1,720,979 research outputs found
Confocal Microscopic Analysis of Cellular Ultrastructure of the Diabetic Heart: Correlation with Mechanical Function
No full textBackground: Diabetic cardiomyopathy is often defined by compromised contractile
function and is associated with changes in excitation-contraction coupling (ECC)
organisation. Changes within diabetic cardiomyocytes often include structural alterations in
the proteins critical to ECC, including the ryanodine receptor (RyR2), junctophilin-2 (JPH2)
and the contractile myofilament. We hypothesise that in the Type 2 diabetic heart, the
ultrastructure’s of RyR2, JPH2 and the filamentous actin (F-actin) are altered and that these
protein alterations lead to contractile functional impairment.
Methodology: Type 2 diabetes was induced in rats with a high-fat diet and with a single
injection of low-dose streptozotocin. To test our hypotheses, we studied isolated
left-ventricular trabeculae. We measured a number of indices of mechanical performance
(active stress production, twitch duration, extent of shortening, shortening velocity,
shortening power, and work output) of the trabeculae. The same trabeculae were then imaged
by immunohistochemistry exploiting the 2D Airyscan confocal microscopy. Images were
segmented and processed to quantify the organisation of the structure including the degree
of spatial periodicity, area fraction, and localization of JPH2, RyR2, and F-actin proteins.
Results: We found that none of the indices of mechanical performance was affected by
diabetes. Effects of diabetes were seen on the three-protein ultrastructures. Whereas protein
spatial periodicity was lower in the diabetic trabeculae for all three proteins, area fraction
was lower in F-actin, preserved in JPH2, but higher in RyR2.
Conclusions: Collectively, these findings show that Type 2 diabetes causes changes in
protein organisation but without affecting cardiac mechanical function. This study provides
a lead for exploring possible compensatory mechanisms for the perseverance of mechanical
functional output in the face of impaired protein structures
Mechano-Energetics of Dynamically-Impeded Cardiac Trabeculae
No full textCardiac muscle contraction results in coordinated stress-length changes that correspond to ventricular pressure-volume changes. Each contraction cycle occurs in four sequential phases: isovolumic contraction, ejection, relaxation, and refilling, the dynamics of which are dictated by both the mechanics of the muscles and the load imposed by the cardiovascular system. Compared with the complex geometry of the whole-heart, isolated tissues allow a simpler 1-dimensional assessment of cardiac function. When subjecting isolated muscles to work-loop contractions that mimic the four phases of the cardiac cycle (isometric contraction, shortening, relaxation and re-stretching), the length-change phases (shortening and re-stretching) are conventionally simplified to result in ‘flat-topped’ work-loops. Such simplification overlooks the dynamic nature of the cardiovascular load as the tissues experienced in vivo.
In this project, I developed three model-based methods to improve the fidelity of mechanoenergetics experiments and measurements during work-loop contraction protocols designed for isolated cardiac tissues:
i. a 3-element Windkessel model to represent the impedance of the arterial system, thereby allowing model-based, dynamic shortening trajectories,
ii. a muscle-specific model to separate active from basal components of heat measurements, and,
iii. a 6-compartment haemodynamic model of the cardiovascular system to achieve dynamic, load-dependent, muscle re-stretching.
This study uncovers two major findings. Firstly, that the change of cardiac basal heat during muscle active contraction is muscle-specific and is not only length-dependent, but is also dependent on the rate of length-change. Secondly, the force-length work and mechanical energy efficiency of cardiac muscles in vitro have previously been restricted by simplified work-loop loading methods. Use of the methods in this thesis reveals that the boundaries of the ‘end-systolic zone’ were maintained, independent of load type, and confirmed the existence of shortening heat. The expansion of the real-time load to enable model-based
dynamic re-stretching allows, for the first time, synchronous variation of preload and afterload for a better representation of ventricular dynamics when performing in vitro work-loops. The real-time impedance-loading methods developed in this project reveal the limitations imposed by conventional, simplified, loading methods, and enable potential avenues for exploring the cardiac force-length space, physiologically and pathophysiologically
Impact of Mitochondrial Organisation on Cardiac Bioenergetics and Force Dynamics in Diabetic Myocardium
No full textFull Text is available to authenticated members of The University of Auckland only.The global prevalence of type 2 diabetes (T2D) remains substantial. T2D is characterised by
insulin resistance and is associated with altered cardiac mitochondrial ultrastructure and cellular
metabolism. However, it is unclear how these impairments collectively affect cardiac
bioenergetics and force dynamics at the cell and tissue levels. To uncover this uncertainty, T2D
was induced in rats with a 13-week high-fat (23.5%) diet and a single low-dose (27.5 mg/kg)
injection of streptozotocin. Left-ventricular wall tissues and trabeculae were dissected,
respectively, for evaluation of ultrastructure and force production. Wall tissue samples were fixed,
heavy metal stained, and resin-embedded. They were imaged using transmission electron
microscopy (TEM) to quantify mitochondrial and myofibril ultrastructure within cardiomyocytes.
