Adrenaline: mechanism of action on the pacemaker potential in cardiac Purkinje fibers

The pacemaker potential in Purkinje fibers is generated by a slow fall in potassium current which allows the inward background currents to depolarize the membrane. Adrenaline shifts the relation between activation of the potassium current and membrane potential in a depolarizing direction. Consequently, during the pacemaker potential, the potassium current falls more rapidly to lower values and the inward currents then depolarize the membrane more quickly. The shift in the potassium activation curve produced by adrenaline is large compared to that produced by calcium ions. The molecular action of adrenaline may involve either a large change in the surface charge of the membrane or a change in the dependence of the potassium permeability on the local electric field.     

Diffuse-double layer at a membrane-aqueous interface measured with x-ray standing waves

The ion distribution in an electrolyte solution in contact with a charged polymerized phospholipid membrane was directly measured with long-period x-ray standing waves. The 27-angstrom-thick lipid monolayer was supported on a tungsten/silicon mirror. X-ray standing waves were generated above the mirror surface by total external reflection of a 9.8-kiloelectron volt x-ray beam from a synchrotron undulator. The membrane surface, which contained negatively charged phosphate headgroups, was bathed in a dilute ZnCl2 solution. The concentration of Zn2+ in the condensed layer at the membrane surface and the Zn2+ distribution in the diffuse layer were measured as a function of headgroup charge. The Debye length of the diffuse layer varied between 3 and 58 angstroms. The results qualitatively agree with the Gouy-Chapman-Stern model.     

A role for the phagosome in cytokine secretion

Membrane traffic in activated macrophages is required for two critical events in innate immunity: proinflammatory cytokine secretion and phagocytosis of pathogens. We found a joint trafficking pathway linking both actions, which may economize membrane transport and augment the immune response. Tumor necrosis factor alpha (TNFalpha) is trafficked from the Golgi to the recycling endosome (RE), where vesicle-associated membrane protein 3 mediates its delivery to the cell surface at the site of phagocytic cup formation. Fusion of the RE at the cup simultaneously allows rapid release of TNFalpha and expands the membrane for phagocytosis.     

The direction of membrane lipid flow in locomoting polymorphonuclear leukocytes

The objective of this study was to determine the direction of membrane lipid flow in locomoting cells. The plasma membrane of human polymorphonuclear leukocytes was stained with a fluorescent lipid analog dihexadecanoyl indocarbocyanine. A line was photobleached on the cell surface perpendicular to the direction of cell motion. Low-light-level fluorescence microscopy and digital image-processing techniques were used to analyze a series of images taken at short intervals after photobleaching. The bleached line remained visible for about 5 seconds before being erased by diffusional recovery. Examination of fluorescence intensity profiles allowed a comparison to be made between the velocities of line and cell movement. Results indicate that the bleached line moves forward with the same velocity as the cell during locomotion, refuting the retrograde lipid flow model of locomotion. Instead, the plasma membrane lipid appears to move forward according to either the unit movement of membrane or the tank track model of locomotion.     

In vivo function and membrane binding properties are correlated for Escherichia coli lamB signal peptides

Wild-type and pseudorevertant signal peptides of the lamB gene product of Escherichia coli interact with lipid systems whereas a nonfunctional deletion mutant signal peptide does not. This conclusion is based on interaction of synthetic signal peptides with a lipid monolayer-water surface, conformational changes induced by presence of lipid vesicles in an aqueous solution of signal peptide, and capacities of the peptides to promote vesicle aggregation. Analysis of the signal sequences and previous conformational studies suggest that these lipid interaction properties may be attributable to the tendency of the functional signal peptides to adopt alpha-helical conformations. Although the possibility of direct interaction between the signal peptide and membrane lipids during protein secretion is controversial, the results suggest that conformationally related amphiphilicity and consequent membrane affinity of signal sequences are important for function in vivo.     

Cell corpse engulfment mediated by C. elegans phosphatidylserine receptor through CED-5 and CED-12

During apoptosis, phosphatidylserine, which is normally restricted to the inner leaflet of the plasma membrane, is exposed on the surface of apoptotic cells and has been suggested to act as an "eat-me" signal to trigger phagocytosis. It is unclear how phagocytes recognize phosphatidylserine. Recently, a putative phosphatidylserine receptor (PSR) was identified and proposed to mediate recognition of phosphatidylserine and phagocytosis. We report that psr-1, the Caenorhabditis elegans homolog of PSR, is important for cell corpse engulfment. In vitro PSR-1 binds preferentially phosphatidylserine or cells with exposed phosphatidylserine. In C. elegans, PSR-1 acts in the same cell corpse engulfment pathway mediated by intracellular signaling molecules CED-2 (homologous to the human CrkII protein), CED-5 (DOCK180), CED-10 (Rac GTPase), and CED-12 (ELMO), possibly through direct interaction with CED-5 and CED-12. Our findings suggest that PSR-1 is likely an upstream receptor for the signaling pathway containing CED-2, CED-5, CED-10, and CED-12 proteins and plays an important role in recognizing phosphatidylserine during phagocytosis.     

