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External Medium (external + medium)
Selected AbstractsEffects of ouabain on contractions induced by manganese ions in Ca2+ -free, isotonic solutions with varying concentrations of K+ in guinea-pig taenia coliFUNDAMENTAL & CLINICAL PHARMACOLOGY, Issue 3 2005Tetsuyuki Nasu Abstract The action of ouabain, a cell membrane Na+, K+ -ATPase blocker, on contractions induced by manganese ions (Mn2+) in Ca2+ -free, isotonic solutions with varying concentrations of K+ in the external medium were investigated in order to evaluate the underlying role of external Na+ in Mn2+ -induced contractions in isolated taenia coli of the guinea-pig. Mn2+ at 5 mm induced greater contractions as external isotonic K+ concentrations progressively increased from 10 to 100 mm. Ouabain (2 × 10,4 m) completely inhibited tension development stimulated by 5 mm Mn2+ in isotonic, 30 mm K+ (96 mm Na+) medium. Whereas, the tension inhibitory effects of ouabain became progressively weaker as isotonic, external K+ concentrations increased to 60 mm, which successively decreased external Na+ concentrations. Eventually, ouabain failed to affect contractions stimulated by Mn2+ in isotonic, 126 mm K+, Na+ -deficient medium. Ouabain caused progressively greater increase in cellular Na+ concentrations as the Na+ concentrations increased in the isotonic, K+ medium. While, pyruvate, which penetrates cell independently of external Na+, reversed the inhibition of tension by ouabain in isotonic, 30 mm K+, Na+ -sufficient (96 mm) medium containing 5 mm Mn2+. These results suggested that Mn2+ induced the contraction, which was maintained by glucose transport depending on external Na+, in the case of Na+ -sufficient medium in K+ -depolarized taenia coli. However, it induced the contraction independent of external Na+, in the case of Na+ -deficient, K+ medium. Ouabain might exhibit greater inhibition of the contraction induced by Mn2+ as the decrease in the Na+ gradient across the cell membranes continues. [source] Dual Mechanism of Intercellular Communication in HOBIT Osteoblastic Cells: A Role for Gap-Junctional HemichannelsJOURNAL OF BONE AND MINERAL RESEARCH, Issue 8 2001Milena Romanello Abstract Intercellular communication allows tissue coordination of cell metabolism and sensitivity to extracellular stimuli. Paracrine stimulation and cell-to-cell coupling through gap junctions induce the formation of complex cellular networks, which favors the intercellular exchange of nutrients and second messengers. Intercellular Ca2+ signaling was investigated in human osteoblast-like initial transfectant (HOBIT) cells, a human osteoblastic cell line in which cells retain most of the osteoblastic differentiation markers. HOBIT cells express connexin43 (Cx43) clustered at the cell-to-cell boundary and display functional intercellular coupling as assessed by the intercellular transfer of Lucifer yellow. Mechanical stimulation of a single cell induced a wave of increased Ca2+ that was radially propagated to surrounding cells. Treatment of cells with thapsigargin blocked mechanically induced signal propagation. Intercellular Ca2+ spreading and dye transfer were inhibited by 18,-glycyrrhetinic acid (18-GA), showing the involvement of gap junctions in signal propagation. Pretreatment of cells with suramin or with apyrase decreased the extent of wave propagation, suggesting that ATP-mediated paracrine stimulation contribute to cell-to-cell signaling. The functional expression of gap-junctional hemichannels was evidenced in experiments of Mn2+ quenching, extracellular dye uptake, and intracellular Ca2+ release, activated by uptake of inositol 1,4,5-trisphosphate (InsP3) from the external medium. Gap-junctional hemichannels were activated by low extracellular Ca2+ concentrations and inhibited by 18-GA. A role for Cx hemichannels in adenosine triphosphate (ATP) release and paracrine stimulation is suggested. [source] PHOTOSYNTHETIC FUNCTION IN DUNALIELLA TERTIOLECTA (CHLOROPHYTA) DURING A NITROGEN STARVATION AND RECOVERY CYCLEJOURNAL OF PHYCOLOGY, Issue 5 2003Erica B. Young Phytoplankton can be exposed to periods of N starvation with episodic N resupply. N starvation in Dunaliella tertiolecta (Butcher) measured over 4 days was characterized by slow reduction in cell chl and protein content and chl/carotenoid ratio and a decline in photosynthetic capacity and maximum quantum yield of photosynthesis (Fv/Fm). In the early stages of N starvation, cell division was maintained despite reduction in cellular chl. Chl content was more sensitive than carotenoids to N deprivation, and cellular chl a was maintained preferentially over chl b under N starvation. NO3, resupply stimulated rapid and