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W D Comper

Publications and source records attributed to W D Comper.

At least 19 recordsLinked to original sources
PubMed1991

Charge selectivity in kidney ultrafiltration is associated with glomerular uptake of transport probes.

The isolated perfused kidney exhibits substantial charge selectivity, as in vivo, in relation to fractional clearance of [3H]dextran sulfate and [3H]dextran. When cycloheximide is present in perfusate, fractional clearance of dextran sulfate is increased and proteinuria becomes significant, but glomerular filtration rate remains essentially unchanged compared with control. The possible role of cells in affecting transglomerular transport was demonstrated when isolated glomeruli from control perfused kidneys showed a very significant resident concentration of [3H]dextran sulfate and [3H]albumin, whereas there was no corresponding accumulation of [3H]dextran or [3H]inulin. Glomerular concentration of dextran sulfate and albumin was significantly reduced by cycloheximide. Kinetics of uptake and release of glomerular dextran sulfate indicated that it had a half-life of glomerular residence of approximately 2-3 min and that this half-life was considerably extended in the presence of cycloheximide. The half-life for glomerular residence of albumin was in the range of 30-40 min. The conclusion from this work is that glomerular charge selectivity for dextran sulfate could be quantitatively rationalized on the basis of transient uptake and release by glomerular cells.

Animals↗
PubMed1990

Extracellular matrix interactions: sulfation of connective tissue polysaccharides creates macroion binding templates and conditions for dissipative structure formation.

Evidence is now accumulating that the post-polymer modification process of sulfation of connective tissue polysaccharides is primarily to provide an interactive macroion for enthalpic interactions rather than influence thermodynamic non-ideality which primarily affects water distribution in biological systems. Metabolic energy considerations also distinguish these physicochemical classifications. Thermodynamic non-ideality is embodied in the carboxyl group and polysaccharide chain which are energetically favoured in biosynthesis, whereas considerable energy input is required for sulfation. The sulfation process gives rise to macroions, with a wide variety of negative charge patterns, that may participate in heterotypic macromolecular interactions. This partial informational specificity is discussed in terms of evolutionary flexibility of the extracellular matrix as rationalized on the qualitative aspects of dissipative structure formation. The concept of multiple binding interactions of varying specificity associated with connective tissue polysaccharides raises the awareness of a more random, less highly ordered, extracellular matrix as compared to the tight machine-like organization generally found for processes in the cell. This is discussed in terms of the physiological adaptation and development of multicellular-tissue systems.

Animals↗
PubMed1990

Osmotic flow caused by chondroitin sulfate proteoglycan across well-defined nuclepore membranes.

Osmotic flows generated by solutions of Swarm rat chondrosarcoma proteoglycan subunit have been analysed using Nuclepore membranes of well-defined straight-through cylindrical pores of known radius rp. Membranes with rp in the range of 27-500 nm were studied. For semipermeable membranes, which are impermeable to the proteoglycan, the flows were consistently related to r2p and not to r4p (Poiseuille's Law) which demonstrates that the flow is a diffusion-controlled process as described previously (R.P.W. Williams and W.D. Comper, J. Phys. Chem. 91 (1987) 3443). We have also identified a characteristic distance, approx. 50% of the average interparticle spacing, out from the pore surface of the membrane over which the proteoglycan has to move to generate flow. The proteoglycan generates similar osmotic permeability coefficients with membranes up to 125 nm in pore diameter which is significantly larger than the average intermolecular distance. These results have been interpreted in terms of the membrane pore recognising dynamic transient aggregates in the proteoglycan solution.

Animals↗
PubMed1990

Hydrodynamic properties of connective-tissue polysaccharides.

The major hydrodynamic properties of the connective-tissue polysaccharides are those that describe polysaccharide-water interaction as embodied in their osmotic-pressure and hydraulic-conductivity properties. This study shows that, for polysaccharides such as chondroitin sulphate, hyaluronate and the heparin-like polysaccharides, their hydrodynamic properties depend primarily on the presence of the uronic residue and the nature of the glycosidic linkage. Other parameters such as the degree of N-acetylation and sulphation were found not to influence these properties to any great extent. These studies particularly delineate structural-functional aspects of the connective-tissue polysaccharides in terms of their primary structure.

Animals↗
PubMed1990

Model anionic polysaccharide matrices exhibit lower charge selectivity than is normally associated with kidney ultrafiltration.

