全文获取类型
收费全文 | 24157篇 |
免费 | 2088篇 |
国内免费 | 2014篇 |
专业分类
28259篇 |
出版年
2024年 | 127篇 |
2023年 | 620篇 |
2022年 | 708篇 |
2021年 | 979篇 |
2020年 | 1066篇 |
2019年 | 1232篇 |
2018年 | 1044篇 |
2017年 | 949篇 |
2016年 | 969篇 |
2015年 | 1206篇 |
2014年 | 1438篇 |
2013年 | 2173篇 |
2012年 | 1005篇 |
2011年 | 1154篇 |
2010年 | 811篇 |
2009年 | 1297篇 |
2008年 | 1354篇 |
2007年 | 1312篇 |
2006年 | 1208篇 |
2005年 | 957篇 |
2004年 | 905篇 |
2003年 | 767篇 |
2002年 | 622篇 |
2001年 | 515篇 |
2000年 | 445篇 |
1999年 | 404篇 |
1998年 | 394篇 |
1997年 | 393篇 |
1996年 | 303篇 |
1995年 | 254篇 |
1994年 | 214篇 |
1993年 | 209篇 |
1992年 | 188篇 |
1991年 | 158篇 |
1990年 | 141篇 |
1989年 | 113篇 |
1988年 | 102篇 |
1987年 | 87篇 |
1986年 | 73篇 |
1985年 | 82篇 |
1984年 | 54篇 |
1983年 | 37篇 |
1982年 | 58篇 |
1981年 | 45篇 |
1980年 | 31篇 |
1979年 | 19篇 |
1978年 | 12篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Shunhua Ding Qingtao Lu Yan Zhang Zhipan Yang Xiaogang Wen Lixin Zhang Congming Lu 《Plant molecular biology》2009,69(5):577-592
To investigate the possible mechanisms of glutathione reductase (GR) in protecting against oxidative stress, we obtained transgenic
tobacco (Nicotiana tabacum) plants with 30–70% decreased GR activity by using a gene encoding tobacco chloroplastic GR for the RNAi construct. We investigated
the responses of wild type and transgenic plants to oxidative stress induced by application of methyl viologen in vivo. Analyses
of CO2 assimilation, maximal efficiency of photosystem II photochemistry, leaf bleaching, and oxidative damage to lipids demonstrated
that transgenic plants exhibited enhanced sensitivity to oxidative stress. Under oxidative stress, there was a greater decrease
in reduced to oxidized glutathione ratio but a greater increase in reduced glutathione in transgenic plants than in wild type
plants. In addition, transgenic plants showed a greater decrease in reduced ascorbate and reduced to oxidized ascorbate ratio
than wild type plants. However, there were neither differences in the levels of NADP and NADPH and in the total foliar activities
of monodehydroascorbate reductase and dehydroascorbate reductase between wild type and transgenic plant. MV treatment induced
an increase in the activities of GR, ascorbate peroxidase, superoxide dismutase, and catalase. Furthermore, accumulation of
H2O2 in chloroplasts was observed in transgenic plants but not in wild type plants. Our results suggest that capacity for regeneration
of glutathione by GR plays an important role in protecting against oxidative stress by maintaining ascorbate pool and ascorbate
redox state. 相似文献
993.
994.
Ali Bouallegue Nihar R. Pandey Ashok K. Srivastava 《Free radical biology & medicine》2009,47(6):858-866
We have shown earlier a requirement for Ca2+ and calmodulin (CaM) in the H2O2-induced activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and protein kinase B (PKB), key mediators of growth-promoting, proliferative, and hypertrophic responses in vascular smooth muscle cells (VSMC). Because the effect of CaM is mediated through CaM-dependent protein kinase II (CaMKII), we have investigated here the potential role of CaMKII in H2O2-induced ERK1/2 and PKB phosphorylation by using pharmacological inhibitors of CaM and CaMKII, a CaMKII inhibitor peptide, and siRNA knockdown strategies for CaMKIIα. Calmidazolium and W-7, antagonists of CaM, as well as KN-93, a specific inhibitor of CaMKII, attenuated H2O2-induced responses of ERK1/2 and PKB phosphorylation in a dose-dependent fashion. Similar to H2O2, calmidazolium and KN-93 also exhibited an inhibitory effect on glucose/glucose oxidase-induced phosphorylation of ERK1/2 and PKB in these cells. Transfection of VSMC with CaMKII autoinhibitory peptide corresponding to the autoinhibitory domain (aa 281–309) of CaMKII and with siRNA of CaMKIIα attenuated the H2O2-induced phosphorylation of ERK1/2 and PKB. In addition, calmidazolium and KN-93 blocked H2O2-induced Pyk2 and insulin-like growth factor-1 receptor (IGF-1R) phosphorylation. Moreover, treatment of VSMC with CaMKIIα siRNA abolished the H2O2-induced IGF-1R phosphorylation. H2O2 treatment also induced Thr286 phosphorylation of CaMKII, which was inhibited by both calmidazolium and KN-93. These results demonstrate that CaMKII plays a critical upstream role in mediating the effects of H2O2 on ERK1/2, PKB, and IGF-1R phosphorylation. 相似文献
995.
