Showing papers by "National Autonomous University of Mexico published in 1994"

A superluminal source in the galaxy

Abstract: APPARENT velocities greater than the speed of light (superluminal motion) have been inferred for radio-emitting components in a number of distant quasars and active galactic nuclei1. These components move away from the central sources (generally thought to be super-massive black holes) at rates that seem to imply velocities greater than c. The accepted explanation is that clouds of plasma are ejected in opposite directions from the central source at speeds close to (but less than) that of light, and that relativistic effects lead to the apparent superluminal motion2. But the extreme distance of the objects observed so far introduces many uncertainties into this interpretation3. Here we present observations of the first apparent superluminal motion ever detected in a source within our own Galaxy. The optical, infrared and X-ray properties4,5 of the counterpart suggest that the source is either a neutron star or a black hole that is ejecting matter in a process similar to, but on a smaller scale than that seen in quasars. Because of its relative proximity, this superluminal microquasar may offer the best opportunity to gain a general understanding of relativistic ejections seen elsewhere in the Universe.

Elemental abundances for a sample of southern galctic planetary nebulae

Abstract: We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with UV data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredgedup primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H II regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O > 0.8. In the current sample, 11 nebulae having N/O > 0.8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H=(4.93±2.22)×10−4 by number for 42 non-Type I PN and O/H=(4.42±1.44)×10−4 for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H II regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN. The helium abundances derived for the non-Type I PN are in accord with those predicted by BeckerI the carbon has been enhanced by He-burning processed material brought up by the third dredge-up.

Are microtremors useful in site response evaluation

Abstract: We have reviewed the applicability of microtremor measurements to evaluate site response of soft soils. To this end, we evaluated three different techniques commonly used to estimate site effects from microtremor measurements: interpretation of Fourier amplitude spectra, computation of spectral ratios relative to a firm reference station, and, finally, computation of spectral ratios of horizontal components relative to the vertical component of ground motion (Nakamura's technique). These techniques are applied to microtremor records obtained in three cities in Mexico: Mexico City, Oaxaca, and Acapulco. These cities differ in their local geological conditions and in their seismotectonic environment. In order to evaluate the results obtained from microtremor measurements, we compare them with standard spectral ratios of the intense, S -wave part of weak or strong motion records obtained at the same sites. Our results showed that microtremor measurements can be used to estimate the dominant period of a site with very acceptable reliability in the range 0.3 to 5 Hz. The best results were obtained with Nakamura's technique, which also gives a rough estimate of amplification of seismic waves when the local geology is relatively simple. Simple numerical simulations indicate that the underlying assumptions of Nakamura's technique are consistent with the propagation of Rayleigh waves. These simple numerical simulations also explain why different researchers have been able to successfully characterize 1D site effects using microtremor records, regardless of whether they consider microtremors to consist of surface or body waves. Our results strongly suggest that the technique by Nakamura effectively compensates for source effects in microtremor measurements, which eliminates a major limitation to their application in earthquake engineering.

