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  • 1
    Electronic Resource
    Electronic Resource
    Oscillatory model of crassulacean acid metabolism: structural analysis and stability boundaries with a discrete hysteresis switch (1998)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 21 (1998), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: We propose a simple oscillatory model of crassulacean acid metabolism (CAM) describing the CO2 uptake and nocturnal acidification of CAM plants by a system of coupled non-linear differential equations. Large differences in the content of metabolite pools are treated using a pseudo-steady-state approach. For the first time, simulations of the CAM cycle investigate its dependence on all three major control parameters simultaneously: temperature, photon flux density and external CO2 concentration. Under stationary conditions in time the model shows either endogenous rhythmicity or two distinct steady states. Stability boundaries are calculated in parameter space.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.1998.00312.x
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  • 2
    Electronic Resource
    Electronic Resource
    Selective electrosynthesis of p-methoxybenzaldehyde at spinel type CoMn2O4/titanium-composite anodes (1998)
    Springer
    Journal of applied electrochemistry 28 (1998), S. 873-880 
    Details
    ISSN: 1572-8838
    Keywords: organic electrosynthesis ; p-methoxybenzaldehyde ; p-methoxytoluene ; cobalt manganese spinel ; titanium composite anodes ; heterogeneous redoxcatalysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Notes: Abstract Activated titanium anodes with a spinel coating of CoMn2O4 of about 1μm thickness were developed. A relative stability in acid electrolytes was found. The novel anode was employed for the anodic oxidation of p-methoxytoluene (PMT) to p-methoxybenzaldehyde (PMB), which is of industrial interest. Batch type electrolyses with 30% theoretical conversion (4Fmol−1) were used for␣a parametric screening. The solvent/electrolyte-system was 1m H2SO4/5m H2O in methanol. Optimum results (60% selectivity, 50–60% current efficiency) were obtained at low concentrations of the educt (0.2m). A part of the current is consumed for the formation of the benzylmethylether, which can be further oxidized to PMB. It was proved that the novel anode operates according to the mechanism of heterogeneous redox catalysis with ter- and heptavalent manganese as the redox species. A turnover factor of 〉2000 is unusual for a spinel in acid solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1003416421327
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  • 3
    Electronic Resource
    Electronic Resource
    Thin Polyheteroaromatic Interlayers on Commodity Metals for Corrosion Protection (1998)
    s.l. ; Stafa-Zurich, Switzerland
    Materials science forum Vol. 289-292 (Aug. 1998), p. 1217-1228 
    Details
    ISSN: 1662-9752
    Source: Scientific.Net: Materials Science & Technology / Trans Tech Publications Archiv 1984-2008
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Type of Medium: Electronic Resource
    URL: http://www.tib-hannover.de/fulltexts/2011/0528/02/03/transtech_doi~10.4028%252Fwww.scientific.net%252FMSF.289-292.1217.pdf
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  • 4
    Electronic Resource
    Electronic Resource
    Effects of long-term changes in medullary osmolality on heat shock proteins HSp25, hsp60, HSP72 and HSP73 in the rat kidney (1998)
    Springer
    Pflügers Archiv 435 (1998), S. 705-712 
    Details
    ISSN: 1432-2013
    Keywords: Key words Diuresis/antidiuresis ; Osmotic stress ; HSP25 ; HSP60 ; HSP72 ; HSP73 ; Transcription ; Translation ; Medullary hypertonicity ; Phosphorylation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The influence of diuresis and antidiuresis on the expression of heat shock proteins (HSP) 25, 60, 72 and 73 in the renal cortex and outer and inner medulla of Wistar rats was analysed. Medullary osmolality was reduced by long-term diuresis (3% sucrose in the drinking water for 3 weeks) and subsequently enhanced by transition to a concentrating state by giving normal drinking water again in combination with deamino-D-arginine vasopressin (dDAVP) for 5 days. Western blot analyses revealed that neither HSP73 nor HSP60 was influenced by any treatment. The HSP72 level in the medulla was markedly reduced (50%) when osmolality was lowered and increased when tonicity was high. RNAse protection assays showed that the effects on HSP72 are parallelled in general by changes in HSP72 mRNA. While levels of HSP25 were not influenced, isoelectric focusing revealed that the degree of phosphorylation of outer and inner medullary HSP25 increased following both treatments. It thus seems that HSP73 and HSP60 are not directly involved in the long-term adaptation to varying medullary osmolalities. The correlation between changes in osmolality and amounts of the major stress-inducible HSP72 in the medulla implies that medullary hypertonicity is stressful for kidney cells. Furthermore, adaptation to pronounced changes in the osmolality of the environment most likely involves phosphorylation of HSP25.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050572
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  • 5
    Electronic Resource
    Electronic Resource