SATHYA K. CHINNADURAI JOSHUA J. MILLSPAUGH WAYNE S. MATTHEWS KIRSTEN CANTER ROB SLOTOW BRIAN E. WASHBURN RAMI J. WOODS 《The Journal of wildlife management》2009,73(6):1014-1020
ABSTRACT Fecal glucocorticoid metabolite (FGM) assays are a popular means of monitoring adrenocortical activity (i.e., physiological stress response) in wildlife. Species-specific differences in glucocorticoid metabolism and excretion require assay validation, including both laboratory and biological components, before assay use in new species. We validated a commercially available radioimmunoassay (MP 125I corticosterone RIA kit [MP Biomedicals, Solon, OH]) for measuring FGMs of several South African herbivores, including giraffe (Giraffa camelopardalis), impala (Aepyceros melampus), nyala (Tragelaphus buxtoni), kudu (Tragelaphus strepsiceros), wildebeest (Connochaetes taurinus), and zebra (Equus burchelli). These herbivores are important in South African parks and reserves for ecotourism and as a prey base for predators and serve an integral role in ecosystem processes. Standard biochemical validations (e.g., recovery of exogenous corticosterone, intra- and interassay variation, and parallelism) demonstrated that the assay accurately and precisely measured FGMs of all 6 herbivore species. Our biological validations demonstrated that the assay was sensitive enough to detect changes in FGM production associated with season. Samples collected during the dry season (Jun-Aug) contained higher FGM concentrations than those from the wet season (Dec-Feb) in all species. We established optimal sample dilutions and reference FGM levels for these 6 herbivores, which can now be used to monitor the effects of management and ecotourism activities on the stress responses of these herbivores. 相似文献
996.
E. Gamalero G. Berta N. Massa B.R. Glick G. Lingua 《Journal of applied microbiology》2010,108(1):236-245
Aims: After the determination of the toxic but nonlethal concentration of NaCl for cucumber, we examined the interaction between an ACC (1‐aminocyclopropane‐1‐carboxylate) deaminase producing bacterial strain and an arbuscular mycorrhizal fungus (AMF) and their effects on cucumber growth under salinity. Methods and Results: In the first experiment, cucumber seedlings were exposed to 0·1, 50, 100 or 200 mmol l?1 NaCl, and plant biomass and leaf area were measured. While seeds exposed to 200 mmol l?1 NaCl did not germinate, plant growth and leaf size were reduced by 50 or 100 mmol l?1 salt. The latter salt cancentration caused plant death in 1 month. In the second experiment, seeds were inoculated with the ACC deaminase‐producing strain Pseudomonas putida UW4 (AcdS+), its mutant unable to produce the enzyme (AcdS?), or the AMF Gigaspora rosea BEG9, individually or in combination and exposed to 75 mmol l?1 salt. Plant morphometric and root architectural parameters, mycorrhizal and bacterial colonization and the influence of each micro‐organism on the photosynthetic efficiency were evaluated. The AcdS+ strain or the AMF, inoculated alone, increased plant growth, affected root architecture and improved photosynthetic activity. Mycorrhizal colonization was inhibited by each bacterial strain. Conclusions: Salinity negatively affects cucumber growth and health, but root colonization by ACC deaminase‐producing bacteria or arbuscular mycorrhizal fungi can improve plant tolerance to such stressful condition. Significance and Impact of the Study: Arbuscular mycorrhizal fungus and bacterial ACC deaminase may ameliorate plant growth under stressful conditions. It was previously shown that, under optimal growth conditions, Ps. putida UW4 AcdS+ increases root colonization by Gi. rosea resulting in synergistic effects on cucumber growth. These results suggest that while in optimal conditions ACC deaminase is mainly involved in the bacteria/fungus interactions, while under stressful conditions this enzyme plays a role in plant/bacterium interactions. This finding is relevant from an ecological and an applicative point of view. 相似文献
997.
