Minimal model for calcium alternans due to SR release refractoriness

In the heart, rapid pacing rates may induce alternations in the strength of cardiac contraction, termed pulsus alternans. Often, this is due to an instability in the dynamics of the intracellular calcium concentration, whose transients become larger and smaller at consecutive beats. This alternation has been linked experimentally and theoretically to two different mechanisms: an instability due to (1) a strong dependence of calcium release on sarcoplasmic reticulum (SR) load, together with a slow calcium reuptake into the SR or (2) to SR release refractoriness, due to a slow recovery of the ryanodine receptors (RyR2) from inactivation. The relationship between calcium alternans and refractoriness of the RyR2 has been more elusive than the corresponding SR Ca load mechanism. To study the former, we reduce a general calcium model, which mimics the deterministic evolution of a calcium release unit, to its most basic elements. We show that calcium alternans can be understood using a simple nonlinear equation for calcium concentration at the dyadic space, coupled to a relaxation equation for the number of recovered RyR2s. Depending on the number of RyR2s that are recovered at the beginning of a stimulation, the increase in calcium concentration may pass, or not, over an excitability threshold that limits the occurrence of a large calcium transient. When the recovery of the RyR2 is slow, this produces naturally a period doubling bifurcation, resulting in calcium alternans. We then study the effects of inactivation, calcium diffusion, and release conductance for the onset of alternans. We find that the development of alternans requires a well-defined value of diffusion while it is less sensitive to the values of inactivation or release conductance.Postprint (author's final draft

Chaotic motion of space charge wavefronts in semiconductors under time-independent voltage bias

A standard drift-diffusion model of space charge wave propagation in semiconductors has been studied numerically and analytically under dc voltage bias. For sufficiently long samples, appropriate contact resistivity and applied voltage - such that the sample is biased in a regime of negative differential resistance - we find chaos in the propagation of nonlinear fronts (charge monopoles of alternating sign) of electric field. The chaos is always low-dimensional, but has a complex spatial structure; this behavior can be interpreted using a finite dimensional asymptotic model in which the front (charge monopole) positions and the electrical current are the only dynamical variables.Comment: 12 pages, 8 figure

Mirandesa breed calves: growth performance and carcass characterization affected by sex and livestock production system

12 páginas, 8 tablas, 2 figuras.-- Under License of Creative Commons Attribution 3.0 LicenseThe effect of sex and livestock production system (LPS) on growth performance and carcass characteristics of Mirandesa breed calves was studied. A total of 2,263 and 60 animals for growth performance and carcass characteristics, respectively were used. Initially, the suitability of several equations to describe the growth of cattle from the Mirandesa variety was studied. In all situations, mathematical models were formulated with parameters of clear biological meaning. Bertalanffy and Gompertz equations were the most satisfactory equations to model the growth of males and females, respectively. Calves from traditional system were 68.61 kg heavier than animals from extensive system. On the other hand, calves from traditional systems showed (P<0.01) higher dressing percentage compared to animals from extensive system. There were significant differences for all morphometric parameters by LPS effect with the exception of internal depth pf chest, whereas sex effect had less importance. The LPS had statistically significant effect in all individual joints in the front quarter and hind quarter with the exception of sirloin and eye of round for front quarter and shoulder and the sum of chuck plus neck in the hind quarter. Finally, the LPS had statistically significant effect in all components of fifth quarter, with the exception of intestines, liver, tail and spleen, whereas sex effect only affected heart proportionPeer reviewe

Análisis de la demanda de antibióticos en farmacia comunitaria con receta privada, prescripción irregular y sin receta (automedicación): perfil de las farmacias y los farmacéuticos comunitarios participantes

Introducción: La resistencia a los antibióticos constituye uno de los mayores desafíos que afronta la medicina en la actualidad. Es una amenaza creciente para la salud pública que afecta a todos los países del mundo debido a que los nuevos mecanismos de resistencia se extienden a escala internacional.Objetivos: Analizar el perfil profesional y el nivel de participación de los farmacéuticos comunitarios que realizan el estudio.Describir el tipo de farmacia comunitaria, localización y nivel de participación.Material y métodos: Estudio descriptivo, observacional, prospectivo, de corte transversal y multicéntrico realizado en farmacias comunitarias españolas durante cuatro semanas, una por cada estación. Se diseñó un cuaderno de recogida de datos que recogía las siguientes variables: tipo de farmacéutico y farmacia, variables sociodemográficas, antibiótico demandado, tipo y razón de la demanda, especialidad del médico prescriptor, tipo de tratamiento, conocimiento del proceso de uso del antibiótico, problemas relacionados con el medicamento y resultados negativos asociados a la medicación identificados, actuación e intervención del farmacéutico.Se diseñó una plataforma web http://investigacionsefac.org/antibioticos/ para la recogida de datos.Resultados: Registraron encuestas 341 farmacéuticos (57,3% titulares y 35,6% adjuntos) pertenecientes a 247 farmacias (65,6% farmacias de barrio).Conclusiones: Los farmacéuticos inscritos en este estudio son mayoritariamente adjuntos del sexo femenino y los más participativos titulares.Las comunidades autónomas (CCAA) con mayor número de farmacias participantes han sido C. Valenciana, Cataluña y Madrid y las que más han registrado han sido las farmacias de la Comunidad Valenciana.Las farmacias tipificadas como de barrio han sido mayoritarias y las que han registrado mayor número de casos

