Mercury in lichens of Nahuel Huapi National Park, Patagonia, Argentina

Mercury and other elements of interest are determined inlichens collected in Nahuel Huapi National Park, Northern Patagonia, Argentina. Pooled samples are analyzed by instrumental neutron activation analysis. Mercury contents in Usnea sp. collected from undisturbed sites range from 0.0558±0.0083 to 1.38±0.18 μg·g⁻¹. Other potential pollutants are identified by the analysis of Usnea sp. samples, namely Sb, As, Br, Zn, and Se. Previous experiments with foliose and fruticose lichens are also discussed. The analysis of mercury contents of foliose lichens sampled from urban and periurban sites of Bariloche city, and from undisturbed regions, demonstrate that the atmosphere of Bariloche city is enriched in mercury compared to the surroundings. The result is confirmed by transplantation experiments from undisturbed zones to urban sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Eine genaue volumetrische Methode zur Bestimmung von Methylenblau

Ohne Zusammenfassung     

Kinetic studies of complex biomedical process by magnetic resonance: Cuban experiences

The potentials of the magnetic resonance (MR) methods in the research of biomedical systems have been demonstrated during the 70 years of its existence. It is presented that the Cuban experience in quantitative magnetic resonance associated with molecular, preclinical and clinical studies of significant diseases and drugs development. MR “in vitro” and “in vivo” studies of sickle cell disease, the diabetic foot ulcer, the brain tumor response and the magnetic nano-particle pharmacokinetics, are presented as example. Furthermore, contributions and restrictions of magnetic resonance to diagnostic and optimization of therapeutic pathways are discussed in some concrete cases.     

Asphaltene precipitation from latin american heavy crude oils. Effects of solvent aromaticity and agitation on particle size reduction

Asphaltenes from three crudes were precipitated with a mixture of n-heptane and toluene, the size of the particles formed under different solvent mixtures and different agitation regimes were studied. The kinetic size reduction of aggregates formed with an excess of precipitant agent is followed, contrary to other published studies where the kinetic followed is of growing particles. It was found that the particle size of precipitated asphaltenes decreases as precipitant aromaticity increases and agitation energy rises, which indicates the formation of aggregates bonded by weak forces, since the agitation applied was not of high energy, except for the ultrasonic device.     

Neutron activation analysis of Cuban soils

Thirty elements were determined by INAA in Cuban soil samples collected in 17 zones from western regions of Cuba, which were affected by neuropathy epidemic in 1992. The presence of toxic elements was evaluated to provide data for the analysis of possible paths of intoxication through local agriculture product consumption. The results obtained for As, Th and U are compared with similar data reported on literature.     

Alternans and higher-order rhythms in an ionic model of a sheet of ischemic ventricular muscle

Life-threatening arrhythmias such as ventricular tachycardia and fibrillation often occur during acute myocardial ischemia. During the first few minutes following coronary occlusion, there is a gradual rise in the extracellular concentration of potassium ions ([K(+)](0)) within ischemic tissue. This elevation of [K(+)](0) is one of the main causes of the electrophysiological changes produced by ischemia, and has been implicated in inducing arrhythmias. We investigate an ionic model of a 3 cmx3 cm sheet of normal ventricular myocardium containing an ischemic zone, simulated by elevating [K(+)](0) within a centrally-placed 1 cmx1 cm area of the sheet. As [K(+)](0) is gradually raised within the ischemic zone from the normal value of 5.4 mM, conduction first slows within the ischemic zone and then, at higher [K(+)](0), an arc of block develops within that area. The area distal to the arc of block is activated in a delayed fashion by a retrogradely moving wavefront originating from the distal edge of the ischemic zone. With a further increase in [K(+)](0), the point eventually comes where a very small increase in [K(+)](0) (0.01 mM) results in the abrupt transition from a global period-1 rhythm to a global period-2 rhythm in the sheet. In the peripheral part of the ischemic zone and in the normal area surrounding it, there is an alternation of action potential duration, producing a 2:2 response. Within the core of the ischemic zone, there is an alternation between an action potential and a maintained small-amplitude response ( approximately 30 mV in height). With a further increase of [K(+)](0), the maintained small-amplitude response turns into a decrementing subthreshold response, so that there is 2:1 block in the central part of the ischemic zone. A still further increase of [K(+)](0) leads to a transition in the sheet from a global period-2 to a period-4 rhythm, and then to period-6 and period-8 rhythms, and finally to a complete block of propagation within the ischemic core. When the size of the sheet is increased to 4 cmx4 cm (with a 2 cmx2 cm ischemic area), one observes essentially the same sequence of rhythms, except that the period-6 rhythm is not seen. Very similar sequences of rhythms are seen as [K(+)](0) is increased in the central region (1 or 2 cm long) of a thin strand of tissue (3 or 4 cm long) in which propagation is essentially one-dimensional and in which retrograde propagation does not occur. While reentrant rhythms resembling tachycardia and fibrillation were not encountered in the above simulations, well-known precursors to such rhythms (e.g., delayed activation, arcs of block, two-component upstrokes, retrograde activation, nascent spiral tips, alternans) were seen. We outline how additional modifications to the ischemic model might result in the emergence of reentrant rhythms following alternans. (c) 2000 American Institute of Physics.