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   3722篇
  免费   314篇
  国内免费   2篇
  2023年   20篇
  2022年   26篇
  2021年   107篇
  2020年   51篇
  2019年   84篇
  2018年   103篇
  2017年   98篇
  2016年   98篇
  2015年   172篇
  2014年   196篇
  2013年   242篇
  2012年   310篇
  2011年   286篇
  2010年   173篇
  2009年   145篇
  2008年   236篇
  2007年   236篇
  2006年   201篇
  2005年   183篇
  2004年   182篇
  2003年   159篇
  2002年   164篇
  2001年   40篇
  2000年   51篇
  1999年   46篇
  1998年   26篇
  1997年   26篇
  1996年   22篇
  1995年   12篇
  1994年   10篇
  1993年   16篇
  1992年   20篇
  1991年   28篇
  1990年   13篇
  1989年   13篇
  1988年   14篇
  1987年   23篇
  1986年   12篇
  1985年   12篇
  1984年   18篇
  1983年   14篇
  1982年   12篇
  1980年   12篇
  1979年   14篇
  1976年   11篇
  1973年   12篇
  1970年   9篇
  1969年   10篇
  1968年   14篇
  1967年   8篇
排序方式: 共有4038条查询结果,搜索用时 15 毫秒
91.
This study presents the results of the identification and quantification of 12 isoflavones (prunetin, irilone, pseudobaptigenin, glycitein, daidzin, genistin, daidzein, pratensein, puerarin, biochanin A, formononetin and genistein) in 23 species of Trifolium (T. arvense, T. pratense, T. ligusticum, T. striatum, T. lappaceum, T. angustifolium, T. hirtum, T. subterraneum, T. isthmocarpum, T. stellatum, T. mutabile, T. strictum, T. fragiferum, T. alexandrinum, T. tomentosum. T. nigrescens subsp. petrisavii, T. nigrescens, T. glomeratum, T. subterraneum subsp. brachycalycinum, T. cherleri, T. resupinatum, T. campestre and T. repens). Isoflavones were extracted by an MSPD method and analyzed with HPLC coupled with a diode-array detector. The evaluation of molecular phylogeny of the IFS gene and the relation with isoflavone content was also performed. Five species (T. subterraneum subsp. brachycalycinum, T. alexandrinum, T. pratense, T. subterraneum and T. lappaceum) were identified with high levels of biochanin A (431–83 mg/kg), formononetin (72–365 mg/kg) and genistein (9–509 mg/kg), which could be utilized as alternative sources for the nutraceutical industry. Genetic phylogeny for the IFS gene was found in the species studied, with 20 out of 23 species having been divided into two clades, while the remaining three were genetically distant. Based on our results, we confirm the direct correlation between IFS gene polymorphism and isoflavones content in species of Trifolium particularly noted for formononetin. Therefore, the IFS gene can be utilized for screening Trifolium genotypes for formononetin. The relation of the three isoflavones' contents and the molecular phylogeny of plants determined by the IFS sequences, as a screening marker for plants with high isoflavone contents in Trifolium species, are to the best of our knowledge described for the first time.  相似文献   
92.
Abstract

This study reports rate-of-dialysis of an iodinated N-methyl-D-aspartate antagonist drug, [125-1] MK-801, from solutions of lipid vesicles and from proteoliposomes containing purified membrane proteins. A 170 kd protein precipitated from proteoliposomes cross reacts with monoclonal antibodies against cloned NMDA-NR2(A) and NR2(B) subunits. Drug binding in proteoliposomes includes contributions from lipid and from protein, in addition to lipid. A significant change in drug binding was observed in proteoliposomes in response to 10 uM agonist, NMDA. Rate-of-dialysis from agonist-stimulated proteoliposomes was sensitive to perturbation by decreased aqueous ligand concentration in a manner consistent with a lipid-mediated receptor/antagonist equilibrium.  相似文献   
93.
Respiratory syncytial virus (RSV) is a major cause of severe lower respiratory infection in infants and young children and causes disease in the elderly and persons with compromised cardiac, pulmonary, or immune systems. Despite the high morbidity rates of RSV infection, no highly effective treatment or vaccine is yet available. The RSV G protein is an important contributor to the disease process. A conserved CX3C chemokine-like motif in G likely contributes to the pathogenesis of disease. Through this motif, G protein binds to CX3CR1 present on various immune cells and affects immune responses to RSV, as has been shown in the mouse model of RSV infection. However, very little is known of the role of RSV CX3C-CX3CR1 interactions in human disease. In this study, we use an in vitro model of human RSV infection comprised of human peripheral blood mononuclear cells (PBMCs) separated by a permeable membrane from human airway epithelial cells (A549) infected with RSV with either an intact CX3C motif (CX3C) or a mutated motif (CX4C). We show that the CX4C virus induces higher levels of type I/III interferon (IFN) in A549 cells, increased IFN-α and tumor necrosis factor alpha (TNF-α) production by human plasmacytoid dendritic cells (pDCs) and monocytes, and increased IFN-γ production in effector/memory T cell subpopulations. Treatment of CX3C virus-infected cells with the F(ab′)2 form of an anti-G monoclonal antibody (MAb) that blocks binding to CX3CR1 gave results similar to those with the CX4C virus. Our data suggest that the RSV G protein CX3C motif impairs innate and adaptive human immune responses and may be important to vaccine and antiviral drug development.  相似文献   
94.
