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  • 1
    Electronic Resource
    Electronic Resource
    Mean residence time of molecules diffusing in a cell bounded by a semi-permeable membrane: Monte Carlo simulations and an expression relating membrane transition probability to permeability (2000)
    Springer
    European biophysics journal 29 (2000), S. 221-227 
    Details
    ISSN: 1432-1017
    Keywords: Key words Membrane permeability ; Parallel planes ; Sphere ; Biconcave disc ; Bounded diffusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Physics
    Notes: Abstract  The rapid exchange of water across erythrocyte membranes is readily measured using an NMR method that entails doping a suspension of cells with a moderately high concentration of Mn2+ and measuring the rate of transverse relaxation of the nuclear magnetisation. Analysis of the data yields an estimate of the rate constant for membrane transport, from which the membrane permeability can be determined. It is assumed in the analysis that the efflux rate of the water is solely a function of the rate of membrane permeation and that the time it takes for intracellular water molecules to diffuse to the membrane is relatively insignificant. The limits of this assumption were explored by using random-walk simulations of diffusion in cells modelled as parallel planes, spheres, and biconcave discs. The rate of membrane transport was specified in terms of a transition probability but it was not initially clear what the relationship should be between this parameter and the diffusional membrane permeability P d. This relationship was derived and used to show that the mean residence time for a water molecule is determined by P d when the diffusion coefficient is above a certain threshold value; it is determined by the distance to the membrane below that value.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002490000081
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    African Grass Invasion in the Americas: Ecosystem Consequences and the Role of Ecophysiology (2000)
    Springer
    Biological invasions 2 (2000), S. 123-140 
    Details
    ISSN: 1573-1464
    Keywords: African grasses ; ecophysiology ; ecosystem effects ; global change ; invasions ; pasture conversion ; tropical America
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Clearing of natural vegetation for pastures and the deliberate introduction of African grasses constitute significant threats to the biological diversity of the tropics, subtropics, and warm temperate regions of the Americas. African grasses have escaped from cultivated pastures and revegetated rangeland sites and invaded natural areas at alarming rates. Invaded ecosystems tend to be biotically impoverished and differ markedly from adjacent non-invaded areas in structure and function. Effects of pasture creation and invasion by African grasses on ecosystem processes (transformation and flux of energy and matter) are primarily related to loss of woody species and changes in the fire regime. However, the ecophysiological attributes of the African grasses (e.g. high biomass allocation to leaves, high growth rate, and high leaf-level gas exchange rates) also have important consequences. Here we describe the extent of pasture creation with African grasses and their invasive spread in the New World and review ecological effects of these land-cover changes. We highlight a number of comparative ecophysiological studies within the context of mechanisms responsible for invasion by African grasses and resulting ecosystem change.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1010040524588
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Use of 6-Mercaptopurine in Patients with Inflammatory Bowel Disease Previously Intolerant of Azathioprine (2000)
    Springer
    Digestive diseases and sciences 45 (2000), S. 1810-1813 
    Details
    ISSN: 1573-2568
    Keywords: H. pylori ; azathioprine ; 6-mercaptopurine ; immunosuppressive agents ; inflammatory bowel disease
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Both azathioprine and its active metabolite, 6-mercaptopurine, are of benefit in the treatment of inflammatory bowel disease, either in resistant cases, or for their steroid-sparing effect. Azathioprine treatment is limited in some patients by hypersensitivity reactions or other side effects. We report our experience in 11 patients previously unable to tolerate azathioprine for a variety of reasons, who were switched to 6-mercaptopurine. Of seven patients with ulcerative colitis and four patients with Crohn's disease who were treated with 6-mercaptopurine following failed azathioprine therapy, six were able to successfully tolerate the substitute medication, with good response. Where patients have previously been intolerant of azathioprine yet ongoing indications for immunosuppressive therapy remain, a trial of 6-mercatopurine may be warranted. Given the similar efficacies of the two drugs in inflammatory bowel disease, these findings also favor the use of 6-mercaptopurine rather than the parent compound as initial therapy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005569808947
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    Paper (German National Licenses)
    Fulltext
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