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   688篇
  免费   33篇
  国内免费   1篇
  722篇
  2023年   6篇
  2022年   8篇
  2021年   22篇
  2020年   8篇
  2019年   17篇
  2018年   25篇
  2017年   17篇
  2016年   27篇
  2015年   42篇
  2014年   47篇
  2013年   50篇
  2012年   61篇
  2011年   48篇
  2010年   35篇
  2009年   29篇
  2008年   44篇
  2007年   50篇
  2006年   40篇
  2005年   29篇
  2004年   23篇
  2003年   28篇
  2002年   21篇
  2001年   4篇
  2000年   4篇
  1999年   4篇
  1998年   5篇
  1997年   3篇
  1996年   6篇
  1995年   3篇
  1994年   3篇
  1993年   2篇
  1992年   3篇
  1991年   1篇
  1988年   2篇
  1987年   1篇
  1986年   1篇
  1985年   1篇
  1984年   1篇
  1982年   1篇
排序方式: 共有722条查询结果,搜索用时 15 毫秒
41.
The aim of our study was to evaluate the effect of BMI on the change in circulating sex hormone in postmenopausal women during 6 months of oral continuous combined low-dose hormone therapy (HT). Fifty postmenopausal women were allocated to receive daily one tablet containing combination of 17β-estradiol (1 mg)/norethindrone acetate (0.5 mg) for 6 months. Serum levels of follicle-stimulating hormone (FSH), estradiol, total testosterone, sex hormone-binding globulin (SHBG), free androgen index (FAI), free estrogen index (FEI), Δ4-androstendione (Δ4A), and dehydroepiandrosterone sulfate were assessed at baseline and at the end of 6 months. Mean absolute values and percent changes from baseline were compared between lean and overweight women. Mean FSH decreased and mean 17β-estradiol increased significantly in both groups (FSH lean: 82.3 ± 26.7 decreased to 45.0 ± 17.0 mIU/ml, P = 0.0001; FSH overweight: 85.5 ± 22.1 decreased to 52.3 ± 23.8 mIU/ml, P = 0.003; P between groups = 0.661; E2 lean: 23.24 ± 12.55 increased to 53.62 ± 28.29 pg/ml, P = 0.006; E2 overweight: 24.17 ± 10.88 increased to 68.36 ± 53.99 pg/ml, P = 0.0001; P between groups = 0.619). Lean individuals had statistically significant higher increments of FAI and specifically FEI compared to overweight (FEI lean; 0.14 ± 0.09 increased to 0.29 ± 0.14, P = 0.009; overweight 0.23 ± 0.18 increased to 0.52 ± 0.40, P = 0.126; P between groups = 0.034). Although BMI does not affect total 17β-estradiol changes, free sex steroid concentrations increase more steeply in lean compared to overweight women receiving oral low-dose HT.  相似文献   
42.
Angular pyrrolocoumarins were synthesized from the reaction of 4-hydroxyindole or 5-hydroxyindole with DMAD and PPh(3) and were tested for anti-inflammatory and antioxidant activity. These compounds significantly inhibited the carrageenin-induced paw edema (60.5%-73.4%) and have important scavenging activity. Although their interaction with the free stable radical DPPH is not high, compound 9 is the most potent (73.4%) in the in vivo experiment. Compound 7 seems to be a potent LOX inhibitor. An attempt was made to correlate the biological results with their structural characteristics and physicochemical parameters.  相似文献   
43.
A lot of emphasis has been placed in screening individuals with exercise-induced bronchospasm in order to avoid persistence bronchial hyperactivity and consequent chronic silent inflammation of the respiratory tract. The purpose of this study was to evaluate the effect of interval training on the respiratory function and endurance in children with exercise-induced asthma (EIA) participating in the sport of soccer. Twenty-nine boys ages 10-14, who developed EIA after a 6-minute free running test (decline in forced expiratory volume in 1 second: FEV(1)10%), participated in the study. They were divided into 2 groups (experimental: n = 18, and control: n = 11), fulfilling the same criteria (i.e., age, body height and weight, and severity of asthma). The experimental group