An isotope dilution model for partitioning of phenylalanine and tyrosine uptake by the liver of lactating dairy cows

An isotope dilution model to describe the partitioning of phenylalanine (PHE) and tyrosine (TYR) in the bovine liver was developed. The model comprises four intracellular and six extracellular pools and various flows connecting these pools and external blood. Conservation of mass principles were applied to generate the fundamental equations describing the behaviour of the system in the steady state. The model was applied to datasets from multi-catheterised dairy cattle during a constant infusion of [1-13C] phenylalanine and [2,3,5,6-2H] tyrosine tracers. Model solutions described the extraction of PHE and TYR from the liver via the portal vein and hepatic artery. In addition, the exchange of free PHE and TYR between extracellular and intracellular pools was explained and the hydroxylation of PHE to TYR was estimated. The model was effective in providing information about the fates of PHE and TYR in the liver and could be used as part of a more complex system describing amino acid metabolism in the whole animal

Description of the first-stage juveniles of Xiphinema cretense and X. herakliense - Distribution of Xiphinema and Longidorus species in olive orchards and grapevines in Crete, Greece

The occurrence of nematodes of the family Longidoridae was investigated in soil samples collected from cultivated and wild olives and grapevines in Crete. The first-stage juveniles of Xiphinema cretense and X. herakliense are described for the first time. The species X. israeliae, X. cretense, X. herakliense and Longidorus pseudoelongatus, previously recorded exclusively from olives in Crete, are herein reported in the rhizosphere of grapevines. Also L. iranicus is reported for the first time in cultivated olive, while X. italiae and L. closelongatus are reported for the first time in wild olive in Crete. Data on the occurrence of phytoparasitic nematode species in cultivated olives, wild olives and grapevines are updated with those previously published.This research was supported by grant KBBE 219262 ArimNET-ERANET FP7 2012-2015 Project PESTOLIVE ‘Contribution of olive history for the management of soilborne parasites in the Mediterranean basin’ from Hellenic Agricultural Organization-DEMETER and Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA), grant AGR-136 from ‘Consejería de Economía, Innvovación y Ciencia’ from Junta de Andalucía, and Union Europea, Fondo Europeo de Desarrollo regional, “Una manera de hacer Europa”. I.G. Birmpilis and E. Nasiou were employed by the ARIMNET-PESTOL-IVE project.Peer reviewe

Universality of the Gunn effect: self-sustained oscillations mediated by solitary waves

The Gunn effect consists of time-periodic oscillations of the current flowing through an external purely resistive circuit mediated by solitary wave dynamics of the electric field on an attached appropriate semiconductor. By means of a new asymptotic analysis, it is argued that Gunn-like behavior occurs in specific classes of model equations. As an illustration, an example related to the constrained Cahn-Allen equation is analyzed.Comment: 4 pages,3 Post-Script figure

Dynamics of Electric Field Domains and Oscillations of the Photocurrent in a Simple Superlattice Model

A discrete model is introduced to account for the time-periodic oscillations of the photocurrent in a superlattice observed by Kwok et al, in an undoped 40 period AlAs/GaAs superlattice. Basic ingredients are an effective negative differential resistance due to the sequential resonant tunneling of the photoexcited carriers through the potential barriers, and a rate equation for the holes that incorporates photogeneration and recombination. The photoexciting laser acts as a damping factor ending the oscillations when its power is large enough. The model explains: (i) the known oscillatory static I-V characteristic curve through the formation of a domain wall connecting high and low electric field domains, and (ii) the photocurrent and photoluminescence time-dependent oscillations after the domain wall is formed. In our model, they arise from the combined motion of the wall and the shift of the values of the electric field at the domains. Up to a certain value of the photoexcitation, the non-uniform field profile with two domains turns out to be metastable: after the photocurrent oscillations have ceased, the field profile slowly relaxes toward the uniform stationary solution (which is reached on a much longer time scale). Multiple stability of stationary states and hysteresis are also found. An interpretation of the oscillations in the photoluminescence spectrum is also given.Comment: 34 pages, REVTeX 3.0, 10 figures upon request, MA/UC3M/07/9