin. The sum of squared error is reduced by 0.1%, from 5.6834 without myoglobin, to 5.6779 with myoglobin. The steady state solution of the RAAD model with myoglobin suggests that, in the presence of axial diffusion, facilitation of oxygen diffusion to tissue is not myoglobin's primary function. No conclusion can be made about the transient function of myoglobin.

Animals↗
PubMed1986

Myoglobin facilitated oxygen diffusion maintains mechanical function of mammalian cardiac muscle.

Myoglobin, an intracellular iron containing protein that binds oxygen reversibly, has been shown in model systems to facilitate the diffusion of oxygen and thereby maintain the mechanical function of exercising canine skeletal muscle and of hypoxic benthic fish hearts. Since no such role has yet been established for mammalian cardiac muscle small diameter (less than or equal to 0.70 mm) isolated kitten papillary muscles were stimulated at 24 X min-1 under isometric conditions in a physiological bath maintained at 30 degrees C with an oxygen tension of approximately equal to 450 mm Hg (59.8 kPa) to obtain a level of oxygenation just adequate to meet the metabolic needs of the muscles, as confirmed experimentally. Myoglobin was inactivated by adding 2 X 10(-3) mol X litre-1 sodium nitrite to the bath to abolish the facilitated diffusion of oxygen in the presence or absence of glycolytic blockade by 10(-4) mol X litre-1 sodium iodoacetate. This resulted in a 22(8)% (with blockade) or 10(3)% (without blockade) decrease (p less than 0.05) in the maximal rate of relaxation (-dT/dtmax) of the papillary muscles. Since the depression in mechanical function was reversible by increasing the bath oxygen tension to approximately equal to 600 mm Hg (79.8 kPa) it is concluded that the myoglobin facilitated diffusion of oxygen plays a role in maintaining the mechanical function of mammalian cardiac muscle under normal conditions. Furthermore, the maximal rate of relaxation of cardiac muscle is a sensitive indicator of the presence of hypoxia.

Animals↗
PubMedDate not supplied

[7 years study of erythrocyte membrane transport of monoamine precursors. 395 patients (335 depressed, 60 schizophrenic patients)].

According to the monoaminergic hypothesis of affective and schizophrenic syndromes, a perturbation of influx of precursors of monoamines such as tyrosine (TYR) and tryptophan (TRYP) into the brain might be correlated with the clinical syndromes. The measure of blood cell membrane transports of TYR and TRYP is an interesting peripheral model for the study of these syndromes. This work concerns 395 patients (335 depressed and 60 schizophrenics); 215 of depressed patients were followed after the antidepressant therapy. The transport of TYR and TRYP into erythrocytes was measured as followed: erythrocytes were incubated in plasma for 10 min. at 0 degree and 37 degrees C with radiolabeled TYR and TRYP; facilitated diffusion was calculated by subtracting from the total amount of radioactivity taken up by the cells at 37 degrees C (total uptake) that accumulated at 0 degree C (passive diffusion). The results showed that: 1) The depressive syndromes were characterised by a decrease of facilitated diffusion of TYR, an increase of facilitated diffusion of TRYP and a decrease of the index diffusion of TYR/TRYP. 2) The means of variables were different according to the diagnostic sub-groups (DSM III). In the depressions with alcoholism, the facilitated diffusions were increased compared to depressive syndromes without others diagnosis. The depressions with dementia were in intermediate position. 3) After treatment with antidepressant drugs, the biological variables improved with the clinical improvement, without complete normalization compared to controls. The patients without clinical improvement showed no change in the biological measures. 4) Patients for whom the choice of antidepressant drug was done according to the facilitated diffusions showed a 87% rate of clinical improvement, while this rate was 60% in the other patients. 5) In schizophrenic patients, the biological variables were different than in depressed or normal controls and showed a little decrease of facilitated diffusion of TYR, a marked decrease of diffusion of TRYP and an increased index of diffusion TYR/TRYP.

Adolescent↗
PubMed2001

Reaction rates of oxygen with hemoglobin measured by non-equilibrium facilitated oxygen diffusion through hemoglobin solutions.

