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71.
The competition for glucose between Escherichia coli ML30, a typical copiotrophic enterobacterium and Chelatobacter heintzii ATCC29600, an environmentally successful strain, was studied in a carbon-limited culture at low dilution rates. First, as a base for modelling, the kinetic parameters μmax and Ks were determined for growth with glucose. For both strains, μmax was determined in batch culture after different precultivation conditions. In the case of C. heintzii, μmax was virtually independent of precultivation conditions. When inoculated into a glucose-excess batch culture medium from a glucose-limited chemostat run at a dilution rate of 0.075 h−1
C. heintzii grew immediately with a μmax of 0.17±0.03 h−1. After five transfers in batch culture, μmax had increased only slightly to 0.18±0.03 h−1. A different pattern was observed in the case of E. coli. Inoculated from a glucose-limited chemostat at D=0.075 h−1 into glucose-excess batch medium E. coli grew only after an acceleration phase of ∼3.5 h with a μmax of 0.52 h−1. After 120 generations and several transfers into fresh medium, μmax had increased to 0.80±0.03 h−1. For long-term adapted chemostat-cultivated cells, a Ks for glucose of 15 μg l−1 for C. heintzii, and of 35 μg l−1 for E. coli, respectively, was determined in 14C-labelled glucose uptake experiments. In competition experiments, the population dynamics of the mixed culture was determined using specific surface antibodies against C. heintzii and a specific 16S rRNA probe for E. coli. C. heintzii outcompeted E. coli in glucose-limited continuous culture at the low dilution rates of 0.05 and 0.075 h−1. Using the determined pure culture parameter values for Ks and μmax, it was only possible to simulate the population dynamics during competition with an extended form of the Monod model, which includes a finite substrate concentration at zero growth rate (smin). The values estimated for smin were dependent on growth rate; at D=0.05 h−1, it was 12.6 and 0 μg l−1 for E. coli and C. heintzii, respectively. To fit the data at D=0.075 h−1, smin for E. coli had to be raised to 34.9 μg l−1 whereas smin for C. heintzii remained zero. The results of the mathematical simulation suggest that it is not so much the higher Ks value, which is responsible for the unsuccessful competition of E. coli at low residual glucose concentration, but rather the existence of a significant smin. 相似文献
72.
The purpose of this paper is to study the stability of steady state solutions of the Monodomain model equipped with Luo-Rudy I kinetics. It is well established that re-entrant arrhythmias can be created in computational models of excitable cells. Such arrhythmias can be initiated by applying an external stimulus that interacts with a partially refractory region, and spawn breaking waves that can eventually generate extremely complex wave patterns commonly referred to as fibrillation. An ectopic wave is one possible stimulus that may initiate fibrillation. Physiologically, it is well known that ectopic waves exist, but the mechanism for initiating ectopic waves in a large collection of cells is poorly understood. In the present paper we consider computational models of collections of excitable cells in one and two spatial dimensions. The cells are modeled by Luo-Rudy I kinetics, and we assume that the spatial dynamics is governed by the Monodomain model. The mathematical analysis is carried out for a reduced model that is known to provide good approximations of the initial phase of solutions of the Luo-Rudy I model. A further simplification is also introduced to motivate and explain the results for the more complicated models. In the analysis the cells are divided into two regions; one region (N) consists of normal cells as model by the standard Luo-Rudy I model, and another region (A) where the cells are automatic in the sense that they would act as pacemaker cells if they where isolated from their surroundings. We let delta denote the spatial diffusion and a denote a characteristic length of the automatic region. It has previously been shown that reducing diffusion or increasing the automatic region enhances ectopic activity. Here we derive a condition for the transition from stable resting state to ectopic wave spread. Under suitable assumptions on the model we provide mathematical and computational arguments indicating that there is a constant eta such that a steady state solution of this system is stable whenever delta approximately > etaa(2), and unstable whenever delta approximately < etaa(2). 相似文献
73.
In order to elucidate the mechanisms of purinergic transmission of calcium (Ca2 + ) waves between microglial cells, we have employed micro-photolithographic methods to form discrete patterns of microglia
that allow quantitative measurements of Ca2 + wave propagation. Microglia were confined to lanes 20–100 wide and Ca2 + waves propagated from a point of mechanical stimulation, with a diminution in amplitude, for about 120 . The number of cells participating in propagation also decreased over this distance. Ca2 + waves could propagate across a cell-free lane from one microglia lane to another if this distance of separation was less
than about 60 , indicating that propagation involved diffusion of a chemical transmitter. This transmitter was identified as ATP since all
Ca2 + wave propagation was blocked by the purinoceptor antagonist suramin, which blocks P2Y2 and P2Y12 at relatively low concentrations. Antibodies to P2Y12 showed these at very high density compared with P2Y2, indicating a role for P2Y12 receptors. These observations were quantitatively accounted for by a model in which the main determinants are the diffusion
of ATP released from a stimulated microglial cell and differences in the dissociation constant of the purinoceptors on the
microglial cells. 相似文献
74.
Native and high pressure-treated (water suspensions, 650 MPa) waxy maize starch, containing mainly amylopectin, and Hylon VII, rich in amylose, were studied for their ability to generate free radicals upon thermal treatment at 180–230 °C. The electron paramagnetic resonance (EPR) spectroscopy was used to characterize the nature, number and stability of radicals. Various stable and short living (stabilized by N-tert-butyl-α-phenylnitrone (PBN) spin trap) radical species were formed. It was found, that at given conditions the waxy maize starch reveals higher ability to generate radicals, than Hylon VII. The presence of water and high pressure pretreatment of starches, both resulted in the reduction of the amount of thermally generated radicals. The decrease in crystallinity of waxy maize starch and of Hylon VII, occurring upon high pressure treatment, leads to the increase of the relative amount of fast rotating component in the EPR spectrum of both types of starches. 相似文献
75.
