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   24817篇
  免费   1749篇
  国内免费   14篇
  26580篇
  2024年   29篇
  2023年   95篇
  2022年   293篇
  2021年   506篇
  2020年   294篇
  2019年   382篇
  2018年   632篇
  2017年   483篇
  2016年   814篇
  2015年   1290篇
  2014年   1452篇
  2013年   1663篇
  2012年   2117篇
  2011年   1966篇
  2010年   1273篇
  2009年   1090篇
  2008年   1571篇
  2007年   1396篇
  2006年   1251篇
  2005年   1112篇
  2004年   1124篇
  2003年   921篇
  2002年   904篇
  2001年   666篇
  2000年   670篇
  1999年   462篇
  1998年   185篇
  1997年   150篇
  1996年   133篇
  1995年   103篇
  1994年   96篇
  1993年   79篇
  1992年   172篇
  1991年   141篇
  1990年   100篇
  1989年   110篇
  1988年   77篇
  1987年   74篇
  1986年   74篇
  1985年   64篇
  1984年   56篇
  1983年   40篇
  1982年   32篇
  1981年   28篇
  1979年   27篇
  1978年   31篇
  1976年   34篇
  1975年   34篇
  1973年   34篇
  1969年   25篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
The n-3 polyunsaturated fatty acids (PUFAs), EPA and DHA, as well as estrogen have been shown to decrease circulating levels of triglyceride (TG), but their underlying mode of action is unclear. The purpose of this study was to determine the effects of n-3 PUFA consumption and estrogen injection on TG metabolism. Rats (n = 48) were fed a modified AIN-93G diet with 0, 1, or 2 % EPA + DHA relative to the total energy intake during 12 weeks. At 8 weeks, rats were ovariectomized (OVX), and after a 1-week recovery, rats were injected with either 17β-estradiol-3-benzoate (E2) or corn oil for the last 3 weeks. The n-3 PUFA consumption and E2 injection independently decreased the hepatic expressions of sterol regulatory element-binding protein 1, acetyl-CoA carboxylase 1, fatty acid synthase (FAS), and diacylglycerol acyltransferase 2 (DGAT2) (P < 0.05). There were interactions between n-3 PUFA consumption and E2 injection on hepatic expression of FAS and DGAT2. In addition, n-3 PUFA consumption and E2 injection up-regulated the expression of AMP-activated protein kinase (AMPK), phosphorylated AMPK, peroxisomal proliferator-activated receptor α, and carnitine palmitoyltransferase 1 in liver and skeletal muscle. E2 injection increased the expression of estrogen receptor α and β in skeletal muscle and liver, but n-3 PUFA consumption increased the expression of both receptors only in skeletal muscle. The present study suggests that the hypotriglyceridemic effects of n-3 PUFA consumption and E2 injection could be due to the down-regulation of hepatic TG synthesis and up-regulation of TG oxidation in liver and skeletal muscle in OVX rats.  相似文献   
82.
83.
Hypertension is a major health problem with increasing prevalence around the world. Tannic acid is water-soluble polyphenol that is present in tea, green tea, coffee, red wine, nuts, fruits and many plant foods. It has been reported to serve as an antioxidant or a pro-oxidant depending on the type of cells and its concentration. The purpose of our study was to evaluate the effect of tannic acid on systolic blood pressure, oxidative stress and some urinary parameters in the rat model of essential hypertension. Blood pressures of all rats were measured using the tail-cuff method. The nitric oxide synthase inhibitor N (omega)-nitro-L-arginine was administered orally at a dose of 0.5 g/l/day for 15 days to rats in order to create an animal model of hypertension. Tannic acid was intraperitoneally injected at a dose of 50 mg/kg for 15 days. Superoxide dismutase, catalase activity and the concentration of malondialdehyde (MDA) were determined in blood plasma and homogenates of heart, liver and kidney. In order to evaluate renal functions, urine pH, urine volume, urine creatine, uric acid, and urea nitrogen values were measured. Compared with the hypertension group, a decrease in MDA concentrations of heart tissue (p < 0.01), urea nitrogen values (p < 0.01) and urine volumes (p < 0.001) were established in hypertension + tannic acid group. There was also a decrease in blood pressure values (20th and 30th days) of this group, but there was no a statistical difference according to hypertension group. The findings of our research show the effect of tannic acid in lowering blood pressure in hypertensive rats.  相似文献   
84.
Despite the prepdominat agent causing severe entero-pathogenic diarrhea in swine, there are no effective therapeutical treatment of porcine epidemic diarrhea virus (PEDV). In this study, we evaluated the antiviral activity of five phlorotannins isolated from Ecklonia cava (E. cava) against PEDV. In vitro antiviral activity was tested using two different assay strategies: (1) blockage of the binding of virus to cells (simultaneous-treatment assay) and (2) inhibition of viral replication (post-treatment assay). In simultaneous-treatment assay, compounds 25 except compound 1 exhibited antiviral activities of a 50% inhibitory concentration (IC50) with the ranging from 10.8 ± 1.4 to 22.5 ± 2.2 μM against PEDV. Compounds 15 were completely blocked binding of viral spike protein to sialic acids at less than 36.6 μM concentrations by hemagglutination inhibition. Moreover, compounds 4 and 5 of five phlorotannins inhibited viral replication with IC50 values of 12.2 ± 2.8 and 14.6 ± 1.3 μM in the post-treatment assay, respectively. During virus replication steps, compounds 4 and 5 exhibited stronger inhibition of viral RNA and viral protein synthesis in late stages (18 and 24 h) than in early stages (6 and 12 h). Interestingly, compounds 4 and 5 inhibited both viral entry by hemagglutination inhibition and viral replication by inhibition of viral RNA and viral protein synthesis, but not viral protease. These results suggest that compounds isolated from E. cava have strong antiviral activity against PEDV, inhibiting viral entry and/or viral replication, and may be developed into natural therapeutic drugs against coronavirus infection.  相似文献   
85.
