Showing papers by "University of Los Andes published in 1993"

On the dynamical sliding mode control of nonlinear systems

Abstract: The consequences of the differential algebraic approach in the sliding mode control of nonlinear single-input single-output systems are reviewed in tutorial fashion. Input-dependent sliding surfaces, possibly including time derivatives of the input signal, are shown to arise naturally from elementary differential algebraic results pertaining to the Fliess's Generalized Controller Canonical Forms of nonlinear systems. This class of switching surfaces generally leads to chattering-free dynamically synthesized sliding regimes, in which the highest time derivative of the input signal undergoes all the bang-bang type discontinuities. Examples illustrating the obtained results are also included.

Use of general circulation model output in the creation of climate change scenarios for impact analysis

Abstract: Many scientific studies warn of a rapid global climate change during the next century. These changes are understood with much less certainty on a regional scale than on a global scale, but effects on ecosystems and society will occur at local and regional scales. Consequently, in order to study the true impacts of climate change, regional scenarios of future climate are needed. One of the most important sources of information for creating scenarios is the output from general circulation models (GCMs) of the climate system. However, current state-of-the-art GCMs are unable to simulate accurately even the current seasonal cycle of climate on a regional basis. Thus the simple technique of adding the difference between 2 × CO2 and 1 × CO2 GCM simulations to current climatic time series cannot produce scenarios with appropriate spatial and temporal details without corrections for model deficiencies. In this study a technique is developed to allow the information from GCM simulations to be used, while accommodating for the deficiencies. GCM output is combined with knowledge of the regional climate to produce scenarios of the equilibrium climate response to a doubling of the atmospheric CO2 concentration for three case study regions, China, Sub-Saharan Africa and Venezuela, for use in biological effects models. By combining the general climate change calculated with several GCMs with the observed patterns of interannual climate variability, reasonable scenarios of temperature and precipitation variations can be created. Generalizations of this procedure to other regions of the world are discussed.

Preparation and Catalytic Activity of Titanium Pillared Montmorillonite

Abstract: Thermally stable titanium pillared montmorillonite has been prepared by the intercalation of four different titanium alkoxides. X-ray diffraction, BET, chemical analysis, themogravimetric analysis, differential thermal analysis, temperature-programmed desorption of ammonia and IR spectra of samples allow the determination of the structure. The catalysts prepared from Ti (OEt)4 present the most interesting structural properties after calcination. The titanium pillared montmorillonite with pillars of ca. 12 angstrom shows high activity in the dehydration of 1-butanol. The selectivity of the catalyst is directly related to its acidic properties.

Effects of long-term administration of clozapine on body weight and food intake in rats

Abstract: Previous reports have shown that long-term administration of typical and atypical neuroleptics induced obesity in female but not in male rats. It has been suggested that impaired ovarian steroidogenesis related to neuroleptic-induced hyperprolactinemia is necessary to observe the body weight changes. This hypothesis was tested with clozapine, an atypical neuroleptic that produces in rats a shorter increase in serum prolactin levels than do other neuroleptics. The effects of clozapine on body weight and food intake were assessed in female and male rats under treatment with any of the following doses: 0.5, 1, 2.5, 5, 10, and 20 mg/kg IP for 21 days. Vaginal cycle under clozapine treatment, as an indirect indicator of ovarian steroidogenesis, was also assessed. Obesity was not observed in any group. By contrast, clozapine at the doses of 10 and 20 mg/kg significantly decreased body weight and feeding in male rats. Clozapine at the doses of 5 and 10 mg/kg IP induced permanent diestrus. The failure of clozapine to induce obesity in female rats, despite impaired vaginal cycle, can be considered indirect evidence that drug-induced hyperprolactinemia is not sufficient to observe neuroleptic-induced obesity in rats.

Changes in extracellular PVN monoamines and macronutrient intake after idazoxan or fluoxetine injection.

