Calcium waves in a model with a random spatially discrete distribution of Ca2+ release sites

We study the propagation of intracellular calcium waves in a model that features Ca2+ release from discrete sites in the endoplasmic reticulum membrane and random spatial distribution of these sites. The results of our simulations qualitatively reproduce the experimentally observed behavior of the waves. When the level of the channel activator inositol trisphosphate is low, the wave undergoes fragmentation and eventually vanishes at a finite distance from the region of initiation, a phenomenon we refer to as an abortive wave. With increasing activator concentration, the mean distance of propagation increases. Above a critical level of activator, the wave becomes stable. We show that the heterogeneous distribution of Ca2+ channels is the cause of this phenomenon.     

The spatial distribution of nucleation sites and its effect on recrystallization kinetics in commercial aluminum alloys

Particle distributions play a major role in the processing response of aluminum alloys. While large constituent particles play an important role in the nucleation of recrystallization, small particles may heavily restrict the growth of recrystallized grains. In the present investigation, a two-dimensional (2-D) tesselation procedure has been used to characterize the particle distribution in commercial aluminum alloys and its relevance to nucleation of recrystallization. This procedure enabled the quantification of the degree of particle clustering in samples rolled to different strain levels. A characteristic aspect seems to be a transition from a rather nonuniform spatial distribution at low rolling strains, toward a more or less random distribution at high strains. Nucleation kinetics has been found to be site saturated, indicating that all nucleation events effectively occur at the start of recrystallization. A simple model is proposed, which explains the development of the spatial particle distribution as a function of rolling strain.     

The effect of constant baroque music on premature infants

A rapid and simple method for the in vitro evaluation of the virulence of F. tularensis is proposed. The optimal parameters--the concentrations of bacteria and the substrate (phenolphthalein phosphate), the time incubation in the serum--have been developed and special conditions for strains of different subspecies have been selected. The proposed method makes it possible to greatly reduce the time of obtaining results and to exclude labor-consuming experiments on laboratory animals when working with large collections of F. tularensis laboratory and natural wild strains. Moreover, the method widens the possibilities of experimenters in the evaluation of the effect of some mutation on the virulence of the strain under study.     

The effect of particle size distribution on the performance of a phosphate flotation process

A 25 cu ft flotation cell was operated continuously to produce an apatite-rich concentrate. The experimental results reported in this paper attempt to elucidate the role of the particle size distribution upon the performance of the cell. The particle size distribution influences performance through its effect on the particle residence time distribution and on the rate of impaction, adherence and detachment of particles and bubbles. A model is developed which is capable of predicting a fractional recovery of apatite in any particular particle size range.     

The topographic distribution of the first sites of diabetic retinal neovascularization

PURPOSE: To examine the topographic distribution of the origin of diabetic retinal neovascularization. METHODS: The eyes of 3,121 patients with background diabetic retinopathy were investigated. These patients were volunteers in systemic medical therapy experiments. Color stereo photographs were obtained annually. The first retinal neovascularization sites were identified and the distances from the optic nerve measured. RESULTS: In 1 year, neovascularization originated in 282 eyes. The superotemporal quadrant, at 6 mm from the optic disk, was the most frequent initial site. CONCLUSIONS: The first retinal neovascularization sites cluster around specific anatomic foci. This information should influence retinopathy monitoring protocols.     

The effect of combolen on the erythrocyte distribution in the dog

The effect of Combolen (3.9 mg/kg bw s.c.) on the distribution of erythrocytes in dogs was investigated. After application of Combolen, the hematocrit of the animals decreased exponentially within 1 h by 21%. The reversal of the reduction began from 2 h after the time of application, amounting to 4% within the first day and finishing by the fourth day. After injection of 51Cr-labelled erythrocytes, radioactivity in the circulation of Combolen-treated dogs decreased exponentially within 2 h by at least 30%. It was concluded that the dog's spleen, under physiological conditions, contains about 10% of the animal's blood. The time-courses of the decrease of hematocrit and radioactivity in the circulation were found to be very similar. In accordance with this observation, a high correlation (r = 0.97) between the level of radioactivity after injection of radio-labelled erythrocytes and the corresponding hematocrit values after application of Combolen was found. After application of erythrocytes, labelled with 99mTc, an extensive distribution from the circulation into the spleen was observed by scintigraphy. This process can be understood by using a closed-compartment model. An equation, based on this model, describes the observed time course of the hematocrit values, as well as the number of 51Cr-labelled erythrocytes, in Combolen-treated dog. Presumably, the observed effect of Combolen is the result of the relaxation of the smooth muscle cells in the trabeculae of the spleen, caused by central-nervous depression of sympathetic tone. Combolen seems to be a suitable tool in pre-clinical testing of a novel blood preserve with dog as a test animal. Its potent ability to eliminate erythrocytes from circulation is distinguishable from the sequestration of damaged red cells. Furthermore, its ability to prevent the spleen from uncontrolled hematocrit modulating actions in addition to its sedative effects is considered to be an invaluable advantage.     

