1,721,008 research outputs found
Biochemical consequences of electrical pacing in ischemic-reperfused isolated rat hearts
Full text linkIt is still unclear if performance recovery in postischemic hearts is related to their tissue level of high-energy phosphates before reflow. To test the existence of this link, we monitored performance, metabolism and histological damage in isolated, crystalloid-perfused rat hearts during 20 min of low-flow ischemia (90% coronary flow reduction) and reflow. To prevent interference from different ischemia times and perfusing media compositions, the ischemic ATP level was varied by changing energy demand (electrical pacing at 330 min-1). Under full coronary flow conditions, work output, as well as ATP and phosphocreatine contents were the same in control, spontaneously contracting (n = 23) and paced (n = 21) hearts. During low-flow ischemia, the higher work output (p < 0.0001) in paced hearts decreased their tissue content of ATP, phosphocreatine and total adenylates and purines (p < 0.05), as opposed to maintained values in control hearts. During reflow, the recovery of mechanical performance and O2 uptake was 94 ± 5% and 110 ± 9% (p = NS vs. baseline) in controls, vs. 71 ± 5% and 74 ± 6% in paced hearts (p < 0.004 vs. baseline). The levels of ATP and total adenylates and purines remained constant in control, but were markedly depressed (p < 0.05 vs. baseline) in paced hearts. Phosphocreatine+creatine was the same in both groups. These data, together with the observed lack of creatine kinase leakage and of structural damage, indicate that myocardial recovery during reflow reflects the tissue level of ATP, phosphocreatine and total adenylates and purines during ischemia, regardless of physical cell damage
High-energy phosphates metabolism and recovery in reperfused ischaemic hearts
Full text linkBackground. The aim of this study was to assess how coronary flow, oxygen supply and energy demand affect myocardial ATP, phosphocreatine and their metabolites during oxygen shortage and recovery. Methods. Isolated rat hearts were exposed for 20 min to either low-flow ischaemia or hypoxaemia at the same oxygen supply, followed by return to baseline conditions (20 min). Seventy-three hearts were divided into four groups: ischaemic or hypoxaemic, spontaneously beating or paced to increase energy demand. Results. During O2 shortage, myocardial performance was less in ischaemic, spontaneously beating hearts (SpIs), than in the other groups (14 ± 1% of baseline vs. 25-48%). Consequently, the tissue levels of ATP, total adenylates and phosphocreatine were maintained in SpIs, in contrast to marked decreases in the other groups. Upon reflow, the recovery of performance and of myocardial ATP was 94 ± 5% in SpIs (P = NS vs. baseline) compared with 64-85% (P < 0.05 vs. baseline) in the other groups. The degree of recovery was positively related to the ischaemic contents of ATP (P = 0.03) and adenylates (P = 0.001), but nor to that of phosphocreatine (P = NS). Conclusion. The maintenance of the ATP pool under low oxygen supply conditions is essential for good recovery. The most important factors that determine the ATP pool size are the energy demand, which increases the formation of diffusible ATP catabolites, and the coronary flow, which removes these catabolites, rather than the oxygen supply per se
REGULATION OF BIOENERGETICS IN O-2-LIMITED ISOLATED RAT HEARTS
Full text linkAssessing the role of O2 supply in the regulation of cardiac function in O2-limited hearts is crucial to understanding myocardial ischemic preconditioning and adaptation to hypoxia. We exposed isolated Langendorff- perfused rat hearts to either ischemia (low coronary flow) or hypoxemia (low PO2 in the perfusing medium) with matched O2 supply (10% of baseline). Myocardial contractile work and ATP turnover were greater in hypoxemic than in ischemic hearts (P < 0.05; n = 12). Thus, the energy demand was higher during hypoxemia than during ischemia, suggesting that ischemic hearts are more downregulated than hypoxemic hearts. Venous PO2 was 12 ± 2 and 120 ± 15 Torr (P < 0.0001) for ischemic and hypoxemic hearts, respectively, but O2 uptake was the same. Lactate release was higher during hypoxemia than during ischemia (9.7 ± 0.9 vs. 1.4 ± 0.2 μmol/min, respectively; P < 0.0001). Electrical stimulation (300 min-1; to increase energy demand) increased performance in ischemic (P < 0.005) but not in hypoxemic hearts without changes in venous PO2 or O2 uptake. However, venous lactate concentration and lactate release increased in ischemic (P < 0.002) but not in hypoxemic hearts, suggesting that anaerobic glycolysis provides the energy necessary to meet the increased energy demand in ischemic hearts only. We conclude that high intracellular lactate or H+ concentration during ischemia plays a major role as a downregulating factor. Downregulation disappears in hypoxemic hearts secondary to enhanced washout of lactate or H+
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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