1,721,025 research outputs found
The Plasma Membrane Ca2+ ATPase and the Plasma Membrane Sodium Calcium Exchanger Cooperate in the Regulation of Cell Calcium
No full textCalcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the mM range)
Spectroscopic and functional properties of subunit III-depleted cytochrome oxidase
No full textBeef heart cytochrome c oxidase has been depleted of subunit I11 by treatment with chymotrypsin. The removal of subunit 111 has been evaluated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel fluorography of preparations of the oxidase labeled with [14C]dicyclohexylcarbodiimide prior to proteolysis. Removal of subunit I11 resulted in a perturbation of the visible spectrum of reduced cytochrome oxidase. Subunit III-depleted oxidase is spectroscopically very similar to the oxidase from Paracoccus denitrificans. When reconstituted into liposomes, the depleted enzyme still pumped protons in response to a pulse of reduced cytochrome c. The H+/e- stoichiometry averaged 0.5. Redox-linked proton translocation could be observed only when respiratory control ratios were higher than 3 and the reductant pulse was of a magnitude that allowed for no more than 5 turnovers of the oxidase
Expression of an active Na+/Ca2+ exchanger isoform lacking the six C-terminal transmembrane segments
No full textThe short isoform of the Na+/Ca2+ exchanger (67 kDa) that is produced by alternative splicing during the expression of the 6 kb canine exchanger cDNA in 293 cells was separately expressed in the same system. The protein consisted of the five N-terminal transmembrane segments and of a large portion of the main hydrophilic loop, but lacked the six C-terminal hydrophobic segments of the regular protein (108 kDa). Very high RNA levels were found after transient cell transfection with plasmid DNA encoding this truncated isoform. The RNA processing, the translation and targeting of the resulting protein to the plasma membrane appeared to be less efficient than those of the 108-kDa polypeptide produced in the same system. The Na(+)-dependent Ca(2+)-uptake activity of 293 cells expressing the short isoform was measured by an isotopic rapid filtration method, whereas the current associated with Ca2+ extrusion was measured in electrophysiological patch-clamp experiments. The results showed that the expressed isoform functioned in the typical reverse and forward Na+/Ca2+ exchange modes. In both the electrophysiological and the isotopic measurements the activity of the short isoform was 6-7-fold lower than that of the 108-kDa protein expressed in the same system. However, lower amounts of the short isoform reached the plasma membrane: its specific activity could thus be significantly higher. Possibly, the short isoform could form a dimer in which a second 67 kDa polypeptide replaces the C-terminal part of the 108-kDa protein
Electrophysiological characterization of ionic transport by the retinal exchanger expressed in human embryionic kidney cells
No full textThe retinal Na+:Ca2+, K+exchanger cDNA was transiently expressed in human embryonic kidney (HEK 293) cells by transfection with plasmid DNA. The correct targeting of the expressed protein to the plasma membrane was confirmed by immunocytochemistry. The reverse exchange offrent (Ca2+ imported per Na+ extruded) was measured in whole-cell voltage-clamp experiments after intracellular perfusion with Na+ (Na+i, 128 mM) and extracellular perfusion with Ca2+ (Ca2o+, 1 mM) and Ko+ (20 mM). As expected, the exchange current was suppressed by removing Ca2o+. Surprisingly, however, it was also abolished by increasing Na+o to almost abolish the Na+ gradient, and it was almost unaffected by the removal of Ko+. Apparently, then, at variance with the exchanger in the rod outer segment, the retinal exchanger expressed in 293 cells acts essentially as a Na+:Ca2+ exchanger and does not require K+ for its electrogenic activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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