Abstract: Norepinephrine (NE) and serotonin (5-HT) in the paraventricular nucleus (PVN) have opposite effects on feeding, with NE stimulating carbohydrate intake through α 2 noradrenergic receptors and 5-HT inhibiting carbohydrate intake. This study examined the action of drugs that affect brain monoaminergic systems, in terms of their impact on nutrient intake and on PVN monoamines measured using microdialysis. The drugs studied were idazoxan, a blocker of α 2 receptors, or fluoxetine, a 5-HT reuptake blocker. In rats maintained on pure macronutrient diets, idazoxan (1 mg/kg) and fluoxetine (10 mg/kg), 120 min after injection both reduced total food intake, and specifically carbohydrate intake. In dialysis experiments, successive 20-min dialysate samples were taken, three samples before and seven samples after intraperitoneal injection of idazoxan (5 and 20 mg/kg), fluoxetine (10 mg/kg), or vehicle. Idazoxan increased NE, homovanillic acid, and hihydroxyphenylacetic acid in the PVN. Fluoxetine induced a significant increment of 5-HT in PVN, while producing a smaller increase in NE, dopamine, and homovanillic acid. These results support the conclusion that the impact of these drugs on macronutrient intake may be a consequence of their action on endogenous monoamine systems in the PVN. Thus, in this nucleus, the blockade of α 2 -noradrenergic receptors, like stimulation of 5-HT receptors, attenuates normal ingestion of carbohydrate.

Changes in the total content of iron, copper, and zinc in serum, heart, liver, spleen, and skeletal muscle tissues of rats infected with Trypanosoma cruzi.

Abstract: The effects of theTrypanosoma cruzi infection on the total content of the essential trace elements iron (Fe), copper (Cu), and zinc (Zn) in serum, heart, liver, spleen, and skeletal muscle were determined in “Wistar” rats inoculated with reticulotropic “Y” strain trypanosoms (Tryps) in their slender blood form. The 250 rats were divided in two groups of 80 rats (L-1 and L-2) and one of 90 (C) used as controls. L-1 and L-2 were inoculated with 2×105 and 5×102 Tryps, respectively. Ten rats of the C group were killed the inoculation day (i), and ten rats of each group chosen at random were killed and blood parasitemia determined at 5, 10, 15, 20, 25, 30, 60, and 90 post-i days covering the infection acute-phase myocarditis. Previously cryohomogenized and lyophilized tissues were digested in an HNO3− H2O2 mixture with the aid of a microwave oven, and the elements Fe, Cu, and Zn were determined by Flame Atomic Absorption Spectrometry. Generally, more intense changes were observed in the L-1 group. Serum Fe and Zn levels are lower and Cu levels higher in groups L-1 and L-2 than in C. However, Fe is not significantly sequestered in the liver during the acute phase of the infection as expected, but of the tissues studied, the spleen was the main site of Fe binding. Zn tended to increase in all tissues, except in the spleen, where during the acute phase of the infection, the total content of Zn in groups L-1 and L-2 was lower than in group C. Cu increased mainly in the spleen and muscle. In general, each tissue presented its own pattern of redistribution related to its nature, functions, and number of parasites inoculated, and these patterns may have been altered by the tropism of the parasite.

Passage of the bracken fern carcinogen ptaquiloside into bovine milk

Abstract: Summary - Ptaquiloside has been discovered for the first time in milk samples 01 a bovine fed a dietary complement (6 kg fresh weighVday) of freshly-eut bracken fronds. This was equivalent to ca 216 mg/day pure ptaquiloside. The compound was found in a quantity of no less than 0.11 mg/I in milk. This was determined by the appearance of pterosin 6 - the decomposition product of ptaquiloside - in prepurified milk sampi es treated with milg 1I1kllii (pH 8.0 and 11.5). The lack of base treatment led to the absence of pterosin B. A mi!kj}rilpurlflcatlon protocol based on meth,anol-acetonitrile precipitations in succession, hexllne and dlchloromethane extractions, alkaline treatment, reextraction and silica gel microcolumn chromatography followed by reverse-phase HPLC was developed for the detection 01 ptaquiloside.

