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   1592篇
  免费   202篇
  2021年   12篇
  2016年   15篇
  2015年   27篇
  2014年   43篇
  2013年   45篇
  2012年   42篇
  2011年   63篇
  2010年   53篇
  2009年   35篇
  2008年   63篇
  2007年   66篇
  2006年   57篇
  2005年   58篇
  2004年   55篇
  2003年   53篇
  2002年   43篇
  2001年   47篇
  2000年   51篇
  1999年   62篇
  1998年   30篇
  1997年   18篇
  1996年   20篇
  1995年   20篇
  1994年   12篇
  1993年   12篇
  1992年   48篇
  1991年   51篇
  1990年   43篇
  1989年   46篇
  1988年   37篇
  1987年   54篇
  1986年   36篇
  1985年   30篇
  1984年   30篇
  1983年   20篇
  1982年   22篇
  1981年   24篇
  1980年   22篇
  1979年   16篇
  1978年   20篇
  1977年   22篇
  1976年   16篇
  1975年   20篇
  1974年   21篇
  1973年   31篇
  1972年   23篇
  1971年   19篇
  1970年   17篇
  1968年   14篇
  1965年   15篇
排序方式: 共有1794条查询结果,搜索用时 15 毫秒
991.
992.
993.
Respiration and excretion were measured relative to pH and aluminum in three nymphal weight groups of a detritivore caddisfly. Although a reduction in respiration rates was consistently observed within all stages exposed to a pH of 4 and 0.3 mg 1−1 of aluminum, the differencewas not statistically significant among treatments. Excretion rates, on the other hand, were significantly increased throughout the low pH treatment, but not when exposed to low pH and aluminum combined. The derived O:N ratios indicate low pH causes a shift to a protein-oriented metabolism which was most pronounced in the smallest size group. Aluminum mitigates this effect.  相似文献   
994.
Research in psychology and affective neuroscience often relies on film as a standardized and reliable method for evoking emotion. However, clip validation is not undertaken regularly. This presents a challenge for research with adolescent and young adult samples who are exposed routinely to high-definition (HD) three-dimensional (3D) stimuli and may not respond to older, validated film clips. Studies with young people inform understanding of emotional development, dysregulated affect, and psychopathology, making it critical to assess whether technological advances improve the study of emotion. In the present study, we examine whether 3D film is more evocative than 2D using a tightly controlled within-subjects design. Participants (n  =  408) viewed clips during a concurrent psychophysiological assessment. Results indicate that both 2D and 3D technology are highly effective tools for emotion elicitation. However, 3D does not add incremental benefit over 2D, even when individual differences in anxiety, emotion dysregulation, and novelty seeking are considered.  相似文献   
995.
996.
997.
Although orphan drug applications required by the EMEA must include assessments of similarity to pre-existing products, these can be difficult to quantify. Here we illustrate a paradigm in comparing nilotinib to the prototype kinase inhibitor imatinib, and equate the degree of structural similarity to differences in properties. Nilotinib was discovered following re-engineering of imatinib, employing structural biology and medicinal chemistry strategies to optimise cellular potency and selectivity towards BCR-ABL1. Through evolving only to conserve these properties, this resulted in significant structural differences between nilotinib and imatinib, quantified by a Daylight-fingerprint—Tanimoto similarity coefficient of 0.6, with the meaning of this absolute measure being supported by an analysis of similarity distributions of similar drug-like molecules. This dissimilarity is reflected in the drugs having substantially different preclinical pharmacology and a lack of cross-intolerance in CML patients, which translates into nilotinib being an efficacious treatment for CML, with a favourable side-effect profile.  相似文献   
998.
A series of 1-(3-aryloxyaryl)benzimidazoles incorporating a sulfone substituent (6) was prepared. High affinity LXR ligands were identified (LXRβ binding IC50 values <10 nM), some with excellent agonist potency and efficacy in a functional assay of LXR activity measuring ABCA1 mRNA increases in human macrophage THP1 cells. The compounds were typically stable in liver microsome preparations and had good oral exposure in mice.  相似文献   
999.