Ecology of Free-Living Amoebae

Abstract: Small free-living amoebae (FLA) are the main predators controlling bacterial populations in soils. They are distributed in the rhizospheric zone and the surrounding bulk soil; however, they may spread deeper, reaching the vadose zone of groundwater systems, especially where bacterial populations get to high densities. Soil texture is the physical factor controlling the distribution of FLA because it determines the mean bore pore of soil aggregates and other important physical factors. FLA help maintain the high bacterial mineralization rate of organic matter through predation. As attachment onto a surface is necessary for feeding, the quantity of available surfaces is very important for developing this activity. However, the role of protozoa on plant growth promotion is still unclear because they may increase this effect by feeding on both fungi and bacteria. Small FLA are found in soils or sediments, as well as attached to suspended particulate matter in water columns, in the first 30 pm of water surface, or on the bodies of submerged animals and plants. These microor- ganisms do not distinguish between terrestrial or aquatic environments because they live in the interfaces between them. However, their importance in aquatic systems has been considered as negligible because they are outcompeted by free swimmers. The water conditions affecting amoebae survival are pH, temperature, concentration of sulfhydric acid and salinity. These factors have a strong influence on the structure of amoebae communities in aquatic environments. FLA are considered cosmopolitan as a group, and they live inside vertebrates, in soils, freshwater, marine waters, and on the aerial parts of plants and animals. These microbes, are spread by wind and water currents. Once in the air, cysts and trophozoites behave like any other suspended particulate matter. Therefore, suspension, transportation, and removal depend on atmospheric dynamics rather than on their own mechanisms. Ultraviolet light and drought are the main causes of losing viability, but much needs to be learned about the effects of air contaminants on amoebal survival. Naked amoebae also live in the phyllosphere as part of phylloplane community, but their importance and participation in this environment remain unknown. Some species belonging to the genera Acantharnoebae, Naegleria, and Balamuthia cause fatal diseases in humans and are carriers of other pathogens such as Legionella pneumophilia. However, FLA communities can be of some utility in sewage treatment works based in soil filters. %A's predatory activity in the root zone method may be of greater importance than previously thought, because this is their natural or more favorable environment. Research on the feeding behavior of FLA species, their rate of biomass transformation, and growth rate changes in response to environmental stresses are still needed in order to clarify their importance in both sewage system works and agricultural fields, where both bacteria and amoebae are involved.

Return of the native

Abstract: structure, and the decentring of narrative authority” (xxvii) 89:;# \" < RN = >? \"#@$ (% )A B2 (Thomas Hardy)-C D EF GHIJ KLM# N$ KO; CD EF -A B2 >?PQ7./012 RS8L T ULV$ >?WX YZ[G\\] ^ 56 LM#\" < RN $ EF _ @ -$ ` 1a bc)dea (Eustacia Vye) fgU-L\"T]Oh< i j$ * G klM#\" ` 1abcPmnop fg-$ B .q)ras B2 t (Damon Wildeve) 8 uv)w txa( (Clym Yeobright) P y ; zhPmnop {\"| (a dull sound) (287) lM#}~ $ ` 1abcPmnop € ‚ƒ-„h L\"< ` 1abc …† C‡Pˆ‰ŠL ‹$ ` 1abc-Œ L h$ Ž 8 @L h8\"O‚‘L!’P KL #“ < ”•>? \"#$ –[8 -—˜L…™ }N$ ` 1abc lO † –[š š”•>? ›f† < 8œ$ (% )

Mesoscopic transport through chaotic cavities: A random S -matrix theory approach

Abstract: We deduce the effects of quantum interference on the conductance of chaotic cavities by using a statistical ansatz for the S matrix. Assuming that the circular ensembles describe the S matrix, we find that the conductance fluctuation and weak-localization magnitudes are universal: they are independent of the size and shape of the cavity if the number of incoming modes, N, is large. The limit of small N is more relevant experimentally; here we calculate the full distribution of the conductance and find striking differences as N changes or a magnetic field is applied.

Cellular and molecular characterization of Ca2+ currents in acutely isolated, adult rat neostriatal neurons