The purpose of this study was to verify the concept of non-equilibrium facilitated oxygen diffusion. This work succeeds our previous study, where facilitated oxygen diffusion by hemoglobin was measured at conditions of chemical equilibrium, and which yielded diffusion coefficients of hemoglobin and of oxygen. In the present work chemical non-equilibrium was induced using very thin diffusion layers. As a result, facilitation was decreased as predicted by theory. Thus, this work presents the first experimental demonstration of non-equilibrium facilitated oxygen diffusion. In addition, association and dissociation rate parameters of the reaction between oxygen and bovine and human hemoglobin were calculated and the effect of the homotropic and heterotropic interactions on each rate parameter was demonstrated. The results indicate that the homotropic interaction--which leads to increasing oxygen affinity with increasing oxygenation--is predominantly due to an increase in the association rate. The heterotropic interaction--which leads to decreasing oxygen affinity by anionic ligands--appears to be effected in two ways. Cl- increases the dissociation rate. In contrast, 2,3-diphosphoglycerate decreases the association rate.

2,3-Diphosphoglycerate↗
PubMed1991

Fatty-acid-binding protein facilitates the diffusion of oleate in a model cytosol system.

The proposed function of fatty-acid-binding proteins in facilitating the diffusion of their ligand has been examined in a cytosolic model using a steady-state diaphragm cell apparatus. The white muscle of ocean pout (Macrozoarces americanus) does not have a fatty-acid-binding protein: nor does it exhibit detectable beta-oxidative capacity, and as such is a good source of cytosol preparation. After determining the diffusion coefficient of oleate in this medium, fatty-acid-binding protein, isolated from the ventricle of this animal, was added and the apparent diffusion coefficient was again determined. The presence of the fatty-acid-binding protein increased the apparent diffusion coefficient of this long-chain fatty acid about 6-fold, from 0.087 x 10(-5) cm2.s-1 to 0.58 x 10(-5)cm2.s-1. This confirms the facilitated diffusion hypothesis as one role of fatty-acid-binding protein. The diffusion coefficients of oleate in 50 mM-sodium phosphate buffer, pH 7.4, at 10 degrees C and 25 degrees C (0.15 x 10(-5) cm2.s-1 and 0.28 x 10(-5) cm2.s-1 respectively) were also measured.

Animals↗
PubMed1997

Diffusion coefficients of oxygen and hemoglobin measured by facilitated oxygen diffusion through hemoglobin solutions.

Diffusion coefficients of oxygen (DO2) and hemoglobin (DHb) were obtained from measuring the oxygen flux through thin layers of hemoglobin solutions at 20 degrees C. The liquid layers were supported by a membrane and not soaked in any filter material. Oxygen fluxes were measured from the changes in oxygen partial pressure in the gas phases at both sides of the layer. A mathematical treatment is presented for correct evaluation of the measurements. Measurements were done for bovine and for human hemoglobin. Hemoglobin concentrations (CHb) were between 11 and 42 g/dl, which covers the concentrations in the erythrocyte. Both DO2 and DHb could be fitted to the empirical equation D = D0(1-CHb/C1)10-CHb/C2. The following parameters were obtained: DO = 1.80 x 10(-9) m2/s, C1 = 100 g/dl, C2 = 119 g/dl, for oxygen and D0 = 7.00 x 10(-11) m2/s, C1 = 46 g/dl, C2 = 128 g/dl, for hemoglobin. No difference between the diffusion coefficients of bovine or human hemoglobin was found. The diffusion coefficients of hemoglobin were higher than most values reported in the literature, probably because in this study the mobility of hemoglobin was not hindered by surrounding filter material.

Animals↗
PubMed1983

Bidirectional transport of glutamine across the cell membrane in rat liver.