exercised with the interval training method for a period of 8 weeks, (3 sessions per week), whereas the control group exercised with the usual football program. Measurements were made for FEV(1) and endurance in both groups, before and after the application of training (8 weeks). Following the implementation of the training program, a significant improvement in FEV(1) and endurance was documented in the experimental group, as well as significant differences between the 2 groups. In conclusion, duration and aerobic training via the interval method seems to be beneficial to soccer players with EIA.  相似文献   
44.
The primary visual cortex (V1) is pre-wired to facilitate the extraction of behaviorally important visual features. Collinear edge detectors in V1, for instance, mutually enhance each other to improve the perception of lines against a noisy background. The same pre-wiring that facilitates line extraction, however, is detrimental when subjects have to discriminate the brightness of different line segments. How is it possible to improve in one task by unsupervised practicing, without getting worse in the other task? The classical view of perceptual learning is that practicing modulates the feedforward input stream through synaptic modifications onto or within V1. However, any rewiring of V1 would deteriorate other perceptual abilities different from the trained one. We propose a general neuronal model showing that perceptual learning can modulate top-down input to V1 in a task-specific way while feedforward and lateral pathways remain intact. Consistent with biological data, the model explains how context-dependent brightness discrimination is improved by a top-down recruitment of recurrent inhibition and a top-down induced increase of the neuronal gain within V1. Both the top-down modulation of inhibition and of neuronal gain are suggested to be universal features of cortical microcircuits which enable perceptual learning.  相似文献   
45.
Antizyme, long known to be a non-competitive inhibitor of ornithine decarboxylase, is encoded by the atoC gene in Escherichia coli. The present study reveals another role for AtoC, that of a response regulator of the AtoS-AtoC two component system regulating the expression of the atoDAEB operon upon acetoacetate induction. This operon encodes enzymes involved in short-chain fatty acid catabolism in E. coli. Evidence is presented to show that AtoS is a sensor kinase that together with AtoC constitutes a two-component signal transduction system. AtoS is a membrane protein which can autophosphorylate and then transfer that phosphoryl group to AtoC. This process can also be reproduced in vitro. AtoC contains in its amino acid sequence a conserved aspartic acid (D55), which is the putative phosphorylation site, as well as an unexpected "H box" consensus sequence (SHETRTPV), common to histidine kinases, with the histidine contained therein (H73) being a second potential target for phosphorylation. Substitution of either D55 or H73 in His10-AtoC diminished but did not abrogate AtoC phosphorylation suggesting that either both residues can be phosphorylated independently or that the phosphate group can be transferred between them. However, the D55 mutation in comparison to H73 had a more pronounced effect in vivo, on the activation of atoDAEB promoter after acetoacetate induction, although it was the presence of both mutations that rendered AtoC totally unresponsive to induction. These data provide evidence that the gene products of atoS and atoC constitute a two-component signal transduction system, with some unusual properties, involved in the regulation of the atoDAEB operon.  相似文献   
46.