complete recovery of Fv/Fm (from 0.4 to 0.7) within 24 h and commencement of cell division after 10 h, although N-replete levels of cell chl and protein were not reestablished within 24 h. Recovery of Fv/Fm was correlated with increases in cell chl and protein and was more related to increases in Fm than to changes in F0. Recovery of Fv/Fm was biphasic with a second phase of recovery commencing 4,6 h after resupply of NO3,. Uptake of NO3, from the external medium and the recovery of Fv/Fm, cell chl, and protein were inhibited when either cytosolic or chloroplastic protein synthesis was inhibited by cycloheximide or lincomycin, respectively; a time lag observed before maximum NO3, uptake was consistent with synthesis of NO3, transporters and assimilation enzymes. When both chloroplastic and cytosolic translation was inhibited, Fv/Fm declined dramatically. Dunaliella tertiolecta demonstrated a capacity to rapidly reestablish photosynthetic function and initiate cell division after N resupply, an important strategy in competing for limiting inorganic N resources. [source] Use of ATP bioluminescence to determine the bacterial sensitivity threshold to a bacteriocinLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 5 2003Charlotte Valat Abstract A new ATP bioluminescence-based method was developed to determine the effectiveness of nisin on a sensitive strain of Lactococcus cremoris. The principle of the method is to quantify the release of adenylic-nucleotides (AN) by a sensitive strain under the action of the bacteriocin, with the complex luciferin,luciferase. Nisin-induced leakage of AN included ATP from a sensitive L. cremoris to the external medium immediately after the contact with the bacteria. The growth of L. cremoris was correlated with the extracellular AN content. The extracellular ATP and AN concentration exhibited a linear correlation to the logarithm of the nisin concentration. For the determination of the effectiveness threshold, the concentration of AN was more sensitive and more reliable than the direct quanti,cation of ATP. The effectiveness threshold, corresponding to a 100% inhibition of L. cremoris growth, was obtained for a null concentration of intracellular nucleotides, i.e. for a ANtot:ANext ratio = 1. For an initial concentration of 1.4 × 107 bacteria/mL, the nisin effectiveness threshold is 3.4 ± 0.01 mg nisin/L. It is possible to detect effectiveness threshold concentration by taking into account the physiological state of the cells. Copyright © 2003 John Wiley & Sons, Ltd. [source] A structure and energy dissipation efficiency of relativistic reconfinement shocksMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009Krzysztof Nalewajko ABSTRACT We present a semi-analytical hydrodynamical model for the structure of reconfinement shocks formed in astrophysical relativistic jets interacting with external medium. We take into account exact conservation laws, both across the shock front and in the zone of the shocked matter, and exact angular relations. Our results confirm a good accuracy of the approximate formulae derived by Komissarov & Falle. However, including the transverse pressure gradient in the shocked jet, we predict an absolute size of the shock to be about twice larger. We calculate the efficiency of the kinetic energy dissipation in the shock and show a strong dependence on both the bulk Lorentz factor and opening angle of the jet. [source] Winds from massive stars: implications for the afterglows of , -ray burstsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2001Enrico Ramirez-Ruiz Recent observations suggest that long-duration , -ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. The pre-explosive ambient medium provides a natural test for the most likely progenitors of GRBs. Those stars that shed their envelopes most readily have short jet crossing times and are more likely to produce a GRB. We construct a simple computational scheme to explore the expected contribution of the presupernova ejecta of single Wolf,Rayet (WR) stars to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main-sequence mass, metallicity, rotation and membership in a binary system have on the ambient medium. We extend the theory of GRB afterglows in winds to consider the effect of the relativistic fireball propagating through the WR ejecta. Specific predictions are made for the interaction of the relativistic blast wave with the density bumps that arise when the progenitor star rapidly loses a large fraction of its initial mass or when the ejected wind interacts with the external medium and decelerates. A re-brightening of the afterglow with a spectrum redder than the typical synchrotron spectrum (as seen in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calculate