1,720,973 research outputs found
Rate dependency of repolarization in rat myocytes paced with linearly changing cycle length sequences
No full textAim: Action potential duration at -60 mV (APD60) of isolated rat cardiomyocytes, displays rate dependency, i.e. increasing in pacing rate results in both APD60 rate-dependent shortening (RDS) or prolongation (RDP) and vice-versa . The aim of this work is to investigate how APD60 correlates with the preceding cycle length (CL) when this is made changing linearly.
Methods: Action potentials were recorded from patch-clamped rat cardiomyocytes in current clamp mode. Starting from a basic cycle length (BCL) of 250 ms, CL was linearly first increased, and then decreased within a range of BCL ±20% by steps of 5 ms (protocol A), or BCL±12% by steps of 3 ms (protocol B), and finally returned to BCL. Both protocols were tested in turn in Normal Tyrode (NT), 5 mM 4-aminopyridine (4AP) or 10 μM Nifedipine.
Results: In 6 cells with RDS following protocol A, the correlation coefficient (R) between APD60 and preceding CL calculated in NT over the entire CL frequency (R=0.5) was significantly increased in 4AP (R=0.84) and decreased in Nifedipine (R= -0.13). Same results were found in protocol B, both for NT vs 4AP (R=0.45 vs 0.8) and NT vs Nifedipine (R=0.45 vs -0.04). Interestingly, even in 2 cells that displayed a slight RDP in NT during protocol A, application of 4AP resulted in a RDS (R= -0.29 vs 0.71).
Conclusion: The tendency of APD60 to correlate with a linearly changing preceding CL can be pharmacologically modulated. Specifically our results suggest a role of Ito and ICaL in respectively decrease and increase correlation between APD60 and CL. These results might be relevant for the understanding of the mechanisms of class III prolonging and class IV shortening antiarrhythmic drugs
Adaptation of rat ventricular action potential repolarization to random fluctuations in pacing rate
No full textAim: Action potential duration (APD) of rat cardiomyocytes displays both adaptation to pacing cycle length (CL) and beat-to-beat variability (BTBV). The aim of this study is to investigate both properties via classical rate-dependency and restitution protocols as well as by means of a new random stimulation protocol.
Methods: Patch-clamped cells were paced in current-clamp mode at a basic cycle length (BCL) of 250 ms, then underwent one of the following protocols: a) CL was made change randomly around BCL with percentages of variability (CLR) from 2 to 10% (step 2%), in turn in Normal Tyrode (NT), 5 mM 4-aminopyridine (4AP) or 10 M Nifedipine; b) CL was switched from BCL to different constant values up to 20% (step 4%). c) BCL trains were followed by pre/postmature stimuli with different delays. APD was measured at -60 mV (APD60).
Results: a) BTBV calculated as coefficient of variability (CV) of APD60 in NT was 1.16 at BCL, and increased for CLR >4%. 4AP decreased CV at BCL (0.81), while Nifedipine increased it (1.59). Correlation between 10 consecutive APD60 and preceding CLs at CLR=10% increased in 4AP (R = 0.72) and decreased in Nifedipine (R = 0.18) with respect to NT (R = 0.43). In b) and c) protocols, changes in pacing CL became detectable as APD60 changes only when >5%.
Conclusion: BTBV masks small random changes (4%) in CL, setting up a limit for the detection of APD60 rate-dependency. 4AP and Nifedipine respectively increase and decrease correlation between APD60 and the preceding CL. This finding might help to clarify the still not completely understood mechanism of class III prolonging and class IV shortening antiarrhythmic drugs, which display in vivo similar electrical effects
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
Temporal variability of repolarization in rat ventricular myocytes paced with time-varying frequencies.
No full textAdaptation of action potential duration (APD) to pacing cycle length (CL) has been previously characterized in isolated cardiomyocytes for sudden changes in constant CL and for pre-/postmature stimuli following constant pacing trains. However, random fluctuations characterize both physiological sinus rhythm (up to 10% of mean CL) and intrinsic beat-to-beat APD at constant pacing rate. We analysed the beat-to-beat sensitivity of each APD to the preceding CL during constant–sudden, random or linearly changing pacing trains in single patch clamped rat left ventricular myocytes, in the absence of the autonomic and electrotonic effects that modulate rate dependency in the intact heart. Beat-to-beat variability of APD at −60 mV (APD−60 mV), quantified as s.d. over 10-beat sequences, increased with corresponding mean APD. When measured as coefficient of variability (CV), APD−60 mV variability was inversely proportional to pacing frequency (from 1.2% at 5 Hz to 3.2% at 0.2 Hz). It was increased, at a basic CL (BCL) of 250 ms, by 55% by the L-type calcium current (ICaL) blocker nifedipine, and decreased by 23% by the transient-outward potassium current (Ito) blocker 4-aminopyridine. Variability of APD at BCL of 250 ms prevented the detection of random changes of CL smaller than ∼5%. Ten per cent random changes in CL were detected as a 40% increase in CV of APD and tended to correlate with it (r = 0.43). Block of ICaL depressed this correlation (r = 0.23), whereas block of Ito significantly increased it (r = 0.67); this was similar with linearly changing CL ramps (ranging ±10% and ±20% of 250 ms). We conclude that beat-to-beat APD variability, a major determinant of the propensity for development of arrhythmia in the heart, is present in isolated myocytes, where it is dependent on mean APD and pacing rate. Action potential duration shows a beat-to-beat positive correlation with preceding randomly/linearly changing CL, which can be pharmacologically modulated
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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