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31.
32.
Preincubation of brain membranes with phospholipase A2 (PLA2) has been shown previously to affect the binding characteristics of various recognition sites associated with the gamma-aminobutyric acid (GABA) receptor complex. In the present study, we have investigated the effects of PLA2 (from Naja naja siamensis venom) on the functional activity of the GABA receptor/chloride ion channel. PLA2 (0.001-0.02 U/mg protein) preincubation decreased pentobarbital-induced 36Cl- efflux and muscimol-induced 36Cl- uptake in rat cerebral cortical synaptoneurosomes. The effect of PLA2 was prevented by EGTA and two nonselective PLA2 inhibitors, mepacrine and bromophenacyl bromide. The removal of free fatty acids by addition of bovine serum albumin both prevented and reversed the effect of PLA2. Products of the catalytic activity of PLA2, such as the unsaturated free fatty acids, arachidonic and oleic acids, mimicked the effect of PLA2. However, the saturated fatty acid, palmitic acid, and lysophosphatidyl choline had no effect on pentobarbital-induced 36Cl- efflux. Because unsaturated free fatty acids are highly susceptible to peroxidation by oxygen radicals, the role of oxygen radicals was investigated. Xanthine plus xanthine oxidase, a superoxide radical generating system, mimicked the effect of PLA2, whereas the superoxide radical scavenger, superoxide dismutase, diminished the effects of PLA2 and arachidonic acid on pentobarbital-induced 36Cl- efflux. Similarly, the effect of PLA2 was also inhibited by methanol (1 mM), a scavenger of the hydroxyl radical, and by catalase. These data indicate that exogenously added PLA2 induces alterations in membrane phospholipids, possibly promoting the generation of oxygen radicals and fatty acid peroxides which can ultimately modulate GABA/barbiturate receptor function in brain. 相似文献
33.
Significant differences in the thermal stability of benzodiazepine receptors were found in cerebral cortical membranes prepared from the long-sleep (LS) and short-sleep (SS) selected mouse lines. Thus, benzodiazepine receptors from LS mice were heat inactivated (55 degrees C) at a significantly faster rate than those from SS mice. Although gamma-aminobutyric acid (GABA) reduced the rate of heat inactivation in both lines, the more rapid rate of inactivation in the LS line was maintained. Furthermore, the potency of GABA to enhance [3H]flunitrazepam binding decreased threefold in membranes from LS mice as the incubation temperature was increased from 0 degrees to 37 degrees C, but was unaltered in membranes from SS mice. These differences in the biophysical properties of the benzodiazepine/GABA receptor chloride channel complex ("supramolecular complex"), together with a higher KD for t-[35S]butylbicyclophosphorothionate in membranes from LS compared to SS mice, suggest that the supramolecular complex may modulate the differential sensitivity to some depressants and convulsants in these lines. 相似文献
34.
Life-history strategies of Australian lizards: a comparison between the tropics and the temperate zone 总被引:1,自引:0,他引:1
Summary Previous studies have suggested that tropical and temperate-zone lizards may differ fundamentally in life histories. We tested the applicability of this idea to Australian species by comparing temperate-zone species of agamid and scincid lizards with their congeners from the seasonal tropics. Data were derived from dissection of 1,941 specimens and from published information. Clutch size and egg size were positively correlated with mean maternal body size in most lizard species from both climatic zones. Mean body size of the lizards studies did not differ between the tropics and the temperate zone, nor did egg or hatchling size. However, tropical skinks showed considerably (approximately 20%) lower clutch size and relative clutch mass than did temperate-zone skinks. This difference was partly due to the higher incidence of species with low, invariant clutch size in the tropical lizard fauna (as seen in other continents as well), but primarily due to a trend for lineages (especially genera) with relatively high fecundity to be more common in the temperate zone than in the tropics. In contrast to studies on African lizards, our data suggested that modification of clutch size between areas has not occurred within genera: congeneric species from the tropics and temperate zone did not differ in clutch size. Production of more than one clutch per annum by individual females was common in both climatic zones. Tropical lizards may differ from temperate-zone species in showing higher reproductive frequencies, more rapid growth and earlier maturation. However, most of these effects may be due to phenotypic responses to environmental conditions (especially longer annual activity season), rather than to genetically based lifehistory adaptations. 相似文献
35.
Craig Loehle 《Plant Ecology》1988,79(3):109-115
Stability analysis of whole forests is proposed as a qualitative tool for the study of forest responses to partial or patchy harvests or mortality. Instead of modeling every tree or stand, aggregate tree biomass is modeled. In order to aggregate stands, spatial effects must be incorporated. It is shown that depensation growth responses (reduced growth at low biomass) are common because forests often modify harch environments to be more suitable for their growth. Depensation can result from thinning, partial mortality, patchy cutting, or clearcutting, depending on forest type and abiotic factors. Examples of these types of behaviors are given. Stability analysis of different growth regimes under exploitation are related to catastrophe theory and to optimal harvesting policies. Such qualitative analysis is shown to be applicable to data-poor regions such as the tropics where there is great concern over responses of forests to exploitation. 相似文献
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The DXS255 locus at Xp11.22 is highly polymorphic due to a 26-bp variable number of tandem repeats (VNTR) motif. In previous studies, one of the MspI sites flanking the VNTR manifested a correlation between methylation and X chromosome inactivation. Here we show, by DNA sequence analysis, that this MspI site is located within the CpG island at the 5' end of a LINE-1 element, which is 2.5 kb from the VNTR. The methylation status of the CpG island was assessed in Southern blotting experiments using the methylation-sensitive enzymes HpaII, HhaI, and BssHII. All these sites were completely methylated on active X chromosomes, consistent with previously reported findings of full methylation of LINE-1 elements throughout the genome. However, on inactive X chromosomes these sites were predominantly unmethylated, although patterns were found to be heterogeneous. The results suggest that LINE-1 elements on the inactive X chromosome are not suppressed by full methylation of their CpG islands. The differential methylation of the DXS255 CpG island provides the basis for a highly informative X inactivation analysis system. 相似文献
38.
Stress modulates calcium mobilization in immune cells 总被引:1,自引:0,他引:1
Both acute and chronic restraint stress modulated mitogen-induced increases in cytoplasmic free-calcium concentrations ([Ca2+]i) in mouse spleen cells. Dual-color analysis of lymphocyte subpopulations demonstrated that acute (2 hour) restraint stress suppressed mitogen-stimulated increases in [Ca2+]i in CD4+ T cells, but enhanced [Ca2+]i in CD8+ T cells. Chronic restraint stress (2 hours daily for up to 21 days) resulted in a significant suppression of mitogen-stimulated increases in [Ca2+]i in CD4+ T cells at 3 and 7 days, but not at 21 days. CD8+ T cells were unaffected by chronic stress. Chronic stress (for 7 days) had a modest suppressive effect on mitogen-induced Ca2+ responses in B cells. Within T lymphocyte subpopulations, both acute and chronic stress predominantly affected CD4+ T cells, which may induce a functional reversal of the CD4/CD8 ratios in vivo. Such a reversal could result in suppression of a variety of immune responses such as lymphocyte proliferation and antigen-specific antibody production. These findings indicate that the inhibitory effects of stress on calcium mobilization in lymphocytes may be an early event mediating stress-induced immunosuppression. 相似文献
39.
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
40.