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   85084篇
  免费   6291篇
  国内免费   2042篇
  2023年   1244篇
  2022年   1308篇
  2021年   2217篇
  2020年   2658篇
  2019年   3029篇
  2018年   2868篇
  2017年   2071篇
  2016年   1972篇
  2015年   2512篇
  2014年   4878篇
  2013年   6679篇
  2012年   3642篇
  2011年   5050篇
  2010年   3820篇
  2009年   4359篇
  2008年   4379篇
  2007年   4488篇
  2006年   4031篇
  2005年   3622篇
  2004年   3216篇
  2003年   2728篇
  2002年   2419篇
  2001年   1572篇
  2000年   1357篇
  1999年   1331篇
  1998年   1163篇
  1997年   989篇
  1996年   962篇
  1995年   952篇
  1994年   892篇
  1993年   819篇
  1992年   721篇
  1991年   666篇
  1990年   498篇
  1989年   491篇
  1988年   434篇
  1987年   410篇
  1986年   378篇
  1985年   608篇
  1984年   967篇
  1983年   705篇
  1982年   771篇
  1981年   604篇
  1980年   552篇
  1979年   478篇
  1978年   382篇
  1977年   328篇
  1976年   300篇
  1975年   271篇
  1973年   245篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
Human mast cells (MCs) contain TG-rich cytoplasmic lipid droplets (LDs) with high arachidonic acid (AA) content. Here, we investigated the functional role of adipose TG lipase (ATGL) in TG hydrolysis and the ensuing release of AA as substrate for eicosanoid generation by activated human primary MCs in culture. Silencing of ATGL in MCs by siRNAs induced the accumulation of neutral lipids in LDs. IgE-dependent activation of MCs triggered the secretion of the two major eicosanoids, prostaglandin D2 (PGD2) and leukotriene C4 (LTC4). The immediate release of PGD2 from the activated MCs was solely dependent on cyclooxygenase (COX) 1, while during the delayed phase of lipid mediator production, the inducible COX-2 also contributed to its release. Importantly, when ATGL-silenced MCs were activated, the secretion of both PGD2 and LTC4 was significantly reduced. Interestingly, the inhibitory effect on the release of LTC4 was even more pronounced in ATGL-silenced MCs than in cytosolic phospholipase A2-silenced MCs. These data show that ATGL hydrolyzes AA-containing TGs present in human MC LDs and define ATGL as a novel regulator of the substrate availability of AA for eicosanoid generation upon MC activation.  相似文献   
3.
4.
Clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated (cas) genes constitute the adaptive immune system in bacteria and archaea. Although the CRISPR-Cas systems have been hypothesized to encode potential toxins, no experimental data supporting the hypothesis are available in the literature. In this work, we provide the first experimental evidence for the presence of a toxin gene in the type I-A CRISPR system of hyperthermophilic archaeon Sulfolobus. csa5, under the control of its native promoter in a shuttle vector, could not be transformed into CRISPR-deficient mutant Sulfolobus solfataricus Sens1, demonstrating a strong toxicity in the cells. A single-amino-acid mutation destroying the intersubunit bridge of Csa5 attenuated the toxicity, indicative of the importance of Csa5 oligomerization for its toxicity. In line with the absence of Csa5 toxicity in S. solfataricus InF1 containing functional CRISPR systems, the expression of csa5 is repressed in InF1 cells. Induced from the arabinose promoter in Sens1 cells, Csa5 oligomers resistant to 1% SDS co-occur with chromosome degradation and cell death, reinforcing the connection between Csa5 oligomerization and its toxicity. Importantly, a rudivirus was shown to induce Csa5 expression and the formation of SDS-resistant Csa5 oligomers in Sulfolobus cells. This demonstrates that the derepression of csa5 and the subsequent Csa5 oligomerization take place in native virus-host systems. Thus, csa5 is likely to act as a suicide gene under certain circumstances to inhibit virus spreading.  相似文献   
