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   38555篇
  免费   125篇
  国内免费   149篇
系统科学   177篇
丛书文集   537篇
教育与普及   63篇
理论与方法论   132篇
现状及发展   16448篇
研究方法   1570篇
综合类   19164篇
自然研究   738篇
  2013年   346篇
  2012年   549篇
  2011年   1279篇
  2010年   241篇
  2008年   635篇
  2007年   799篇
  2006年   741篇
  2005年   765篇
  2004年   794篇
  2003年   676篇
  2002年   706篇
  2001年   1283篇
  2000年   1229篇
  1999年   769篇
  1992年   728篇
  1991年   557篇
  1990年   618篇
  1989年   608篇
  1988年   569篇
  1987年   571篇
  1986年   630篇
  1985年   778篇
  1984年   569篇
  1983年   502篇
  1982年   432篇
  1981年   440篇
  1980年   513篇
  1979年   1245篇
  1978年   953篇
  1977年   922篇
  1976年   753篇
  1975年   770篇
  1974年   1147篇
  1973年   931篇
  1972年   1002篇
  1971年   1125篇
  1970年   1521篇
  1969年   1149篇
  1968年   1068篇
  1967年   1024篇
  1966年   993篇
  1965年   709篇
  1964年   212篇
  1959年   370篇
  1958年   689篇
  1957年   474篇
  1956年   372篇
  1955年   352篇
  1954年   376篇
  1948年   258篇
排序方式: 共有10000条查询结果,搜索用时 652 毫秒
51.
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma.  相似文献   
52.
Activating and inactivating mutations of SHP-2 are responsible, respectively, for the Noonan (NS) and the LEOPARD (LS) syndromes. Clinically, these developmental disorders overlap greatly, resulting in the apparent paradox of similar diseases caused by mutations that oppositely influence SHP-2 phosphatase activity. While the mechanisms remain unclear, recent functional analysis of SHP-2, along with the identification of other genes involved in NS and in other related syndromes (neurofibromatosis-1, Costello and cardio-facio-cutaneous syndromes), strongly suggest that Ras/MAPK represents the major signaling pathway deregulated by SHP-2 mutants. We discuss the idea that, with the exception of LS mutations that have been shown to exert a dominant negative effect, all disease-causing mutations involved in Ras/MAPK-mediated signaling, including SHP-2, might lead to enhanced MAPK activation. This suggests that a narrow range of MAPK signaling is required for appropriate development. We also discuss the possibility that LS mutations may not simply exhibit dominant negative activity. Received 30 November 2006; received after revision 8 February 2007; accepted 13 March 2007  相似文献   
53.
Targeted inhibition of Livin resensitizes renal cancer cells towards apoptosis   总被引:10,自引:0,他引:10  
Cancer cells are typically characterized by apoptosis deficiency. In order to investigate a possible role for the anti-apoptotic livin gene in renal cell cancer (RCC), we analyzed its expression in tumor tissue samples and in RCC-derived cell lines. In addition, we studied the contribution of livin to the apoptotic resistance of RCC cells by RNA interference (RNAi). Livin gene expression was detected in a significant portion of RCC tumor tissue specimens (13/14, 92.9%) and tumor-derived cell lines (12/15, 80.0%). Moreover, targeted inhibition of livin by RNAi markedly sensitized RCC cells towards proapoptotic stimuli, such as UV irradiation or the chemotherapeutic drugs etoposide, 5-fluorouracil, and vinblastine. These effects were specific for livin expressing tumor cells. We conclude that livin can contribute significantly to the apoptosis resistance of RCC cells. Targeted inhibition of livin could represent a novel therapeutic strategy to increase the sensitivity of renal cancers towards pro-apoptotic agents. Received 30 November 2006; received after revision 22 February 2007; accepted 20 March 2007  相似文献   
54.
The recycling of the amyloid precursor protein (APP) from the cell surface via the endocytic pathways plays a key role in the generation of amyloid beta peptide (Abeta) in Alzheimer disease. We report here that inherited variants in the SORL1 neuronal sorting receptor are associated with late-onset Alzheimer disease. These variants, which occur in at least two different clusters of intronic sequences within the SORL1 gene (also known as LR11 or SORLA) may regulate tissue-specific expression of SORL1. We also show that SORL1 directs trafficking of APP into recycling pathways and that when SORL1 is underexpressed, APP is sorted into Abeta-generating compartments. These data suggest that inherited or acquired changes in SORL1 expression or function are mechanistically involved in causing Alzheimer disease.  相似文献   
55.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n...  相似文献   
56.
