用异龄移植研究有尾类胚胎表皮传导能力的出现和消失

两栖类胚胎的表皮,只是在一定的发育阶段具有传播兴奋的能力。有尾类胚胎表皮显示可兴奋性的期间,从分期26(早尾芽期)开始,直到分期37末(第一对外鳃出现分枝),某些部位甚至更晚些。无尾类的爪蟾和黑斑蛙胚胎,虽然其体形以及以此为准的分期与蝾螈的有所不同,表皮传播兴奋的期间,自分期24到分期40,和后者基本一致。     

Lipopolysaccharide promoted opening of the porin channel

We show here that the imipenem (a carbapenem, β-lactam antibiotic)-permeable porin channels (protein D2 or OprD2) of Pseudomonas aeruginosa were closed mostly in the lipopolysaccharide (LPS)-free membrane and were openable by adding LPS to the membrane as assayed by ion conductivity measurements using planar lipid bilayers. Open and closed states of the OprD2 channels exhibited conductivities of about 400 and 30 pS, respectively, in 1 M NaCl. The OprD2 channel in the LPS-containing membrane showed very rapid opening and closing events in a second order and the duration of closure became longer at low membrane potentials.     

The synthesis of some peptides related to the amyloid β peptide 25–35: Use of N-(2-hydroxy-4-methoxybenzyl) protection

Summary The sequence from residues 25–35 of the amyloid peptide has been identified as an important factor in the neurotoxicity displayed by this peptide. Attempts to synthesize analogues of this sequence using Fmoc-protected amino acids in solid phase synthesis were initially unsuccessful, with coupling stopping after Ala³⁰. By incorporating N-(2-hydroxy-4-methoxybenzyl)alanine at this position, total synthesis of the undecapeptide was accomplished. Success was also achieved by switching the synthesis to a commercial synthesiser which uses a flow system and improved solvent conditions.     

How the anisotropy of the intracellular and extracellular conductivities influences stimulation of cardiac muscle

The bidomain model, which describes the behavior of many electrically active tissues, is equivalent to a multi-dimensional cable model and can be represented by a network of resistors and capacitors. For a two-dimensional sheet of tissue, the intracellular and extracellular conductivity tensors can be visualized as two ellipses. For any pair of conductivity tensors, a coordinate transformation can be found that reduces the extracellular ellipse to a circle and aligns the intracellular ellipse with the coordinate axes. The eccentricity of the intracellular ellipse in this new coordinate system is an important parameter. It can have two special values: zero (in which case the tissue has equal anisotropy ratios) or one (in which case the tissue is comprised of one-dimensional fibers coupled through the two-dimensional extracellular space). Thus the bidomain model provides a unifying framework within which the electrical behavior of a wide variety of nerve and muscle tissues can be studied.When the anisotropy ratios in the intracellular and extracellular domains are not equal, stimulation with an anode always causes depolarization of some region of tissue. An analogous effect occurs in models that describe one-dimensional fibers, in which an activating function determines the site of stimulation. Experiments indicate that cardiac muscle does not have equal anisotropy ratios. Therefore, models developed to describe stimulation of axons may also help in understanding stimulation of two- or three-dimensional cardiac tissue, and may explain the concept of anodal stimulation of cardiac tissue through a virtual cathode.     

Dynamic hydrochemical and vegetation gradients in fens

The mixing of groundwater, river water and precipitation was studied in a discharge and a recharge fen in the Vechtplassen area, the Netherlands. The aim of the study was to characterize relationships between vegetation in fens, the chemical composition of the fen water, and the hydrologic regimes. We were particularly interested in the influence of polluted water from the river Vecht on vegetation and nutrient dynamics. Analyses were made for Ionic Ratio (IR = 2[Ca]/(2[Ca] + [Cl]), molar concentrations) and Electrical Conductivity (EC), all indices of the relative importance of each of the three main water sources: groundwater, river water and precipitation.During winter, water in the discharge fen was strongly influenced by calcium-rich groundwater (high IR, moderate EC), while the recharge fen was entirely fed by precipitation (low IR and EC). During summer, river water with a moderate IR and high EC infiltrated the fens and caused a dramatic change in the chemical composition of the fen water. In the discharge fen, infiltration occurred predominantly as sheetflow, causing inundation of the entire fen surface. River water infiltration affected the surface peat of this fen more severely than deeper peat layers. Spatial variation in water chemistry along the transect was small, and only one type of plant community was found. In the recharge fen, river water infiltration occurred in subsurface water, and rainwater that had accumulated during the winter persisted in the central parts of the fen. Gradients in fen water chemistry were correlated with the observed distribution of three plant associations. Differences in water chemistry could also be attributed to intra-site variation in the relative importance of the three water sources. In both fens, there was considerable temporal variation in the chemical composition of the fen water, clearly related to dynamics of the hydrologic regime, particularly infiltration of river water. There are indications that characteristic mesotraphent fen plant communities are negatively affected by water from the river Vecht. River water supply should therefore be avoided as much as possible if these fen plant communities are to be maintained.     

Mechanism of freeze-induced embolism in Fagus sylvatica L.

