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   1587篇
  免费   147篇
  国内免费   51篇
  2023年   10篇
  2022年   13篇
  2021年   13篇
  2020年   21篇
  2019年   42篇
  2018年   48篇
  2017年   41篇
  2016年   31篇
  2015年   51篇
  2014年   74篇
  2013年   93篇
  2012年   56篇
  2011年   91篇
  2010年   61篇
  2009年   63篇
  2008年   63篇
  2007年   86篇
  2006年   65篇
  2005年   88篇
  2004年   76篇
  2003年   51篇
  2002年   54篇
  2001年   43篇
  2000年   35篇
  1999年   39篇
  1998年   42篇
  1997年   43篇
  1996年   29篇
  1995年   33篇
  1994年   23篇
  1993年   29篇
  1992年   20篇
  1991年   16篇
  1990年   24篇
  1989年   18篇
  1988年   12篇
  1987年   11篇
  1986年   13篇
  1985年   19篇
  1984年   19篇
  1983年   15篇
  1982年   24篇
  1981年   15篇
  1980年   9篇
  1979年   17篇
  1978年   10篇
  1977年   6篇
  1976年   6篇
  1974年   7篇
  1973年   7篇
排序方式: 共有1785条查询结果,搜索用时 15 毫秒
941.
Guanidinoacetate Methyltransferase deficiency is an inborn error of metabolism that results in decreased creatine and increased guanidinoacetate (GAA) levels. Patients present neurological symptoms whose mechanisms are unclear. We investigated the effects of an intrastriatal administration of 10 μM of GAA (0.02 nmol/striatum) on energy metabolism, redox state, inflammation, glutamate homeostasis, and activities/immunocontents of acetylcholinesterase and Na+,K+-ATPase, as well as on memory acquisition. The neuroprotective role of creatine was also investigated. Male Wistar rats were pretreated with creatine (50 mg/kg) or saline for 7 days underwenting stereotactic surgery. Forty-eight hours after surgery, the animals (then sixty-days-old) were divided into groups: Control, GAA, GAA + Creatine, and Creatine. Experiments were performed 30 min after intrastriatal infusion. GAA decreased SDH, complexes II and IV activities, and ATP levels, but had no effect on mitochondrial mass/membrane potential. Creatine totally prevented SDH and complex II, and partially prevented COX and ATP alterations. GAA increased dichlorofluorescein levels and decreased superoxide dismutase and catalase activities. Creatine only prevented catalase and dichlorofluorescein alterations. GAA increased cytokines, nitrites levels and acetylcholinesterase activity, but not its immunocontent. Creatine prevented such effects, except nitrite levels. GAA decreased glutamate uptake, but had no effect on the immunocontent of its transporters. GAA decreased Na+,K+-ATPase activity and increased the immunocontent of its α3 subunit. The performance on the novel object recognition task was also impaired. Creatine partially prevented the changes in glutamate uptake and Na+,K+-ATPase activity, and completely prevented the memory impairment. This study helps to elucidate the protective effects of creatine against the damage caused by GAA.  相似文献   
942.

Background

Acute hyperglycemia is regarded as a risk factor for critically ill patients; however, insufficient understanding of its nature and underlying mechanisms hinders widespread adoption of glycemic control in critical care units.

Methods

A single center, prospective cohort study recruiting 107 burn patients and 62 controls was conducted to characterize the early phase of acute hyperglycemia in burn patients. A total of 1643 blood samples were collected and analyzed over the entire postburn 200?h. A mouse severe burn model was used to study the underlying mechanisms of acute hyperglycemia postburn.

Results

The dynamic change of postburn blood glucose represented a distinctive pattern in amplitude and duration that was in parallel with the degree of burn injury. Multiple linear regressions revealed that serum insulin, glucagon and glucocorticoid were the major factors affecting blood glucose postburn. Particularly, extensive burns impaired capacity and responsiveness of pancreatic insulin secretion, which was associated with increased serum IL-1β in burn patients. Mechanistically, acute IL-1β elevation specifically induced pancreatic beta cell apoptosis and dampened capacity of insulin secretion, leading to postburn hyperglycemia in burned mice. More importantly, inhibition of IL-1β not only alleviated pancreatic beta cell apoptosis, but also attenuated hyperglycemia and improved survival of burned mice.

