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   9429篇
  免费   758篇
  国内免费   369篇
  2023年   109篇
  2022年   119篇
  2021年   186篇
  2020年   309篇
  2019年   375篇
  2018年   387篇
  2017年   294篇
  2016年   274篇
  2015年   335篇
  2014年   595篇
  2013年   936篇
  2012年   343篇
  2011年   604篇
  2010年   308篇
  2009年   466篇
  2008年   453篇
  2007年   533篇
  2006年   447篇
  2005年   438篇
  2004年   324篇
  2003年   350篇
  2002年   287篇
  2001年   212篇
  2000年   172篇
  1999年   148篇
  1998年   150篇
  1997年   147篇
  1996年   114篇
  1995年   121篇
  1994年   111篇
  1993年   128篇
  1992年   133篇
  1991年   97篇
  1990年   68篇
  1989年   79篇
  1988年   55篇
  1987年   57篇
  1986年   33篇
  1985年   47篇
  1984年   38篇
  1983年   25篇
  1982年   39篇
  1981年   26篇
  1980年   13篇
  1979年   17篇
  1978年   13篇
  1977年   12篇
  1976年   10篇
  1975年   5篇
  1974年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Peripheral blood mononuclear cell (PBMNC) is one of powerful tools for therapeutic angiogenesis in hindlimb ischemia. However, traditional approaches with transplanted PBMNCs show poor therapeutic effects in severe ischemia patients. In this study, we used autograft models to determine whether hypoxic pretreatment effectively enhances the cellular functions of PBMNCs and improves hindlimb ischemia. Rabbit PBMNCs were cultured in the hypoxic condition. After pretreatment, cell adhesion, stress resistance, and expression of angiogenic factor were evaluated in vitro. To examine in vivo effects, we autografted preconditioned PBMNCs into a rabbit hindlimb ischemia model on postoperative day (POD) 7. Preconditioned PBMNCs displayed significantly enhanced functional capacities in resistance to oxidative stress, cell viability, and production of vascular endothelial growth factor. In addition, autologous transplantation of preconditioned PBMNCs significantly induced new vessels and improved limb blood flow. Importantly, preconditioned PBMNCs can accelerate vessel formation despite transplantation on POD 7, whereas untreated PBMNCs showed poor vascularization. Our study demonstrated that hypoxic preconditioning of PBMNCs is a feasible approach for increasing the retention of transplanted cells and enhancing therapeutic angiogenesis in ischemic tissue.  相似文献   
2.
目的:研究精神疲劳状态下脑组织血氧饱和度的变化规律。方法:从某军校随机抽取25名被试,采用模拟飞行任务负荷构建精神疲劳模型,采用NASA-TLX量表评价模型。实验全程使用近红外光谱技术对被试脑组织血氧饱和度进行实时监测;实验前后分别对被试的作业绩效水平进行评估。结果:任务后NASA-TLX量表评分明显高于任务前(P0.01);任务前后作业绩效发生变化,反应能力测试的正确率下降(P0.01),错误率上升(P0.01);任务负荷后脑组织血氧饱和度相对于静息状态升高(P0.05)。结论:精神疲劳状态会影响被试的作业绩效。疲劳后脑组织血氧饱和度水平受被试动机以及代偿机制的影响高于静息水平。  相似文献   
3.
4.
5.
We investigated whether sitagliptin, a dipeptidyl peptidase‐4 (DPP‐4) inhibitor, attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression in post‐infarcted normoglycemic rats, focusing on adenosine and reactive oxygen species production. DPP‐4 bound adenosine deaminase has been shown to catalyse extracellular adenosine to inosine. DPP‐4 inhibitors increased adenosine levels by inhibiting the complex formation. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline or sitagliptin in in vivo and ex vivo studies. Post‐infarction was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle‐treated infarcted rats compared with sham. Compared with vehicle, infarcted rats treated with sitagliptin significantly increased interstitial adenosine levels and attenuated oxidative stress. Sympathetic hyperinnervation was blunted after administering sitagliptin, as assessed by immunofluorescent analysis and western blotting and real‐time quantitative RT‐PCR of NGF. Arrhythmic scores in the sitagliptin‐treated infarcted rats were significantly lower than those in vehicle. Ex vivo studies showed a similar effect of erythro‐9‐(2‐hydroxy‐3‐nonyl) adenine (an adenosine deaminase inhibitor) to sitagliptin on attenuated levels of superoxide and NGF. Furthermore, the beneficial effects of sitagliptin on superoxide anion production and NGF levels can be reversed by 8‐cyclopentyl‐1,3‐dipropulxanthine (adenosine A1 receptor antagonist) and exogenous hypoxanthine. Sitagliptin protects ventricular arrhythmias by attenuating sympathetic innervation via adenosine A1 receptor and xanthine oxidase‐dependent pathways, which converge through the attenuated formation of superoxide in the non‐diabetic infarcted rats.  相似文献   
6.
A facile synthesis strategy to control the porosity of ionothermal nitrogen doped carbons is demonstrated. Adenine is used as cheap and biomass based precursor and a mixture of NaCl/ZnCl2 as combined solvent‐porogen. Variation of the ratio between the two salt influences the pore structure over a wide range. The eutectic mixture leads to micro‐ and mesoporous material with high total pore volume (TPV) of 3.0 cm3 g?1 and very high surface area of 2900 m2 g?1 essentially rendering the product an “all‐surface‐area” nitrogen doped carbon. Increasing NaCl contents cause a continuous increase of the mesopore size and the formation of additional macropores resulting in a very high maximal TPV of 5.2 cm3 g?1, showing 2540 m2 g?1 specific surface area using 60 mol% NaCl. Interestingly, the electrocatalytic activity of the samples toward oxygen reduction is strongly affected by the detailed pore structure. The different—however, chemically equivalent—catalysts vary up to 70 mV in their half wave potentials (E 1/2).The sample with optimized pore system shows a high selectivity toward the favored four electron process and an outstanding E 1/2 of ≈880 mV versus reversible hydrogen electrode (RHE), which is one of the best values reported for nitrogen doped carbons so far.  相似文献   
7.
Background: Oxidative stress has been identified as an important pathogenesis mechanism in the development of renal interstitial fibrosis in unilateral ureteral obstruction (UUO). Previous studies have demonstrated increased expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOXs) in response to UUO. We aimed to investigate whether NOXs activation was involved in the development of renal fibrosis in UUO by contribution to oxidative stress and the potential mechanism in the present study.