Li Fei Hu Tianojiang Wang Guang-ming Shen Lin-cheng 《仿生工程学报(英文版)》2005,2(3):115-121
In addition to forward undulatory swimming, Gymnarchus niloticus can swim via undulations of the dorsal fin while the body axis remains straight; furthermore, it swims forward and backward in a similar way, which indicates that the undulation of the dorsal fin can simultaneously provide bidirectional propulsive and maneuvering forces with the help of the tail fin. A high-resolution Charge-Coupled Device (CCD) imaging camera system is used to record kinematics of steady swimming as well as maneuvering in G. niloticus. Based on experimental data, this paper discusses the kinematics (cruising speed, wave speed, cycle frequency, amplitude, lateral displacement) of forward as well as backward swimming and maneuvering. During forward swimming, the propulsive force is generated mainly by undulations of the dorsal fin while the body axis remains straight. The kinematic parameters (wave speed, wavelength, cycle frequency, amplitude) have statistically significant correlations with cruising speed. In addition, the yaw at the head is minimal during steady swimming. From experimental data, the maximal lateral displacement of head is not more than 1% of the body length, while the maximal lateral displacement of the whole body is not more than 5% of the body length. Another important feature is that G. niloticus swims backwards using an undulatory mechanism that resembles the forward undulatory swimming mechanism. In backward swimming, the increase of lateral displacement of the head is comparatively significant; the amplitude profiles of the propulsive wave along the dorsal fin are significantly different from those in forward swimming. When G. niloticus does fast maneuvering, its body is first bent into either a C shape or an S shape, then it is rapidly unwound in a travelling wave fashion. It rarely maneuvers without the help of the tail fin and body bending. 相似文献
76.
77.
《Cell calcium》2016,59(6):549-557
BackgroundThe role of the serotonin receptor 4 (5-HT4R) pathway in cardiac excitation-contraction coupling (ECC) remains unclear. In the brain, induction of the calcium (Ca2+)-binding protein p11 enhances 5-HT4R translocation and signaling and could therefore be considered as a modulator of the 5-HT4R pathway in the myocardium. p11 expression is increased by brain-derived neurotrophic factor (BDNF) or antidepressant drugs (imipramine). Thus, we investigated whether p11 regulates the 5-HT4R pathway in the heart in physiological conditions or under pharmacological induction and the effects on calcium handling.Methods and resultsp11 expression was induced in vivo in healthy Wistar rats by imipramine (10 mg/kg/21 days) and in vitro in left ventricular cardiomyocytes exposed to BDNF (50 ng/ml/8 h). Cell shortening and real-time Ca2+ measurements were processed on field-stimulated intact cardiomyocytes with the selective 5-HT4R agonist, prucalopride (1 μM). Both imipramine and BDNF-induced cardiomyocyte p11 expression unmasked a strong response to prucalopride characterized by an increase of both cell shortening and Ca2+ transient amplitude compared to basal prucalopride associated with a high propensity to trigger diastolic Ca2+ events. Healthy rats treated with BDNF (180 ng/day/14 days) exhibited a sustained elevated heart rate following a single injection of prucalopride (0.1 mg/kg) which was not observed prior to treatment.ConclusionsWe have identified a novel role for p11 in 5-HT4R signaling in healthy rat ventricular cardiomyocytes. Increased p11 expression by BDNF and imipramine unraveled a 5-HT4R-mediated modulation of cardiac Ca2+ handling and ECC associated with deleterious Ca2+ flux disturbances. Such mechanism could partly explain some cardiac adverse effects induced by antidepressant treatments. 相似文献
78.
79.
M. Wall L. Putchim G. M. Schmidt C. Jantzen S. Khokiattiwong C. Richter 《Proceedings. Biological sciences / The Royal Society》2015,282(1799)
Tropical scleractinian corals are particularly vulnerable to global warming as elevated sea surface temperatures (SSTs) disrupt the delicate balance between the coral host and their algal endosymbionts, leading to symbiont expulsion, mass bleaching and mortality. While satellite sensing of SST has proved a reliable predictor of coral bleaching at the regional scale, there are large deviations in bleaching severity and mortality on the local scale that are poorly understood. Here, we show that internal waves play a major role in explaining local coral bleaching and mortality patterns in the Andaman Sea. Despite a severe region-wide SST anomaly in May 2010, frequent upslope intrusions of cold sub-pycnocline waters due to breaking large-amplitude internal waves (LAIW) mitigated coral bleaching and mortality in shallow waters. In LAIW-sheltered waters, by contrast, bleaching-susceptible species suffered severe bleaching and total mortality. These findings suggest that LAIW benefit coral reefs during thermal stress and provide local refugia for bleaching-susceptible corals. LAIW are ubiquitous in tropical stratified waters and their swash zones may thus be important conservation areas for the maintenance of coral diversity in a warming climate. Taking LAIW into account can significantly improve coral bleaching predictions and provide a valuable tool for coral reef conservation and management. 相似文献
80.
《Current biology : CB》2020,30(18):3657-3663.e5