首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   95669篇
  免费   502篇
  国内免费   890篇
  97061篇
  2022年   27篇
  2021年   40篇
  2020年   25篇
  2019年   32篇
  2018年   11875篇
  2017年   10684篇
  2016年   7492篇
  2015年   699篇
  2014年   395篇
  2013年   424篇
  2012年   4382篇
  2011年   12981篇
  2010年   12083篇
  2009年   8308篇
  2008年   9900篇
  2007年   11473篇
  2006年   400篇
  2005年   645篇
  2004年   1086篇
  2003年   1115篇
  2002年   849篇
  2001年   336篇
  2000年   240篇
  1999年   90篇
  1998年   39篇
  1997年   49篇
  1996年   31篇
  1995年   23篇
  1994年   28篇
  1993年   47篇
  1992年   78篇
  1991年   73篇
  1990年   45篇
  1989年   50篇
  1988年   46篇
  1987年   42篇
  1986年   31篇
  1985年   35篇
  1984年   27篇
  1983年   47篇
  1982年   15篇
  1981年   13篇
  1979年   21篇
  1977年   18篇
  1975年   16篇
  1972年   267篇
  1971年   290篇
  1965年   17篇
  1962年   24篇
  1944年   12篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Chen C  Chou C  Sun Y  Huang W 《Cellular signalling》2001,13(8):543-553
TNF-alpha induced an increase in intercellular adhesion molecule-1 (ICAM-1) expression in human A549 epithelial cells and immunofluorescence staining confirmed this result. The enhanced ICAM-1 expression was shown to increase the adhesion of U937 cells to A549 cells. Tyrosine kinase inhibitors (genistein or tyrphostin 23) or phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor (D 609) attenuated TNF-alpha-induced ICAM-1 expression. TNF-alpha produced an increase in protein kinase C (PKC) activity and this effect was inhibited by D 609. PKC inhibitors (staurosporine, Ro 31-8220, calphostin C, or Go 6976) also inhibited TNF-alpha-induced response. 12-O-Tetradecanoylphorbol-13-acetate (TPA), a PKC activator, stimulated ICAM-1 expression, this effect was inhibited by genistein or tyrphostin 23. Treatment of cells with TNF-alpha resulted in stimulation of p44/42 MAPK, p38, and JNK. However, TNF-alpha-induced ICAM-1 expression was not affected by either MEK inhibitor, PD 98059, or p38 inhibitor, SB 203580. A cell-permeable ceramide analog, C(2) ceramide, also stimulated the activation of these three MAPKs, but had no effect on ICAM-1 expression. NF-kappaB DNA-protein binding and ICAM-1 promoter activity were enhanced by TNF-alpha and these effects were inhibited by D 609, calphostin C, or tyrphostin 23, but not by PD 98059 or SB 203580. TPA also stimulated NF-kappaB DNA-protein binding and ICAM-1 promoter activity, these effects being inhibited by genistein or tyrphostin 23. TNF-alpha- or TPA-induced ICAM-1 promoter activity was inhibited by dominant negative PKCalpha or IKK2, but not IKK1 mutant. IKK activity was stimulated by both TNF-alpha and TPA, and these effects were inhibited by Ro 31-8220 or tyrphostin 23. These data suggest that, in A549 cells, TNF-alpha activates PC-PLC to induce activation of PKCalpha and protein tyrosine kinase, resulting in the stimulation of IKK2, and NF-kappaB in the ICAM-1 promoter, then initiation of ICAM-1 expression and neutrophil adhesion. However, activation of p44/42 MAPK, p38, and JNK is not involved in this event.  相似文献   
992.
993.
Interleukin-1 (IL-1) has been implicated in the regulation of the expression of various matrix metalloproteinases (MMPs) in many mesenchymal cell types, but its role in liver myofibroblasts (MFs) has not been elucidated. A myofibroblast-like cell line, MG2, was derived from an isolate of rat hepatic stellate cells (HSCs). These cells expressed desmin, vimentin, smooth muscle -actin, and fibulin-2. Using a recombinant IL-1 at 5 ng/ml, it was shown that IL-1 would upregulate, while IL-1Ra, an IL-1 receptor antagonist, would down-regulate the expression of IL-1 mRNA in MG2 cells, indicating the presence of an autostimulatory loop of IL-1 in these cells. Besides, a paracrine source of IL-1 may be produced from Kupffer cells, as we showed primarily cultured Kupffer cells responded much more remarkably than MG2 cells to lipopolysaccharide stimuli to produce both IL-1 and IL-1. Recombinant IL-1 upregulated the expression of both MMP-9 and -13, and the induction of MMP-13 but not MMP-9 could be inhibited by SB203580, an inhibitor of p38. Similarly, in primarily cultured human liver MFs, upregulation of MMP-1 by IL-1 was also shown to be inhibited by SB203580. All of these data suggested that, during liver inflammation, IL-1 produced by an autocrine model from MFs or by a paracrine model from Kupffer cells might play a crucial role in the remodeling of liver fibrosis through an either p38-dependent or p38-independent pathway to regulate the expression of various MMPs by liver MFs.  相似文献   
994.

Background  

There is an ongoing discussion about age limits for deep brain stimulation (DBS). Current indications for DBS are tremor-dominant disorders, Parkinson's disease, and dystonia. Electrode implantation for DBS with analgesia and sedation makes surgery more comfortable, especially for elderly patients. However, the value of DBS in terms of benefit-risk ratio in this patient population is still uncertain.  相似文献   
995.
