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21.
Anatoli I. Yashin Kenneth G. Manton Eric Stallard 《Journal of mathematical biology》1986,24(2):119-140
Analyses of human mortality data classified according to cause of death frequently are based on competing risk theory. In particular, the times to death for different causes often are assumed to be independent. In this paper, a competing risk model with a weaker assumption of conditional independence of the times to death, given an assumed stochastic covariate process, is developed and applied to cause specific mortality data from the Framingham Heart Study. The results generated under this conditional independence model are compared with analogous results under the standard marginal independence model. Under the assumption that this conditional independence model is valid, the comparison suggests that the standard model overestimates by 4% the effect on life expectancy at age 30 due to the hypothetical elimination of cancer and by 7% the effect for cardiovascular/cerebrovascular disease. By age 80 the overestimates were 11% for cancer and 16% for heart disease. These results suggest the importance of avoiding the marginal independence assumption when appropriate data are available — especially when focusing on mortality at advanced ages. 相似文献
22.
Paola Cescutti Neil Ravenscroft Stephen Ng Zamas Lam Guy G.S. Dutton 《Carbohydrate research》1983,244(2):325-340
The capsular polysaccharide of Klebsiella SK1 was investigated by methylation analysis, Smith degradation, and 1H NMR spectroscopy. The oligosaccharides (P1 and P2) obtained by bacteriophage ΦSK1 degradation of the polymer were studied by methylation analysis, and 1D- and 2D-NMR spectroscopy. The resulting data showed that the patent repeating unit is a branched pentasaccharide having a structure identical to the revised structure recently proposed for Klebsiella serotype K8 capsular polysaccharide. The 2D-NMR data showed that one third of the glucuronic acid residues in the SK1 polymer are acetylated at O-2, O-3, or O-4. FABMS studies confirmed the presence of monoacetylated glucuronic acid residues. Thus, the relationship between the Klebsiella K8 and SK1 polymers is akin to that found for Klebsiella polysaccharides K30 and K33, which have been typed as serologically distinct yet their structures differ only in the degree of acetylation. 相似文献
23.
Chinese hamster V79 cells were mutagenized with ethyl methanesulfonate at various concentrations. Clones resistant to 8-azaguanine (20 and 80 micrograms/ml) or 6-thioguanine (4 micrograms/ml) were selected at different times after the treatments. The total yield of induced mutations was only slightly affected by the kind and concentration of purine analog used in the selection. However, full phenotypic expression of the mutants selected with 8-azaguanine was achieved earlier than that of mutants resistant to 6-thioguanine. This result seems to be best explained by the reported lower affinity of 8-azaguanine for the wild-type HGPRT enzyme, thus providing evidence that, in this gene-mutation assay, the phenotypic expression time has a physiological component. 相似文献
24.
The conformation in solution of derivatives of methyl hexopyranosides has been studied by n.m.r. The esters of methyl 2,3-O-isopropylidene-α-D-manno- and -talopyranosides as well as their 4-deoxy-4-C-methyl analog having a manno configuration exist mainly in a flattened (4,0F) chair conformation (4C1). The presence in the talo epimer of the 4-deoxy-4-C-methyl analog of the bulky methyl group on the endo side of the bicyclic system results in a skew form (3S1). The methyl 4-deoxy-2,3-O-isopropylidene-4-C-methylene-α-D-lyxo-hexopyranosides monosubstituted at C-4′ adopt, in solution, a conformation close to 3S1, whichever their configuration (cis or trans) at the double bond, as indicated by their allylic coupling constants. 相似文献
25.
Sheryl M. Winston Michael D. Hayward Eric J. Nestler Ronald S. Duman 《Journal of neurochemistry》1990,54(6):1920-1925
Acute seizures and other stimuli that increase neuronal activity cause a rapid induction of the immediate-early genes c-fos and c-jun, also referred to as nuclear proto-oncogenes, in the nervous system. In the present study, rats were administered one or more electroconvulsive seizures (ECS) and the responsiveness of c-fos and c-jun to an acute, "test" seizure was examined. Four hours after a single ECS, the induction of c-fos mRNA by a test seizure was blocked, in agreement with earlier findings, but by 18 h the levels of c-fos mRNA could be reinduced by the test seizure, suggesting that 1 day is sufficient to "reset" the responsiveness of this system. However, it was found that chronic, daily ECS treatments resulted in a time-dependent decrease in the expression of c-fos mRNA in response to a test seizure administered 18 h after the last daily ECS; this effect was maximal after 8-10 days of treatment, at which time the induction of c-fos mRNA by the test seizure was blocked dramatically. Chronic ECS also blocked the induction of c-jun in response to an acute, test seizure. The effect of chronic ECS on levels of Fos protein was also investigated. It was found that basal levels of Fos protein were reduced after chronic (10 days) ECS and were not induced by a test seizure. Because levels of Fos protein remain elevated 4 h after a single seizure this finding suggests that the mechanisms by which acute (4 h) and chronic (8-10 days) ECS block the induction of c-fos may differ. 相似文献
26.
