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21.
Nils A. Øristland 《Bulletin of mathematical biology》1990,52(5):643-655
A computer model of body mass and composition in relation to gross energy balance is constructed. The model is built using
conventional empirical physiological formulae rather than statistical or analytical mathematical techniques. The model is
applied to the Minnesota and other experiments and produces as good or better simulations of observed values of changes in
body weight than reported for other formulae or models. Alternative physiological mechanisms concerning metabolic adaptions
to starvation, changes in time activity budgets and the energy equivalents of weight loss offer equally good simulations of
experimental results. The present analysis highlights the survival value of a basal metabolic depression during starvation
and indicates an optimal body composition of 10% mobilizable fat for starvation survival for a 70 kg man. Proper quantification
of the effects of the physiological mechanisms involved depends on new experimental data, however. Long term continuous monitoring
of time activity budgets are a necessary part of such experiments. 相似文献
22.
23.
Simulation Model of the Coupling between Nitrification and Denitrification in a Freshwater Sediment 总被引:1,自引:1,他引:0
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T. Henry Blackburn Nicholas D. Blackburn Kim Jensen Nils Risgaard-Petersen 《Applied microbiology》1994,60(9):3089-3095
A model was constructed to simulate the results of experiments which investigated nitrification and denitrification in the freshwater sediment of Lake Vilhelmsborg, Denmark (K. Jensen, N. P. Sloth, N. Risgaard-Petersen, S. Rysgaard, and N. P. Revsbech, Appl. Environ. Microbiol. 60:2094-2100, 1994). The model output faithfully represented the profiles of O2 and NO3- and rates of nitrification, denitrification, and O2 consumption as the O2 concentration in the overlying water was increased from 10 to 600 μM. The model also accurately predicted the response, to increasing O2 concentrations, of the integrated (micromoles per square meter per hour) rates of nitrification and denitrification. The simulated rates of denitrification of NO3- diffusing from the overlying water (Dw) and of NO3- generated by nitrification within the sediment (Dn) corresponded to the experimental rates as the O2 concentration in the overlying water was altered. The predicted Dw and Dn rates, as NO3- concentration in the overlying water was changed, closely resembled those determined experimentally. The model was composed of 41 layers 0.1 mm thick, of which 3 represented the diffusive boundary layer in the water. Large first-order rate constants for nitrification and denitrification were required to completely oxidize all NH4+ diffusing from the lower sediment layers and to remove much of the NO3- produced. In addition to the flux of NH4+ from below, the model required a flux of an electron donor, possibly methane. Close coupling between nitrification and denitrification, achieved by allowing denitrification to tolerate some O2 (~10 μM), was necessary to reproduce the real data. Spatial separation of the two processes (no toleration by denitrification of O2) resulted in too high NO3- concentrations and too low rates of denitrification. 相似文献
24.
Lars Iversen †Eileen Mulvihill †Betty Haldeman ‡Nils Henrik Diemer Frank Kaiser Malcolm Sheardown Peter Kristensen 《Journal of neurochemistry》1994,63(2):625-633
Abstract: Metabotropic glutamate receptors mediate their intracellular response by coupling to G proteins and may be divided into three subfamilies: mGluR1 and mGluR5, which stimulate phosphatidylinositol hydrolysis; mGluR2 and mGluR3, which are negatively coupled to cyclic AMP formation; and mGluR4 and mGluR6, which also inhibit forskolin-stimulated cyclic AMP formation. The mGluR4 subtypes may represent l -2-amino-4-phosphonobutyrate-sensitive presynaptic autoreceptors, and two alternatively spliced variants of the mGluR4 coding for two receptors with different C termini have been identified. Using in situ hybridization, we measured the levels of mGluR1–mGluR5 mRNA in regions of the rat brain 24 h after transient global ischemia, a time point when no neuronal damage can yet be observed morphologically. In the hippocampus, the mRNA levels for mGluR1, mGluR2, and mGluR5 were decreased, mGluR3 mRNA levels were unchanged, and the mGluR4 mRNA levels were strongly increased. The strongest increase appeared to be in the mRNA encoding mGluR4b. The mGluR4 mRNA was also increased in the parietal cortex, whereas the ventral posteromedial thalamic nucleus showed a small decrease in its mRNA content. These results suggest that vulnerable neurons react to an increased extracellular glutamate concentration by differential regulation of the mRNA for pre- and postsynaptically located metabotropic glutamate receptors. 相似文献
25.