The influence of the anionic polysaccharide matrix (APM) of the glomerular basement membrane (GBM) on water transport and macromolecular transport charge selectivity has been studied in model systems which enable the analysis of the specific properties of the APM. Various APMs were studied including heparin and heparin-like polysaccharides, chondroitin sulfate and dextran sulfate. Upper estimates of the APM concentration in the GBM were obtained by relating measurements of the specific hydraulic conductivity of the heparin-like polysaccharides to measurements of single nephron glomerular filtration rate. This gave values of 40-45 mg ml-1 of polysaccharide, which was then used to analyse factors contributing to macromolecular charge selectivity. Transport analysis of the test dextran probes, used for previous in vivo clearance studies, in APMs demonstrated that there were no differential charge effects at APM concentrations in the range predicted above in the GBM. This was also found to be the case for the partitioning of anionic test probes at the APM-solution interface as measured directly using frontal gel chromatography and through thermodynamic analysis of sedimentation and diffusion data. These studies demonstrated that previous biophysical interpretations of glomerular charge selectivity have severely overestimated the partitioning effect due to the electrostatic effects of polyion-polyion interaction.

Anions↗
PubMed1990

'Generic' physical mechanisms of morphogenesis and pattern formation.

The role of 'generic' physical mechanisms in morphogenesis and pattern formation of tissues is considered. Generic mechanisms are defined as those physical processes that are broadly applicable to living and non-living systems, such as adhesion, surface tension and gravitational effects, viscosity, phase separation, convection and reaction-diffusion coupling. They are contrasted with 'genetic' mechanisms, a term reserved for highly evolved, machine-like, biomolecular processes. Generic mechanisms acting upon living tissues are capable of giving rise to morphogenetic rearrangements of cytoplasmic, tissue and extracellular matrix components, sometimes leading to 'microfingers', and to chemical waves or stripes. We suggest that many morphogenetic and patterning effects are the inevitable outcome of recognized physical properties of tissues, and that generic physical mechanisms that act on these properties are complementary to, and interdependent with genetic mechanisms. We also suggest that major morphological reorganizations in phylogenetic lineages may arise by the action of generic physical mechanisms on developing embryos. Subsequent evolution of genetic mechanisms could stabilize and refine developmental outcomes originally guided by generic effects.

Animals↗
PubMed1990

The inhibitory action of oxygen radical scavengers on proteinuria and glomerular heparan sulphate loss in the isolated perfused kidney.

The perfused isolated kidney is a partial ischemic system that is characterised by glomerular proteinuria and release of glomerular heparan sulfate. Metabolic changes associated with the levels of glutathione, xanthine oxidase and glyceraldehyde 3-dehydrogenase indicated that oxygen radical metabolites were being produced during the perfusion. We have demonstrated that a mixture of oxygen metabolite scavengers containing mannitol, superoxide dismutase and catalase included in the perfusion medium significantly reduced protein excretion. Similar results were obtained with the administration of allopurinol to the rat 24h prior to kidney removal and allopurinol in the perfusion medium. [35S]Heparan sulfate loss from the glomerulus was totally inhibited by the scavenger mixture. These results suggest that reactive oxygen metabolites may be involved in damage to renal capillaries, specifically to heparan sulfate proteoglycan, which leads to proteinuria as a result of partial ischemia produced during perfusion.

Animals↗
PubMed1989

Passive loss of proteoglycan from articular cartilage explants.

The addition of proteinase inhibitors (1 mM phenylmethylsulfonyl fluoride, 10 mM N-ethylmaleimide, 0.25 mM benzamidine hydrochloride, 6.25 mM EDTA, 12.5 mM 6-aminohexanoic acid and 2 mM iodoacetic acid) to explant cultures of adult bovine articular cartilage inhibits proteoglycan synthesis as well as the loss of the macromolecule from the tissue. Those proteoglycans lost to the medium of explant cultures treated with proteinase inhibitors were either aggregates or monomers with functional hyaluronic acid-binding regions, whereas proteoglycans lost from metabolically active tissue also included a population of monomers that were unable to aggregate with hyaluronate. Analysis of the core protein from proteoglycans lost into the medium of inhibitor-treated cultures showed the same size distribution as the core proteins of proteoglycans present in the extracellular matrix of metabolically active cultures. The core proteins of proteoglycans appearing in the medium of metabolically active cultures showed that proteolytic cleavage of these macromolecules occurred as a result of their loss from the tissue. Explant cultures of articular cartilage maintained in medium with proteinase inhibitors were used to investigate the passive loss of proteoglycan from the tissue. The rate of passive loss of proteoglycan from the tissue was dependent on surface area, but no difference in the proportion of proteoglycan aggregate to monomer appearing in the medium was observed. Furthermore, proteoglycans were lost at the same rate from the articular and cut surfaces of cartilage. Proteoglycan aggregates and monomer were lost from articular cartilage over a period of time, which indicates that proteoglycans are free to move through the extracellular matrix of cartilage. The movement of proteoglycans out of the tissue was shown to be temperature dependent, but was different from the change of the viscosity of water with temperature, which indicates that the loss of proteoglycan was not solely due to diffusion. The activation energy for the loss of proteoglycans from articular cartilage was found to be similar to the binding energies for electrostatic and hydrogen bonds.

