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   517篇
  免费   43篇
  560篇
  2023年   3篇
  2022年   4篇
  2021年   6篇
  2020年   10篇
  2019年   6篇
  2018年   6篇
  2017年   15篇
  2016年   11篇
  2015年   28篇
  2014年   25篇
  2013年   28篇
  2012年   41篇
  2011年   33篇
  2010年   26篇
  2009年   15篇
  2008年   21篇
  2007年   24篇
  2006年   24篇
  2005年   23篇
  2004年   30篇
  2003年   23篇
  2002年   27篇
  2001年   6篇
  1999年   9篇
  1998年   8篇
  1997年   6篇
  1996年   6篇
  1995年   6篇
  1994年   6篇
  1993年   4篇
  1992年   7篇
  1991年   7篇
  1989年   12篇
  1988年   1篇
  1987年   4篇
  1986年   2篇
  1985年   8篇
  1984年   3篇
  1983年   4篇
  1982年   4篇
  1981年   7篇
  1980年   4篇
  1979年   1篇
  1978年   4篇
  1977年   1篇
  1976年   2篇
  1975年   2篇
  1974年   2篇
  1972年   1篇
  1956年   1篇
排序方式: 共有560条查询结果,搜索用时 0 毫秒
121.
Mutations in the CETP gene resulting in defective CETP activity have been shown to cause remarkable elevations of plasma HDL-C levels, with the accumulation in plasma of large, buoyant HDL particles enriched in apolipoprotein E. Genetic CETP deficiency thus represents a unique tool to evaluate how structural alterations of HDL impact on HDL atheroprotective functions. Aim of the present study was to assess the ability of HDL obtained from CETP-deficient subjects to protect endothelial cells from the development of endothelial dysfunction. HDL isolated from one homozygous and seven heterozygous carriers of CETP null mutations were evaluated for their ability to down-regulate cytokine-induced cell adhesion molecule expression and to promote NO production in cultured endothelial cells. When compared at the same protein concentration, HDL and HDL3 from carriers proved to be as effective as control HDL and HDL3 in down-regulating cytokine-induced VCAM-1, while carrier HDL2 were more effective than control HDL2 in inhibiting VCAM-1 expression. On the other hand, HDL and HDL fractions from carriers of CETP deficiency were significantly less effective than control HDL and HDL fractions in stimulating NO production, due to a reduced eNOS activating capacity, likely because of a reduced S1P content. In conclusion, the present findings support the notion that genetic CETP deficiency, by affecting HDL particle structure, impacts on HDL vasculoprotective functions. Understanding of these effects might be important for predicting the outcomes of pharmacological CETP inhibition.  相似文献   
122.
In discrete tomography, a scanned object is assumed to consist of only a few different materials. This prior knowledge can be effectively exploited by a specialized discrete reconstruction algorithm such as the Discrete Algebraic Reconstruction Technique (DART), which is capable of providing more accurate reconstructions from limited data compared to conventional reconstruction algorithms. However, like most iterative reconstruction algorithms, DART suffers from long computation times. To increase the computational efficiency as well as the reconstruction quality of DART, a multiresolution version of DART (MDART) is proposed, in which the reconstruction starts on a coarse grid with big pixel (voxel) size. The resulting reconstruction is then resampled on a finer grid and used as an initial point for a subsequent DART reconstruction. This process continues until the target pixel size is reached. Experiments show that MDART can provide a significant speed-up, reduce missing wedge artefacts and improve feature reconstruction in the object compared with DART within the same time, making its use with large datasets more feasible.  相似文献   
123.
Global environmental changes are expected to alter ecosystem carbon and nitrogen cycling, but the interactive effects of multiple simultaneous environmental changes are poorly understood. Effects of these changes on the production of nitrous oxide (N2O), an important greenhouse gas, could accelerate climate change. We assessed the responses of soil N2O fluxes to elevated CO2, heat, altered precipitation, and enhanced nitrogen deposition, as well as their interactions, in an annual grassland at the Jasper Ridge Global Change Experiment (CA, USA). Measurements were conducted after 6, 7 and 8?years of treatments. Elevated precipitation increased N2O efflux, especially in combination with added nitrogen and heat. Path analysis supported the idea that increased denitrification due to increased soil water content and higher labile carbon availability best explained increased N2O efflux, with a smaller, indirect contribution from nitrification. In our data and across the literature, single-factor responses tended to overestimate interactive responses, except when global change was combined with disturbance by fire, in which case interactive effects were large. Thus, for chronic global environmental changes, higher order interactions dampened responses of N2O efflux to multiple global environmental changes, but interactions were strongly positive when global change was combined with disturbance. Testing whether these responses are general should be a high priority for future research.  相似文献   
124.
Kramer  Kees J. M.  Jenner  Henk A.  de Zwart  Dick 《Hydrobiologia》1989,188(1):433-443
Biological sensors are becoming more important to monitor the quality of the aquatic environment. In this paper the valve movement response of freshwater (Dreissena polymorpha) and marine (Mytilus edulis) mussels is presented as a tool in monitoring studies. Examples of various methods for data storage and data treatment are presented, elucidating easier operation and lower detection limits. Several applications are mentioned, including an early warning system based on this valve movement response of mussels.  相似文献   
125.

