Tortuous Cardiac Intercalated Discs Modulate Ephaptic Coupling

Cardiac ephaptic coupling, a mechanism mediated by negative electric potentials occurring in the narrow intercellular clefts of intercalated discs, can influence action potential propagation by modulating the sodium current. Intercalated discs are highly tortuous due to the mingling of plicate and interplicate regions. To investigate the effect of their convoluted structure on ephaptic coupling, we refined our previous model of an intercalated disc and tested predefined folded geometries, which we parametrized by orientation, amplitude and number of folds. Ephaptic interactions (assessed by the minimal cleft potential and amplitude of the sodium currents) were reinforced by concentric folds. With increasing amplitude and number of concentric folds, the cleft potential became more negative during the sodium current transient. This is explained by the larger resistance between the cleft and the bulk extracellular space. In contrast, radial folds attenuated ephaptic interactions and led to a less negative cleft potential due to a decreased net cleft resistance. In conclusion, despite limitations inherent to the simplified geometries and sodium channel distributions investigated as well as simplifications regarding ion concentration changes, these results indicate that the folding pattern of intercalated discs modulates ephaptic coupling

Localization of Na + channel clusters in narrowed perinexi of gap junctions enhances cardiac impulse transmission via ephaptic coupling: a model study

ABSTRACT: It has been proposed that when gap junctional coupling is reduced in cardiac tissue, action potential propagation can be supported via ephaptic coupling, a mechanism mediated by negative electric potentials occurring in narrow intercellular clefts of intercalated discs (IDs). Recent studies showed that sodium (Na(+)) channels form clusters near gap junction plaques in nanodomains called perinexi, where the ID cleft is even narrower. To examine the electrophysiological relevance of Na(+) channel clusters being located in perinexi, we developed a 3D finite element model of two longitudinally abutting cardiomyocytes, with a central Na(+) channel cluster on the ID membranes. When this cluster was located in the perinexus of a closely positioned gap junction plaque, varying perinexal width greatly modulated impulse transmission from one cell to the other, with narrow perinexi potentiating ephaptic coupling. This modulation occurred via the interplay of Na(+) currents, extracellular potentials in the cleft and patterns of current flow within the cleft. In contrast, when the Na(+) channel cluster was located remotely from the gap junction plaque, this modulation by perinexus width largely disappeared. Interestingly, the Na(+) current in the ID membrane of the pre‐junctional cell switched from inward to outward during excitation, thus contributing ions to the activating channels on the post‐junctional ID membrane. In conclusion, these results indicate that the localization of Na(+) channel clusters in the perinexi of gap junction plaques is crucial for ephaptic coupling, which is furthermore greatly modulated by perinexal width. These findings are relevant for a comprehensive understanding of cardiac excitation. KEY POINTS: Ephaptic coupling is a cardiac conduction mechanism involving nanoscale‐level interactions between the sodium (Na(+)) current and the extracellular potential in narrow intercalated disc clefts. When gap junctional coupling is reduced, ephaptic coupling acts in conjunction with the classical cardiac conduction mechanism based on gap junctional current flow. In intercalated discs, Na(+) channels form clusters that are preferentially located in the periphery of gap junction plaques, in nanodomains known as perinexi, but the electrophysiological role of these perinexi has never been examined. In our new 3D finite element model of two cardiac cells abutting each other with their intercalated discs, a Na(+) channel cluster located inside a narrowed perinexus facilitated impulse transmission via ephaptic coupling. Our simulations demonstrate the role of narrowed perinexi as privileged sites for ephaptic coupling in pathological situations when gap junctional coupling is decreased

Magnetic loop emergence within a granule

We investigate the temporal evolution of magnetic flux emerging within a granule in the quiet-Sun internetwork at disk center. We combined IR spectropolarimetry performed in two Fe I lines at 1565 nm with speckle-reconstructed G-band imaging. We determined the magnetic field parameters by a LTE inversion of the full Stokes vector using the SIR code, and followed their evolution in time. To interpret the observations, we created a geometrical model of a rising loop in 3D. The relevant parameters of the loop were matched to the observations where possible. We then synthesized spectra from the 3D model for a comparison to the observations. We found signatures of magnetic flux emergence within a growing granule. In the early phases, a horizontal magnetic field with a distinct linear polarization signal dominated the emerging flux. Later on, two patches of opposite circular polarization signal appeared symmetrically on either side of the linear polarization patch, indicating a small loop-like structure. The mean magnetic flux density of this loop was roughly 450 G, with a total magnetic flux of around 3x10^17 Mx. During the ~12 min episode of loop occurrence, the spatial extent of the loop increased from about 1 to 2 arcsec. The middle part of the appearing feature was blueshifted during its occurrence, supporting the scenario of an emerging loop. The temporal evolution of the observed spectra is reproduced to first order by the spectra derived from the geometrical model. The observed event can be explained as a case of flux emergence in the shape of a small-scale loop.Comment: 10 pages, 13 figures; accepted for Astronomy and Astrophysics; ps and eps figures in full resolution are available at http://www.astro.sk/~koza/figures/aa2009_loop

Active force generation contributes to the complexity of spontaneous activity and to the response to stretch of murine cardiomyocyte cultures.