Abstract: Ca2+ currents in acutely isolated, adult rat neostriatal neurons were studied with whole-cell voltage-clamp techniques. In the vast majority of neurons (approximately 90%, n > 250), currents were exclusively of the high-voltage-activated (HVA) type. HVA currents activated near -40 mV and reached their maximum amplitude near 0 mV. Quasi-steady-state inactivation curves in many neurons were well fitted only with a sum of Boltzmann functions, suggesting that the HVA currents were heterogeneous. Although the block of whole-cell current by Cd2+ was well fitted with a single isotherm having an IC50 of near 1 microM, experiments with organic channel antagonists suggested that at least four types of HVA channels were expressed by most cells. On average, the L-channel antagonist nifedipine (5–10 microM) blocked 31 +/- 10% of the whole-cell current (n = 20), the N-channel antagonist omega- conotoxin GVIA (omega-CgTx) (2–5 microM) blocked 27 +/- 11% (n = 20), and the P-channel antagonist omega-agatoxin IVA (100–500 nM) blocked 21 +/- 10% (n = 18). In many neurons, the block by omega-CgTx was partially or completely reversible. In cells tested with a combination of these antagonists, 34 +/- 17% of the peak Ca2+ current remained unblocked (n = 13). Single-cell expression profiling of medium-sized neurons revealed the presence of rbA and rbB Ca2+ channel alpha 1 subunit mRNAs but low or undetectable levels of rbC mRNA (n = 12). These findings suggest that although adult neostriatal projection neurons do not express significant levels of LVA Ca2+ current, they do express a pharmacologically and structurally heterogeneous population of HVA currents.

Nucleologenesis: U3 snRNA-containing prenucleolar bodies move to sites of active pre-rRNA transcription after mitosis.

Abstract: We have investigated the distribution of U3 snRNA and rRNA in HeLa cells and normal rat kidney cells during interphase and mitosis. U3 snRNA, known to be involved in pre-rRNA processing, was detected in nucleoli and coiled bodies during interphase, whereas rRNA was distributed in the nucleoli and throughout the cytoplasm. By comparison, ribosomal protein S6 was detected in nucleoli, coiled bodies, and in the cytoplasm. During nucleologenesis, pre-rRNA was observed in newly forming nucleoli during late telophase but not in prenucleolar bodies (PNBs), whereas U3 snRNA was detected in forming nucleoli and PNBs. Similar findings to those reported here for the localization of U3 snRNA have been reported previously for the U3 small nuclear ribonucleoprotein fibrillarin. These results suggest that components involved in pre-rRNA processing localize to discrete PNBs at the end of mitosis. The nucleolus is formed at specific telophase domains (nucleolar organizing regions) and the PNBs, containing factors essential for pre-rRNA processing, are recruited to these sites of rRNA transcription and processing.

Non flying mammals and landscape changes in the tropical rain forest region of Los Tuxtlas, Mexico

Abstract: The rapid and extensive conversion of Neotropical rain forests to a landscape consisting of pasture lands and other agricultural habitats has meant the local disappearance fragmentation and isolation of vertebrate populations To avoid further losses in biodiversity in Neotropical rain forests, it is imperative to determine how the different species that constitute the different vertebrate communities have responded to the anthropogenic alterations of their natural habitats To provide data in this direction we sampled non flying mammals in 35 forest fragments, in six disturbed forest sites in 12 regenerating forests, in 20 arboreal agricultural habitats (cacao coffee, mixed, citrus and allspice), in four live fence sites and in four pasture sites at Los Tuxtlas, Veracruz Mexico Sampling indicated that area of forest fragments and isolating distance were important variables influencing species richness Disturbed forest had fewer species and individuals present and supported a lower non flying mammal biomass than undisturbed and regenerating forests Shaded plantations (cacao coffee and mixed) were richer in species and in individuals than unshaded ones (citrus, allspice) Species richness in forests and agricultural habitats were associated to horizontal plant diversity and vertical foliage diversity Pasture habitats were the poorest man-made habitats in non flying mammal species Live fences supported a higher biomass and species of non flying mammals than pastures and were similar to forest and shaded man-made habitats in species assemblages

Seroepidemiology of amebiasis in Mexico.