Pulmonary involvement is common in sarcoidosis, an immune-mediated inflammatory disorder that is characterized by non-caseating granulomas in tissue. Sarcoid patients with advanced pulmonary disease, especially end-stage pulmonary fibrosis, risk developing pulmonary hypertension (World Health Organization group III pulmonary hypertension secondary to hypoxic lung disease). Increased levels of endothelin (ET)-1 in plasma and bronchoalveolar lavage of some sarcoid patients suggest that ET-1 may be driving pulmonary fibrosis and sarcoidosis-associated pulmonary hypertension. Although a relationship between raised levels of ET-1 and clinical phenotype is yet to be identified, early evidence from studies of ET-1 blockade with drugs such as bosentan is encouraging. Such therapy possibly could be combined with standard anti-inflammatory agents to improve outcome.  相似文献   
996.
Achievement of transposon mediated germline transgenesis in a basal chordate, Ciona intestinalis, is discussed. A Tc1/mariner superfamily transposon, Minos, has excision and transposition activities in Ciona. Minos enables the creation of stable transgenic lines, enhancer detection, and insertional mutagenesis.  相似文献   
997.
Nucleosides are neuromodulators that have a wide range of biological roles in the brain. In order to better understand the function of nucleosides in the human central nervous system (CNS), we constructed a nucleoside map showing the concentration of various nucleosides and their metabolites using post mortem samples from 61 human brain areas and 4 spinal cord areas. We evaluated in vivo tissue levels of four nucleosides (uridine, inosine, guanosine, and adenosine) and three of their metabolites (uracil, hypoxanthine, and xanthine). The concentrations of nucleosides were unevenly distributed across different brain regions, where the highest levels were found in the cerebral cortex and basal ganglia, whereas the lowest concentrations were located in the locus coeruleus, the zona incerta, the substantia nigra, and the inferior colliculus. The regional differences in nucleoside levels in the CNS may reflect the distinct physiological functions adopted by these compounds in different brain areas.  相似文献   
998.
An efficient shoot organogenesis system has been developed from mature plants of selected elite clones of Eucalyptus tereticornis Sm. Cultures were established using nodal explants taken from freshly coppice shoots cultured on Murashige and Skoog medium containing 58 mM sucrose, 0.7% (w/v) agar (MS medium) and supplemented with 2.5 μM benzyladenine (BA) and 0.5 μM α-naphthaleneacetic acid (NAA). Shoot organogenesis was achieved from leaf segments taken from elongated microshoots on MS medium supplemented with 5.0 μM BA and 1.0 μM 2,4-dichlorophenoxyacetic acid (2,4-D). The addition of cefotaxime to the medium promoted shoot differentiation, whereas carbenicillin and cephalexin inhibited shoot differentiation. Maximum shoot bud organogenesis (44.6%) occurred in explants cultured on MS medium supplemented with 5.0 μM BA, 1.0 μM 2,4-D and 500 mg/l cefotaxime. Leaf maturity influenced shoot regeneration, with maximum shoot organogeneisis (40.5%) occurring when the source of explants was the fifth leaf (14–16 days old) from the top of microshoot. Shoot organogenic potential also varied amongst the different clones of E. tereticornis. Random amplified polymorphic DNA (RAPD) and inter-simple sequence repeat (ISSR) analyses indicated clonal uniformity of the newly formed shoots/plants, and these were also found to be true-to-type.  相似文献   
999.
Singapore is located near the equator, off the southern tip of the Malay Peninsula. The whole country consists mostly of lowland. It has many interesting types of natural habitats such as primary rain forest, freshwater swamp forest, mangroves, secondary forests, shrub, grasslands, and urban parks and fields. The climate is equatorial with relatively uniform temperature and high humidity. Unfortunately, many of the natural habitats and the native orchids which thrive there have disappeared due to habitat destruction. Some 226 species of native orchids have been recorded in Singapore. However, of these 178 are considered to be extinct, and only five are common. The orchid conservation programme aims to monitor existing species, explore ways to conserve their germplasm, and increase their numbers in natural, semi-natural, and urban environments through ex-situ seedling culture and subsequent re-introduction into appropriate habitats, including roadside trees, parks and natural areas. In the first phase of the programme, we have successfully propagated and carried out experiments of re-introduction on five species of native orchids, namely, Grammatophyllum speciosum, Bulbophyllum vaginatum, Bulbophyllum membranaceum, Cymbidium finlaysonianum and Cymbidium bicolor. Survival percentages 8-yr after the reintroduction events ranged from 10 to 95 for G. speciosum, the target species of the earliest re-introduction experiments. Size of the seedlings at reintroduction, host trees, and relative humidity seemed to play significant roles in the success rate of the reintroductions.  相似文献   
1000.
The European bitterling, Rhodeus amarus, is a non-indigenous fish species in British fresh waters. It lays its eggs in unionid mussels which themselves are vulnerable to fouling by the non-indigenous zebra mussel, Dreissena polymorpha. Observations from an unmanipulated natural system showed that only 27% of zebra mussel-fouled Unio pictorum hosted bitterling, while 47% of unfouled U. pictorum hosted bitterling. We conducted a field experiment in the River Great Ouse catchment, Cambridgeshire, England in May–June 2007 and 2008 to quantify the impact of zebra mussels on bitterling load in host mussels. Zebra mussel-fouled unionids were significantly less likely to host bitterling than unfouled unionids. The number of unionids hosting bitterling did not differ significantly whether the zebra mussels fouling the unionid were alive or dead. Bitterling appeared to discriminate against zebra mussel-fouled unionids less as the 2007 breeding season advanced, potentially because preferred unfouled unionids had a higher bitterling load, and were therefore relatively lower quality hosts than at the start of the breeding season.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号