5.
Eukaryotic protein kinases are typically strictly controlled by second messenger binding, protein/protein interactions, dephosphorylations or similar processes. None of these regulatory mechanisms is known to work for protein kinase CK2 (former name “casein kinase 2”), an acidophilic and constitutively active eukaryotic protein kinase. CK2 predominantly exists as a heterotetrameric holoenzyme composed of two catalytic subunits (CK2α) complexed to a dimer of non-catalytic subunits (CK2β). One model of CK2 regulation was proposed several times independently by theoretical docking of the first CK2 holoenzyme structure. According to this model, the CK2 holoenzyme forms autoinhibitory aggregates correlated with trans-autophosphorylation and driven by the down-regulatory affinity between an acidic loop of CK2β and the positively charged substrate binding region of CK2α from a neighboring CK2 heterotetramer. Circular trimeric aggregates in which one-half of the CK2α chains show the predicted inhibitory proximity between those regions were detected within the crystal packing of the human CK2 holoenzyme. Here, we present further in vitro support of the “regulation-by-aggregation” model by an alternative crystal form in which CK2 tetramers are arranged as approximately linear aggregates coinciding essentially with the early predictions. In this assembly, the substrate binding region of every CK2α chain is blocked by a CK2β acidic loop from a neighboring tetramer. We found these crystals with CK2Andante that contains a CK2β variant mutated in a CK2α-contact helix and described to be responsible for a prolonged circadian rhythm in Drosophila. The increased propensity of CK2Andante to form aggregates with completely blocked active sites may contribute to this phenotype.  相似文献   
6.
Intraplantar injection of 0.4% formalin into the rat hind paw leads to a biphasic nociceptive response; an ‘acute’ phase (0–15 min) and ‘tonic’ phase (16–120 min), which is accompanied by significant phosphorylation of extracellular signal‐regulated kinase (ERK)1/2 in the contralateral striatum at 120 min post‐formalin injection. To uncover a possible relationship between the slow‐onset substance P (SP) release and increased ERK1/2 phosphorylation in the striatum, continuous infusion of SP into the striatum by reverse microdialysis (0.4 μg/mL in microdialysis fiber, 1 μL/min) was performed to mimic volume neurotransmission of SP. Continuous infusion for 3 h of SP reduced the duration of ‘tonic’ phase nociception, and this SP effect was mediated by neurokinin 1 (NK1) receptors since pre‐treatment with NK1 receptor antagonist CP96345 (10 μM) blocked the effect of SP infusion. However, formalin‐induced ‘tonic’ phase nociception was significantly prolonged following acute injection of the MAP/ERK kinase 1/2 inhibitor PD0325901 (100 pmol) by microinjection. The coinfusion of SP and PD0325901 significantly increased the ‘tonic’ phase of nociception. These data demonstrate that volume transmission of striatal SP triggered by peripheral nociceptive stimulation does not lead to pain facilitation but a significant decrease of tonic nociception by the activation of the SP‐NK1 receptor–ERK1/2 system.