    Cellular response to osmotic stress in the renal medulla (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 814-827 
    Details
    ISSN: 1432-2013
    Keywords: Key words Antidiuresis ; Diuresis ; Heat shock proteins ; Ionic strength ; Organic osmolytes ; Osmotic stress ; Renal medulla
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  Cells of the renal medulla, which are exposed under normal physiological conditions to widely fluctuating extracellular solute concentrations, respond to hypertonic stress by accumulating the organic osmolytes glycerophosphorylcholine (GPC), betaine, myo-inositol, sorbitol and free amino acids. Increased intracellular contents of these osmolytes are achieved by a combination of increased uptake (myo-inositol and betaine) and synthesis (sorbitol, possibly GPC), decreased degradation (GPC) and reduced osmolyte release. In the medulla of the concentrating kidney, accumulation of organic osmolytes, which do not perturb cell function even at high concentrations, allows the maintenance of ”normal” intracellular concentrations of inorganic electrolytes. Adaptation to decreasing extracellular solute concentrations, e.g. diuresis, is achieved primarily by activation of pathways allowing the efflux of organic osmolytes, and secondarily by inactivation of production (sorbitol) and uptake (betaine, myo-inositol) and stimulation of degradation (GPC). Apart from modulation of the osmolyte content, osmolality-dependent reorganization of the cytoskeleton and expression of specific stress proteins (heat shock proteins) may be further, as yet poorly characterized, components of the regulatory systems involved in the adaptation of medullary cells to osmotic stress.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050710
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  • 6
    Electronic Resource
    Electronic Resource
    Expression of Na+/Cl–/betaine and Na+/myo-inositol transporters, aldose reductase and sorbitol dehydrogenase in macula densa cells of the kidney (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 807-809 
    Details
    ISSN: 1432-2013
    Keywords: Key words Aldose reductase ; In situ hybridization ; Macula densa ; Na+/Cl ; /betaine cotransporter ; Na+/myo-inositol cotransporter ; Osmolytes ; Sorbitol dehydrogenase
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  It has been suggested that macula densa cells may be exposed to hyperosmotic stress. Since chronic exposure to hypertonic stress causes the amount of intracellular organic osmolytes to increase, the expression of transporters and enzymes that participate in the intracellular accumulation of organic osmolytes was examined using non-radioactive in situ hybridization in the macula densa region of control rats and furosemide-treated animals. Both the sodium- and chloride-dependent betaine transporter (BGT) and sodium-dependent myo-inositol transporter (SMIT) were expressed preferentially in macula densa cells and for both mRNAs the signal intensity was visibly reduced by furosemide. The enzymes aldose reductase (which mediates the conversion of glucose to sorbitol) and sorbitol dehydrogenase (which converts sorbitol into fructose) were expressed not only in macula densa cells but also in the surrounding tubular cells, and the expression was insensitive to furosemide. Thus it remains unclear whether the expression of BGT and SMIT is related to a putative hypertonic juxtaglomerular region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050707
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  • 7
    Electronic Resource
    Electronic Resource
    Pretreatment with hypertonic NaCl protects MDCK cells against high urea concentrations (1998)
    Springer
    Pflügers Archiv 435 (1998), S. 407-414 
    Details
    ISSN: 1432-2013
    Keywords: Key words MDCK cells ; Hypertonic stress ; NaCl ; Urea ; Organic osmolytes ; Heat shock proteins ; Cell viability
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  In antidiuresis, the cells of the renal medulla are exposed to high extracellular concentrations of NaCl and urea. Since urea equilibrates with the intracellular compartment and is known to perturb intracellular macromolecules, high urea concentrations may well disturb the structure and function of cell proteins. Two types of organic substances are believed to counteract the adverse effects of high intracellular urea concentrations: specific organic osmolytes of the trimethylamine family [betaine and glycerophosphorylcholine (GPC)], which accumulate in renal medullary cells during prolonged periods of antidiuresis and cytoprotective heat shock proteins (HSPs), the tissue content of two of which (HSPs 27 and 72) is much higher in the inner medulla than in the iso-osmotic renal cortex. To evaluate the contribution of trimethylamines and HSPs to cytoprotection in the presence of high urea concentrations, the effect of HSP induction and osmolyte accumulation prior to exposure to high urea concentrations was examined in Madin-Darby canine kidney (MDCK) cells. Accumulation of organic osmolytes and synthesis of HSP27 and HSP72 was initiated by hypertonic stress (increasing the osmolality of the medium from 290 to 600 mosmol/kg H2O by NaCl addition). Control, non-conditioned cells remained in the isotonic medium for the same period. Upon subsequent exposure to an additional 600 mM urea in the medium for 24 h, 