Molybdenum uptake in Escherichia coli K12

Abstract: SUMMARY: Molybdenum uptake was examined in Escherichia coli K12 using the radionuclide 99Mo. The molybdenum uptake system was characterized in an unusual chlD strain, which appeared to be normal in uptake of the MoO2- 4 ion but altered in subsequent molybdenum processing. As a consequence, molybdenum could be chased from cells in the chlD strain, while it was irreversibly assimilated in the wild-type strain. Molybdenum uptake showed a biphasic kinetic curve, with a very rapid binding followed by a slow uptake phase. The uptake appeared to involve an active transport system. Molybdenum, probably in the form of molybdate, accumulated by a factor of about 30 in the cells. An energy source was necessary and uptake was inhibited by arsenate, but not by CCCP (carbonyl cyanide m-chlorophenylhydrazone). The uptake system saturated with a Km of 2.5-2.7 × 10-8 M. Uptake seemed to depend on a periplasmic binding protein, since cold shock treatment and arsenate abolished uptake. A molybdate binding protein activity was detected in the periplasmic fluid with a KD of 9 nM. Sulphate inhibited uptake and the uptake activity was pH dependent, with an apparent pK of 6.7. These results imply that molybdate transport belongs to the family of energy-dependent periplasmic binding protein systems. An explanation for the peculiar behaviour of the chlD strain used in this work is proposed.

Fever and feeding: Differential actions of macrophage inflammatory protein-1 (MIP-1), MIP-1α and MIP-1β on rat hypothalamus

Abstract: Changes in body temperature (Tb) and feeding were characterized in unrestrained rats following the micro-injection into the anterior hypothalamic preoptic area (AH/POA) of macrophage inflammatory protein-1 (MIP-1), MIP-1α or MIP-1β. After the rats recovered from the stereotaxic implantation of a single guide tube placed in the AH/POA, either one of the MIP-1 compounds or control CSF was micro-injected in a volume of 1.0 μl into this area. Changes in body temperature (Tb) and food and water intakes were monitored throughout each experiment. When micro-injected into the AH/POA in a dose of 28 or 280 pg, doublet MIP-1 and MIP-1β evoked a monophasic fever which increased above baseline to a mean maximum of 2.17±0.14°C and 2.1±0.24°C, respectively. MIP-1α micro-injected similarly evoked a biphasic fever, with the Tb declining transiently at the 30 min point ≥0.4°C lower than the congruent rises in Tb evoked by doublet MIP-1 or MIP-1β. The secondary rise in Tb induced by MIP-1α had a latency of 1.5–2.0 hrs and reached a maximum of 1.56±0.16°C. Although all three cytokines significantly attentuated the rats' mean intake of food during the 24 hr interval after their micro-injection into the AH/POA, doublet MIP-1 exerted the most potent anorexic effect in comparison to that of the saline control rats. However, neither body weight nor intake of water was altered significantly by the three cytokines. These results demonstrate a functional distinction between the febrile actions of MIP-1α and MIP-1β on cells of the AH/POA. It is envisaged that MIP-1β underlies the initial phase of a pyrogen-induced fever, whereas MIP-1α could mediate the secondary phase of a biphasic fever. Clearly, the localized elevation of either MIP-1α or MIP-1β in the AH/POA perturbs the neuronal mechanism underlying the “set-point” for body temperature. Thus, both MIP-1α and MIP-1β could play a functional role in the pathological events occurring in neurons of the AH/POA in response to an endotoxin or other pyrogen challenge.

Elastic stiffness constants of copper indium diselenide determined by neutron scattering

Abstract: From the acoustic-dispersion curves determined by inelastic neutron scattering, we obtain at room temperature the elastic stiffness constants of CuInSe 2 : at 10 10 Nm -1 , c 11 =9.70, c 12 =5.97, c 13 =8.60, c 33 =10.89, and c 44 =3.62. A theoretical calculation based upon the sphalerite approximation gives good agreement with the values of c 11 and c 44 . Using the elastic constants, the isothermal compressibility is calculated along the c axis and the ab plane

Effects of acute and chronic lithium treatment on amphetamine-induced dopamine increase in the nucleus accumbens and prefrontal cortex in rats as studied by microdialysis.

Abstract: The effects of acute and chronic administration of lithium (Li) on the basal levels of dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxy-indoleacetic acid (5-HIAA), and the amphetamine-induced DA increase were assessed in the Nucleus Accumbens (NAC) and Prefrontal Cortex (PFC) by brain dialysis in freely-moving rats. Acute Li (2meq/L) was locally administered by reverse dialysis. Chronic Li (2 meq/kg) was intragastrically administered for 14 days. No effect was observed after acute Li administration. However, after chronic Li administration, the basal levels of DOPAC and the amphetamine-induced DA increase in the NAC were significantly higher in the Li-treated rats than in the saline-treated controls. In the PFC, while the amphetamine-induced DA increase was not affected by chronic Li, the basal levels of DA and DOPAC were significantly decreased after Li administration. The effects of chronic Li in the NAC could be due to increased synthesis and/or decreased release of DA, whereas in the PFC the effects could be due to a decreased synthesis of DA. The absence of effects of acute Li administration is in agreement with the therapeutic inefficacy of the acute use of the cation. The changes observed after chronic treatment in the NAC and the PFC could be related to the effects of Li on mood disorders and cognitive functions, respectively.