 The mechanism of freeze stress-induced embolism in Fagus sylvatica L. branches was analyzed under controlled conditions. Excised branches were exposed to successive freeze-thaw cycles in temperature controlled chambers. Thermocouples were placed on the bark to detect sap freezing exotherms. The degree of xylem embolism was estimated after each cycle by the loss of hydraulic conductivity. After one freeze-thaw cycle the degree of embolism was found to decrease with xylem specific hydraulic conductivity, small apical shoots being more susceptible to embolism. Exotherms revealed that distal shoots were freezing first and exuded sap as a result of water expansion. The lower water content in apical shoots upon freezing probably induced higher sap tensions which promoted air bubble expansion and vessel cavitation preferentially near the apices. When the decrease in water content was experimentally prevented, embolism developed to a lesser extent. The higher vulnerability of shoot apices may protect the rest of the branch from winter damage. Received: 29 May 1998 / Accepted: 15 August 1998     

Assessing aging impact on growth potential of Vitamin E primed soybean seeds via biochemical profiling

Soybean [Glycine max (L.) Merr.] is a high value crop owing to its nutrient rich profile, consisting of some of the largest reserve of proteins and oils among all plant crops. High yielding soybean variety seeds are of great intrinsic value as part of strategies to gain larger yield outputs over the years. These seeds often tend to lose their viability and corresponding storage period. The study is primarily focused on understanding and estimating the impact of storage conditions and influence of biochemical changes that leads to deteriorating seed health. Vitamin E is an essential compound that provides shielding effect to plant seeds against environmental stress. For this purpose seed priming of vitamin E was performed with a concentration of 300mgL^-1 applied on to seeds. A total of seven promising cultivars were accessed for this; including Swat-20, Swat-84, NARC-2. Malakand, Rawal, Ajmeri and FaisalSoy. Results shows that all cultivars tend to lose their yield potential which is greatly in line with storage induced biochemical changes. Among the cultivars, Swat-84 and Swat-20 were resilient to an extent towards harmful storage stress impact. The present study has shown that application of vitamin E seed coating tend to enhance positive traits in stored seeds (including concentrations of CAT, SOD, TSS, etc.) in comparison to untreated seeds showing a healthy impact of the treatment on seed health under storage conditions. It is suggested that storing vitamin E treated seeds under optimum conditions as an effective method for attaining viable seeds after long terms storage. Findings of the present study can be used for future studies, assessment and designing of seed storage system in a manner to reduce negative impact on seed growth potential during long term storages.     

A new approach to the determination of cardiac potential distributions: application to the analysis of electrode configurations

This paper presents a mathematical model and new solution technique for studying the electric potential in a slab of cardiac tissue. The model is based on the bidomain representation of cardiac tissue and also allows for the effects of fibre rotation between the epicardium and the endocardium. A detailed solution method, based on Fourier Series and a simple one-dimensional finite difference scheme, for the governing equations for electric potential in the tissue and the blood, is also presented. This method has the advantage that the potential can be calculated only at points where it is required, such as the measuring electrodes. The model is then used to study various electrode configurations which have been proposed to determine cardiac tissue conductivity parameters. Three electrode configurations are analysed in terms of electrode spacing, placement position and the effect of including fibre rotation: the usual surface four-electrode configuration; a single vertical analogue of this and a two probe configuration, which has the current electrodes on one probe and the measuring electrodes on the other, a fixed distance away. It is found that including fibre rotation has no effect on the potentials measured in the first two cases; however, in the two probe case, non-zero fibre rotation causes a significant drop in the voltage measured. This leads to the conclusion that it is necessary to include the effects of fibre rotation in any model which involves the use of multiple plunge electrodes.     

Study on fish embryo responses to the treatment of cryoprotective chemicals using impedance spectroscopy

Investigations using electrical impedance spectroscopy to measure the responses of fish embryos to the cryoprotective chemicals, methanol and dimethyl sulphoxide (DMSO), were carried out. Zebrafish (Danio rerio) embryos were used as a model to study the newly proposed technique. The normalised permittivity and conductivity changes of the embryos were measured continuously over a 20-min period in a customised embryo-holding chamber. The normalised permittivity and conductivity spectra were obtained during embryo exposure to different concentrations of methanol (1.0, 2.0 and 3.0 M) and DMSO (0.5, 1.0 and 2.0 M) solutions. The results showed significant permittivity and conductivity changes after embryo exposure to methanol and DMSO at the optimum embryo loading level (six embryos). Embryos in different concentrations of methanol and DMSO also resulted in quantitative responses shown in the normalised permittivity and conductivity spectra. The results demonstrated that fish embryo membrane permeability to cryoprotective chemicals could be monitored in real-time. The measurement of permittivity at a lower frequency range (10–10³ Hz) and conductivity at a higher frequency range (10⁴–10⁶ Hz) during fish embryo exposure to cryoprotective chemicals using impedance spectroscopy can be used as a new tool for the fast screening of most effective cryoprotective chemicals. The results from the present study also demonstrated the possibility of quantifying the level of cryoprotective chemicals penetrating the fish embryos.     

极端低温分布模型在邓恩桉抗寒性标准定量化应用的研究

依据闽北建阳市1971~2005年的极端低温资料构建极值分布数学模型, 用电导率指标确定邓恩桉(Eucalyptus dunnii)优株的半致死温度,预测0~5年、5~10年和10年以上可能出现的极端临界低温分别是-7.47℃、-7.47~-8.5℃和-8.68℃。首次提出利用极端低温分布模型结合电导率指标的方法定量划分林木的抗寒性, 称为“抗寒性极端低温分布法”, 该方法可广泛应用于不同区域、树种的抗寒适应性定量评价和林木个体的抗寒适应性筛选。