This study sought to understand the origin and fate of one of the bitumen mounds found on the bottom of Lake Baikal. These mounds are located at a depth of 900 m beneath oil spots detected on the surface of Lake Baikal (53° 18′24, 108° 23′20). The two mounds were sampled with a manipulator from a “MIR” deep-water manned submersible. Mature mound No. 8 was subjected to chemical and microbiological studies. Mound No. 3 was subjected only to chemical studies; we failed to perform microbiological analyses of this mound for logistic reasons. Oil spots collected from the water surface, samples of mound No. 3 and No. 8, were subjected to GC/MS analysis. The water contained aliphatic hydrocarbons with chains between C8 and C23, with the most abundant chain length being C18. Mound No. 3 with the most abundant chain length being C18 actively released oil droplets into the water. It contained 770 mg/g of C13-C32 n-alkanes, with a maximum at C23 (160 mg/g). Mound No. 8 was inactive and contained 148 mg/g of aliphatic C22-C34 n-alkanes, with a maximum at C25. Mound No. 8 also consisted of 3% inorganic matter, 48% unresolved complex mixture (UCM) and less than 1% other compounds (polyaromatic hydrocarbons, isoprenoids, carotenoids, and hopanes). The core of this sample used as inoculate, yielded Rhodococci when cultivated on oil as the only source of carbon. Cultivation of the sample on agar-containing Raymond inorganic medium with crude West Siberian oil as the only source of carbon revealed colonies of these bacteria, which all appeared identical. PCR was performed with DNA isolated from 5 colonies, using primers for 16S rRNA genes. Comparison of the sequences of the 5 PCR products over a length of 714 bp revealed that they were almost identical. Phylogenetic analysis of these homologous sequences showed that they were similar to the corresponding sequences of the genus Rhodococcus. Substrate demands, the morphology of the colonies, and SEM and TEM data confirmed that the isolates obtained could indeed be Rhodococci. All of the isolates could grow in bulk cultures with inorganic medium supplemented with crude oil. Moreover, all of the isolates degraded aliphatic hydrocarbons with lengths between C11 and C29. C23-C29 hydrocarbons were degraded completely. The isolates could grow at 4–37°C. The most unexpected finding was that of the many microorganisms capable of consuming oil, only Rhodococci exhibited this ability in the inactive bitumen mound. The possible mechanisms of how crude oil is transformed into bitumen mounds and mature bitumen are discussed.  相似文献   
95.
We have shown previously that estrogen (estradiol, E2) supplementation enhances voluntary alcohol consumption in ovariectomized female rodents and that increased alcohol consumption impairs ischemic hind limb vascular repair. However, the effect of E2-induced alcohol consumption on post-infarct myocardial repair and on the phenotypic/functional properties of endothelial progenitor cells (EPCs) is not known. Additionally, the molecular signaling of alcohol-estrogen interactions remains to be elucidated. This study examined the effect of E2-induced increases in ethanol consumption on post-infarct myocardial function/repair. Ovariectomized female mice, implanted with 17β-E2 or placebo pellets were given access to alcohol for 6 weeks and subjected to acute myocardial infarction. Left ventricular functions were consistently depressed in mice consuming ethanol compared with those receiving only E2. Alcohol-consuming mice also displayed significantly increased infarct size and reduced capillary density. Ethanol consumption also reduced E2-induced mobilization and homing of EPCs to injured myocardium compared with the E2-alone group. In vitro, exposure of EPCs to ethanol suppressed E2-induced proliferation, survival, and migration and markedly altered E2-induced estrogen receptor-dependent cell survival signaling and gene expression. Furthermore, ethanol-mediated suppression of EPC biology was endothelial nitric oxide synthase-dependent because endothelial nitric oxide synthase-null mice displayed an exaggerated response to post-acute myocardial infarction left ventricular functions. These data suggest that E2 modulation of alcohol consumption, and the ensuing EPC dysfunction, may negatively compete with the beneficial effects of estrogen on post-infarct myocardial repair.  相似文献   
96.
