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   2641篇
  免费   127篇
  国内免费   2篇
  2770篇
  2023年   8篇
  2022年   15篇
  2021年   33篇
  2020年   30篇
  2019年   49篇
  2018年   70篇
  2017年   44篇
  2016年   105篇
  2015年   130篇
  2014年   124篇
  2013年   176篇
  2012年   215篇
  2011年   202篇
  2010年   137篇
  2009年   92篇
  2008年   164篇
  2007年   172篇
  2006年   203篇
  2005年   161篇
  2004年   129篇
  2003年   138篇
  2002年   117篇
  2001年   18篇
  2000年   11篇
  1999年   15篇
  1998年   22篇
  1997年   10篇
  1996年   11篇
  1995年   15篇
  1994年   11篇
  1993年   9篇
  1992年   6篇
  1991年   11篇
  1990年   15篇
  1989年   10篇
  1988年   12篇
  1987年   3篇
  1986年   3篇
  1985年   7篇
  1984年   5篇
  1983年   3篇
  1982年   6篇
  1981年   10篇
  1980年   14篇
  1979年   7篇
  1978年   4篇
  1977年   5篇
  1972年   2篇
  1971年   2篇
  1968年   3篇
排序方式: 共有2770条查询结果,搜索用时 0 毫秒
111.
On the basis of our earlier studies with the serotonin receptor ligands in the group of 1,3-dimethyl-3,7-dihydropurine-2,6-dione derivatives, a series of new arylpiperazinylalkyl and tetrahydroisoquinolinylalkyl analogs of 8-alkoxy-1,3-dimethyl-3,7-dihydropurine-2,6-dione (10-25) and 1,3-dimethyl-7,9-dihydro-3H-purine-2,6,8-trione (26-30) were synthesized and their 5-HT(1A), 5-HT(2A), and 5-HT(7) receptor affinities were determined. The new compounds 17, 18, 20, and 21 were found to be highly active 5-HT(1A) receptor ligands (K(i)=11-19nM) with diversified affinity for 5-HT(2A) receptors (K(i)=15-253nM). Compounds 12, 13, 15, and 19 were moderately potent 5-HT(2A) ligands (K(i)=23-57nM), whereas 17, 18, 24, and 25 showed distinct affinity for 5-HT(7) receptors (K(i)=51-83nM). Purine-2,6,8-triones showed weak affinities for 5-HT(1A) and 5-HT(7) receptors; among them, 27 and 29 were classified as 5-HT(2A) receptor ligands. The selected compounds 17 and 21 were pharmacologically evaluated to determine their functional activities at pre-(hypothermia in mice) and post-(lower lip retraction in rats) synaptic 5-HT(1A) receptors. Compound 17 showed features of a potential agonist of pre- and post-synaptic 5-HT(1A) receptors, whereas 21 was classified as a potential, weak partial agonist of postsynaptic sites. Last of all, the most interesting compound 17 tested in behavioral models showed potential anxiolytic and antidepressant activities.  相似文献   
112.
As part of our continuing research for anticancer compounds from the Walloon Region forest, EtOAc extract from Carpinus betulus leaves was phytochemically studied, leading to the bioguided isolation of pheophorbide a, which is responsible of anticancer properties of C. betulus young leaves. This compound was identified using nuclear magnetic resonance and mass spectrophotometric data and comparison with a commercial standard. Evaluation of the growth inhibitory activities of pheophorbide a using MTT colorimetric assay and phase-contrast microscopy in various human cancer cell lines confirmed the photoactivable properties of this compound. Our research showed, for the first time, the presence of pheophorbide a, a chlorophyll derived compound, which we quantified in high quantities in young leaves of C. betulus. This is in contrast with the literature which generally describes pheophorbide a as a catabolic product of chlorophyll, then preferentially present in old leaves.  相似文献   
113.