  相似文献   

7.
Toxoplasma 3 main clonal lineages are designated as type I, II, and III; however, atypical and mixed genotypes were also reported. This study was conducted for detection of Toxoplasma gondii genotypes in rats (Rattus rattus) in Riyadh region, Saudi Arabia. PCR test on T. gondii B1 gene was conducted on ELISA IgM positive samples for confirmation of the infection. However, genetic analysis of the SAG2 locus was performed to determine T. gondii genotypes using PCR-RFLP technique. PCR test on T. gondii B1gene showed that 22 (81.5%) out of the 27 ELISA IgM positive samples have T. gondii DNA. Genotypic analysis shows that, of the total 22 PCR positive samples, only 13 (59.1%) were of type II, 7 (31.8%) were of type III, and 2 (9.1%) were of an unknown genotype. It is obvious that the prevalence of both type II and III is high in rats. No reports have been available on T. gondii genotypes among rats in Riyadh region, and only little is known about its seroprevalence in rats. Future studies on T. gondii genotypes in rats using multi-locus markers is needed in Riyadh region, Saudi Arabia for better understanding of T. gondii pathogenesis and treatment in humans and animals.  相似文献   
8.

Aims

Septic shock, the severe form of sepsis, is associated with development of progressive damage in multiple organs. Kidney can be injured and its functions altered by activation of coagulation, vasoactive-peptide and inflammatory processes in sepsis. Endothelin (ET)-1, a potent vasoconstrictor, is implicated in the pathogenesis of sepsis and its complications. Protease-activated receptors (PARs) are shown to play an important role in the interplay between inflammation and coagulation. We examined the time-dependent alterations of ET-1 and inflammatory cytokine, such as tumor necrosis factor (TNF)-α in kidney tissue in lipopolysaccharide (LPS)-induced septic rat model and the effects of PAR2 blocking peptide on the LPS-induced elevations of renal ET-1 and TNF-α levels.

Main methods

Male Wistar rats at 8 weeks of age were administered with either saline solution or LPS at different time points (1, 3, 6 and 10 h). Additionally, we treated LPS-administered rats with PAR2 blocking peptide for 3 h to assess whether blockade of PAR2 has a regulatory role on the ET-1 level in septic kidney.

Key findings

An increase in ET-1 peptide level was observed in kidney tissue after LPS administration time-dependently. Levels of renal TNF-α peaked (around 12-fold) at 1 h of sepsis. Interestingly, PAR2 blocking peptide normalized the LPS-induced elevations of renal ET-1 and TNF-α levels.

Significance

The present study reveals a distinct chronological expression of ET-1 and TNF-α in LPS-administered renal tissues and that blockade of PAR2 may play a crucial role in treating renal injury, via normalization of inflammation, coagulation and vaso-active peptide.  相似文献   
9.
目的:探讨颈围用于筛选糖调节受损人群的临床价值。方法:选取哈尔滨市第一医院2017年3月至11月收治的糖调节受损患者共100例,男性颈围以38 cm为临界值,女性以35 cm为临界值,分为颈围正常组和颈围异常组,比较两组患者空腹及餐后血糖、空腹胰岛素、糖化血红蛋白、总胆固醇、甘油三酯、低密度脂蛋白等相关指标水平,并分析颈围与以上指标的相关性。结果:颈围异常组患者的餐后2 h血糖、空腹胰岛素、糖化血红蛋白、总胆固醇、甘油三酯、低密度脂蛋白数值高于颈围正常组,尤以甘油三酯明显,高密度脂蛋白数值低于颈围正常组,差异有统计学意义(P0.05)。颈围与餐后2 h血糖、总胆固醇、甘油三酯、低密度脂蛋白水平呈正相关,差异均有统计学意义(P0.05),颈围与高密度脂蛋白水平呈负相关,差异均有统计学意义(P0.05)。结论:颈围可以作为临床预测、评估糖调节受损患者的早期指标,为早期社区筛查及诊断提供相关的参考依据。  相似文献   
10.
目的:探究人髓细胞白血病基因-1(myeloid cell leukemia-1,Mcl-1)是否参与非小细胞肺癌对吉非替尼的耐药。方法:应用Western blot检测Mcl-1在吉非替尼敏感细胞PC-9和耐药细胞H1975表达差异;梯度浓度的吉非替尼作用于PC-9细胞后,Western blot实验检测B淋巴细胞瘤-2基因(B-cell lymphoma-2,Bcl-2)家族中抗凋亡蛋白的表达变化;应用Western blot实验检测Mcl-1在PC-9和H1975细胞内降解速度差异。结果:Mcl-1在PC-9细胞内的表达明显低于H1975细胞,差异有统计学意义(P0.05),并且随着吉非替尼作用浓度的升高,Mcl-1表达逐渐降低,而Bcl-2和Bcl-x L表达基本不变,并且PC-9细胞内Mcl-1降解更迅速,半衰期明显缩短。结论:非小细胞肺癌对吉非替尼耐药可能与Mcl-1的表达量上调,降解速度减慢,半衰期延长有关。  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