MAPPING CARTESIAN COORDINATES INTO EMISSION COORDINATES: SOME TOY MODELS

After briefly reviewing the relativistic approach to positioning systems based on the introduction of the emission coordinates, we show how explicit maps can be obtained between the Cartesian coordinates and the emission coordinates, for a suitably chosen set of emitters, whose world lines are supposed to be known by the users. We consider Minkowski space–time and the space–time where a small inhomogeneity is introduced (i.e. a small a "gravitational" field), in 1 + 1 and 1 + 3 dimensions.

Intracellular pattern of cytosolic Ca2+ changes during adhesion and multiple phagocytosis in human neutrophils. Dynamics of intracellular Ca2+ stores

The subcellular pattern of cytosolic free Ca2+ ([Ca2+]i) changes in human polymorphonuclear neutrophils (PMNs) was studied using imaging of fura-2 fluorescence (time resolution 12.5 ratios/s) to determine whether PMNs could obtain directional information from the [Ca2+]i signal. [Ca2+]i changes were observed during initial adherence, the subsequent chemotactic movement, and the phagocytosis of opsonized yeast particles. Initial adherence was followed by a rapid increase in [Ca2+]i (from 90 +/- 10 to 290 +/- 40 nmol/L in 6.5 +/- 2.5 seconds; +/- SEM, n = 10), apparently homogeneously distributed over the entire cytoplasm, which preceded the spreading of the PMNs. [Ca2+]i increases after the contact of the PMNs with yeast particles were of lower mean amplitude; [Ca2+]i increased simultaneously throughout the cytosol. In the absence of extracellular Ca2+, multiple phagocytotic events could proceed normally without a mandatory [Ca2+]i transient. In PMNs polarized on phagocytosis, gradients in [Ca2+]i could be observed. [Ca2+]i was more elevated in the periphagosomal area than in the remaining parts. Taken together, these data show that [Ca2+]i waves do not provide the neutrophil with directional information during chemotaxis and phagocytosis. Sustained small inhomogeneity of [Ca2+]i levels are consistent with a proposed redistribution of releasable Ca2+ stores on phagocytosis.

Interaction Between Colloidal Particles in A.C. Electric Fields in the Relaxation Region of Double Layers

When an electric field is applied to a colloidal suspension of micronsize particles, the particles are attracted to each other in the direction of the external field. They line up to form chains and columns across the gap of the electrodes and therefore drastically change the rheological properties of the suspension. These electrorheological phenomena have been studied extensively by recent computer simulation work [1,2], and it is generally accepted that the interactions between the induced electric dipole moments of the particles are responsible for their alignment in the external fields. To avoid net migrations of particles in electric fields of small inhomogeneity, alternating fields are generally used in experiments.