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41.
Dawson K 《Bioethics》1988,2(1):1-14
A basic consideration in research on human embryos is the controversy about when the embryo acquires moral status. The author refutes the contention that segmentation is the determinant of moral status. She notes that segmentation, as a stage in embryonic development, does not coincide with the development of "irreversible individuality" upon which the segmentation argument depends. Dawson also finds a lack of clarity in the meaning of "individuality." These problems, she maintains, prevent segmentation from being morally important and render the proposed 14-day limit on embryo research unnecessary. Dawson concludes that to introduce a time restriction on embryo research is premature because it is based on an inadequate philosophical argument. 相似文献
42.
Hypoxanthine-guanine phosphoribosyltransferase (HPRT, EC 2.4.2.8) is a purine salvage enzyme that catalyses the conversion of hypoxanthine and guanine to their respective mononucleotides. Partial deficiency of this enzyme can result in the overproduction of uric acid leading to a severe form of gout, whilst a virtual absence of HPRT activity causes the Lesch-Nyhan syndrome which is characterised by hyperuricaemia, mental retardation, choreoathetosis and compulsive self-mutilation. The HPRT-encoding gene is located on the X chromosome in the region q26–q27 and consists of nine exons and eight introns totalling 57 kb. This gene is transcribed to produce an mRNA of 1.6 kb, which contains a protein encoding region of 654 nucleotides. With the advent of increasingly refined techniques of molecular biology, it has been possible to study the HPRT gene of individuals with a deficiency in HPRT activity to determine the genetic basis of the enzyme deficiency. Many different mutations throughout the coding region have been described, but in the absence of precise information on the three-dimensional structure of the HPRT protein, it remains difficult to determine any consistent correlation between the structure and function of the enzyme. 相似文献
43.
Bettina M. Diekamp H. Carl Gerhardt 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1992,171(2):245-250
1. We derived audiograms from recordings of multiunit activity in the torus semicircularis of 10 males and 6 females of the spring peeper from central Missouri, USA. We used free-field stimulation with tone bursts that had temporal properties similar to typical advertisement calls and that ranged in frequency from 500-6000 Hz. 2. Audiograms from different electrode positions in the same animal had the same general shape. There was no evidence of tonotopy. 3. Audiograms showed two regions of maximal sensitivity: a low-frequency region (500-700 Hz); and a high-frequency region (2000-4000 Hz). Absolute thresholds and frequencies of maximum sensitivity varied considerably from individual to individual. 4. Audiograms derived from all individuals of each sex indicated that in the high-frequency region, corresponding to the frequency range of advertisement calls, males were more broadly tuned than females. However, tuning in both sexes was relatively weak, and the data predict relatively little selectivity in behavioral responses over the entire range of variation in frequency of the advertisement call in local populations. 5. The results are discussed in terms of behavioral experiments with both males and females from the same populations in central Missouri. We show that merely summarizing the audiograms based on estimates of minimum thresholds of a population or species may mask significant individual differences in tuning. Moreover, most behavioral studies are conducted at playback levels considerably above threshold. For these reasons, behavioral selectivity is not always accurately predicted by inspection of "average" audiograms. 相似文献
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The regulation of PAG by ammonia in whole brain (Sprague-Dawley) and regional (Fischer-344) synaptosomal preparations from adult and aged animals was assessed. Whole brain synaptosomal preparations from both age groups displayed a significant decrease in PAG activity with increasing ammonium chloride concentrations, however, the aged rats exhibited a significant attenuation in ammonia-induced PAG inhibition. PAG activity measured in synaptosomes prepared from the striatum (STR), temporal cortex (TCX) and hippocampus (HIPP) was also inhibited by ammonium chloride. The STR showed the greatest degree of ammonia-induced PAG inhibition (55%) followed by the HIPP (30–35%) and the TCX (25–30%). This reduction in PAG activity was significantly attenuated in STR from aged rats at ammonium chloride concentrations greater than 50 M and in the TCX, PAG activity was significantly attenuated in the aged rats at ammonia concentrations of 0.5 and 1.0 mM. Ammonia regulation of PAG activity in the HIPP appeared to be unaffected by age. Ammonium chloride concentrations up to 5 mM had no effect on GLU release from cortical slices, although GLN efflux was significantly enhanced. These findings suggest that isozymes of PAG may exist in different brain regions based on their differential sensitivity to ammonia. The attenuation of ammonia-induced PAG inhibition seen in aged rats may have deleterious effects in the aged brain.Abbreviations PAG
phosphate-activated glutaminase: L-glutamine amidohydrolase; EC 3.5.1.2
- STR
striatum
- TCX
temporal cortex
- HIPP
hippocampus 相似文献
46.
Seasonal carbon isotope discrimination in a grassland community 总被引:11,自引:0,他引:11
Mark P. Smedley Todd E. Dawson Jonathan P. Comstock Lisa A. Donovan Dorothy E. Sherrill Craig S. Cook James R. Ehleringer 《Oecologia》1991,85(3):314-320
Summary Grassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture. 相似文献
47.
Summary We investigated the effects of thorns and spines on the feeding of 5 herbivore species in arid Australia. The herbivores were the rabbit (Oryctolagus cuniculus), euro kangaroo (Macropus robustus), red kangaroo (Macropus rufus), sheep (Ovis aries), and cattle (Bos taurus). Five woody plants without spines or thorns and 6 woody plants with thorns were included in the study. The spines and thorns were not found to affect the herbivores' rates of feeding (items ingested/min), but they did reduce the herbivores' rates of biomass ingestion (g-dry/item). The reduction in biomass ingested occurred in two ways: at a given diameter, twigs with spines and thorns had less mass than undefended plants, and the herbivores consumed twigs with smaller diameters on plants with spines and thorns. The relative importance of the two ways that twigs with spines and thorns provided less biomass varied with herbivore body mass. Reduced twig mass was more important for small herbivores, while large herbivores selected smaller diameters. The effectiveness of spines and thorns as anti-herbivore defenses did not vary with the evolutionary history of the herbivores (i.e. native vs. introduced). Spines and thorns mainly affected the herbivores' selection of maximum twig sizes (reducing diameter and mass), but the minimum twig sizes selected were also reduced. 相似文献
48.
Summary Semi-thin sections of three-dimensional reaggregates from adult female rat pituitary, cultured in serum-free defined medium, were stained for prolactin, gonadotropin, thyrotropin, growth hormone and S-100, using the double immunolabelling technique. The frequency of juxtaposition between lactotrophs and gonadotrophs was enumerated and compared with the expected frequency at random distribution of polygonal cell profiles in a hexagonal configuration. The proportions of lactotrophs and gonadotrophs in the aggregate sections were determined using stereometrical analysis. The observed frequency of juxtaposition did not differ significantly from the expected frequency. Hence, no reason was found to assume a selective adhesion between lactotrophs and gonadotrophs in adult female rat pituitary reaggregates. A constant proportion of lactotrophs was found to meet the criteria of a cup-shaped morphology, and 70%±9% (mean ±S.D.) of these so-called cupshaped lactotrophs were found to be juxtaposed at their concave side to gonadotrophs. Administration of 0.01 nM 17-oestradiol to the culture medium resulted in a significant reduction of the proportion of cup-shaped lactotrophs but did not affect the selectivity of juxtaposition to gonadotrophs. The selectivity of juxtaposition between cup-shaped lactotrophs and gonadotrophs may be the morphological correlate of the functional relationship between these cells, which are known to be involved in an intra-pituitary paracrine communication system. 相似文献
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