Small-Angle Scattering and Diffusion: Application to Relativistic Shock Acceleration

We investigate ways of accurately simulating the propagation of energetic charged particles over small times where the standard Monte Carlo approximation to diffusive transport breaks down. We find that a small-angle scattering procedure with appropriately chosen step-lengths and scattering angles gives accurate results, and we apply this to the simulation of propagation upstream in relativistic shock acceleration.Comment: 4 pages, 2 figures, proceedings of World Space Environment Forum (WSEF2002) to appear in Space Science Reviews, accepte

Answers in a flash; optical analysis of exocytosis in human cultured endothelial cells

Endothelial cells line all of our blood vessels. They monitor and respond to signals generated during injury, infection and disease by releasing a wide range of molecules that regulate blood flow, coagulation, inflammatory responses and vessel growth. Protein mediators are released by exocytosis of intracellular organelles, and a major trigger for this type of secretion is an increase in intracellular free calcium ion concentration ([Ca2+]i). Mitochondria are thought to influence Ca2+ homeostasis through local Ca2+ buffering. Due to a lack of sensitive and time-resolved assays for endothelial exocytosis little is known about the precise relationship between Ca2+-signalling and exocytosis, and the influence of Ca2+ buffering by mitochondria. Using fluorescence and biochemical techniques I have investigated the relationship between secretagogue-evoked Ca2+-signalling and the influence of mitochondrial function on the exocytosis of two distinct organelle populations in cultures of Human Umbilical Vein Endothelial Cells (HUVEC); 1) the Weibel-Palade body (WPB) the main storage organelle for pro-coaguland and inflammatory mediators, and 2) the non-WPB, a non-stored and morphologically distinct organelle that can contain a range of inflammatory and anti-coagulant molecules. These two distinct organelle populations were labeled for fluorescence microscopy by targeted expression of chimeras of green (EGFP) or red (mRFP) fluorescent proteins in living HUVEC. Exocytosis was evoked by both physiological and pharmacological secretogogues that increase [Ca2+]i. The times of exocytosis of individual organelles were monitored by flashes of light from granule EGFP, produced by pH changes within the organelle upon fusion. In the same experiments, [Ca2+]i and intra-mitochondrial Ca2+ concentration ([Ca2+]m) were monitored using fluorescent Ca2+-indicators. The data obtained has defined more precisely the relationship between agonist-evoked changes in [Ca2+]i and secretory vesicle exocytosis in HUVEC. These studies will contribute to a better understanding of the processes that regulate secretion of biomolecules from the endothelium

Particle-in-cell simulation of a mildly relativistic collision of an electron-ion plasma carrying a quasi-parallel magnetic field: Electron acceleration and magnetic field amplification at supernova shocks

Plasma processes close to SNR shocks result in the amplification of magnetic fields and in the acceleration of electrons, injecting them into the diffusive acceleration mechanism. The acceleration of electrons and the B field amplification by the collision of two plasma clouds, each consisting of electrons and ions, at a speed of 0.5c is investigated. A quasi-parallel guiding magnetic field, a cloud density ratio of 10 and a plasma temperature of 25 keV are considered. A quasi-planar shock forms at the front of the dense plasma cloud. It is mediated by a circularly left-hand polarized electromagnetic wave with an electric field component along the guiding magnetic field. Its propagation direction is close to that of the guiding field and orthogonal to the collision boundary. It has a low frequency and a wavelength that equals several times the ion inertial length, which would be indicative of a dispersive Alfven wave close to the ion cyclotron resonance frequency of the left-handed mode (ion whistler), provided that the frequency is appropriate. However, it moves with the super-alfvenic plasma collision speed, suggesting that it is an Alfven precursor or a nonlinear MHD wave such as a Short Large-Amplitude Magnetic Structure (SLAMS). The growth of the magnetic amplitude of this wave to values well in excess of those of the quasi-parallel guiding field and of the filamentation modes results in a quasi-perpendicular shock. We present evidence for the instability of this mode to a four wave interaction. The waves developing upstream of the dense cloud give rise to electron acceleration ahead of the collision boundary. Energy equipartition between the ions and the electrons is established at the shock and the electrons are accelerated to relativistic speeds.Comment: 16 pages, 18 figures, Accepted for publication by Astron & Astrophy

New perspectives on the potential role of aquaporins (AQPs) in the physiology of inflammation

Aquaporins (AQPs) are emerging, in the last few decades, as critical proteins regulating water fluid homeostasis in cells involved in inflammation. AQPs represent a family of ubiquitous membrane channels that regulate osmotically water flux in various tissues and sometimes the transport of small solutes, including glycerol. Extensive data indicate that AQPs, working as water channel proteins, regulate not only cell migration, but also common events essential for inflammatory response. The involvement of AQPs in several inflammatory processes, as demonstrated by their dysregulation both in human and animal diseases, identifies their new role in protection and response to different noxious stimuli, including bacterial infection. This contribution could represent a new key to clarify the dilemma of host-pathogen communications, and opens up new scenarios regarding the investigation of the modulation of specific AQPs, as target for new pharmacological therapies. This review provides updated information on the underlying mechanisms of AQPs in the regulation of inflammatory responses in mammals and discusses the broad spectrum of options that can be tailored for different diseases and their pharmacological treatment