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   13272篇
  免费   765篇
  国内免费   5篇
  2023年   41篇
  2022年   81篇
  2021年   206篇
  2020年   137篇
  2019年   130篇
  2018年   202篇
  2017年   164篇
  2016年   312篇
  2015年   438篇
  2014年   503篇
  2013年   615篇
  2012年   1069篇
  2011年   1196篇
  2010年   515篇
  2009年   454篇
  2008年   972篇
  2007年   994篇
  2006年   952篇
  2005年   826篇
  2004年   775篇
  2003年   687篇
  2002年   670篇
  2001年   325篇
  2000年   357篇
  1999年   217篇
  1998年   106篇
  1997年   42篇
  1996年   56篇
  1995年   58篇
  1994年   55篇
  1993年   51篇
  1992年   47篇
  1991年   48篇
  1990年   35篇
  1989年   39篇
  1988年   35篇
  1987年   46篇
  1986年   29篇
  1985年   42篇
  1984年   37篇
  1983年   51篇
  1982年   39篇
  1981年   35篇
  1980年   25篇
  1979年   25篇
  1978年   20篇
  1977年   34篇
  1976年   22篇
  1971年   18篇
  1970年   19篇
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
961.
962.
963.
Delivery of recombinant superoxide dismutase to the lung is limited by its short half-life and poor tissue penetration. We hypothesized that a chimeric protein, SOD2/3, containing the enzymatic domain of manganese superoxide dismutase (SOD2) and the heparan-binding domain of extracellular superoxide dismutase (SOD3), would allow for the delivery of more sustained lung and pulmonary vascular antioxidant activity compared to SOD2. We administered SOD2/3 to rats by intratracheal (i.t.), intraperitoneal (i.p.), or intravenous (i.v.) routes and evaluated the presence, localization, and activity of lung SOD2/3 1 day later using Western blot, immunohistochemistry, and SOD activity gels. The effect of i.t. SOD2/3 on the pulmonary and systemic circulation was studied in vivo in chronically catheterized rats exposed to acute hypoxia. Active SOD2/3 was detected in lung 1 day after i.t. administration but not detected after i.p. or i.v. SOD2/3 administration or i.t. SOD2. The physiologic response to acute hypoxia, vasoconstriction in the pulmonary circulation and vasodilation in the systemic circulation, was enhanced in rats treated 1 day earlier with i.t. SOD2/3. These findings indicate that i.t. administration of SOD2/3 effectively delivers sustained enzyme activity to the lung as well as pulmonary circulation and has a longer tissue half-life compared to native SOD2. Further testing in models of chronic lung or pulmonary vascular diseases mediated by excess superoxide should consider the longer tissue half-life of SOD2/3 as well as its potential systemic vascular effects.  相似文献   
964.
Chronic inflammation plays an important role in the initiation and progression of various human diseases including benign prostatic hyperplasia or prostate cancer. Here we show that the proinflammatory cytokine interleukin-6 (IL-6) has prosurvival effects and chronically activates the Jak2/STAT3 signalling pathway in a model of benign prostatic hyperplasia (BPH-1). We demonstrate that the antiinflammatory cytokine transforming growth factor-β1 (TGF-β1), which also permanently activates its canonical signalling pathway through SMAD proteins in BPH-1 cells, modifies the effects of IL-6 on cell proliferation. Importantly, TGF-β1 inhibits IL-6 signal transduction by decreasing the phosphorylation levels of STAT3. This effect is associated with decreased expression of Jak2 at both mRNA and protein levels. Moreover, we showed that TGF-β1 inhibits IL-6-induced expression of the cancer-associated gene MUC1. These observations demonstrated a novel interaction between TGF-β1 and IL-6 signalling and suggested another mechanism of how defects in TGF-β signalling, frequently associated with prostate pathologies, can contribute to the disruption of tissue homeostasis.  相似文献   
965.
The morphology, ontogenesis, encystment, and 18S rRNA gene sequence of a new soil hypotrich ciliate, Kahliella matisi, were studied. Main characteristics of K. matisi are: (1) two short and six longitudinal cirral rows right of the adoral zone of membranelles and four longitudinal rows left of it; (2) three dorsal kineties, of which kinety 1 extends along the left cell margin, kinety 2 runs in a slightly sigmoidal line, and kinety 3 is distinctly shortened posteriorly. Ontogenesis is similar to that in congeners, especially in the development of the marginal rows and long dorsal kineties, the preservation of some old cirral rows after division, and the direction of the neokinetal wave. However, there are some peculiarities: (1) reorganization of the proximal parental adoral membranelles; (2) splitting of opisthe's anlage II into the cirral streak II and III; and (3) formation of the parental cirral row R3 from anlagen IV and V. During encystment, the body diminishes and becomes globular, the nuclear apparatus is reorganized, and the ciliature is resorbed. In our molecular phylogenies, the family Kahliellidae is polyphyletic and the position of K. matisi is rather poorly resolved, indicating a relationship with oxytrichids.  相似文献   
966.
