Dependence of Lattice Hadron Masses on External Magnetic Fields

We study the variation of the hadron masses in the presence of external magnetic fields of strength of the order of the masses themselves. We identify the main factors affecting the lattice simulation results: - the boundary discontinuities for $eB << 2\pi / L^2 a^2$. - the SU(6) choice of the hadron wave-function. We confirm qualitatively the earlier theoretical ansatz on the linear behaviour of the masses with the magnetic field and, as a by-product, we improve the lattice measurements of the nucleon magnetic moments. However our systematic and statistical errors preclude us from measuring the theoretically predicted field strength at which the proton becomes heavier than the neutron.Comment: 18 pages, compressed uuencoded postscript fil

Osmotic cell swelling-induced ATP release mediates the activation of extracellular signal-regulated protein kinase (Erk)-1/2 but not the activation of osmo-sensitive anion channels

Human intestine 407 cells respond to hypo-osmotic stress by the rapid release of ATP into the extracellular medium. A difference in the time course of activation as well as in the sensitivity to cytochalasin B treatment and BAPTA-AM [1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester] loading suggests that ATP leaves the cell through a pathway distinct from volume-regulated anion channels. To evaluate a putative role for nucleotides as autocrinic/paracrinic factors in osmotic signalling, the effects of extracellular ATP on the regulation of volume-sensitive anion channels as well as on the hypotonicity-induced activation of extracellular signal-regulated protein kinases (Erk-1/2) were investigated. Micromolar concentrations of ATP were unable to elicit an isotope efflux from (125)I(-)-loaded cells by itself, but strongly potentiated the hypotonicity-provoked anion efflux through a Ca(2+)-dependent mechanism. The order of potency of nucleotides (ATP = UTP = ATP[S] > ADP = AMP >> adenosine = cAMP) indicated the involvement of P2Y(2) receptors. In contrast, millimolar concentrations of ATP markedly inhibited both the osmotically induced isotope efflux and whole-cell Cl(-) currents. Inhibition of whole-cell Cl(-) currents, not only by millimolar ATP but also by the purinoceptor antagonists suramin and reactive blue, was observed most prominently at depolarizing holding potentials, suggesting a direct interaction with volume-sensitive Cl(-) channels rather than interaction with purinoceptors. Both ATP and UTP, at submicromolar levels, were found to act as potent activators of Erk-1/2 in intestine 407 cells. Addition of the ATP hydrolase apyrase to the bath greatly reduced the hypotonicity-induced Erk-1/2 activation, but did not affect the swelling-induced isotope efflux or whole-cell Cl(-) currents. Furthermore, pre-treatment with suramin or reactive blue almost completely prevented the hypo-osmotic activation of Erk-1/2. The results indicate that extracellularly released ATP functions as an autocrinic/paracrinic factor that mediates hypotonicity-induced Erk-1/2 activation but does not serve as an activator of volume-sensitive compensatory Cl(-) currents

Large-scale structural organization of social networks

The characterization of large-scale structural organization of social networks is an important interdisciplinary problem. We show, by using scaling analysis and numerical computation, that the following factors are relevant for models of social networks: the correlation between friendship ties among people and the position of their social groups, as well as the correlation between the positions of different social groups to which a person belongs.Comment: 5 pages, 3 figures, Revte

Hypo-osmotic cell swelling activates the p38 MAP kinase signalling cascade

Hypo-osmotic swelling of human Intestine 407 cells leads to a significant increase of intracellular MAPKAP-kinase 2 activity and Hsp27 phosphorylation. Pre-treatment of the cells with the p38 MAP kinase inhibitor SB-203580 blocks this activation, indicating that the hypotonicity-induced activation of MAPKAP kinase 2 is, similarly to that described for hyperosmotic treatment, the result of an activated p38 MAP kinase cascade. The activation of MAPKAP kinase 2 proceeds with kinetics similar to that of one of the first physiological responses of hypo-osmotic treatment, the opening of compensatory Cl- channels. However, inhibition of the p38 MAP kinase cascade does not block the osmo-sensitive anion efflux and, vice versa, activation of p38 MAP kinase by cytokines and anisomycin does not increase the efflux. These results indicate that the p38 MAP kinase cascade is not directly involved in Cl- channel activation but instead may play a role in subsequent cellular repair processes

Protein tyrosine phosphorylation is involved in osmoregulation of ionic conductances

Using the human Intestine 407 cell line as a model, we investigated a possible role for tyrosine kinase(s) in regulating the ion efflux pathways induced by hyposmotic stimulation (regulatory volume decrease, RVD). Pretreatment of 125I(-)-and 86Rb(+)-loaded cells with the phosphotyrosine phosphatase inhibitor sodium orthovanadate (200 microM) potentiated isotope efflux triggered by mild hypotonicity (10-20%) but did not further increase the efflux in response to more vigorous osmotic stimulation (30% hypotonicity). The tyrosine kinase inhibitors herbimycin A and genistein largely reduced the osmoshock-induced efflux in both control and vanadate-pretreated cells, while not affecting calcium-activated 86Rb+ efflux. Potentiation of the RVD response by vanadate was confirmed by direct measurements of hypotonicity-induced changes in cell volume. Hypotonic shock alone triggered a rapid and transient increase in tyrosine phosphorylation of several proteins as well as phosphorylation of mitogen-activated protein kinase. Furthermore, the potentiating effects of vanadate on hypotonicity-induced ion efflux and mitogen-activated protein (MAP) kinase phosphorylation were mimicked by epidermal growth factor. Neither vanadate nor epidermal growth factor provoked a RVD-like ionic response under isotonic conditions. These results indicate that tyrosine phosphorylation is an essential step in the RVD response and suggest a novel role of growth factors in the cellular defense against osmotic stress