86.
87.
The effects of urban pollution from Hanoi city on the benthic diatom communities of the Nhue–Tolich river system were studied during the 2003 dry season. Benthic diatoms were allowed to grow on glass slides suspended in the water flow for 4 weeks. To reveal the relationship between water quality and diatom communities, Canonical correspondence analysis (CCA) was used on data concerning relative abundances of diatom species and environmental variables. Two diatom indices, IPS and DAIpo, were applied to evaluate water quality in the three rivers. A total of 291 diatom taxa were found in the Red, Nhue and Tolich Rivers. These were mainly cosmopolitan taxa, with some tropical, subtropical and endemic taxa. The most abundant taxa at the Red site were Aulacoseira granulata, Achnanthidium minutissimum, Encyonema minutum, Navicula recens and other halophilous taxa such as Nitzschia kurzii, Seminavis strigosa, Entomoneis paludosa, Bacillaria paradoxa. Diatom assemblages at the Tolich site consisted mainly of Nitzschia umbonata, Nitzschia palea and Eolimna minima. Diatom density ranged from 660 to 30,000 cells/cm2. Environmental variables and diatom assemblage composition at all sites were significantly correlated. Two diatom indices gave similar results and indicate the Tolich River with the lowest values as a highly polluted site.  相似文献   
88.
Members of the Pumilio (Pum) family of RNA-binding proteins act as translational repressors and are required for germ cell development and asymmetric division. We identified the chicken Pum1 and Pum2 genes and analyzed their expression patterns in various tissues. Comparative sequence analysis of the Pum1 and Pum2 proteins from the drosophila, chicken, mouse, and human revealed a high degree of evolutionary conservation in terms of the levels of homology of the peptide sequences and the structure of Pumilio homology domain (PUM-HD), C-terminal RNA-binding domain, with similar spacing between the adjacent Pum eight tandem repeats. In addition, phylogenetic patterns of pumilio family showed that Pum 1 and 2 of chicken are more closely related to those of mouse and human than other species and Pum1 is more conserved than Pum2. Using real-time RT-PCR, the expression levels of the Pum1 and Pum2 genes were found to be highest in hatched female gonads, and high-level expression of Pum2 was detected in 12-day and hatched gonads among the various chicken embryonic tissues tested. In adult tissues, the expression levels of Pum1 and Pum2 were expressed at higher levels in the testis and muscle than in any other tissue. The characteristics of the tissue-specific expression of Pum genes suggest that Pum1 and Pum2 have effects crucially in particular stage during development of chicken gonads depending on sexual maturation.  相似文献   
89.
The monogonont rotifer, Brachionus ibericus (S type), is considered to be a promising model species for developmental biology, evolution, and environmental genomics. In an attempt to accelerate the molecular understanding of B. ibericus, we sequenced 680.5 Mb of genomic DNA using the genome sequencer GS-FLX-Titanium. We obtained 2,062,621 reads (average read length 329.9 bp) and 145,418 contigs (total contigs length 125.7 Mb) after excluding small reads (less than 200 bp) from the assembly, and finally obtained 10,133 unigenes (E value ?? 9.00E?04) after non-redundant (NR) BLAST search. In this article, we summarize the genomic DNA sequences of B. ibericus and discuss their potential use in the study of reproductive biology, endocrinology, environmental genomics, and ecotoxicological studies, and for providing insight into the genetic basis of mechanisms such as egg formation, antioxidant stress defense, and xenobiotic metabolism.  相似文献   
90.
In our ongoing effort of discovering anticancer and chemopreventive agents, a series of 2-arylindole derivatives were synthesized and evaluated toward aromatase and quinone reductase 1 (QR1). Biological evaluation revealed that several compounds (e.g., 2d, IC50?=?1.61?μM; 21, IC50?=?3.05?μM; and 27, IC50?=?3.34?μM) showed aromatase inhibitory activity with half maximal inhibitory concentration (IC50) values in the low micromolar concentrations. With regard to the QR1 induction activity, 11 exhibited the highest QR1 induction ratio (IR) with a low concentration to double activity (CD) value (IR?=?8.34, CD?=?2.75?μM), while 7 showed the most potent CD value of 1.12?μM. A dual acting compound 24 showed aromatase inhibition (IC50?=?9.00?μM) as well as QR1 induction (CD?=?5.76?μM) activities. Computational docking studies using CDOCKER (Discovery Studio 3.5) provided insight in regard to the potential binding modes of 2-arylindoles within the aromatase active site. Predominantly, the 2-arylindoles preferred binding with the 2-aryl group toward a small hydrophobic pocket within the active site. The C-5 electron withdrawing group on indole was predicted to have an important role and formed a hydrogen bond with Ser478 (OH). Alternatively, meta-pyridyl analogs may orient with the pyridyl 3′-nitrogen coordinating with the heme group.  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