Monolayer cultures of cardiac cells exhibit spontaneous electrical and contractile activity, as in a natural cardiac pacemaker. Beating variability in these preparations recapitulates the power-law behavior of heart rate variability in vivo. However, the effects of mechano-electrical feedback on beating variability are not yet fully understood. Using stretchable microelectrode arrays, we examined the effects of the contraction uncoupler blebbistatin and the non-specific stretch activated channel blocker streptomycin on beating variability and on stretch-induced changes of beat rate. Without stretch, blebbistatin decreased the spatial complexity of beating variability, while streptomycin had no effects. Both stretch and release transiently increased beat rate; blebbistatin attenuated the increase of beat rate upon stretch, while streptomycin had no effects. Active force generation contributes to the complexity of spatiotemporal patterns of beating variability and to the increase of beat rate upon mechanical deformation. Our study contributes to understanding how mechano-electric feedback influences heart rate variability. Cardiomyocyte cultures exhibit spontaneous electrical and contractile activity, as in a natural cardiac pacemaker. In such preparations, beat rate variability exhibits features similar to those of heart rate variability in vivo. Mechanical deformations and forces feedback on the electrical properties of cardiomyocytes, but it is not fully elucidated how this mechano-electrical interplay affects beating variability in such preparations. Using stretchable microelectrode arrays, we assessed the effects of the myosin inhibitor blebbistatin and the nonselective stretch-activated channel blocker streptomycin on beating variability and on the response of neonatal or foetal murine ventricular cell cultures against deformation. Spontaneous electrical activity was recorded without stretch and upon predefined deformation protocols (5% uniaxial and 2% equibiaxial strain, applied repeatedly for 1 min every 3 min). Without stretch, spontaneous activity originated from the edge of the preparations, and its site of origin switched frequently in a complex manner across the cultures. Blebbistatin did not change mean beat rate, but it decreased the spatial complexity of spontaneous activity. In contrast, streptomycin did not exert any manifest effects. During the deformation protocols, beat rate transiently increased upon stretch, but paradoxically also upon release. Blebbistatin attenuated the response to stretch, while this response was not affected by streptomycin. Therefore, our data support the notion that in a spontaneously firing network of cardiomyocytes, active force generation, rather than stretch-activated channels, is mechanistically involved in the complexity of the spatiotemporal patterns of spontaneous activity and in the stretch-induced acceleration of beating. Abstract figure legend Mechano-electric feedback modulates myocardial electrical function, including pacemaking. By growing monolayer cultures of spontaneously active murine cardiac cells on stretchable microelectrode arrays, we examined whether active contractions influence the spatiotemporal characteristics of beating variability and the effects of stretching on beat rate. Under control conditions (no stretch and no pharmacological agent), the origin of the electrical activity changed frequently. After blocking contractions with blebbistatin, the spatiotemporal pattern of electrical activity became less variable and less complex. Under control conditions (no pharmacological agent), stretching (and also releasing) the cardiomyocyte monolayers transiently increased beat rate. Blebbistatin attenuated the acceleration of beating upon stretch. In contrast, streptomycin had no detectable effects. Thus, active force generation is involved in determining beating variability in spontaneously active cardiac tissue. Possible mechanisms may include cellular processes that sense contraction and chemical messengers. Our study contributes to understanding how mechano-electric feedback influences heart rate variability. This article is protected by copyright. All rights reserved

Open questions on prominences from coordinated observations by IRIS, Hinode, SDO/AIA, THEMIS, and the Meudon/MSDP

Context. A large prominence was observed on September 24, 2013, for three hours (12:12 UT -15:12 UT) with the newly launched (June 2013) Interface Region Imaging Spectrograph (IRIS), THEMIS (Tenerife), the Hinode Solar Optical Telescope (SOT), the Solar Dynamic Observatory Atmospheric Imaging Assembly (SDO/AIA), and the Multichannel Subtractive Double Pass spectrograph (MSDP) in the Meudon Solar Tower. Aims. The aim of this work is to study the dynamics of the prominence fine structures in multiple wavelengths to understand their formation. Methods. The spectrographs IRIS and MSDP provided line profiles with a high cadence in Mg II and in Halpha lines. Results. The magnetic field is found to be globally horizontal with a relatively weak field strength (8-15 Gauss). The Ca II movie reveals turbulent-like motion that is not organized in specific parts of the prominence. On the other hand, the Mg II line profiles show multiple peaks well separated in wavelength. Each peak corresponds to a Gaussian profile, and not to a reversed profile as was expected by the present non-LTE radiative transfer modeling. Conclusions. Turbulent fields on top of the macroscopic horizontal component of the magnetic field supporting the prominence give rise to the complex dynamics of the plasma. The plasma with the high velocities (70 km/s to 100 km/s if we take into account the transverse velocities) may correspond to condensation of plasma along more or less horizontal threads of the arch-shape structure visible in 304 A. The steady flows (5 km/s) would correspond to a more quiescent plasma (cool and prominence-corona transition region) of the prominence packed into dips in horizontal magnetic field lines. The very weak secondary peaks in the Mg II profiles may reflect the turbulent nature of parts of the prominence.Comment: 15 pages, 14 figure