Membrane insertion of a potassium-channel voltage sensor

The mechanism of voltage gating in K+ channels is controversial. The paddle model posits that highly charged voltage-sensor domains move relatively freely across the lipid bilayer in response to membrane depolarization; competing models picture the charged S4 voltage-sensor helix as being shielded from lipid contact by other parts of the protein. We measured the apparent free energy of membrane insertion of a K+-channel S4 helix into the endoplasmic reticulum membrane and conclude that S4 is poised very near the threshold of efficient bilayer insertion. Our results suggest that the paddle model is not inconsistent with the high charge content of S4.     

Tetrodotoxin does not block excitation from inside the nerve membrane

Tetrodotoxin does not block the action potential or membrane sodium current when internally perfused through the giant axon of a squid at much higher concentrations than those required for blocking by external application. It is suggested that the gate for the sodium channel is located on the exterior surface of the axon, because tetrodotoxin is not lipid soluble.     

Sodium current in ventricular myocardial fibers

Membrane currents were measured in thin bundles of dog ventricular myocardium under voltage-clamp conditions. A rather large initial inward current which had an equilibrium potential at about + 55 millivolts could be recorded. When the external sodium concentration was reduced, the equilibrium potential for this current was shifted by the amount predicted theoretically for a current carried solely by sodium ions. The size of the sodium inward current (I(Na)) was largely dependent on the preceding membrane potential. The I(Na) was completely inactivated if the membrane potential was as low as -45 millivolts. Sodium ions are the main carrier of charge during the rapid depolarization phase of the action potential.     

Molecular basis of gating charge immobilization in Shaker potassium channels

Voltage-dependent ion channels respond to changes in the membrane potential by means of charged voltage sensors intrinsic to the channel protein. Changes in transmembrane potential cause movement of these charged residues, which results in conformational changes in the channel. Movements of the charged sensors can be detected as currents known as gating currents. Measurement of the gating currents of the Drosophila Shaker potassium channel indicates that the charge on the voltage sensor of the channels is progressively immobilized by prolonged depolarizations. The charge is not immobilized in a mutant of the channel that lacks inactivation. These results show that the region of the molecule responsible for inactivation interacts, directly or indirectly, with the voltage sensor to prevent the return of the charge to its original position. The gating transitions between closed states of the channel appear not to be independent, suggesting that the channel subunits interact during activation.     

Membrane phosphatidylserine regulates surface charge and protein localization

Electrostatic interactions with negatively charged membranes contribute to the subcellular targeting of proteins with polybasic clusters or cationic domains. Although the anionic phospholipid phosphatidylserine is comparatively abundant, its contribution to the surface charge of individual cellular membranes is unknown, partly because of the lack of reagents to analyze its distribution in intact cells. We developed a biosensor to study the subcellular distribution of phosphatidylserine and found that it binds the cytosolic leaflets of the plasma membrane, as well as endosomes and lysosomes. The negative charge associated with the presence of phosphatidylserine directed proteins with moderately positive charge to the endocytic pathway. More strongly cationic proteins, normally associated with the plasma membrane, relocalized to endocytic compartments when the plasma membrane surface charge decreased on calcium influx.     

Restricted lateral diffusion of PH-20, a PI-anchored sperm membrane protein

The rate of lateral diffusion of integral membrane proteins is constrained in cells, but the constraining factors for most membrane proteins have not been defined. PH-20, a sperm surface protein involved in sperm-egg adhesion, was shown to be anchored in the plasma membrane by attachment to the lipid phosphatidylinositol and to have a diffusion rate that is highly restricted on testicular sperm, being more than a thousand times slower than lipid diffusion. These results support the hypothesis that lateral mobility of a membrane protein can be regulated exclusively by interactions of its ectodomain.     

Compartmentalization of cyclic AMP during phagocytosis by human neutrophilic granulocytes

Immunocytochemistry shows that early during phagocytosis of zymosan, adenosine 3',5'-monophosphate (cyclic AMP) appears on the cell surface before the phagosome is internalized. The appearance of cyclic AMP on the cell surface is coincident with that of granule products and regulatory subunit of type I cyclic AMP-dependent protein kinase. Guanosine 3',5'-monophosphate is not associated with the initiation site of phagocytosis, but is observed throughout the granular cytoplasmic region. This sharply localized accumulation of cyclic AMP may serve as a signal for the initiation of phagocytosis.     