Methods: Apocynin, a NOXs inhibitor, was initiated immediately by gavage after UUO was performed on Wistar rats and continued until 7 days after UUO. Changes of markers of oxidative stress, renal macrophage infiltration and fibrosis, TGF-β1 expression, NOXs expression and activity, and ERK activation were evaluated.

Results: Apocynin significantly attenuated the activity of NOXs, accompanied with decreased expression of NOX2, NOX4, and oxidative stress markers in the obstructed kidneys of UUO. Additionally, collagen deposition and renal fibrosis induced by UUO were attenuated by apocynin treatment. Furthermore, apocynin treatment significantly attenuated the phosphorylation of ERK, accumulation of myofibroblast and infiltration of macrophage in obstructed kidneys. No significant effect of apocynin on UUO-induced increased TGF-β1 expression could be observed. And there was no significant change of anti-oxidants enzyme activities in the obstructed kidneys of apocynin-treated rats.

Conclusions: These results suggested that apocynin might exert beneficial effects on renal fibrosis by inhibition of NOXs activation and subsequent reduction of oxidative stress, ERK activation, and myofibroblast accumulation in UUO rats. Targeting NOXs may serve as a therapeutic strategy for the treatment of renal fibrosis.  相似文献   

8.
We previously reported the involvement of serotonin (5-HT) metabolism in cigarette smoke-induced oxidative stress in rat lung in vivo. Here, we report cigarette smoke as a source of serotonin (5-HT) to the airways and aim at investigating the effects of 5-HT on oxidative stress and inflammation in human bronchial epithelial cells (BEAS-2B). A 5-HT analog was identified to be present in aqueous phase cigarette smoke using the LC-MS/MS approach, which was later confirmed by a 5-HT enzyme-linked immune assay (EIA). Furthermore, exposure to 5-HT caused a time-dependent elevation of intracellular ROS level, which was blocked in the presence of apocynin (a NOX inhibitor). In support, the immunoblot analysis indicated that there was an increase in the expression of NOX2 time-dependently. 5-HT-induced elevation of IL-8 at both mRNA and protein levels was observed, which was inhibited by TEMPOL (a free radical scavenger), and inhibitors for p38 MAPK (SB203580) and ERK (U0126), in line with the time-dependent phosphorylation of p38 MAPK and ERK. In conclusion, our findings suggest that 5-HT presented in bronchial epithelium of smokers may be involved in cigarette smoke-induced oxidative stress and inflammation via activation of p38 MAPK and ERK pathway after the formation of free radicals.  相似文献   
9.
We report an analysis of the effect of two different times of day (noon and dusk), body weight and sex on the oxygen consumption rate (OCR) of the freshwater crabs Trichodactylus kensleyi (Trichodactylidae), Aegla singularis and A. platensis (Aeglidae) in their natural environment. Both families are sympatric in the studied locations, with a co-occurrence found between T. kensleyi and A. singularis or T. kensleyi and A. platensis. The mean OCR was highest in A. singularis and lowest in A. platensis. The OCR was higher in Aegla species at noon, and Trichodactylus consumed more oxygen at dusk. In aeglids, there was no difference in oxygen consumption between noon and dusk. T. kensleyi exhibited statistically significant differences in the OCR between noon and dusk. The oxygen uptake of all three species analysed was not influenced by the sex of the individual but varied according to the animals’ weight. The families evaluated share some biological and ecological characteristics, such as diet and habitats, but the strategies used in the regulation of gas exchange at different times of the day were different. Environmental factors may be influencing the oxygen consumption of each morphotype differently.  相似文献   
10.
The kinetic constraints that are imposed on cytochrome oxidase in its dual function as the terminal oxidant in the respiratory process and as a redox-linked proton pump provide a unique opportunity to investigate the molecular details of biological O2 activation. By using flow/flash techniques, it is possible to visualize individual steps in the O2-binding and reduction process, and results from a number of spectroscopic investigations on the oxidation of reduced cytochrome oxidase by O2 are now available. In this article, we use these results to synthesize a reaction mechanism for O2 activation in the enzyme and to simulate time-concentration profiles for a number of intemediates that have been observed experimentally. Kinetic manifestation of the consequences of coupling exergonic electron transfer to endergonic proton translocation emerge from this analysis. Energetic efficiency in this process apparently requires that potentially toxic intermediate oxidation states of dioxygen accumulate to substantial concentration during the reduction reaction.  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