Nils Welsh 《Bioscience reports》1994,14(1):43-50
The aim of this investigation was to study the putative role of protein phosphorylation in interleukin-1 (IL-1) induced signal transduction in insulin producing cells. For this purpose, insulin producing RINm-5F cells were exposed to IL-1 for 7 hours with or without different agonists and antagonists to protein kinases and phosphatases and the production of nitrite was subsequently determined. It has been shown earlier that IL-1 will stimulate the production of nitrite in such cells. It was found that EDTA, TPA and staurosporine did not affect IL-1 induced nitrite production. However, the tyrosine kinase antagonist tyrphostin inhibited, whereas sodium orthovanadate, okadaic acid and cyclosporin A, all inhibitors of protein phosphatases, potentiated IL-1 induced nitrite release to the medium. The tyrosine kinase antagonist genistein potentiated at a low concentration and inhibited at a high concentration the IL-1 effect. It is concluded that protein phosphorylation events, mediated either by protein kinases or phosphatases on both tyrosine and serine/threonine residues, may mediate or antagonize IL-1 induced signal transduction in insulin producing cells. 相似文献
26.
Arne O. Smals Eldbjrg S. Heimstad Asbjrn Hordvik Nils P. Willassen Rune Male 《Proteins》1994,20(2):149-166
The crystal structure of an anionic form of salmon trypsin has been determined at 1.82 Å resolution. We report the first structure of a trypsin from a phoikilothermic organism in a detailed comparison to mammalian trypsins in order to look for structural rationalizations for the cold-adaption features of salmon trypsin. This form of salmon trypsin (T II) comprises 222 residues, and is homologous to bovine trypsin (BT) in about 65% of the primary structure. The tertiary structures are similar, with an overall displacement in main chain atomic positions between salmon trypsin and various crystal structures of bovine trypsin of about 0.8 Å. Intramolecular hydrogen bonds and hydrophobic interactions are compared and discussed in order to estimate possible differences in molecular flexibility which might explain the higher catalytic efficiency and lower thermostability of salmon trypsin compared to bovine trypsin. No overall differences in intramolecular interactions are detected between the two structures, but there are differences in certain regions of the structures which may explain some of the observed differences in physical properties. The distribution of charged residues is different in the two trypsins, and the impact this might have on substrate affinity has been discussed. © 1994 Wiley-Liss, Inc. 相似文献
27.
28.
This study describes the population structure of Norway spruce (Picea abies) as revealed by protein polymorphisms and morphological variation. Electrophoretically detectable genetic variability was examined at 22 protein loci in 70 populations from the natural range of the species in Europe. Like other conifers, Norway spruce exhibits a relatively large amount of genetic variability and little differentiation among populations. Sixteen polymorphic loci (73%) segregate for a total of 51 alleles, and average heterozygosity per population is 0.115. Approximately 5% of the total genetic diversity is explained by differences between populations (GST = 0.052), and Nei's standard genetic distance is less than 0.04 in all cases. We suggest that the population structure largely reflects relatively recent historical events related to the last glaciation and that Norway spruce is still in a process of adaptation and differentiation. There is a clear geographic pattern in the variation of allele frequencies. A major part of the allelefrequency variation can be accounted for by a few synthetic variables (principal components), and 80% of the variation of the first principal component is “explained” by latitude and longitude. The central European populations are consistently depauperate of genetic variability, most likely as an effect of severe restrictions of population size during the last glaciation. The pattern of differentiation at protein loci is very similar to that observed for seven morphological traits examined. This similarity suggests that the same evolutionary forces have acted upon both sets of characters. 相似文献
29.
Nils Empacher Volker Mosbrugger Anita Roth Matthias Wolf Arne Wunderlin 《Journal of biological physics》1995,21(4):241-264
We shall present several qualitative mathematical models to describe the early evolution of water transport systems in plants. To perform this in a systematic way we apply methods which have been developed in phenomenological synergetics. These methods rest on the fact that it becomes possible to describe the macroscopic behavior of a complex system by a set of control and order parameters when they are suitably identified. Our presentation is addressed to community with interdisciplinary interests. 相似文献
30.
Summary The effects of acetic acid and specific growth rate on acetic acid tolerance and trehalose content of Saccharomyces cerevisiae CBS 2806 were studied using anaerobic chemostat cultures. Cells grown in the presence of acetic acid at a defined specific growth rate showed a higher acetic acid tolerance and a slightly lower trehalose content. Cells grown at a low specific growth rate showed a lower energy demand, a higher acetic acid tolerance, and a higher trehalose content. These results indicate that trehalose plays a growth rate dependent role in the tolerance of S. cerevisiae to acetic acid. 相似文献