1,567 research outputs found

    Geochemistry and origin of explosive alkaline volcanism in the early Rukwa Rift, southwestern Tanzania

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    Leigh Lawrence studied the nature and origin of early rift magmatism in the Western Branch of the East African Rift. He discovered and modelled an entirely new mechanism of explosive volcanism and provided new insights into the geodynamic evolution of continental rift segments in sub-equatorial Eastern Africa

    Retinol improves development of heat-stressed oocytes during maturation

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    Addition of retinol may improve development of maturing oocytes stressed by culture. Objective was to evaluate retinol for improving development of heat-stressed oocytes. Cumulus oocyte complexes (COC) were matured for 24 h at 38.5 or 41.0°C (first 12 h) in 0 or 5 μM all trans-retinol. Ability of putative zygotes (268-297/treatment) to cleave and develop to blastocyst was assessed on days 3 and 8 after fertilization, respectively. Fixed effects in statistical model included temperature, retinol, and the interaction. Data were tested to ensure normality. Culture of COC at 4l.0°C did not affect lysis (1.6 and 2.1% for 38.5 and 41.0°C; SEM=0.8) or cleavage after fertilization (75.4 and 72.2% for 38.5 and 41.0°C; SEM=3.8). Within cleaved embryos however, culture of COC at 41.0°C increased 2-cell (10.6 versus 6.2% for 41.0 and 38.5°C; SEM= 1.3; P\u3c 0.04) but decreased 8-16 cell (41.4 versus 51.1% for 41.0 and 38.5°C, SEM=4.7; P\u3c 0.05) embryos and development to blastocyst (20.0 vs 27.5% for 41.0 and 38.5°C, SEM= 2.2; P\u3c0.007). Blastocysts derived from heat-stressed COC had fewer nuclei (85.3 versus 94.0 for 41.0 and 38.5°C; SEM=6.7; P \u3c 0.005; Hoechst staining). Addition of retinol during maturation did not improve development of COC regardless of culture temperature. In three replicates, effects of 41.0°C for reducing development of COC to blastocyst were minimal. When these replicates were deleted from analysis, a significant temperature x retinol interaction was observed (n= 4 reps; 189-197 COC/treatment) for development to blastocyst (P\u3c0.01) and number of nuclei (P\u3c0.004). Addition of retinol to COC compromised by heat stress during maturation prevented heat-induced reductions in development to blastocyst (24.6 versus 14.7% for 38.5 and 41.0°C; and 23.2 and 22.0% for 38.5 and 41.0°C cultured in presence of 5 μM retinol; SEM = 2.4). Also, culture of COC at 38.5°C in 5 μM retinol increased number of nuclei in blastocysts (102.6 versus 98.7 for 38.5 C and 41.0° C; and 120.5 versus 95.1 for 38.5 and 41.0°C cultured in 5 μM retinol; SEM= 5.0). Retinol may protect oocytes that are heat-stressed during maturation from the deleterious effects of elevated temperature

    The effect of chronic propranolol hydrochloride therapy on circulating free fatty acids and glucose during moderate, long-term exercise in post-myocardial infarct subjects.

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    Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1978 .L455. Source: Masters Abstracts International, Volume: 40-07, page: . Thesis (M.H.K.)--University of Windsor (Canada), 1978

    Joint evolution of a Lorentz-covariant massless scalar field and its point-charge source in one space dimension

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    In this paper we prove that the static solution of the Cauchy problem for a massless real scalar field that is sourced by a point charge in 1+11+1 dimensions is asymptotically stable under perturbation by compactly-supported incoming radiation. This behavior is due to the process of back-reaction. Taking the approach of Kiessling, we rigorously derive the expression for the force on the particle from the principle of total energy-momentum conservation. We provide a simple, closed form for the self-force resulting from back-reaction, and show that it is restorative, i.e. proportional to negative velocity, and causes the charge to return to rest after the radiation passes through. We establish these results by studying the joint evolution problem for the particle-scalar field system, and proving its global well-posedness and the claimed asymptotic behavior.Comment: 19 pages, 1 figur

    An investigation of the influence of silver doping on the intergranular 'weak-link' properties of the superconducting system Y1Ba2Cu307-x.

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    Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2006.High-temperature superconducting materials have found considerable technological application and still have a largely unrealised potential. The key to unlocking this potential depends on a better understanding of their properties; in particular, the maximum 'critical current density' which these materials, in the form of wires, tapes, thin-films and bulk monolithic forms, are able to support for high-current applications. The 'critical current density' in a polycrystalline high-temperature oxide superconductor system is determined by a percolation process of the super current through a three dimensional grain-boundary network. Grain-boundaries in these systems behave as Josephson junction 'weak-links' and they severely limit the critical current density in the presence of even moderate self or applied magnetic fields. In the present work, isothermal quasi-static magnetisation measurements on the polycrystalline YIBa2Cu307-x system are presented and analysed. An effective granular penetration depth in conjunction with a critical state model, which includes an approximate treatment for the percolation process, is used to obtain many of the salient physical parameters of the grain-boundary Josephson junctions and of the three-dimensional grain boundary junction network. Determination of the temperature and magnetic field dependence of several of these parameters, in particular a magnetic field-independent critical current which depends on the micro structure of the grain-boundary junction network, allows for testing and verification of models of the weak-link and network behaviour. This treatment has been carried out specifically on various silver doped polycrystalline Y1Ba2Cu30 7-x specimens in order to determine and quantify the effects of silver doping. An improvement in the critical current density with silver doping is explained in terms of silver scavenging and ridding grain-boundaries of impurities, and a proximity effect where trace amounts of silver residing in the grain-boundaries decreases the normal resistance of the grain-boundary Josephson junction. The insight gained from silver doping experimentation led to a macroscopic investigation into the joining of large single-domain YIBa2Cu307-x specimens for large-scale applications

    Use of Placebo in Supplementation Studies-Vitamin D Research Illustrates an Ethical Quandary.

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    History has shown that without explicit and enforced guidelines, even well-intentioned researchers can fail to adequately examine the ethical pros and cons of study design choices. One area in which consensus does not yet exist is the use of placebo groups in vitamin supplementation studies. As a prime example, we focus on vitamin D research. We aim to provide an overview of the ethical issues in placebo-controlled studies and guide future discussion about the ethical use of placebo groups. Research in the field of vitamin D shows variation in how placebo groups are used. We outline four types of control groups in use: active-control, placebo-control with restrictions on supplementation, placebo-control without supplementation restrictions, and placebo-control with rescue repletion therapy. The first two types highlight discrete ethical issues: active-control trials limit the ability to detect a difference; placebo-control trials that restrict supplementation potentially place subjects at risk of undue harm. The final two, placebo-control without supplementation restrictions or with rescue repletion therapy, offer potential solutions to these ethical challenges. Building on this, guidelines should be established and enforced on the use of placebo in supplementation studies. Furthermore, the field of vitamin D research has the potential to set an example worthy of emulation
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