We found a significantly lower mitochondrial fractional area (15%) and a complementary higher
myofibrillar fractional area (15%) in the diabetic tissues. These structural findings were not
translatable to the functional performance at the trabecula level, where isometric force production
was found to be preserved in diabetes.
We thus combined structural image analysis and computational modelling in an attempt to explain
these structure-function inconsistencies. We developed cell-specific finite element models of
cardiomyocyte ultrastructure based on our TEM images. We embedded a model of mitochondrial
respiration within our structural cell models, parameterised using data collected from oxidative
phosphorylation experiments, and a model of cross-bridge contraction. We used the integrated
model to explore whether observed diabetes-induced alterations in mitochondrial organisation and
respiration can reach the threshold for inducing contractile dysfunction.
Our simulation results reveal that mitochondrial structural derangement and depressed respiration
led to larger gradients of metabolite concentrations across the diabetic cell. However, these
alterations are not sufficient to bring about contractile dysfunction in diabetes
Mechanics and Energetics of Diabetic Human Cardiac Tissues
No full textDiabetic cardiomyopathy is a multifactorial disease associated with both mechanical and energetic dysfunction. At the cellular level, the interplay between myofilament force production and changes in metabolic state arising from diabetes is not well understood. Thus, the primary aim of this research was to explore the effects of diabetes on cross-bridge kinetics and metabolite sensitivity through the development of a human cardiac cross-bridge model that is responsive to cellular metabolic state.
In achieving this aim, I established an experimental-modelling pipeline centred around the measurement of cardiac active complex modulus: cross-bridge stiffness as a function of frequency. I used a purpose-built experimental apparatus to measure this in permeabilised isolated muscles across a range of metabolite conditions, first from rat hearts and then from human atria. Using linearisation techniques to uncover underlying physiological mechanisms, I constructed a mean-field cross-bridge model that was sensitive to ATP and Pi concentration, based on rat experimental data. Using the same framework, I then parameterised a human cross-bridge model to the data that I had collected from cardiac tissues of non-diabetic and diabetic patients. The human cross-bridge model was incorporated into a muscle model to explore the mechanical and energetic performance of diabetic muscle under isometric and work-loop contractions.
My development of this experimental-modelling pipeline produced insights into the complex modulus measurement and cross-bridge model properties, including those associated with metabolite sensitivity. From the experimental measurements and model simulations, I found several key differences in the cellular and tissue function of diabetic human atrial trabeculae. Diabetic muscles produced lower active stress and stiffness, with structural imaging linking this to lower myocyte density. They also exhibited a leftward shift in the complex modulus, identified in model fitting to be driven by slower cross-bridge cycling rates. Reflecting these parameter differences, muscle model simulations of isometric contractions revealed a prolonged relaxation phase in diabetes, which was exacerbated under reduced ATP concentration. Work-loop simulations showed that diabetes reduced work and shortening power but increased cross-bridge efficiency. A lower sensitivity to Pi in diabetic muscles diminished the extent to which muscle power was decreased under conditions of raised Pi. This reduced sensitivity and the increase in efficiency suggest the presence of compensatory mechanisms that mitigate the effects of metabolic dysfunction in the diabetic heart.
As well as identifying several mechanisms that underlie myofilament dysfunction in diabetic cardiomyopathy, in this project I have developed a suite of robust experimental and modelling methods that are suitable and will be broadly applicable for future studies of cross-bridge dysfunction in cardiac and metabolic disorders
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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