Background

The CUPID (Cultural and Psychosocial Influences on Disability) study was established to explore the hypothesis that common musculoskeletal disorders (MSDs) and associated disability are importantly influenced by culturally determined health beliefs and expectations. This paper describes the methods of data collection and various characteristics of the study sample.

Methods/Principal Findings

A standardised questionnaire covering musculoskeletal symptoms, disability and potential risk factors, was used to collect information from 47 samples of nurses, office workers, and other (mostly manual) workers in 18 countries from six continents. In addition, local investigators provided data on economic aspects of employment for each occupational group. Participation exceeded 80% in 33 of the 47 occupational groups, and after pre-specified exclusions, analysis was based on 12,426 subjects (92 to 1018 per occupational group). As expected, there was high usage of computer keyboards by office workers, while nurses had the highest prevalence of heavy manual lifting in all but one country. There was substantial heterogeneity between occupational groups in economic and psychosocial aspects of work; three- to five-fold variation in awareness of someone outside work with musculoskeletal pain; and more than ten-fold variation in the prevalence of adverse health beliefs about back and arm pain, and in awareness of terms such as “repetitive strain injury” (RSI).

Conclusions/Significance

The large differences in psychosocial risk factors (including knowledge and beliefs about MSDs) between occupational groups should allow the study hypothesis to be addressed effectively.  相似文献   
47.
Convergent extension (CE) is a conserved morphogenetic movement that drives axial lengthening of the primary body axis and depends on the planar cell polarity (PCP) pathway. In Drosophila epithelia, a polarised subcellular accumulation of PCP core components, such as Dishevelled (Dvl) protein, is associated with PCP function. Dvl has long been thought to accumulate in the mediolateral protrusions in Xenopus chordamesoderm cells undergoing CE. Here we present a quantitative analysis of Dvl intracellular localisation in Xenopus chordamesoderm cells. We find that, surprisingly, accumulations previously observed at mediolateral protrusions of chordamesodermal cells are not protrusion-specific but reflect yolk-free cytoplasm and are quantitatively matched by the distribution of the cytoplasm-filling lineage marker dextran. However, separating cell cortex-associated from bulk Dvl signal reveals a statistical enrichment of Dvl in notochord–somite boundary-(NSB)-directed protrusions, which is dependent upon NSB proximity. Dvl puncta were also observed, but only upon elevated overexpression. These puncta showed no statistically significant spatial bias, in contrast to the strongly posteriorly-enriched GFP-Dvl puncta previously reported in zebrafish. We propose that Dvl distribution is more subtle and dynamic than previously appreciated and that in vertebrate mesoderm it reflects processes other than protrusion as such.  相似文献   
48.
Lecithin/cholesterol acyltransferase (LCAT) is responsible for the esterification of the free cholesterol of plasma lipoproteins. Here, we investigated the involvement of LCAT in mechanisms associated with diet-induced hepatic triglyceride accumulation in mice. LCAT-deficient (LCAT?/?) and control C57BL/6 mice were placed on a Western-type diet (17.3% protein, 48.5% carbohydrate, 21.2% fat, 0.2% cholesterol, 4.5 kcal/g) for 24 weeks, then histopathological and biochemical analyses were performed. We report that, in our experimental setup, male LCAT?/? mice are characterized by increased diet-induced hepatic triglyceride deposition and impaired hepatic histology and architecture. Mechanistic analyses indicated that LCAT deficiency was associated with enhanced intestinal absorption of dietary triglycerides (3.6±0.5 mg/dl per minute for LCAT?/? vs. 2.0±0.7 mg/dl per minute for C57BL/6 mice; P<.05), accelerated clearance of postprandial triglycerides and a reduced rate of hepatic very low density lipoprotein triglyceride secretion (9.8±1.1 mg/dl per minute for LCAT?/? vs. 12.5±1.3 mg/dl per minute for C57BL/6 mice, P<.05). No statistical difference in the average daily food consumption between mouse strains was observed. Adenovirus-mediated gene transfer of LCAT in LCAT?/? mice that were fed a Western-type diet for 12 weeks resulted in a significant reduction in hepatic triglyceride content (121.2±5.9 mg/g for control infected mice vs. 95.1±5.8 mg/g for mice infected with Ad-LCAT, P<.05) and a great improvement of hepatic histology and architecture. Our data extend the current knowledge on the functions of LCAT, indicating that LCAT activity is an important modulator of processes associated with diet-induced hepatic lipid deposition.  相似文献   
49.
50.
The impact of different diets on the gut microbiota of reared Nephrops norvegicus was investigated based on bacterial 16S rRNA gene diversity. Specimens were collected from Pagasitikos Gulf (Greece) and kept in experimental rearing tanks, under in situ conditions, for 6 months. Treatments included three diets: frozen natural (mussel) food (M), dry formulated pellet (P) and starvation (S). Gut samples were collected at the initiation of the experiment, and after 3 and 6 months. Tank water and diet samples were also analyzed for bacterial 16S rRNA gene diversity. Statistical analysis separated the two groups fed or starved (M and P vs. S samples). Most gut bacteria were not related to the water or diet bacteria, while bacterial diversity was higher in the starvation samples. M and P samples were dominated by Gammaproteobacteria, Epsilonproteobacteria and Tenericutes. Phylotypes clustering in Photobacterium leiognathi, Shewanella sp. and Entomoplasmatales had high frequencies in the M and P samples but low sequence frequencies in S samples. The study showed that feeding resulted in the selection of specific species, which also occurs in the natural population, and might be associated with the animal's nutrition.  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