Alternative splicing (AS) of precursor mRNAs is a widespread phenomenon in plants; however, many questions, especially regarding its regulation and functional implications, remain to be elucidated. In vertebrates, polypyrimidine tract‐binding proteins (PTBs) have been identified as key splicing factors influencing splice site selection and orchestrating coordinated splicing programmes during developmental processes. Here, we analysed three PTB homologues from Arabidopsis thaliana and provide evidence for their gene regulatory potential based on AS and a splicing‐independent mechanism. Our data reveal that Arabidopsis PTB homologues are subject to extensive auto‐ and cross‐regulation via AS‐coupled nonsense‐mediated decay, thereby establishing a basis for interlinking their expression. Furthermore, the multiple modes of action of Arabidopsis PTB homologues are reflected in their subcellular localization in the nucleus, cytosol and processing bodies. This work provides insight into the regulation of AS in plants and highlights the regulatory potential of the multifunctional plant PTB homologues, which might have important implications in diverse biological processes.  相似文献   
967.
A series of 3-aryl-5-acyloxymethyl-5,6-dihydro-2H-pyran-2-ones, related to highly antifungally active butenolides, was synthesized via cyclization of substituted δ-hydroxy acids as the key step, and evaluated for their in vitro antifungal activity and cytostatic activity. While the extension of the furanone ring to pyranone led to a complete loss of the antifungal effect, some of the compounds displayed promising effect against several cell lines, including the resistant colorectal carcinoma cells.  相似文献   
968.
Agonists of the 5-HT2C receptor have been shown to suppress appetite and reduce body weight in animal models as well as in humans. However, agonism of the related 5-HT2B receptor has been associated with valvular heart disease. Synthesis and biological evaluation of a series of novel and highly selective dihydroquinazolinone-derived 5-HT2C agonists with no detectable agonism of the 5-HT2B receptor is described. Among these, compounds (+)-2a and (+)-3c were identified as potent and highly selective agonists which exhibited weight loss in a rat model upon oral dosing.  相似文献   
969.
ATP‐dependent proteases are crucial for cellular homeostasis. By degrading short‐lived regulatory proteins, they play an important role in the control of many cellular pathways and, through the degradation of abnormally misfolded proteins, protect the cell from a buildup of aggregates. Disruption or disregulation of mammalian mitochondrial Lon protease leads to severe changes in the cell, linked with carcinogenesis, apoptosis, and necrosis. Here we present the structure of the proteolytic domain of human mitochondrial Lon at 2 Å resolution. The fold resembles those of the three previously determined Lon proteolytic domains from Escherichia coli, Methanococcus jannaschii, and Archaeoglobus fulgidus. There are six protomers in the asymmetric unit, four arranged as two dimers. The intersubunit interactions within the two dimers are similar to those between adjacent subunits of the hexameric ring of E. coli Lon, suggesting that the human Lon proteolytic domain also forms hexamers. The active site contains a 310 helix attached to the N‐terminal end of α‐helix 2, which leads to the insertion of Asp852 into the active site, as seen in M. jannaschii. Structural considerations make it likely that this conformation is proteolytically inactive. When comparing the intersubunit interactions of human with those of E. coli Lon taken with biochemical data leads us to propose a mechanism relating the formation of Lon oligomers with a conformational shift in the active site region coupled to a movement of a loop in the oligomer interface, converting the proteolytically inactive form seen here to the active one in the E. coli hexamer.  相似文献   
970.
OBJECTIVE:: This purpose of this consensus conference was to determine whether surgical atrial fibrillation (AF) ablation during cardiac surgery improves clinical and resource outcomes compared with cardiac surgery alone in adults undergoing cardiac surgery for valve or coronary artery bypass grafting. METHODS:: Before the consensus conference, the consensus panel reviewed the best available evidence, whereby systematic reviews, randomized trials, and nonrandomized trials were considered in descending order of validity and importance. Evidence-based statements were created, and consensus processes were used to determine the ensuing recommendations. The American Heart Association/American College of Cardiology system was used to label the level of evidence and class of recommendation. RESULTS:: The consensus panel agreed on the following statements in patients with AF undergoing cardiac surgery concomitant surgical ablation: CONCLUSIONS:: Given these evidence-based statements, the consensus panel stated that, in patients with persistent and permanent AF undergoing cardiac surgery, concomitant surgical ablation is recommended to increase incidence of sinus rhythm at short- and long-term follow-up (class 1, level A); to reduce the risk of stroke and thromboembolic events (class 2a, level B); to improve EF (class 2a, level A); and to exercise tolerance (class 2a, level A) and long-term survival (class 2a, level B).  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