Observations of ozone production in a dissipating tropical convective cell during TC4

From 13 July–9 August 2007, 25 ozonesondes were launched from Las Tablas, Panama as part of the Tropical Composition, Cloud, and Climate Coupling (TC4) mission. On 5 August, a strong convective cell formed in the Gulf of Panama. World Wide Lightning Location Network (WWLLN) data indicated 563 flashes (09:00–17:00 UTC) in the Gulf. NO2 data from the Ozone Monitoring Instrument (OMI) show enhancements, suggesting lightning production of NOx. At 15:05 UTC, an ozonesonde ascended into the southern edge of the now dissipating convective cell as it moved west across the Azuero Peninsula. The balloon oscillated from 2.5–5.1 km five times (15:12–17:00 UTC), providing a unique examination of ozone (O3) photochemistry on the edge of a convective cell. Ozone increased at a rate of 1.6–4.6 ppbv/hr between the first and last ascent, resulting cell wide in an increase of (2.1–2.5)×106 moles of O3. This estimate agrees to within a factor of two of our estimates of photochemical lightning O3 production from the WWLLN flashes, from the radar-inferred lightning flash data, and from the OMI NO2 data (1.2, 1.0, and 1.7×106 moles, respectively), though all estimates have large uncertainties. Examination of DC-8 in situ and lidar O3 data gathered around the Gulf that day suggests 70–97% of the O3 change occurred in 2.5–5.1 km layer. A photochemical box model initialized with nearby TC4 aircraft trace gas data suggests these O3 production rates are possible with our present understanding of photochemistry

Evaluating the performance of low-energy feed forward osmosis system for desalination using impaired and saline water sources

Forward Osmosis (FO) is a natural process of treating water or wastewater due to the difference in osmotic pressures. FO is a membrane separation technology, applicable to food processing, industrial wastewater treatment and seawater or brackish water desalination. The phenomena of FO processes occur whereby water molecules are driven across a semi-permeable membrane by an osmotic pressure gradient that is generated from a higher concentrate draw solution. FO processes can recover potable water resources from wastewater streams through the flow of pure water from a lower concen-trated feed solution towards higher concentrated draw solutions leaving behind pollutants, impurities, and salts in the semi-permeable membrane. This paper assesses the design, build and testing of a laboratory scaled Feed Forward Osmosis (FFO) system for treating river water collected from the River Medway, Kent, England. The FO process was a highly effective form of river water treatment and able to treat the River Water with high rejection rates of solutes (>90%). Experimental results showed that the FFO system can achieve a better performance when the molarity of the draw solution is higher. The average solute rejection rate of the FO membrane for both inorganic and organic compounds was 94.83 %. Moreover, the operation of the forward osmosis membrane illustrated that it has a lower fouling propensity and higher solute rejection ca-pabilities. The pilot scaled FFO system has the ability for greater salt rejection and lower electronic conductivity levels which resulted from the successful desalination of river water. A sodium chloride (NaCl) or saltwater draw solution performed posi-tively in inducing higher osmotic pressures with a substantial effect of lower energy requirements for the system. Lower en-ergy consumptions of the FFO system allow similar water treatment possibilities with energy savings potential. The FFO system showed to be an environmentally viable and economically feasible river water treatment technology

Untersuchungen zum Einfluß ausgewählter Strukturparameter von Spanplatten auf die Schallemission bei Biegebelastung

Zusammenfassung: An einschichtigen, labortechnisch hergestellten Spanpiatten ausPinus radiata wurde der Einfluß der mittleren Plattenrohdichte und des Festharzanteiles auf die mechanischen Eigenschaften und die Schallemission geprüft. Die Impulssumme steigt mit zunehmendem Belastungsgrad an. Eine Erhöhung von Rohdichte und Festharzanteil bewirken einen Anstieg der freigesetzten Impulse. Zwischen der Impulssumme bei 40% der Bruchlast und dem E-Modul besteht eine straffe Korrelation, die allerdings auch durch die starke Dichte- bzw. Festigkeitsspreizung des Versuchsmaterials mitbedingt is