M. G. Corda O. Giorgi B. Longoni M. Orlandi G. Biggio 《Journal of neurochemistry》1990,55(4):1216-1221
The acute administration of pentylenetetrazol (PTZ; 25-75 mg/kg i.p.) failed to modify the specific binding of t-[35S]butylbicyclophosphorothionate ([35S]TBPS) to membrane preparations from the cerebral cortex of the rat. In contrast, the repeated administration of PTZ (30 mg/kg i.p., three times a week for 12 weeks) reduced by 26% the density of [35S]TBPS binding sites without modifying the dissociation constant. This effect was observed 3 days after the last PTZ administration. A parallel reduction of gamma-aminobutyric acid (GABA)-stimulated 36Cl- uptake was measured in the cerebral cortex of PTZ-treated rats 3 days after the last injection. The repeated administration of PTZ produced sensitization to the drug, or chemical kindling. In fact, no convulsions were observed in the first week of treatment, but all the animals became sensitized to PTZ by the 12th week. The results are consistent with the hypothesis that chronic treatment with PTZ at a subconvulsant dose causes a decrease in GABA-coupled chloride channel activity that may be related to the chemical kindling produced by this compound. 相似文献
27.
W. Gibson Wood† Christine Gorka Friedhelm Schroeder‡ 《Journal of neurochemistry》1989,52(6):1925-1930
Alcohols, including ethanol, have a specific effect on transbilayer and lateral membrane domains. Recent evidence has shown that alcohols in vitro have a greater effect on fluidity of one leaflet as compared to the other. The present study examined effects of chronic ethanol consumption on fluidity of synaptic plasma membrane (SPM) exofacial and cytofacial leaflets using trinitrobenzenesulfonic acid (TNBS) labeling and differential polarized fluorometry of 1,6-diphenyl-1,3,5-hexatriene (DPH). Mice were administered ethanol or a control liquid diet for 3 weeks. Animals were killed and SPM prepared. The exofacial leaflet of SPM was significantly more fluid than the cytofacial leaflet in both groups, as indicated by limiting anisotropy of DPH. However, differences between the two leaflets were much smaller in the ethanol-treated group. Ethanol at concentrations seen clinically had a greater effect in vitro on the more fluid exofacial leaflet. This asymmetric effect of ethanol was significantly diminished in the exofacial leaflet of the ethanol-treated mice. Chronic ethanol consumption has a specific effect on membranes. Membrane functions that may be regulated by asymmetry of fluidity and lipid distribution may be altered by chronic ethanol consumption. 相似文献
28.
对我国52种微茎类吸虫的18项成虫形态学特征进行主成分分析,结果表明:卵巢位置、子宫延伸位置等7项性状对第一主成分贡献较大,提示描述器官位置的指标是重要的分类依据。52个虫种在前三个主成分上的排序图显示应将其划分成4个亚科。 相似文献
29.
30.
为了验证转基因烟草中表达的外壳蛋白(CP)能够重新包被侵入的烟草花叶病毒(TMV)的假设,利用抗原表位标记的方法观察CP亚单位在病毒5′端的交换。通过PCR 方法将来源于鼠肝炎病毒(MHV) S蛋白的两个小肽段(11 a.a.和15 a.a.)的DNA序列分别插入TMV-U1 CP基因邻近3′端的两个位点,并构建了带有外源序列的TMV 侵染克隆V9 (11 a.a.)和E15 (15 a.a.)。通过体外转录反应,得到V9 RNA 及E15 RNA。突变病毒RNA 侵染烟草(Nicotiana tabacum )后表现不同特性。V9 和E15 侵染XanthiNN烟草后同野生型TMV一样产生枯斑。但是,当它们侵染Xanthinn 烟草时,V9 产生同侵染XanthiNN 烟草相同的枯斑,而E15的特性同TMV-U1几乎完全相同,能对Xanthinn 烟草进行系统侵染并在叶片中聚集大量的带有外源片段的外壳蛋白,而且病毒的结构极其稳定。V9 和E15 特性的差异可能是由于外源片段在外壳蛋白中存在位置的不同影响了外壳蛋白的结构所致 相似文献