1,720,960 research outputs found
O2 availability modulates transmembrane Ca2+ flux via second-messenger pathways in anoxia-tolerant hepatocytes
No full textTransmembrane Ca(2+)-flux was studied from single isolated turtle hepatocytes by using a noninvasive Ca(2+)-selective self-referencing microelectrode. Cells in Ca(2+)-reduced culture medium demonstrated a vanadate- and lanthanum-inhibitable Ca(2+)-efflux of 4 x 10(-17) mol Ca2+. microns-2. s-1 continuously over 170 h. This flux diminished with 50 nM phorbol 12-myristate 13-acetate, a protein kinase C (PKC) activator, and was reinstated on PKC deactivation with sphingosine. Progressive hypoxia resulted in a reversible suppression of Ca2+ efflux to 90% of normoxic controls with an apparent Michaelis constant for oxygen of 145 microM. PKC activation was critical in this suppression, as anaerobic administration of sphingosine caused a Ca2+ influx and cell rupture. Hypoxia was also associated with an altered pattern of adenosine-mediated control over Ca2+ efflux. Adenosine (100 microM) elevated Ca2+ efflux twofold in normoxia, but neither adenosine nor the A1-purinoreceptor antagonist 8-phenyltheophylline altered the observed anaerobic suppression. Aerobic administration of 2-10 mM KCN failed to reproduce the anaerobic suppression; however, in conjunction with 10 mM iodoacetate, complete metabolic blockade caused a Ca2+ influx and cell rupture. These observations suggest modulatory control by oxygen over transmembrane Ca2+ efflux involving second-messenger systems in the hypoxic transition
The self-referencing oxygen-selective microelectrode: Detection of transmembrane oxygen flux from single cells
No full textA self-referencing, polarographic, oxygen-selective microelectrode was developed for measuring oxygen fluxes from single cells. This technique is based on the translational movement of the microelectrode at a known frequency through an oxygen gradient, between known points, The differential current of the electrode was converted into a directional measurement of flux using the Fick equation. Operational characteristics of the technique were determined using artificial gradients. Calculated oxygen flux values matched theoretical values derived from static measurements. A test preparation, an isolated neuron, yielded an oxygen flux of 11.46+/-1.43 pmol cm(-2) s(-1) (mean +/- S.E.M.), a value in agreement with those available in the literature for single cells. Microinjection of metabolic substrates or a metabolic uncoupler increased oxygen flux, whereas microinjection of KCN decreased oxygen flux. In the filamentous alga Spirogyra greveilina, the probe could easily differentiate a 16.6 % difference in oxygen flux with respect to the position of the spiral chloroplast (13.3+/-0.4 pmol cm(-2) s(-1) at the chloroplast and 11.4+/-0.4 pmol cm(-2) s(-1) between chloroplasts), despite the fact that these positions averaged only 10.6+/-1.8 mu m apart (means +/- S.E.M.). A light response experiment showed realtime changes in measured oxygen flux correlated with changes in lighting. Taken together, these results show that the self-referencing oxygen microelectrode technique can be used to detect local oxygen fluxes with a high level of sensitivity and spatial resolution in real time. The oxygen fluxes detected reliably correlated with the metabolic state of the cell
Dielectrophoretic assembly of insulinoma cells and fluorescent nanosensors into three-dimensional pseudo-islet constructs
Full text linkDielectrophoretic forces, generated by radio-frequency voltages applied to micromachined, transparent, indium tin oxide electrodes, have been used to condense suspensions of insulinoma cells (BETA-TC-6 and INS-1) into a 10times10 array of three-dimensional cell constructs. Some of these constructs, measuring ~150 mum in diameter, 120 mum in height and containing around 1000 cells, were of the same size and cell density as a typical islet of Langerhans. With the dielectrophoretic force maintained, these engineered cell constructs were able to withstand mechanical shock and fluid flow forces. Reproducibility of the process required knowledge of cellular dielectric properties, in terms of membrane capacitance and membrane conductance, which were obtained by electrorotation measurements. The ability to incorporate fluorescent nanosensors, as probes of cellular oxygen and pH levels, into these 'pseudo-islets' was also demonstrated. The footprint of the 10times10 array of cell constructs was compatible with that of a 1536 microtitre plate, and thus amenable to optical interrogation using automated plate reading equipment
Electrokinetic measurements of membrane capacitance and conductance for pancreatic -cells
Full text linkAuthor Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of IEE for personal use, not for redistribution. The definitive version was published in IEE Proceedings - Nanobiotechnology 152 (2005): 189-193, doi:10.1049/ip-nbt:20050040.Membrane capacitance and membrane conductance values are reported for insulin secreting cells (primary β-cells and INS-1 insulinoma cells) determined using the methods of dielectrophoresis and electrorotation. The membrane capacitance value of 12.57 (± 1.46) mF/m2 obtained for β-cells, and the values 9.96 (± 1.89) mF/m2 to 10.65 (± 2.1) mF/m2 obtained for INS-1 cells, fall within the range expected for mammalian cells. The electrorotation results for the INS-1 cells lead to a value of 36 (± 22) S/m2 for the membrane conductance associated with ion channels, if values in the range 2nS to 3 nS are assumed for the membrane surface conductance. This membrane conductance value falls within the range reported for INS cells obtained using the whole-cell patch-clamp technique. However, the total ‘effective’ membrane conductance value of 601 (± 182) S/m2 obtained for the INS-1 cells by dielectrophoresis is significantly larger (by a factor of around three-fold) than the values obtained by electrorotation. This could result from an increased membrane surface conductance, or increased passive conduction of ions through membrane pores, induced by the larger electric field stresses experienced by cells in the dielectrophoresis experiments.This study was financed by a gift from the Denis Robinson Memorial Fund to RP and NIH grants NCRR RR001395 and DK06984 to PJSS
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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