AimsWe investigated the effects of ketogenic diet (KD) on levels of tumor necrosis factor alpha (TNF-α, a classical pro-inflammatory cytokine), BDNF (brain-derived neurotrophic factor, commonly associated with synaptic plasticity), and S100B, an astrocyte neurotrophic cytokine involved in metabolism regulation.Main methodsYoung Wistar rats were fed during 8 weeks with control diet or two KD, containing different proportions of omega 6 and omega 3 polyunsaturated fatty acids. Contents of TNF-α, BDNF and S100B were measured by ELISA in two brain regions (hippocampus and striatum) as well as blood serum and cerebrospinal fluid.Key findingsOur data suggest that KD was able to reduce the levels of BDNF in the striatum (but not in hippocampus) and S100B in the cerebrospinal fluid of rats. These alterations were not affected by the proportion of polyunsaturated fatty acids offered. No changes in S100B content were observed in serum or analyzed brain regions. Basal TNF-α content was not affected by KD.SignificanceThese findings reinforce the importance of this diet as an inductor of alterations in the brain, and such changes might contribute to the understanding of the effects (and side effects) of KD in brain disorders.  相似文献   
97.
The emergence of the highly virulent Ug99 race complex of the stem rust fungus (Puccinia graminis Pers. f. sp. tritici Eriks. and Henn.) threatens wheat (Triticum aestivum L.) production worldwide. One of the effective genes against the Ug99 race complex is Sr44, which was derived from Thinopyrum intermedium (Host) Barkworth and D.R. Dewey and mapped to the short arm of 7J (designated 7J#1S) present in the noncompensating T7DS-7J#1L?7J#1S translocation. Noncompensating wheat-alien translocations are known to cause genomic duplications and deficiencies leading to poor agronomic performance, precluding their direct use in wheat improvement. The present study was initiated to produce compensating wheat-Th. intermedium Robertsonian translocations with Sr44 resistance. One compensating RobT was identified consisting of the wheat 7DL arm translocated to the Th. intermedium 7J#1S arm resulting in T7DL?7J#1S. The T7DL?7J#1S stock was designated as TA5657. The 7DL?7J#1S stock carries Sr44 and has resistance to the Ug99 race complex. This compensating RobT with Sr44 resistance may be useful in wheat improvement. In addition, we identified an unnamed stem rust resistance gene located on the 7J#1L arm that confers resistance not only to Ug99, but also to race TRTTF, which is virulent to Sr44. However, the action of the second gene can be modified by the presence of suppressors in the recipient wheat cultivars.  相似文献   
98.
Sawadalepis n. g. is erected for Sawadalepis prima n. sp. in Schreiber’s bent-winged bat Miniopterus schreibersii Kuhl (Chiroptera: Vespertilionidae) from southern China. Diagnostic features of the currently monotypic genus include attributes of the osmoregulatory system and female genital organs. The dorsal osmoregulatory canals are shifted bilaterally towards the margins of proglottides in relation to the ventral canals. The genital pores are unilateral and sinistral. Among female attributes, the copulatory part of the vagina is covered externally by a dense layer of intensely stained cells; the conductive part of the vagina is clearly distinguishable from the seminal receptacle; the uterus has ventral and dorsal diverticula, extending bilaterally beyond the longitudinal osmoregulatory canals; and the eggs are spherical with thick outer coat.  相似文献   
99.
A road map for molecular ecology   总被引:1,自引:0,他引:1  
The discipline of molecular ecology has undergone enormous changes since the journal bearing its name was launched approximately two decades ago. The field has seen great strides in analytical methods development, made groundbreaking discoveries and experienced a revolution in genotyping technology. Here, we provide brief perspectives on the main subdisciplines of molecular ecology, describe key questions and goals, discuss common challenges, predict future research directions and suggest research priorities for the next 20 years.  相似文献   
100.
The biological functions of myotonic dystrophy protein kinase (DMPK), a serine/threonine kinase whose gene mutations cause myotonic dystrophy type 1 (DM1), remain poorly understood. Several DMPK isoforms exist, and the long ones (DMPK-A/B/C/D) are associated with the mitochondria, where they exert unknown activities. We have studied the isoform A of DMPK, which we have found to be prevalently associated to the outer mitochondrial membrane. The kinase activity of mitochondrial DMPK protects cells from oxidative stress and from the ensuing opening of the mitochondrial permeability transition pore (PTP), which would otherwise irreversibly commit cells to death. We observe that DMPK (i) increases the mitochondrial localization of hexokinase II (HK II), (ii) forms a multimeric complex with HK II and with the active form of the tyrosine kinase Src, binding its SH3 domain and (iii) it is tyrosine-phosphorylated by Src. Both interaction among these proteins and tyrosine phosphorylation of DMPK are increased under oxidative stress, and Src inhibition selectively enhances death in DMPK-expressing cells after HK II detachment from the mitochondria. Down-modulation of DMPK abolishes the appearance of muscle markers in in vitro myogenesis, which is rescued by oxidant scavenging. Our data indicate that, together with HK II and Src, mitochondrial DMPK is part of a multimolecular complex endowed with antioxidant and pro-survival properties that could be relevant during the function and differentiation of muscle fibers.  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