烤烟烘烤过程中叶片膜脂过氧化特性的研究

采用温湿度自动控制电热烤烟箱，研究了烘烤过程中烟叶膜脂过氧化及其内源保护酶活性的变化。结果表明，随烘烤时间延长，叶内超氧物歧化酶（ＳＯＤ）和过氧化物酶（ＰＯＤ）活性下降，丙二醛（ＭＤＡ）含量增加。烘烤４８ｈ和７２ｈ后，两种酶先后失活，ＭＤＡ积累达最大值，证明烟叶烘烤过程中叶片膜脂过氧化水平不断提高。不同处理以高温快烤烟叶膜脂过氧化水平最高，低温慢烤最低。而且变黄温度对烟叶膜脂过氧化的作用比定色期不同升温处理的作用大。     

金属离子-质膜相互作用的分子动力学模拟研究

为研究植物根表质膜与土壤溶液界面上的双电层结构对金属离子在质膜上吸附以及吸收的影响,从分子水平探讨质膜表面电势下金属离子与质膜的相互作用,建立了质膜与金属溶液的界面模型,采用分子动力学方法模拟了质膜界面K~+、Na~+、Ca~(2+)、Mg~(2+)等碱/碱土金属阳离子与Cu~(2+)、Cd~(2+)等重金属离子的结合与分布过程。通过对吸附构型、浓度分布等统计分析,发现阳离子在质膜表面主要与质膜头部的羰基和磷酸基团等极性基团结合;二价阳离子吸附作用比一价阳离子更为强烈,并造成质膜表面电势的反转,直接控制着扩散层中离子的分布特征。分子动力学模拟结果与描述界面双电层结构的Gouy-Chapman-Stern(GCS)经典理论很好地吻合,可以从分子水平预测和解释金属阳离子与质膜相互作用的动态过程。     

Surface area of human erythrocyte lipids: reinvestigation of experiments on plasma membrane

Ratios of the lipid monolayer area to the erythrocyte surface area are 2:1 at low surface pressures and approach 1: 1 at collapse pressures. Un saturated phospholipids in cholesterol-phospholipid complexes of membrane ex tracts resemble their saturated derivatives at collapse pressures. Area ratio and phospholipid area data are related by an equation that tests hypothetical values for molecular areas used in membrane models.     

植物膜脂过氧化作用和内源激素对UV-B辐射和稀土的响应

[目的]为稀土农用拓展及环境污染防治提供策略。[方法]采用水培法研究了大豆幼苗（Glycinemax）膜脂过氧化作用和内源激素对UV-B辐射和稀土的响应。[结果]UV-B（T1-0.15W/m2和T2-0.45W/m2）辐射下,大豆幼苗叶绿素（Chl）和吲哚乙酸（IAA）含量在胁迫期（1～5d）逐渐下降,在恢复期（6～11d）上升;丙二醛（MDA）和脱落酸（ABA）含量先增（1～5d）后减（6～11d）。20mg/LLa（Ⅲ）减缓了上述指标在胁迫期的下降/上升趋势,加快了恢复期的上升/下降速度。[结论]La（Ⅲ）对大豆幼苗膜脂过氧化作用和内源激素的调控能提高叶绿素和IAA含量,改善活性氧的新陈代谢,抑制膜脂过氧化作用,减少ABA积累,且对低剂量紫外辐射（T1）的防护效果优于高剂量紫外辐射（T2）。 更多还原     

Surface charge--induced ordering of the au(111) surface

Synchrotron surface x-ray scattering (SXS) studies have been carried out at the Au(lll)/electrolyte interface to determine the influence of surface charge on the microscopic arrangement of gold surface atoms. At the electrochemical interface, the surface charge density can be continuously varied by controlling the applied potential. The top layer of gold atoms undergoes a reversible phase transition between the (1 x 1) bulk termination and a (23 x radical3) reconstructed phase on changing the electrode potential. In order to differentiate the respective roles of surface charge and adsorbates, studies were carried out in 0.1 M NaF, NaCl, and NaBr solutions. The phase transition occurs at an induced surface charge density of 0.07 +/- 0.02 electron per atom in all three solutions.     

复配浸种对干旱胁迫下谷子萌发及幼苗生长的影响

以晋谷21号为试材,以含水量为9%的土壤模拟干旱胁迫,通过对谷子发芽率、发芽势、株高及渗透调节物质、膜脂过氧化、活性氧和保护酶活性等指标进行测定分析,研究GA3、稀土和脯氨酸复配浸种后在干旱胁迫下谷子萌发及幼苗生理特性的影响。结果表明,混合药剂浸种处理能够缓解干旱胁迫对种子萌发的抑制,提高种子发芽率和发芽势;同时提高谷子脯氨酸和可溶性蛋白含量,降低渗透势,促进种子吸水;诱导抗氧化系统相关酶活性上升;抑制干旱胁迫下谷子幼苗叶片O2-.的产生速率和H2O2的积累,降低MDA含量。说明GA3、稀土以及脯氨酸复配浸种能够提高谷子叶片抗氧化能力,减轻干旱胁迫诱导的膜脂过氧化损伤并改善干旱胁迫下谷子幼苗的渗透调节,从而提高谷子幼苗对干旱胁迫的适应性。     

Temperature and charge transfer in a receptor membrane

The rate of rise and the amplitude of a mechanically elicited generator potential in a receptor membrane (Pacinian corpuscle) increases markedly with temperature. By contrast, the amplitude of the action potential of the Ranvier node adjacent to the receptor membrane remains practically unchanged over a wide range of temperature. The activation energy of the rate-limiting process in excitation of the receptor membrane is high; it indicates the existence of a high potential energy barrier for charge transfer.