Background  

FYVE domains have emerged as membrane-targeting domains highly specific for phosphatidylinositol 3-phosphate (PtdIns(3)P). They are predominantly found in proteins involved in various trafficking pathways. Although FYVE domains may function as individual modules, dimers or in partnership with other proteins, structurally, all FYVE domains share a fold comprising two small characteristic double-stranded β-sheets, and a C-terminal α-helix, which houses eight conserved Zn2+ ion-binding cysteines. To date, the structural, biochemical, and biophysical mechanisms for subcellular targeting of FYVE domains for proteins from various model organisms have been worked out but plant FYVE domains remain noticeably under-investigated.  相似文献   
114.
Summary AllAspergillus nidulans 5S rRNA pseudogenes known so far are the result of integration of an approx. 0.2-kbp-long DNA sequence into the 5S rRNA genes. This sequence, called block C, is present in at least five copies in theA. nidulans genome and seems to be associated either with 5S rRNA genes or pseudogenes. In contrast to the 78% sequence conservation of the C-block in pseudogenes, the truncated 5 halves of the pseudogenes are very highly conserved (96.9–100%). We postulate that the 5S rRNA pseudogenes are still a subject of concerted evolution. The C-block sequence shows similarity to the switch region of the mouse heavy chain immunoglobulin gene. A characteristic motif GGGTGAG is repeated several times in both sequences; the sequence conservation is 63%.  相似文献   
115.
The aim of the investigations was the determination of the Cu contents in the liver, kidneys and skeletal muscles of canine females. Material for research was collected post mortem from 45 animals aged 1 to 18 years coming from the Warsaw area. The effect of the health state, age and life conditions on the distribution of copper in the investigated organs was estimated. That element was determined using the method of inductively coupled plasma-atomic emission spectrometry (ICP-AES). In the liver, the average Cu contents amounted to 24.04 mg kg−1 wet weight, in kidneys to 2.90 mg kg−1 wet weight and in muscles to 0.94 mg kg−1 wet weight. The highest values of copper content in particular tissues and organs were noted in the group of animals with neoplastic changes. In respect to the animal age the highest mean values of the copper content were noted in the oldest animals. They amounted to 30.97 mg kg−1 in the liver, 3.34 mg kg−1 in kidneys and 1.18 mg kg−1 wet weight in muscles. Considering life conditions of the dogs it was observed that the higher mean values in all the investigated organs occurred in dogs coming from the urban areas.  相似文献   
116.
The objective of the present experiments was to produce the intergeneric hybrids of domesticated and wild goose via artificial insemination with fresh and frozen-thawed semen. The experiments were carried out during two successive goose reproductive seasons, on eight five-year-old Canada Goose (Branta canadensis L.) males used as semen donors and 16 two-year-old White Ko?uda geese designated to fertility tests. Pooled semen was collected twice a week by the dorso-abdominal massage. In freshly collected semen, ejaculate volume, color, consistency, degree of fecal or blood contamination, spermatozoa concentration, motility, and morphology were evaluated. Part of the semen collected in the first year of the experiment (Experiment 1) was used for geese insemination with fresh semen, while the remainder was frozen. In Experiment 2 all samples were subjected exclusively to freezing procedure. Geese were inseminated once a week with fresh semen in a dose of 80 μl or 160 μl, and twice a week with frozen-thawed semen in a dose of 80 μl (160 μl per wk) or 100 μl (200 μl per wk). Eggs were set weekly and incubated up to hatching.The volume of ejaculates varied from 0.100 to 0.470 ml; spermatozoa concentration from 140 to 310 million ml−1; progressive movement was observed in 40 to 60% of spermatozoa; the percentage of total live spermatozoa ranged from 69.3 to 92.0%, the highest percentage (34.0-68.3) was represented by live normal spermatozoa and those with bulb-head (13.3-41.0). Cryopreservation caused a decrease in percentage of motile cells to 30%; total live spermatozoa contribution by 27.2%p, including those live normal by 15.9%p (in relation to the fresh semen), bulb-head spermatozoa by 10.9%p, and increase (by 5.9%p) in number of spermatozoa with other deformations. Goose insemination 1×/week with fresh semen containing about 10.3 million live normal spermatozoa resulted in 66.7% of fertile eggs and with dose higher by 2.8 million spermatozoa (on average) the fertility increased by 20.9%p (up to 87.6% on average). Hatchability from set and fertile eggs was 55.9% and 83.9% vs. 66.3% and 75.6%, respectively. After twice a week insemination with frozen-thawed semen containing about 10.2 million live normal cells 58.2% eggs were fertile; hatchability from set eggs was 42.8% and from fertile eggs 71.7%, while insemination dose increase by 2.7 million spermatozoa per week caused a fertilization increase by 3.8%p (62.0% on average), this increase was not statistically significant, but hatchability from the fertile eggs (95.4%), was significantly (P < 0.05) higher.The use of AI with fresh semen in the creation of intergeneric hybrids of Canada goose males and White Ko?uda females allows a high level of egg fertility to be obtained. Furthermore, one limitation which is the short reproductive season of the Canada goose may be overcome by the use of cryopreserved semen.  相似文献   
117.