Aminocaproic Acid↗
PubMed1989

Hydraulic conductivity of chondroitin sulfate proteoglycan solutions.

The hydraulic conductivity of solutions of Swarm rat chondrosarcoma proteoglycan subunit and of chondroitin 4- and 6-sulfate up to concentrations of 80 mg ml-1 have been measured under physiological conditions using sedimentation velocity and membrane ultrafiltration techniques. This study establishes the very high flow resistance of the proteoglycan and that this resistance is due to its constituent chondroitin sulfate chains. We have also demonstrated little difference in the hydraulic conductivity of chondroitin 4-sulfate as compared to chondroitin 6-sulfate. Studies of hydraulic conductivity of chondroitin sulfate and proteoglycan subunit over a range of salt concentrations demonstrate that the chondroitin sulfates exhibit only a small degree of electrolyte dissipation indicating that their constituent charge groups do not significantly contribute to flow resistance at high mechanical pressures. It appears that the shape and conformation of the polysaccharide backbone and its glycosidic linkages are the factors that primarily govern flow resistance. This is also consistent with the fact that hydraulic conductivity of the proteoglycans and chondroitin sulfates is considerably lower than that of its more charged counterpart heparin but has similar values to hyaluronate. Qualitative agreement between sedimentation analysis and ultrafiltration measurements is also established although the latter technique suffers from not knowing over what distance, adjacent to the membrane, ultrafiltration takes place. It is predicted that the proteoglycans will significantly contribute to flow resistance of cartilagenous tissues which confirms the Maroudas correlation that high proteoglycan concentration in cartilage yields high flow resistance. Further, we establish through a comparison of hydraulic conductivity measurements on hyaluronate, desulfated chondroitin sulfate, chondroitin sulfate, and proteoglycan subunit and osmotic pressure measurements of hyaluronate and proteoglycan that the sulfate groups of the chondroitin sulfate chain play only a small role in the net movement of water relative to the proteoglycan.

Animals↗
PubMed1989

Evaluation of nonideality from gel chromatographic partition coefficients. A technique with greater versatility than equilibrium dialysis.

Frontal gel chromatography has been used to measure partition coefficients which enable a quantitative evaluation of the thermodynamic nonideality of small solutes generated by the presence of high concentrations of macromolecular solutes. Equivalence of results obtained by the present method and by equilibrium dialysis is demonstrated in a comparison of results for dextran sulfate-NaCl and dextran-sorbitol systems. Interaction coefficients obtained for dextran-sorbitol and protein-polyethylene glycol 4000 systems yields results which are in reasonable agreement with those predicted on the statistical-mechanical basis of excluded volume. Because of its greater versatility in regard to the range of systems that may be studied, the frontal gel chromatographic procedure is likely to be of particular value for the quantitative characterization of thermodynamic nonideality arising from excluded volume effects in concentrated mixtures of macromolecular solutes.

Biopolymers↗
PubMed1989

Hydraulic conductivity of polymer matrices.

We demonstrate that the chondroitin sulfate proteoglycan exhibits enhanced sensitivity to the flow of water compared to other macromolecules which is in accord with their functional role in conferring compressive resistance to cartilage. In order to understand factors that may contribute to its low hydraulic conductivity, a comparative study of hydraulic conductivity, as measured by the sedimentation velocity technique is made of various macromolecules representing variations in charge density, chemical composition, thermodynamic nonideality, size and flexibility. The polymers examined were dextran, poly(ethylene glycol), poly(vinyl alcohol), albumin, and dextran sulfate. The differences in hydraulic conductivity between the various macromolecules could not be explained by conventional theories which included prediction of hydraulic conductivity related to the radius of the molecule regarded as a uniform cylinder, nor the absolute charge density of the molecule and nor to the steric hindrance offered by the macromolecule to the diffusion of tritiated water. A qualitative relationship is established, however, between the noncounterion polymer contribution to osmotic activity and the resistance to water flow for polymers with high osmotic activity.

Biopolymers↗
PubMed1989

Partial ischemia and proteinuria during isolated kidney perfusion is accompanied by the release of vascular [35S]heparan sulfate.

The isolated kidney perfused with modified Krebs-Henseleit buffer with amino acids yields heavy proteinuria associated with reduced ATP levels characteristic of partial ischemia. These conditions are associated with a similar perfusion time dependent release of degraded vascular [35S]heparan sulfate proteoglycan into the perfusate solution which included a 60% loss of [35S]macromolecular material from the glomerulus after 2h of perfusion. Small amounts of [35S]macromolecular material were found in the urine and lymph. These results demonstrate that partial ischemia promotes a specific response in the overall renal vasculature, probably involving oxygen reactive metabolites, that results in the preferential release of heparan sulphate from the basement membrane and endothelial cells on the luminal side of the capillary wall.