90% of the osmotically conditioned cells but only 15% of non-conditioned cells survived. The HSP72 and trimethylamine contents of the NaCl-conditioned MDCK cells, but not HSP27 content, correlated positively with cell survival. To separate the effects of organic osmolytes and HSP72, chronically NaCl-adapted MDCK cells were returned to isotonic medium for 1 or 2 days, so depleting them of trimethylamine osmolytes. HSP72, with its longer half life, remained elevated. Subsequent exposure of these cells to 600 mM urea in the medium resulted in about 80% survival. These results suggest that in MDCK cells and probably in the renal medulla, HSP72 and perhaps additional protective factors contribute substantially to the resistance against high urea concentrations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050531
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  • 8
    Electronic Resource
    Electronic Resource
    Cellular site of active K absorption in the guinea-pig distal colonic epithelium (1998)
    Springer
    Pflügers Archiv 436 (1998), S. 280-288 
    Details
    ISSN: 1432-2013
    Keywords: Key words Unidirectional Rb fluxes ; Electron microprobe analysis ; Luminal Rb uptake ; Cellular element concentrations ; Ouabain ; Ethoxzolamide ; Amiloride
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract  The mammalian distal colon, which is composed of different cell types, actively transports Na, K and Cl in absorptive and K and Cl in secretory directions. To further characterize the K absorption process and to identify the cells involved in K absorption, unidirectional Rb fluxes and luminal Rb uptake into different epithelial cell types were determined in isolated guinea-pig distal colon. Net Rb absorption (1.5–2.5 µmol·h–1·cm–2) was not influenced by inhibition of Na transport with amiloride or by incubating both sides of the epithelium with Na-free solutions, but was almost completely abolished by luminal ouabain, ethoxzolamide or by incubating both sides of the epithelium with Cl-free solutions. Luminal Rb uptake, blockable by luminal ouabain, preferentially occurred in columnar surface and neck cells, to a lesser extent in surface goblet cells and to an insignificant degree in lower crypt cells. Employing a luminal Rb-Ringer (5.4 mM Rb) the Rb concentration increased within 10 min in columnar surface and neck, surface goblet and lower crypt cells to 70, 32 and about 10 mmol·kg–1 wet weight, respectively. The presence of 5.4 mM K in the luminal incubation solution reduced Rb uptake almost completely indicating a much higher acceptance of the luminal H-K-ATPase for K than for Rb. The increase in Na and decrease in K concentrations in surface and neck cells induced by luminal ouabain might indicate inhibition of the basolateral Na-K-ATPase or drastic enhancement of cellular Na uptake by the Na-H exchanger. Bilateral Na-free incubation did not alter Rb uptake, but bilateral Cl-free incubation drastically reduced it. Inhibition of net Rb absorption by ethoxzolamide and inhibition of both Rb absorption and Rb uptake by bilateral Cl-free incubation support the notion that cellular CO2 hydration is a necessary prerequisite for K absorption and that HCO3 leaves the cell via a Cl-HCO3 exchanger. Since ouabain-inhibitable transepithelial Rb flux and luminal Rb uptake rate by surface and neck cells were about the same, Rb(K) absorption seems to be accomplished mainly by columnar surface cells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004240050633
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  • 9
    Electronic Resource
    Electronic Resource
    Thermodynamics and Energetics of the Tonoplast Membrane Operating as a Hysteresis Switch in an Oscillatory Model of Crassulacean Acid Metabolism (1998)
    Springer
    The journal of membrane biology 165 (1998), S. 37-43 
    Details
    ISSN: 1432-1424
    Keywords: Key words: Crassulacean acid metabolism — Endogenous rhythm — Lipid membrane structure — Phase transition — Osmotic cell pressure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract. The observed endogenous circadian rhythm in plants performing Crassulacean acid metabolism is effected by malate transport at the tonoplast membrane. Experimental and theoretical work asks for a hysteresis switch, regulating this transport via the ordering state of the membrane. We apply a schematic molecular model to calculate the thermally averaged order parameter of the membrane lipid structure in its dependence on external parameters temperature and area per molecule. The model shows a first order structural phase transition in a biologically relevant temperature range. Osmotic consequences of malate accumulation can trigger a transition between the two phases by changing the surface area of the cell vacuole. Estimation of the energy needed to expand the vacuole under turgor pressure because of osmotic changes while acidifying shows that energy needed as latent heat for the calculated change between phases can easily be afforded by the cell. Thus, malate content and the coexisting two phases of lipid order, showing hysteretic behavior, can serve as a feedback system in an oscillatory model of Crassulacean acid metabolism, establishing the circadian clock needed for endogenous rhythmicity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002329900418
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