Background

BCG, the only licensed vaccine against tuberculosis, provides some protection against disseminated disease in infants but has little effect on prevention of adult pulmonary disease. Newer parenteral immunization prime boost regimes may provide improved protection in experimental animal models but are unproven in man so that there remains a need for new and improved immunization strategies.

Methods and Findings

Mice were immunized parenterally, intranasally or simultaneously by both routes with BCG or recombinant mycobacterial antigens plus appropriate adjuvants. They were challenged with Mycobacterium tuberculosis (Mtb) and the kinetics of Mtb growth in the lungs measured. We show that simultaneous immunization (SIM) of mice by the intranasal and parenteral routes is highly effective in increasing protection over parenteral BCG administration alone. Intranasal immunization induces local pulmonary immunity capable of inhibiting the growth of Mtb in the early phase (the first week) of infection, while parenteral immunization has a later effect on Mtb growth. Importantly, these two effects are additive and do not depend on priming and boosting the immune response. The best SIM regimes reduce lung Mtb load by up to 2 logs more than BCG given by either route alone.

Conclusions

These data establish SIM as a novel and highly effective immunization strategy for Mtb that could be carried out at a single clinic visit. The efficacy of SIM does not depend on priming and boosting an immune response, but SIM is complementary to prime boost strategies and might be combined with them.  相似文献   
126.
Twenty two species of the paper wasp genus Ropalidia Guérin-Méneville, 1831, are listed from China. Among them, R. malaisei van der Vecht, 1962, R. cyathiformis (Fabricius, 1804), R. santoshae Das & Gupta, 1989, R. scitula (Bingham, 1897), R. obscura Gusenleitner, 1996 and R. ornaticeps (Cameron, 1900) are new records from China. A new species, R. parartifex Tan & van Achterberg, is described. Their diagnostic characteristics are summarized in an illustrated key and 36 colourplates.  相似文献   
127.
Since the proposition in 1975 of the European Neogene Mammal (MN) scale by Pierre Mein, the amount of taxonomical, stratigraphical and chronological information around Europe has increased exponentially. In this paper, the stratigraphical schemes of three of the best studied areas for the Lower and Middle Miocene, the Aragonian type area in Spain and the Upper Freshwater Molasse from the North Alpine Foreland Basin in Switzerland and Bavaria, are compared. The correlation of their local biostratigraphies are discussed. Sixteen rodent's events are studied and ranked in the three areas according to their local biostratigraphy. This study shows, and quantifies for the first time, the significant asynchronies of the different included rodent events. The MN-system is discussed in the light of those results. In accordance, we propose that it is still useful but only in a biochronological way, as a sequence of time-ordered reference localities allowing coarse long-distance correlations. In order to obtain better temporal resolution, this system has to be combined with local biostratigraphies that are well calibrated to the time scale, implementing the information about synchrony and diachrony of mammal events in different areas.  相似文献   
128.
A growing group of migrants age in an environment in which the dominant language (L2) differs from their mother tongue (L1). This study considers the occurrence of a language barrier in accessing (information on) health and healthcare provisions, and under which circumstances a limited proficiency in the L2 negatively influences well-being for a group of older Turkish women in the Netherlands. Data from interviews with 39 Turkish females reveal that a limited L2 proficiency does not automatically result in a lower level of well-being. When individuals are well-embedded in a social network (and feel belonging to it), a limited L2 proficiency can be alleviated by either L1 assistance from their environment or by facilitating an interpreter in L2 situations. However, when such a network is absent, L2 situations can cause anxiety, which may have repercussions for well-being. Even though there is no clear one-on-one relationship between language and well-being, language does play a role in many (social) processes that influence well-being.  相似文献   
129.
The abundance of the human intestinal symbiont Akkermansia muciniphila has found to be inversely correlated with several diseases, including metabolic syndrome and obesity. A. muciniphila is known to use mucin as sole carbon and nitrogen source. To study the physiology and the potential for therapeutic applications of this bacterium, we designed a defined minimal medium. The composition of the medium was based on the genome‐scale metabolic model of A. muciniphila and the composition of mucin. Our results indicate that A. muciniphila does not code for GlmS, the enzyme that mediates the conversion of fructose‐6‐phosphate (Fru6P) to glucosamine‐6‐phosphate (GlcN6P), which is essential in peptidoglycan formation. The only annotated enzyme that could mediate this conversion is Amuc‐NagB on locus Amuc_1822. We found that Amuc‐NagB was unable to form GlcN6P from Fru6P at physiological conditions, while it efficiently catalyzed the reverse reaction. To overcome this inability, N‐acetylglucosamine needs to be present in the medium for A. muciniphila growth. With these findings, the genome‐scale metabolic model was updated and used to accurately predict growth of A. muciniphila on synthetic media. The finding that A. muciniphila has a necessity for GlcNAc, which is present in mucin further prompts the adaptation to its mucosal niche.  相似文献   
130.
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