The placenta plays a major role in embryo-fetal defects and intrauterine growth retardation after maternal alcohol consumption. Our aims were to determine the oxidative status and cellular and molecular oxidative stress effects on uterine myometrium and trophoblast-decidual tissue following perigestational alcohol intake at early organogenesis. CF-1 female mice were administered with 10% alcohol in drinking water for 17 days prior to and up to day 10 of gestation. Control females received ethanol-free water. Treated mice had smaller implantation sites compared to controls (p < 0.05), diminished maternal vascular lumen, and irregular/discontinuous endothelium of decidual vessels. The trophoblast giant cell layer was disorganized and presented increased abnormal nuclear frequency. The myometrium of treated females had reduced nitrite content, increased superoxide dismutase activity, and reduced glutathione (GSH) content (p < 0.05). However, the trophoblast-decidual tissue of treated females had increased nitrite content (p < 0.05), increased GSH level (p < 0.001), increased thiobarbituric acid-reactive substance concentration (p < 0.001), higher 3-nitrotyrosine immunoreaction, and increased apoptotic index (p < 0.05) compared to controls. In summary, perigestational alcohol ingestion at organogenesis induced oxidative stress in the myometrium and trophoblast-decidual tissue, mainly affecting cells and macromolecules of trophoblast and decidual tissues around early organogenesis, in CF-1 mouse, and suggests that oxidative-induced abnormal early placental formation probably leads to risk of prematurity and fetal growth impairment at term.  相似文献   
118.
We evaluated in a double-blind study the bronchodilatory properties of 2-decarboxy-2-hydroxymethyl prostaglandin E1 (PGE1-carbinol), described recently as a nonirritant bronchodilator in animals. Fifteen asthmatic patients received by inhalation single doses of 1, 10, and 30 μg PGE1-carbinol, 55 μg PGE2, and placebo (10% ethanol in normal saline, which was also used as diluent for the PGs). Such pulmonary function tests as forced expiratory volume in 1 second, forced vital capacity, and maximal expiratory flow were monitored during 2 hours following inhalation of each compound. 10 and 30 μg PGE1-carbinol produced significant but short-acting bronchodilation, similar to that caused by 55 μg PGE2. One-third of the patients reported mild cough and throat irritation during and shortly after inhalation of 30 μg PGE1-carbinol or 55 μg PGE2. Placebo and 1 μg PGE1-carbinol produced minimal side effects, but neither agent caused bronchodilation. In an adjunctive, unblinded trial, the same patients received 400 μg fenoterol. Fenoterol caused greater bronchodilation 15 and 30 minutes after inhalation than did the PGs in the double-blind study.  相似文献   
119.
This paper reports an investigation on the effects of the hydrophobic, bifunctional SH group reagent phenylarsine oxide (PhAsO) on mitochondrial membrane permeability. We show that PhAsO is a potent inducer of the mitochondrial permeability transition in a process which is sensitive to both the oxygen radical scavanger BHT and to cyclosporin A. The PhAsO-induced permeability transition is stimulated by Ca2+ but takes place also in the presence of EGTA in a process that maintains its sensitivity to BHT and cyclosporin A. Our findings suggest that, at variance from other known inducers of the permeability transition, PhAsO reacts directly with functional SH groups that are inaccessible to hydrophilic reagents in the absence of Ca2+.  相似文献   
120.
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