Background

Long- and short-term exposures to air pollution, especially fine particulate matter (PM2.5), have been linked to cardiovascular morbidity and mortality. One hypothesized mechanism for these associations involves microvascular effects. Retinal photography provides a novel, in vivo approach to examine the association of air pollution with changes in the human microvasculature.

Methods and Findings

Chronic and acute associations between residential air pollution concentrations and retinal vessel diameters, expressed as central retinal arteriolar equivalents (CRAE) and central retinal venular equivalents (CRVE), were examined using digital retinal images taken in Multi-Ethnic Study of Atherosclerosis (MESA) participants between 2002 and 2003. Study participants (46 to 87 years of age) were without clinical cardiovascular disease at the baseline examination (2000–2002). Long-term outdoor concentrations of PM2.5 were estimated at each participant''s home for the 2 years preceding the clinical exam using a spatio-temporal model. Short-term concentrations were assigned using outdoor measurements on the day preceding the clinical exam. Residential proximity to roadways was also used as an indicator of long-term traffic exposures. All associations were examined using linear regression models adjusted for subject-specific age, sex, race/ethnicity, education, income, smoking status, alcohol use, physical activity, body mass index, family history of cardiovascular disease, diabetes status, serum cholesterol, glucose, blood pressure, emphysema, C-reactive protein, medication use, and fellow vessel diameter. Short-term associations were further controlled for weather and seasonality. Among the 4,607 participants with complete data, CRAE were found to be narrower among persons residing in regions with increased long- and short-term levels of PM2.5. These relationships were observed in a joint exposure model with −0.8 µm (95% confidence interval [CI] −1.1 to −0.5) and −0.4 µm (95% CI −0.8 to 0.1) decreases in CRAE per interquartile increases in long- (3 µg/m3) and short-term (9 µg/m3) PM2.5 levels, respectively. These reductions in CRAE are equivalent to 7- and 3-year increases in age in the same cohort. Similarly, living near a major road was also associated with a −0.7 µm decrease (95% CI −1.4 to 0.1) in CRAE. Although the chronic association with CRAE was largely influenced by differences in exposure between cities, this relationship was generally robust to control for city-level covariates and no significant differences were observed between cities. Wider CRVE were associated with living in areas of higher PM2.5 concentrations, but these findings were less robust and not supported by the presence of consistent acute associations with PM2.5.

Conclusions

Residing in regions with higher air pollution concentrations and experiencing daily increases in air pollution were each associated with narrower retinal arteriolar diameters in older individuals. These findings support the hypothesis that important vascular phenomena are associated with small increases in short-term or long-term air pollution exposures, even at current exposure levels, and further corroborate reported associations between air pollution and the development and exacerbation of clinical cardiovascular disease. Please see later in the article for the Editors'' Summary  相似文献   
1000.
The sequencing of the genomes of 12 Drosophila species has created an opportunity for much in the way of comparative molecular analyses amongst these species. To aid that endeavor, we have made several transformation vectors based on the piggyBac transposon with 3xP3-EGFP and -ECFP transgenic markers that should be useful for mutagenesis and establishing the GAL4/UAS system in these species. We have tested the ability of mini-white to be used as a marker for insertional mutagenesis, and have observed mini-white derived pigmentation of the testes sheath in a subset of lines from D. pseudoobscura and D. virilis. We have incorporated a source of piggyBac transposase into nine Drosophila species, and have demonstrated the functionality of these transposase lines for mobilization of marked inserts in vivo. Additionally, we tested the ability of a D. melanogaster nanos enhancer element to drive expression of GAL4 in D. melanogaster, D. simulans, D. erecta, D. yakuba, D. pseudoobscura, and D. virilis. The efficacy of the nos-Gal4 transgene was determined by measuring the response of UAS-EGFPtub in all six species. Our results show that D. melanogaster nos-Gal4 drives expression in other species, to varying degrees, in similar spatiotemporal domains in the ovaries, testes, and embryos as seen in D. melanogaster. However, expression levels are variable, demonstrating the possible need to use species-specific promoters in some cases. In summary, we hope to provide a set of guidelines and basic tools, based upon this work, for both insertional mutagenesis and GAL4/UAS system-based experiments in multiple species of Drosophila.  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