The distribution and mobility of anionic sites on the surfaces of baby hamster kidney cells

The distribution and mobility of anionic sites on the surfaces of baby hamster kidney cells were studied by utilizing the multivalent ligand, polycationic ferritin, as a visual probe. Our observations revealed that anionic sites are distributed over the entire cell surface, with the highest density of sites being located on cell surface microextensions. Following the initial binding of polycationic ferritin to the surface of unfixed cells, the ligand-bound anionic sites redistributed by migrating from the surface of microextensions to the surface of the cell body. In 20 min, this migration resulted in a total clearing of anionic sites from the surface of microextensions concomitant with the formation of patches of anionic sites on the surface of the cell body. Polycationic ferritin-induced migration and patch formation of anionic sites was not prevented by 2,4-dinitrophenol, N-ethylmaleimide, colchicine, or cytochalasin B. However, the ligand-induced redistribution of cell surface anionic sites was prevented by prefixation of cells with glutaraldehyde.     

The effect of stimulus sequence on the waveform of the cortical event-related potential

The waveform of the cortical event-related potential is extremely sensitive to variations in the sequence of stimuli preceding the eliciting event. The waveform changes were manifested primarily in the amplitudes of the negative component of the potential that peaked at 200 milliseconds, the positive component that peaked at 300 milliseconds, and the slow-wave components. A quantitative model was developed relating the waveform changes to changes in event expectancy. Expectancy is assumed to depend on a decaying memory for events within the prior sequence, the specific structure of the sequence, and the global probability of event occurrence. For stimuli relevant to the task, the less expected the stimulus the larger the amplitudes of late components of the event-related potentials.     

The detection of linkage disequilibrium in molecular sequence data

Studies of genetic variation in natural populations at the sequence level usually show that most polymorphic sites are very asymmetrical in allele frequencies, with the rarer allele at a site near fixation. When the rarer allele at a site is present only a few times in the sample, say below five representatives, it becomes very difficult to detect linkage disequilibrium between sites from tests of association. This is a consequence of the numerical properties of even the most powerful test of association, Fisher's exact test. Sites with fewer than five representatives in the sample should be excluded from association tests, but this generally leaves few site pairs eligible for testing. A test for overall linkage disequilibrium, based on the sign of the observed linkage disequilibria, is derived which can use all the data. It is shown that more power can be achieved by increasing the length of sequence determined than by increasing the number of genomes sampled for the same total work.     

The effect of random vaccine response on the vaccination coverage required to prevent epidemics

The response people have to vaccination varies because their immune systems differ and vaccine failures occur. Here we consider the effect that a random response, independent for each vaccinee, has on the vaccination coverage required to prevent epidemics in a large community. For a community of uniformly mixing individuals an explicit expression is found for the critical vaccination coverage (CVC) and the effect of the vaccine response is determined entirely by the mean E(AB), where A and B, respectively, reflect the infectivity and susceptibility of a vaccinated individual. This result shows that the usual concept of vaccine efficacy, which focuses on the amount of protection the vaccine provides the vaccinee against infection, is not adequate to describe the requirements for preventing epidemics when vaccination affect infectivity. The estimation of E(AB) poses a problem because A and B refer to the vaccine response of the same individual. Similar results are found when there are different types of individual, but now the mean E(AB) may differ between types. However, for a community made up of households it is shown that the CVC also depends on other characteristics of the vaccine response distribution. In practice this means that estimating a single measure of vaccine effectiveness is generally not enough to determine the CVC. For a specific community of households it is found that the vaccination coverage required to prevent epidemics decreases as the variation in the vaccine response increases.     

The effect of high-frequency random mutagenesis on in vitro protein evolution: a study on TEM-1 beta-lactamase

For a number of years a major limitation in genetic analysis of protein function has been the inability to introduce multiple substitutions at distant sites that would enable the selection of clusters of mutations required for improved or novel biological functions. In order to achieve this, we have recently developed a novel mutagenesis procedure in which the triphosphate derivatives of a pyrimidine (6-(2-deoxy-beta-d-ribofuranosyl)-3, 4-dihydro-8H-pyrimido-[4,5-c][1,2]oxazin-7-one; dP) and a purine (8-oxo-2'-deoxyguanosine; 8-oxodG) nucleoside analogue are employed in DNA synthesis reactions in vitro. The procedure allows control of the mutational load and can yield frequencies of amino acid residue substitutions at least one order of magnitude greater than those previously achieved. Here we report the results of an experiment in which we have hypermutated the bacterial enzyme TEM-1 beta-lactamase and selected small pools (<1.5x10(5)) of clones for enzymatic activity against the beta-lactam antibiotic cefotaxime. The experiment resulted in the isolation of a number of TEM-1 mutants with greatly improved activity against cefotaxime. Among these, clone 3D.5 (E104K:M182T:G238S) exhibited a minimum inhibitory concentration for cefotaxime 20,000-fold higher than wild-type TEM-1 and a catalytic efficiency (kcat/Km) 2383 times higher than the wild-type enzyme. Thus, small pools of hypermutated sequences enabled the selection of one of the most active extended beta-lactamases described so far. These results argue against the accepted view that multiple rounds of low-rate mutagenesis and stepwise selection are essential for in vitro protein evolution and extend the scope of directed molecular evolution to proteins for which no genetic selection is available.     

The effect of tin on the tissue distribution of 99mTc-sodium pertechnetate

When tin complex is administered prior to the injection of 99mTc-sodium pertechnetate, the distribution of the tracer is altered such that: (a) the concentration at foci of cerebral pathology is drastically reduced; (b) the concentration in the stomach (mucus cells), thyroid and salivary glands, and choroid plexus is greatly increased; and (c) there is a shift of the tracer from the plasma to the red blood cells. Bone studies utilizing a tin complex should be done after other organs have been evaluated.