Hepatocytes isolated from fed rats were used to investigate glutamine transport. Glutamine transport appears as a composite process involving at least two saturable components. The Na+-dependent component probably represents the entry through the N system. The Na+-independent component was also inhibited by histidine and exhibited trans-stimulation, suggestive of a facilitated diffusion process. Kinetic parameters for both systems suggest that facilitated diffusion only plays a minor role in glutamine influx. In contrast, the Km for glutamine efflux was consistent with a physiological role of the facilitated-diffusion component in glutamine release. In Na+ medium, relatively constant distribution ratios (about 8) between intra- and extra-cellular concentrations were observed, with external glutamine ranging from 0.5 to 5 mM. The present observations suggest that glutamine influx might largely be mediated by the N system, whereas facilitated diffusion allows hepatocytes to release glutamine when intracellular concentrations are elevated. The physiological consequences of this bidirectional transfer of glutamine across the liver cell membrane is discussed.

Amino Acids↗
PubMed1989

Calcium buffer injections block fucoid egg development by facilitating calcium diffusion.

The polarity of fucoid eggs is fixed either when tip growth starts or a bit earlier. A steady flow of calcium ions into the incipient tip is thought to establish a high calcium zone that is needed for its localization and formation. To test this hypothesis, we have injected seven different 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-type calcium buffers into Pelvetia eggs many hours before tip growth normally starts. Critical final cell concentrations of each buffer prove to block outgrowth (as well as cell division) for up to 2 weeks. This critical inhibitory concentration is lowest for two buffers with dissociation constants or Kd values of 4-5 x 10(-6) M and increases steadily as the buffers' Kd values shift either below or above this optimal value to ones as low as 4 x 10(-7) M or as high as 9.4 x 10(-5) M. To analyze these results, we have derived an equation (based on the concept of facilitated diffusion) for the effects of diffusable calcium buffers on steady-state calcium gradients. The data fit this equation quite well if it is assumed that cytosolic free calcium at the incipient tip is normally kept at about 7 microM and, thus, far above the general cytosolic level.

Buffers↗
PubMed2000

Studies of L-arginine transport in bovine aortic endothelial cells.

We have previously demonstrated that p(1),p(4)-diadenosine 5'-tetraphosphate induces the release of NO and modulates the uptake of L-arginine by bovine aortic endothelial cells (BAEC) [Hilderman, R. H., and Christensen, E. F. (1998) FEBS Lett. 407, 320-324; Hilderman, R. H., Casey, T. E., and Pojoga, L. H. (2000) Arch. Biochem. Biophys. 375, 124-130]. In this communication we characterize the uptake of L-Arg by BAEC. L-Arg is transported into BAEC by at least two different transporter systems. One transporter system is protein synthesis dependent, and L-Arg transported by this system is incorporated into proteins. The second transporter system involved in L-Arg uptake is protein synthesis independent, and uptake occurs by facilitated diffusion. The L-Arg transported by facilitated diffusion is metabolized into L-argininosuccinate. Homologous and heterologous competition uptake studies were performed using a fixed concentration of radiolabeled L-Arg, L-lysine, and L-leucine with varying concentrations of competing nonradiolabeled amino acids. The results of these competition uptake studies are consistent with the protein-synthesis-dependent uptake of L-Arg taking place through a transporter system that is highly specific for L-Arg and with the facilitated diffusion uptake taking place through a transporter that is specific for L-Arg and L-Leu.

Adenosine Triphosphate↗
PubMed1985

Numerical simulation of pulmonary O2 and CO2 exchange.

The process of gas exchange leading to the oxygenation of blood in pulmonary capillaries is simulated numerically, taking into account the main transport mechanisms of molecular diffusion, convection and the facilitated diffusion due to the presence of haemoglobin, as well as physiologically relevant boundary conditions and variable initial data. An algorithmic program to solve the relevant equations is run on a computer. It is found that, in the immediate neighbourhood of the entry, the amount of dissolved oxygen decreases, whereas the amount of carbaminohaemoglobin increases and the facilitated diffusion is more dominant over the molecular diffusion. Further, it is shown that (i) O2 takes longest and CO2 is the fastest to attain equilibration, (ii) the blood is completely oxygenated with one fifth part of its transit. Finally, the effect of various physiological parameters on equilibration length is examined.

Humans↗
PubMed1992

5'-Nucleotidase activity and adenosine production in rat liver mitochondria.