Adenosine Triphosphate↗
PubMed1988

Oriented fibrillogenesis of collagen in vitro by ordered convection.

Oriented fibers, arising from the self assembly of collagen, may be produced from in vitro fibrillogenesis of soluble collagen with gradients of fibrillogenic potential. These gradients may be introduced by either concentration gradients of pronase-treated collagen or of arginine which, in turn, create concentration gradients of collagen aggregate intermediates during the fiber formation process. These gradients are gravitationally unstable and generate ordered convective flows which guide fiber growth.

Animals↗
PubMed1987

Hydrodynamics of concentrated proteoglycan solutions.

The dynamics of water transport in proteoglycan compartments has been studied in relation to osmotic flow (proteoglycan diffusion) and hydraulic permeability (proteoglycan sedimentation) in concentrated solutions of proteoglycan subunit and native proteoglycan aggregate isolated from Swarm rat chondrosarcoma. A central parameter that describes the kinetics of both types of water movement is the hydrodynamic frictional coefficient of water with proteoglycan. The frictional coefficient is markedly concentration dependent, increasing with increasing concentration, and highlights important structural features and types of organization of the proteoglycans in concentrated solutions. These include the requirements that proteoglycans in the extracellular matrix not to be immobilized but to have translational diffusive mobility and concentration gradients to be osmotically active, that chondroitin sulfate segmental mobility describing translational motion largely determines osmotic flow and hydraulic permeability of the proteoglycans, and that the proteoglycans exhibit an enhanced ability to resist flow as compared to other macromolecules. Additional dynamic studies suggest the formation of transient super-aggregate structures may occur at high concentrations which endows the proteoglycan subunit hydrodynamic properties similar to proteoglycan aggregate.

Animals↗
PubMed1987

Cell transport in model extracellular matrices.

The rapid transport of cells has been shown to occur by ordered countercurrent convection. This convection can be created by mixtures of macromolecules which make up the extracellular matrix and by the degradation and aggregation products of these macromolecules. The ordered countercurrent convection is manifested in the form of structured flows and arises in isothermal systems with small concentration gradients of solutes. The flows are gravity driven but may rapidly move at angles close to the horizontal axis if they are mechanically constrained to do so. These flows have been shown to rapidly transport cells at rates ranging from 1 to 100 mm h-1, depending on the conditions of the experiment. The transport of cells is nonspecific in that various cell types (chondrocytes, fibroblasts, endothelial cells, and red blood cells) as well as inert particles of similar size (latex beads 6-microns diam) are transported at similar rates. Latex bead transport by structured flow has also been demonstrated to occur in confined spaces in the form of Teflon tubing down to 200 microns in diameter and at angles in the range of 45-90 degrees to the horizontal axis. The flows may also occur over relatively long distances for a prolonged period of time. The conditions for flow formation are simple and widespread. It is suggested that it may contribute to the forces involved in the movement of cells in the extracellular matrix in vivo especially during remodeling and embryogenesis.

Animals↗
PubMed1984

Dissipative structures in proteoglycan solutions.

Diffusion in multicomponent solutions containing proteoglycan is shown to result in the formation of coherent, fluid structures (known as dissipative structures) and induction of rapid polymer transport. These phenomena occur over a wide range of conditions (i.e. varying solute distribution, concentration, size, and chemical composition) which are envisaged to occur in the extracellular matrix of connective tissues. A concentration gradient of chondroitin sulfate in a proteoglycan matrix of uniform concentration yields dissipative structures which transport the proteoglycan up to 300-fold faster than its transport in the absence of the gradient component. Similar behavior was observed with other polysaccharide and monosaccharide concentration gradient components. Amplification of structure formation and rapid transport was achieved by 1) increasing the concentration of proteoglycan matrix, 2) increasing the magnitude of the concentration gradient, 3) decreasing the molecular weight of the gradient-forming component, and 4) decreasing the concentration gradient of proteoglycan in the matrix. Dissipative structure morphology exhibits a marked dependence on the initial component distribution. Non-specific, excluded volume interactions between the proteoglycan and the gradient component are believed to induce coupled diffusive transport of the proteoglycan. This leads to microscopic density inversions which nucleate and develop into macroscopic convective flows. These results are similar to those previously observed in ternary solutions of uncharged polymers (i.e. dextran/polyvinylpyrrolidone). We have demonstrated that dissipative structures may transport Micrococcus luteus cells as well as various solutes. Flows were also observed in proteoglycan solutions after localized addition of small amounts of either a proteolytic enzyme or hyaluronic acid. It is likely that the prerequisites for this spontaneous macroscopic self-organization, as manifested by the flow phenomenon, are present in the extracellular matrix of connective tissues.

Animals↗