During its lifetime, the mammary gland undergoes many phases of development and differentiation. Much of this occurs during puberty, when the ductal epithelium expands by branching morphogenesis, invading the surrounding fat pad to form an organised mammary tree. Throughout its existence, the epithelium will go through several cycles of proliferation and cell death during pregnancy, lactation and involution. Many of the signalling mechanisms which control the initial invasion of the fat pad by the epithelium, and regulate its continuing plasticity, can be harnessed or corrupted by tumour cells in order to support their aberrant growth and progression towards invasion. This is true not just for the epithelial cells themselves but also for cells in the surrounding microenvironment, including fibroblasts, macrophages and adipocytes. This review examines the complex web of signalling and adhesion interactions controlling branching morphogenesis, and how their alteration can promote malignancy. Current in vivo and in vitro mammary gland models are also discussed. (Part of a Multi-author Review)  相似文献   
57.
煤矸石空心砖是一种新型环保墙体材料,符合国家及山西省出台的限制生产和使用实心黏土砖,鼓励发展新型建筑材料的政策。利用煤矸石制造的空心砖可以作为保温材料,既可节约建筑能耗,又可节约土地资源,减少环境污染。煤矸石空心砖同黏土实心砖相比,具有高强、轻质、隔音、隔热、保温、防震等优点,市场前景良好。  相似文献   
58.
Obese postmenopausal female volunteers were given timed daily oral dosages of bromocriptine, and tested for reduction of body fat stores. This dopamine agonist has been shown to reset circadian rhythms that are altered in obese animals and to reduce body fat levels in several animal models. The participants were instructed not to alter their existing exercise and eating behavior during treatment. Skinfold measurements were taken on 33 subjects as indices of body fat. The measurements (e.g., suprailiac) were reduced after six weeks by about 25%, which represents a reduction of 11.7% of the total body fat. These dramatic decreases in body fat, which are equivalent to that produced by severe caloric restriction, were accompanied by more modest reductions of body weight (2.5%), indicating a possible conservation of protein that is usually lost as a consequence of such caloric restriction. The effects of bromocriptine treatment on body fat and hyperglycemia were also examined in non-insulin dependent diabetics being treated with oral hypoglycemics (7 subjects) or insulin (7 subjects). Total body fat was reduced by 10.7% and 5.1% in diabetics on oral hypoglycemics and insulin, respectively, without any significant reductions in body weight. Hyperglycemia was reduced in most of the 15 diabetic subjects treated leading to euglycemia and even cessation of hypoglycemic drugs in 3 of the 7 subjects during 4-8 weeks of bromocriptine treatment. These findings support the hypothesis that obesity and type II diabetes may be treated effectively with bromocriptine when administered at the proper times and dosages.  相似文献   
59.
A Kowluru  R A Kowluru 《Experientia》1992,48(5):486-488
Urinary excretion of glycated albumin was quantitated in genetically hyperglycemic mice (C57BL-Ks-J, db/db mice), a model for non-insulin-dependent diabetes mellitus, and compared with their non-diabetic littermates. The data indicated a preferential excretion of glycated albumin in non-diabetic mice. This phenomenon of 'editing' of glycated albumin is decreased significantly in diabetic mice. Quantitative measurements of overall excretion of glycated albumin suggested that the loss of editing in diabetic mice is due to the dilution of glycated albumin by the unmodified albumin which is excreted in large amounts in diabetic mice. Therefore, the loss of editing observed in this model resembled the one we characterized in insulin-dependent diabetic humans and a streptozotocin-diabetic rat model.  相似文献   
60.
E Littler  A D Stuart  M S Chee 《Nature》1992,358(6382):160-162
Human cytomegalovirus (HCMV, a betaherpes virus) is the cause of serious disease in immunologically compromised individuals, including those with acquired immunodeficiency syndrome. One of the compounds used in the chemotherapy of HCMV infections is the nucleoside analogue 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (ganciclovir). The mechanism of action of this drug is dependent on the formation of the nucleoside triphosphate, which is a strong inhibitor of the viral DNA polymerase. Thymidine kinase, which is encoded by many of the herpesviruses, catalyses the initial phosphorylation of ganciclovir. But there is no evidence for the coding of this enzyme by HCMV, and DNA sequence analysis of the HCMV genome has shown that there is no open reading frame characteristic of a herpesvirus thymidine kinase. Here we present biochemical and immunological evidence that the HCMV UL97 open reading frame codes for a protein capable of phosphorylating ganciclovir. This protein seems to be responsible for the selectivity of ganciclovir and will be useful tool in the understanding and refinement of the antiviral activity of new selective anti-HCMV compounds.  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