The controversial subject of mitochondrial 5'-nucleotidase in the liver was studied employing density gradient fractionation combined with a method for analyzing the distribution profiles of marker enzymes based on multiple regression analysis. Triton WR-1339 was used to improve the separation of mitochondria from lysosomes by the gradient centrifugation technique. Adenosine production was examined further using acetate to increase intramitochondrial AMP, and thus adenosine production, in incubations with gradient centrifugation-purified mitochondria. Distribution analysis of the crude homogenate showed that 5'-nucleotidase activity exists in the mitochondrial fraction. To increase the resolution of this approach with respect to mitochondria, a crude mitochondrial fraction was also studied. In this case the relative mitochondrial activity decreased but 5'-nucleotidase activity was still clearly detectable. The mitochondrial 5'-nucleotidase exhibited a Km of 94 microM and a Vmax of 31 nmol/min per mg protein for AMP. The kinetic data for the Mg2+, ATP, ADP and AOPCP sensitivity of the enzyme showed that it differs from the plasma membrane, lysosome and cytosol 5'-nucleotidases. AOPCP was only a moderate inhibitor, and ATP was a more potent inhibitor than ADP at a 1 mM concentration. The enzyme also showed a requirement of Mg2+. Acetate caused the conversion of intramitochondrial adenylates to AMP and the formation of adenosine. Adenosine concentration increased in the extramitochondrial space in a time-dependent manner, but only trace amounts of nucleotides were detected. The data show that 5'-nucleotidase activity producing adenosine exists in rat liver mitochondria and a concentration-dependent adenosine output from mitochondria by diffusion or facilitated diffusion is also suggested.

5'-Nucleotidase↗
PubMed1978

The pH-dependence of sugar-transport and glycolysis in cultured Ehrlich ascites-tumour cells.

1. pH-dependence of glycolysis has generally been ascribed to the effects of pH on the activities of glycolytic enzymes. The present study shows that sugar transport is pH-dependent in cultured Ehrlich ascites-tumour cells. 2. The rates of glucose consumption, of 3-O-methylglucose transport, and of 2-deoxyglucose transport and phosphorylation increased as linear functions of pH, as the pH of the cell culture medium was increased from 6.1 to 8.5. Transport of glucose, as measured in ATP-depleted cells, was pH-dependent to the same extent as transport of the non-metabolizable sugars. 3. Glucose consumption rates were about 8-fold higher at pH 8.5 than at pH 6.4. About 65-85% of glucose was converted into lactate. Sugar transport rates were 2.5-fold higher at pH 8.5 than at pH 6.3. 4. pH affected both simple diffusion and facilitated diffusion. pH effect was mainly on the Vmax. of 2-deoxyglucose uptake, and on the rapid-uptake phase of 3-O-methylglucose transport. 5. It was estimated that about 70% of the pH effect on the rates of glucose consumption may be due to the effect on sugar transport and the remainder to the effect on the activities of glycolytic enzymes.

Biological Transport↗
PubMed1991

A mathematical model for the computation of carboxyhaemoglobin in human blood as a function of exposure time.

A mathematical model is developed for the carbon monoxide (CO) uptake by the blood by taking into account the molecular diffusion, convection, facilitated diffusion and the non-equilibrium kinetics of CO with haemoglobin. The overall rate for the combination of CO with haemoglobin is derived by including the dissociation of CO from carboxyhaemoglobin (COHb). The resulting coupled system of nonlinear partial differential equation with physiologically relevant initial, entrance and boundary conditions is solved numerically. A fixed point iterative technique is used to deal with nonlinearities. The concentration of COHb in the blood is computed as a function of exposure time and ambient CO concentration. The COHb levels computed from our model are in good agreement with those measured experimentally. Also, results computed from our model give better approximation to the experimental values compared with the results from other models. The time taken by the blood COHb to attain 95% of its equilibrium value is computed. The COHb concentration in the blood increases with the increase in ventilation rate, association rate coefficient of CO with haemoglobin and total haemoglobin content in the blood, and with the decrease in dissociation rate coefficient of CO with haemoglobin and mean capillary blood PO2. It is found that the COHb level in the blood is not affected significantly because of endogenous production of CO in the body under normal condition. However, the effect may be significant in the patients with haemolytic anaemia.

Capillaries↗
PubMed2000

Hypothesis: do voltage-gated H(+) channels in alveolar epithelial cells contribute to CO(2) elimination by the lung?