the luminosity of the reflected echo that arises when circumstellar material Compton-scatters the prompt radiation, and examine the spectral signatures expected from the interaction of the GRB afterglow with the ejected medium. [source] The environments of FRII radio sourcesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2000M. J. Hardcastle Using ROSAT observations, we estimate gas pressures in the X-ray-emitting medium surrounding 63 FRII radio galaxies and quasars. We compare these pressures with the internal pressures of the radio-emitting plasma estimated by assuming minimum energy or equipartition. In the majority of cases (including 12/13 sources with modelled, spatially resolved X-ray emission) radio sources appear to be underpressured with respect to the external medium, suggesting that simple minimum-energy arguments underestimate the internal energy density of the sources. We discuss possible departures from the minimum-energy condition and the consequences of our result for models of the dynamics of radio galaxies, in particular self-similar models. [source] Involvement of Electron Transfer in the Photoreaction of Zebrafish Cryptochrome-DASH,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2008Kazunori Zikihara Photoreaction of a blue-light photoreceptor Cryptochrome-DASH (Cry-DASH), a new member of the Cryptochrome family, from zebrafish was studied by UV,visible absorption spectroscopy in aqueous solutions at 293 K. Zebrafish Cry-DASH binds two chromophores, a flavin adenine dinucleotide (FAD) and a N5,N10-methenyl-5,6,7,8-tetrahydrofolate (MTHF) noncovalently. The bound FAD exists in the oxidized form (FADox) in the dark. Blue light converts FADox to the neutral radical form (FADH?). Formed FADH? is transformed to the fully reduced form FADH2 (or FADH,) by successive light irradiation, or reverts to FADox. FADH2 (or FADH,) reverts to FADH? or possibly to FADox directly. The effect of dithiothreitol suggests a possible electron transfer between FAD in zebrafish Cry-DASH and reductants in the external medium. This is the first report on the photoreaction pathway and kinetics of a vertebrate Cry-DASH family protein. [source] Protein Matrix Elasticity Determined by Fluorescence Anisotropy of Its Tryptophan Residues,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2003Christian Zentz ABSTRACT Rotational motions of Trp residues embedded within human hemoglobin matrix have been measured by using their steady-state fluorescence anisotropy. The mean square angular displacement ,2 of Trp residues, depending on the temperature, can be expressed by where W is the thermal energy acting on the Trp residues and C the resilient torque constant of the protein matrix. To study the external medium influencing the protein dynamics, comparative experiments were made with protein in aqueous buffer and in the presence of 32% glycerol. The data show that between 5°C and 25°C, external medium acts on the protein matrix elasticity. [source] Phosphate uptake in Chara: membrane transport via Na/Pi cotransportPLANT CELL & ENVIRONMENT, Issue 2 2000R. J. Reid ABSTRACT Phosphate uptake in the freshwater charophyte plant Chara corallina was found to be strongly dependent on the presence of Na in the external medium. Based on the reciprocal stimulations of 32Pi uptake by Na and 22Na uptake by Pi, the logical mechanism for Pi uptake appears to be a nNa/Pi symport with a half-maximal stimulation (Km) for Na of approximately 300 ,M and a Km for Pi of approximately 10 ,M. Comparison of the stimulations of 32Pi and 22Na influxes at pH 6 gives a stoichiometry of Na : Pi of 5·68. The reduction in Pi influx with increasing pH is consistent with the transported species being the monovalent H2PO4,. In voltage-clamp experiments, currents elicited by Pi in the presence of Na were equivalent to an influx of positive charge which exceeded the measured influxes of 32P by a factor of 6·26. Intracellular perfusion was used to examine the dependence of Pi influx on ATP and Na. In perfused cells, Pi influx was low when ATP was absent from the internal medium or Na was absent from the external medium. Addition of ATP alone had little effect whereas addition of Na alone increased the 32Pi influx slightly. Addition of both ATP and Na together restored Pi influx to rates comparable to those of intact cells. It is suggested that the ATP is required for membrane hyperpolarization which in turn drives the highly electrogenic flux of Pi with up to 6 Na. However, consideration of the electrochemical potential differences for Na and Pi at pH less than 6 shows that nNa/Pi would not be feasible. It is suggested that at low pH, H+ may substitute for Na. [source] | |||||||||||||