Abstract: The presence of serum antibodies to Entamoeba histolytica was detected by indirect hemagglutination in a representative sample of the 32 federal entities of the Mexican Republic. This study was designed to include biologic, geographic, social, economic, and educational variables. The total percentage of positive sera was 8.41%. Seroprevalence varied with geographic zones, with the South Central, South Pacific, and Yucatan Peninsula areas showing the highest values (> or = 9%), and the North, Northeast, and Gulf of Mexico areas showing the lowest values (< or = 8.0%). Seroprevalence of anti-E. histolytica antibodies seemed to increase from the northern regions to the southern areas of Mexico. These results indicated that amebiasis is endemic in the Mexican Republic, with areas of high seroprevalence not related to climatic conditions. Exposure to infectious contact with E. histolytica occurred at all ages, with a higher frequency at school age.

Dynamical density functional theory of gas-liquid nucleation

Abstract: We present a consistent dynamical nucleation theory based on density functional theory By considering the properties of stable droplets in closed volumes, the height and shape of the barrier to nucleation are calculated Contributions from fluctuations in the center of mass of the nucleating cluster are taken into account Forward and backward rates for cluster dynamics are obtained, and nucleation rates are then evaluated under steady‐state conditions We test the quantitative effects of several shortcuts to calculating nucleation rates The predictions of the full theory presented here show very modest changes from those of the simpler nonclassical theory proposed earlier by Oxtoby and co‐workers

PCR analysis of the cryI insecticidal crystal family genes from Bacillus thuringiensis.

Abstract: A method allowing rapid and accurate identification of different subgroups within the insecticidal crystal CryI protein-producing family of Bacillus thuringiensis strains was established by using PCR technology. Thirteen highly homologous primers specific to regions within genes encoding seven different subgroups of B. thuringiensis CryI proteins were described. Differentiation among these strains was determined on the basis of the electrophoretic patterns of PCR products. B. thuringiensis strains, isolated from soil samples, were analyzed by PCR technology. Small amounts of bacterial lysates were assayed in two reaction mixtures containing six to eight primers. This method can be applied to rapidly detect the subgroups of CryI proteins that correspond with toxicity to various lepidopteran insects. Images

Contribución al conocimiento de las cactáceas mexicanas amenazadas

Abstract: Mexico is the most important center of concentration of genera and species of cacti, most of which are endemic (73 and 78% respectively). Most of the species occur in the arid or semiarid regions of the country, especially in the southeastern portion of the Chihuahuan Desert, including the Queretaro-Hidalgo Arid Zone. Many of the species are under collecting pressures and their habitats are being destroyed. In addition, these have a limited ability to reestablish demographically after a disturbance event. Consequently, a significant number of mexican cacti are listed as endangered.

How long did it take for life to begin and evolve to cyanobacteria

Abstract: There is convincing paleontological evidence showing that stromatolite-building phototactic prokaryotes were already in existence 3.5 x 10(9) years ago. Late accretion impacts may have killed off life on our planet as late as 3.8 x 10(9) years ago. This leaves only 300 million years to go from the prebiotic soup to the RNA world and to cyanobacteria. However, 300 million years should be more than sufficient time. All known prebiotic reactions take place in geologically rapid time scales, and very slow prebiotic reactions are not feasible because the intermediate compounds would have been destroyed due to the passage of the entire ocean through deep-sea vents every 10(7) years or in even less time. Therefore, it is likely that self-replicating systems capable of undergoing Darwinian evolution emerged in a period shorter than the destruction rates of its components (<5 million years). The time for evolution from the first DNA/protein organisms to cyanobacteria is usually thought to be very long. However, the similarities of many enzymatic reactions, together with the analysis of the available sequence data, suggest that a significant number of the components involved in basic biological processes are the result of ancient gene duplication events. Assuming that the rate of gene duplication of ancient prokaryotes was comparable to today's present values, the development of a filamentous cyanobacterial-like genome would require approximately 7 x 10(6) years--or perhaps much less. Thus, in spite of the many uncertainties involved in the estimates of time for life to arise and evolve to cyanobacteria, we see no compelling reason to assume that this process, from the beginning of the primitive soup to cyanobacteria, took more than 10 million years.

Population genetic structure of cecropia obtusifolia, a tropical pioneer tree species.