Although alveolar epithelial cells were the first mammalian cells in which voltage-gated H(+) currents were recorded, no specific function has yet been proposed. Here we consider whether H(+) channels contribute to one of the main functions of the lung: CO(2) elimination. This idea builds on several observations: 1) some cell membranes have low CO(2) permeability, 2) carbonic anhydrase is present in alveolar epithelium and contributes to CO(2) extrusion by facilitating diffusion, 3) the transepithelial potential difference favors selective activation of H(+) channels in apical membranes, and 4) the properties of H(+) channels are ideally suited to the proposed role. H(+) channels open only when the electrochemical gradient for H(+) is outward, imparting directionality to the diffusion process. Unlike previous facilitated diffusion models, HCO(-)(3) and H(+) recombine to form CO(2) in the alveolar subphase. Rough quantitative considerations indicate that the proposed mechanism is plausible and indicate a significant capacity for CO(2) elimination by the lung by this route. Fully activated alveolar H(+) channels extrude acid equivalents at three times the resting rate of CO(2) production.

Acid-Base Equilibrium↗
PubMed1992

[Membrane transport system of amino acids in the small intestine].

The membrane transport system of amino acids in the small intestine is reviewed. Amino acids are transported by three processes: 1) simple diffusion, 2) facilitated diffusion and 3) active transport. In the brush border membrane, there are at least six Na(+)-dependent systems (NBB, PHE, IMINO, beta, X-G,A, Y+) and the Na(+)-independent systems (L, beta, Y+). On the other hand, the basolateral membrane has at least four Na(+)-dependent systems (A, ASC, X-G,A, N) and three Na(+)-independent systems (asc, L, y+). These systems are discussed.

Amino Acids↗
PubMed1991

Local generation of triiodothyronine by the mammary gland as a source of measurable quantities of the hormone in milk.

Simple diffusion or facilitated diffusion, by a carrier system represented by binding sites on plasma membranes, are thought to be responsible for thyroid hormones (TH) uptake by tissues. The concept implies the existence of an equilibrium ratio between plasma and cellular hormone content. Studies of TH presence in milk have shown that the passage of TH through mammary gland differs between T4, rT3, and T3, and only T3 exists in the milk in measurable amounts (without definite relationship between the levels of T3 in milk and the blood plasma). The present experiments have revealed that the whole milk or its cellular components (namely macrophages, lymphocytes and granulocytes) possess deiodinating enzyme system converting T4 into T3. The milk from animals displaying mastitis show lower T3 levels than controls. Of different enzyme systems, the outer ring deiodinase (5'D-II) dominates in mammary secretion. It plays an important role in galactopoiesis yielding biologically potent T3. The presence of TH deiodinating system(s) in mammary gland is thought to lower T4 and T3 in milk, already in small concentrations due to a poor permeability of the blood-mammary gland barrier for these compounds. On the other hand measurable amount of T3 in milk is due to a local enzyme mechanism generating T3 from T4. It has been concluded that the ultimate THs level in milk depends on the rate of transport of the hormones, their degradation and simultaneous generation of T3 from T4 in mammocytes.

Animals↗
PubMed1987

Role of de novo protein synthesis in the interconversion of glucose transport systems in the yeast Pichia ohmeri.

Glucose-repressed cells of the yeast Pichia ohmeri IGC 2879 transported glucose by facilitated diffusion. Derepression led to the formation of a glucose/proton symport and the simultaneous reduction of the facilitated diffusion capacity by about 70%. Cycloheximide prevented this interconversion indicating its dependence on de novo protein synthesis (proteosynthetic interconversion). In buffer with 2% glucose the glucose/proton symport suffered irreversible inactivation while the facilitated diffusion system was simultaneously restored. This reverse interconversion process did not require de novo protein synthesis as indicated by its lack of sensitivity to cycloheximide (degradative interconversion). Thus the glucose/proton symport system appeared to consist of about 70% of the facilitated diffusion proteins turned silent through association with additional protein(s) the latter being sensitive to glucose-induced repression and glucose-induced inactivation.

3-O-Methylglucose↗