998.
Green PS Mendez AJ Jacob JS Crowley JR Growdon W Hyman BT Heinecke JW 《Journal of neurochemistry》2004,90(3):724-733
Myeloperoxidase, a heme protein expressed by professional phagocytic cells, generates an array of oxidants which are proposed to contribute to tissue damage during inflammation. We now report that enzymatically active myeloperoxidase and its characteristic amino acid oxidation products are present in human brain. Further, expression of myeloperoxidase is increased in brain tissue showing Alzheimer's neuropathology. Consistent with expression in phagocytic cells, myeloperoxidase immunoreactivity was present in some activated microglia in Alzheimer brains. However, the majority of immunoreactive material in brain localized with amyloid plaques and, surprisingly, neurons including granule and pyramidal neurons of the hippocampus. Confirming neuronal localization of the enzyme, several neuronal cell lines as well as primary neuronal cultures expressed myeloperoxidase protein. Myeloperoxidase mRNA was also detected in neuronal cell lines. These results reveal the unexpected presence of myeloperoxidase in neurons. The increase in neuronal myeloperoxidase expression we observed in Alzheimer disease brains raises the possibility that the enzyme contributes to the oxidative stress implicated in the pathogenesis of the neurodegenerative disorder. 相似文献
999.
Aminoacetone (AA) is a threonine metabolite accumulated in threoninemia, cri-du-chat, and diabetes, where it contributes toward the formation of cytotoxic and genotoxic methylglyoxal (MG). Oxyradicals yielded from iron-catalyzed AA aerobic oxidation to MG are shown here to promote Ca2+ -mediated mitochondrial membrane permeabilization in an AA dose-dependent way. The inhibitory effect of added EGTA, cyclosporin A, Mg2+, and DTT observed in this study suggests the formation of transition pores in the inner mitochondrial membrane by AA, associated with thiol protein aggregation. That the mitochondrial iron pool plays a coadjutant role in the transition of mitochondrial permeability is indicated by the dramatic inhibitory effect of added o-phenanthroline. Iron released from ferritin by AA oxidation products--superoxide anion and AA enolyl radicals--is shown to act as an alternative source of ferrous iron, intensifying the mitochondrial damage. These findings may contribute to clarify the role of accumulated AA and iron overload in the mitochondrial oxidative damage reportedly occurring in diabetes mellitus. 相似文献
1000.
Vitamin E reduces amyloidosis and improves cognitive function in Tg2576 mice following repetitive concussive brain injury 总被引:3,自引:0,他引:3
Conte V Uryu K Fujimoto S Yao Y Rokach J Longhi L Trojanowski JQ Lee VM McIntosh TK Praticò D 《Journal of neurochemistry》2004,90(3):758-764
Traumatic brain injury is a well-recognized environmental risk factor for developing Alzheimer's disease. Repetitive concussive brain injury (RCBI) exacerbates brain lipid peroxidation, accelerates amyloid (Abeta) formation and deposition, as well as cognitive impairments in Tg2576 mice. This study evaluated the effects of vitamin E on these four parameters in Tg2576 mice following RCBI. Eleven-month-old mice were randomized to receive either regular chow or chow-supplemented with vitamin E for 4 weeks, and subjected to RCBI (two injuries, 24 h apart) using a modified controlled cortical impact model of closed head injury. The same dietary regimens were maintained up to 8 weeks post-injury, when the animals were killed for biochemical and immunohistochemical analyses after behavioral evaluation. Vitamin E-treated animals showed a significant increase in brain vitamin E levels and a significant decrease in brain lipid peroxidation levels. After RBCI, compared with the group on regular chow, animals receiving vitamin E did not show the increase in Abeta peptides, and had a significant attenuation of learning deficits. This study suggests that the exacerbation of brain oxidative stress following RCBI plays a mechanistic role in accelerating Alphabeta accumulation and behavioral impairments in the Tg2576 mice. 相似文献