Abstract: Theoretical analyses of the genetic organization of pioneer species have postulated two very different scenarios. Some models have predicted that such species would show strong population substructuring, whereas other models have suggested that extinction and recolonization can augment gene flow and reduce interpopulation differentiation. We tested these alternative scenarios by analyzing the genetic structure of eight loci from populations of the pioneer dioecious tree, Cecropia obtusifolia, in the tropical rain forest region of Los Tuxtlas, Mexico. The populations studied exhibit low overall FST values, no clear pattern of isolation by distance, and high estimates of gene flow. These results suggest either that the species is not at a genetic equilibrium under present levels of gene flow with populations derived from each other in the recent past, or that pollen and seed dispersal in this species occur over long distances (up to more than 100 km). Mating among relatives appears higher than expected by chance based on significantly positive fixation indices (F) and FIS values at some loci. However, no direct evidence for biparental inbreeding was found. The multilocus and single-locus outcrossing rates for C. obtusifolia were estimated at tm = 0.974 (SE = 0.024) and ts = 0.980 (SE = 0.035), respectively. These are not significantly different from 1, and the difference, tm - ts = - 0.006 (SE = 0.018), is not significantly different from 0. These estimates, however, could be biased because in all enzymes, except PGM-1, we found statistically significant departures from the mixed-mating model used to estimate them. Two rare alleles were found only in seeds collected from the soil, and the greatest number of different alleles were found also in soil seeds. It is hypothesized that the seed bank may play an important role in the genetic buffering of C. obtusifolia. Significantly positive or negative fixation indices in adults at some loci and significantly different heterozygosities among different life stages (from seeds to adults) suggest the action of selection at some loci.

Regulatory volume decrease in cultured astrocytes. II. Permeability pathway to amino acids and polyols

Abstract: The permeability of the hyposmolarity-activated pathway to amino acids and polyols in cultured astrocytes was examined following the change in rate and direction of regulatory volume decrease (RVD) when the extracellular concentration of the osmolytes was increased to reverse their intracellular-extracellular concentration gradient. Activation of the pathway by swelling would allow those permeable osmolytes to enter the cell and inhibit RVD. The pathway was found to be permeable to neutral amino acids, with beta-amino acids (beta-alanine = taurine > gamma-aminobutyric acid) more permeable than alpha-amino acids. Glycine, alanine, threonine, phenylalanine, and asparagine, but not glutamine, were permeable through this pathway. Aspartate was more permeable than glutamate, and K+ and not Na+ must be the accompanying cation. Basic amino acids were excluded. The dimension of the amino acid pore activated by hyposmolarity seems to be at the limit of glutamate-glutamine size. Influx rather than efflux of amino acids was observed when extracellular concentration was greater than intracellular concentration, with differences in the amount accumulated by cells correlating with their efficiency as RVD blockers. Influx of taurine (as representative of permeable amino acids) was inhibited by the Cl- channel blockers/exchangers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (40%) and dipyridamole (85%) , and it is suggested that amino acids permeate through an anion channel. Sorbitol and mannitol, but not inositol, exhibited a small inhibitory effect on the later phase of RVD, whereas inositol slightly accelerated RVD.

A proposal for classification of neurocysticercosis.

Abstract: The complicated pathophysiological and immunological changes in the central nervous system of patients with neurocysticercosis produce a variety of signs and symptoms, which complicate the clinical and surgical management of this disease. A complete and objective classification is needed, to improve the medical approach as a whole. We studied 336 patients, in whom we classified neurocysticerosis according to criteria of viability and location of the parasite in the CNS: active form (37.2%) when the cysticercus is alive, transitional form (32.8%) when it is in the degenerative phase, and inactive form (30%) when the parasite is dead. This classification establishes the correlation between the different forms of neurocysticerosis and its clinical manifestations, and can be used for planning therapeutic strategies.