The attitudes toward psychiatry of physicians and medical students in Venezuela

Abstract: Attitudes toward psychiatry were assessed in 490 medical students and 362 physicians from two universities in Venezuela: Universidad de Los Andes (ULA) and Universidad Centro-Occidental Lisandro Alvarado (UCLA). A modified version of the Attitude Toward Psychiatry-30 scale was used. The attitude toward psychiatry of all the students and physicians was moderately positive. Students from ULA displayed a more positive attitude than UCLA students, whereas physicians from UCLA displayed a more positive attitude than ULA physicians. The most negative attitude toward psychiatry was observed in students planning to select obstetrics and gynecology (ULA + UCLA), in physicians working in Surgery, obstetrics and gynecology (ULA + UCLA) and in basic sciences (ULA). Female students tended to display a more positive attitude than male students. The attitude toward psychiatry was similar in students with low and high academic achievement in the courses in psychiatry. A large number of years in the profession did not modify the attitude of the physicians. It is recommended to improve teaching and communication with the students and physicians that displayed the most negative attitudes toward psychiatry.

Radiative transfer for a three-dimensional raining cloud

Abstract: Satellite-sensor-based microwave brightness temperatures for a three-dimensional raining cloud over a reflecting surface are computed by using a radiative transfer model based on the discrete-ordinates solution procedure. The three-dimensional model applied to a plane layer is validated by comparison with results from a one-dimensional model that is available in the literature. Results examining the effects of cloud height, rainfall rate, surface reflectance, rainfall footprint area, and satellite viewing position on one- and three-dimensional brightness temperature calculations are reported. The numerical experiments indicate that, under certain conditions, three-dimensional effects are significant in the analysis of satellite-sensor-based rainfall retrieval algorithms. The results point to the need to consider carefully three-dimensional effects as well as surface reflectance effects when interpreting satellite-measured radiation data.

Pulse width modulated control of robotic manipulators

Abstract: We propose a practical discontinuous feedback control scheme for the regulation of joint positions of robotic manipulators. A robust on-off switching control strategy based on a pulse-width-modulation (PWM) feedback scheme is proposed for the joint torques. The discontinuous PWM controller design is carried out on the basis of a suitable controller designed for an average model which is of continuous nature. Simulations of the closed-loop performance of the proposed control scheme are presented for a two-link robotic manipulator.

Analysis of standard reference materials after microwave-oven digestion in open vessels using graphite furnace atomic absorption spectrophotometry and Zeeman-effect background correction

Abstract: Heating-covered teflon digestion vials located inside a reheatable container in the presence of different acid mixtures with microwave oven dissolve the metals from biological and environmental certified reference materials. Pb, Cd, Cu, Mn and Fe from the dissolved samples are determined by graphite furnace atomic absorption spectrophotometry and Zeeman-effect background correction. The method allows the treatment of about 100 samples per operation.

Flexible intravenous microdialysis probe for blood sampling in freely moving rats

Abstract: A flexible intravenous microdialysis probe was constructed from Silastic tubing (0.5 mm ID and 1.0 mm OD), with a cellulose hollow fiber tip 0.2 mm in diameter and 25 mm long with a 6,000 mol wt cut off. In vitro tests showed relative recovery rates of 39.1 +/- 1.9% for epinephrine. In vivo tests in freely moving rats, 36 h and 7 days after surgery, showed stable amounts of epinephrine and glucose. After intraperitoneal injections of 2-deoxy-D-glucose, circulating levels of epinephrine and glucose increased significantly. Similar results were obtained several days after implantation of the probe. We conclude that in situations where prolonged blood sampling is necessary, the flexible microdialysis probe provides a reliable means of accessing circulating levels of neuroactive compounds, nutrients, metabolites, and drugs.