Conclusions

Our findings reveal a novel mechanism of acute hyperglycemia postburn in which impaired insulin secretory capacity mediated by IL-1β leads to acute hyperglycemia. These data suggest that targeting IL-1β to restore endogenous insulin secretory function may be a novel glycemic control strategy to improve outcomes for burn patients.  相似文献   
943.
We previously described the entity of cold-induced apoptosis to rat hepatocytes and characterized its major, iron-dependent pathway. However, after cold incubation in some solutions, e.g. cell culture medium, hepatocytes show an additional, yet uncharacterized component of cold-induced injury. We here assessed the effects of organ preservation solutions on both components of cold-induced injury and tried to further characterize the iron-independent component. None of the preservation solutions (University of Wisconsin, histidine-tryptophan-ketoglutarate, Euro-Collins, histidine-lactobionate, sodium-lactobionate-sucrose and Celsior solutions) provided significant protection against cold-induced cell injury (LDH release after 24-h cold incubation/3h rewarming >65% for all solutions); three solutions even enhanced cold-induced injury. However, when the predominant iron-dependent mechanism was eliminated by the addition of iron chelators, all preservation solutions yielded hepatocyte protection that was clearly superior to the one obtainable in cell culture medium or Krebs-Henseleit buffer with iron chelators (LDH release after 24-h cold incubation/3h rewarming 相似文献   
944.
盆栽条件下暗褐网柄牛肝菌人工菌塘及其子实体的培养   总被引:1,自引:0,他引:1  
在土壤中添加促进暗褐网柄牛肝菌(Phlebopus portentosus)菌丝及其宿主咖啡生长的营养液,用暗褐网柄牛肝菌纯培养种接种盆栽件下的咖啡苗根系,结果表明:92.5%以上的小粒咖啡(Coffea arabica)苗形成菌根,菌根上外延菌丝向根尖、侧根及根系周围的土壤延伸生长,与土粒相互交结形成菌塘。接种20天后,菌塘中子实体紧贴咖啡苗茎基或于咖啡苗株间生长,共生长162个子实体,发育成熟52个。子实体菌柄基部菌索与咖啡苗主根表面菌套连接。  相似文献   
945.
In this article we construct Lyapunov functions for models described by piecewise-continuous and independent differential equations. Because these models are described by discontinuous differential equations, the theory of Lyapunov functions for smooth dynamical systems is not applicable. Instead, we use a geometrical approach to construct a Lyapunov function. Then we apply the general approach to analyze population dynamics describing exploitative competition of two species in a two-patch environment. We prove that for any biologically meaningful parameter combination the model has a globally stable equilibrium and we analyze this equilibrium with respect to parameters.   相似文献   
946.
In this paper, we propose an iterative learning rule that allows the imprinting of correlated oscillatory patterns in a model of the hippocampus able to work as an associative memory for oscillatory spatio-temporal patterns. We analyze the dynamics in the Fourier domain, showing how the network selectively amplify or distort the Fourier components of the input, in a manner which depends on the imprinted patterns. We also prove that the proposed iterative local rule converges to the pseudo-inverse rule generalized to oscillatory patterns.  相似文献   
947.
A pot experiment was conducted in a growth chamber to investigate the effects of dandelion (Taraxacum officinale) and quackgrass (Elymus repens) on the growth of hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker). Single hybrid poplar seedlings were grown in pots either alone (SHP) or with four or eight dandelion plants per pot or with one or three quackgrass plants per pot in two soils collected from sites previously managed for alfalfa and pasture near Meadow Lake, Saskatchewan, Canada. Hybrid poplar and weed species were harvested approximately 7 and 14 weeks after planting. Approximately 14 weeks after planting, hybrid poplar shoot biomass in the SHP treatment was 28 g for the pasture soil and 22 g for the alfalfa soil. Corresponding shoot biomass for hybrid poplar grown with the dandelion and quackgrass treatments varied from 0.54 to 0.81 g and 0.3 to 3.66 g, respectively. Other hybrid poplar growth parameters including stem height, root collar diameter and fresh root biomass were similarly reduced by competition with the weed species. During the growing period, soil solution N and K concentrations decreased several-fold in both soils; however, the magnitude of decrease was comparatively higher in the weed treatments. Nitrogen, P and K uptake by hybrid poplar was greater in the SHP treatment in both the soils (337–425, 38–49 and 396–463 mg pot−1, respectively) compared to the weed treatments (4–28, 0.4–6.2 and 0.6–54.0 mg pot−1, respectively) by the end of the experiment. The presence of quackgrass and dandelion severely affects the growth of hybrid poplar by causing intense below-ground competition for nutrients. Responsible Editor: Ismael Cakmak.  相似文献   
948.
We report the recombinant preparation from Escherichia coli cells of samples of two closely related, small, secreted cysteine‐rich plant peptides: rapid alkalinization factor 1 (RALF1) and rapid alkalinization factor 8 (RALF8). Purified samples of the native sequence of RALF8 exhibited well‐resolved nuclear magnetic resonance (NMR) spectra and also biological activity through interaction with a plant receptor kinase, cytoplasmic calcium mobilization, and in vivo root growth suppression. By contrast, RALF1 could only be isolated from inclusion bodies as a construct containing an N‐terminal His‐tag; its poorly resolved NMR spectrum was indicative of aggregation. We prepared samples of the RALF8 peptide labeled with 15N and 13C for NMR analysis and obtained near complete 1H, 13C, and 15N NMR assignments; determined the disulfide pairing of its four cysteine residues; and examined its solution structure. RALF8 is mostly disordered except for the two loops spanned by each of its two disulfide bridges.  相似文献   
949.
950.
目的:定量探究1%~80%酒精任氏液对蛙坐骨神经干双相复合动作电位(AP)的作用及其恢复情况。方法:制备长为6~8 cm的蛙坐骨神经干标本,分别将含有0%、1%、2%、4%、8%、16%、32%、48%、64%、80%酒精的标准任氏液通过新加装在神经屏蔽盒中的加药液槽浸泡位于刺激电极和接地电极之间的神经干各5 min,用BL-420F系统分别记录由刺激所诱发的双相AP;之后用标准任氏液冲洗5遍,浸泡5 min, 分别记录冲洗后AP。结果:与标准任氏液相比, ≤4%酒精任氏液对AP峰值和传导速度无影响,≥8%酒精任氏液作用后可使AP峰值和传导速度均下降;16%、32%和≥48%酒精任氏液作用后分别使神经干失去产生AP能力的比例为30%、90%和100%。≤32%酒精任氏液作用后冲洗,AP峰值可完全恢复;48%、64%和80%酒精任氏液作用后冲洗,能恢复产生动作电位能力的比例分别为90%、40%和0%,平均峰值分别下降至正常的60%、36%和0%;冲洗后: ≤8%酒精任氏液作用后的AP速度与正常无异,≥16%酒精任氏液作用后的AP速度无法完全恢复。结论:不同浓度酒精对AP峰值和传导速度影响不同,这对酒精的合理使用及过度使用后的损伤恢复有参考意义。  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