Regulatory volume decrease in cultured astrocytes. I. Potassium- and chloride-activated permeability

Abstract: Regulatory volume decrease (RVD) in detached cerebellar astrocytes in culture after acute exposure to hyposmolarity was characterized in this and the accompanying paper [H. Pasantes-Morales, R. A. ...

The prolactin gene is expressed in the hypothalamic-neurohypophyseal system and the protein is processed into a 14-kDa fragment with activity like 16-kDa prolactin

Abstract: The 23-kDa form of prolactin (PRL) has been proposed to function as both a mature hormone and a prohormone precursor for different uniquely bioactive forms of the molecule. We have shown that the 16-kDa N-terminal fragment of PRL (16K PRL) inhibits angiogenesis via a specific receptor. In addition, 16K PRL stimulates natriuresis and diuresis in the rat, and kidney membranes contain high-affinity specific binding sites for this PRL fragment. 16K PRL can be derived from an enzymatically cleaved form of PRL (cleaved PRL). With the use of a specific 16K PRL antiserum, we have localized a 14-kDa immunoreactive protein in the paraventricular and supraoptic nuclei of the hypothalamus and in the neurohypophysis. Reverse transcription-polymerase chain reaction of RNA from isolated paraventricular nuclei showed the expression of the full-length PRL mRNA. The neurohypophysis was found to contain the enzymes that produce cleaved PRL, small amounts of PRL, and cleaved PRL. Medium conditioned by neurohypophyseal cultures, enriched with the 14-kDa immunoreactive protein, has antiangiogenic effects that are blocked by the 16K PRL antiserum. These results are consistent with the expression of PRL in the hypothalamic-neurohypophyseal system, and the preferential processing of the protein into a 14-kDa fragment with biological and immunological properties of 16K PRL.

Motivations and obstacles to university industry cooperation (UIC): a Mexican case

Abstract: Within the context of linkage between universities and the productive sector in Mexico, this article seeks to answer the following questions: i) which are the principal motivations for researchers at universities to carry out technological research and to seek ties to industry? ii) which are the main motivations for industrial entrepreneurs to establish cooperation with universities to develop technology? and iii) which are the main barriers to a healthy relationship between both institutions? Our answers are based on data from two sources: (a) an exploratory survey of 31 researchers at the National University of Mexico and of 28 Mexican entrepreneurs, half of whom had previous experience in collaborative projects; and, (b) a subsequent panel discussion among questionnaire respondents designed to elicit in-depth qualitative data concerning motivations and obstacles to university-industry cooperation. The authors present the factors identified in the study as key elements in the development of liaisons between the two sectors in Latin American countries, and argue the need to rethink our conceptions of motivations and barriers within a theoretical framework of interinstitutional communications and organizational cultural change.

Lymphocyte replicating ability in individuals exposed to arsenic via drinking water.

Abstract: A human monitoring study was carried out to explore the effect on lymphocyte proliferation of chronic exposure to arsenic (As) via drinking water. Blood and urine samples were taken from volunteers from a town where levels of As in the drinking water averaged 412 micrograms/l, and from a matched group of individuals, with similar socioeconomic status, that drank water with As average levels of 37.2 micrograms/l. Exposure was assessed by questionnaires and by determining the levels of As in urine and water samples. The evaluation of the peripheral blood lymphocyte proliferation was done at different culture times using labelling (LI), mitotic (MI) and replication indexes (RI) as endpoints. No significant differences were seen for either LI or MI, except for MI in 72 h cultures and in LI in males and females with skin lesions vs. those without lesions. Significant differences in RI were seen for exposed females but not for males. Correlations between LI and MI showed that progression from the initial S-to M-phase is altered in exposed individuals. Arsenic exposure as well as lead and mercury affect cellular immune response, making the endpoints of cell proliferation variables of interest in population monitoring study design, since they might provide information in health impairment due to exposure, which is important in risk assessment.