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   11032篇
  免费   1004篇
  国内免费   1493篇
  2023年   138篇
  2022年   166篇
  2021年   274篇
  2020年   283篇
  2019年   410篇
  2018年   408篇
  2017年   366篇
  2016年   346篇
  2015年   330篇
  2014年   600篇
  2013年   720篇
  2012年   499篇
  2011年   676篇
  2010年   508篇
  2009年   593篇
  2008年   592篇
  2007年   712篇
  2006年   592篇
  2005年   503篇
  2004年   376篇
  2003年   395篇
  2002年   352篇
  2001年   245篇
  2000年   239篇
  1999年   225篇
  1998年   194篇
  1997年   222篇
  1996年   172篇
  1995年   144篇
  1994年   147篇
  1993年   180篇
  1992年   145篇
  1991年   103篇
  1990年   98篇
  1989年   97篇
  1988年   96篇
  1987年   72篇
  1986年   91篇
  1985年   142篇
  1984年   200篇
  1983年   131篇
  1982年   148篇
  1981年   106篇
  1980年   115篇
  1979年   103篇
  1978年   59篇
  1977年   51篇
  1976年   44篇
  1975年   44篇
  1974年   33篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
PurposeTo observe the effect of cynomorium total flavone on the depression model of perimenopausal rat and to analyze the action characteristics of cynomorium total flavone on depression of rat with perimenopausal syndrome.MethodDuplicate the model of rat with perimenopausal depression based on the combined method of incomplete castration and chronic stimulation, and keep drug administration for 35d. And then measure related behavior indicators and the change of biochemical index level in serum and brains; measure the estrogen/androgen receptor (ER/AR) in related tissues and the ERmRNA expression in hypothalamus.ResultIt can be seen that cynomorium total flavone can significantly improve the behavior indicators of rat with perimenopausal depression; obviously or significantly change the level of related biomedical indexes in serum and brains of perimenopausal depressed rat; obviously or significantly increase the expression of ER/AR in related tissues of perimenopausal depressed rat; obviously or significantly increase the ERmRNA expression in hypothalamus.ConclusionCynomorium total flavone can adjust hypothalamic-pituitary-gonadal axis by increasing E2, and make related biomedical indexes and hormone receptors tend to be normal, so as to relieve perimenopausal syndrome and perimenopausal syndrome with depression.  相似文献   
2.
3.
An investigation on the influence of lead toxicity on some of the hepatic enzymes was studied in rats both after a shorter interval of 15 d and after longer intervals of 60 and 90 d. Three different doses of lead as 5, 10, and 50 mg/kg body wt were administered orally on every alternate day. Whereas significant inhibition of succinic dehydrogenase was seen following lead poisoning, the activity acid and alkaline phosphatase increased with lead intoxication. The histoarchitecture of the liver was grossly intact. Liver accumulated less lead compared to kidney at 60 and 90 d.  相似文献   
4.
Indole acetic acid (IAA) production in Azospirillum brasilense strain Sp245 is controlled by a 85 MDa plasmid naturally present in this bacterium. In the presence of L-tryptophan, anthranilic acid production and almost no IAA production occurs in a derivative strain harbouring a Tn5-Mob insertion in the 85 MDa plasmid. Agrobacterium tumefaciens strain GM19023, upon transfer of Tn5-Mob labelled 85 MDa plasmid of A. brasilense Sp245, gains the ability to produce anthranilic acid.  相似文献   
5.
Photosynthetic characteristics, specific leaf mass, chlorophyll and total leaf nitrogen concentrations of four herbaceous plants (Dicranopteris linearis, Hanguana malayana, Pentaphragma ellipticum, Tacca integrifolia) from nutrient-poor tropical forests showed that all these plants were well-adapted to their natural growth environments. No photoinhibition was observed even in the understorey plants. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
6.
《Cryobiology》2016,72(3):419-431
Cryoprotection of bulky organs is crucial for their storage and for subsequent transplantation. In this work we demonstrate the capability of the X-ray computed tomography (CT) as a non-invasive method to measure the cryoprotectant (cpa) concentration inside a tissue or an organ, specifically for the case of dymethil sulfoxide (Me2SO). It is remarkable that the use of Me2SO has been leader in techniques of cells and tissues cryopreservation. Although CT technologies are mainly based in density differences, and many cpas are alcohols with densities similar to water, the use of very low energies as acceleration voltage (∼70 kV) and the sulfur atom in the molecule of Me2SO makes possible the visualization of this cpa inside tissues. As result we obtain a CT signal proportional to the Me2SO concentration with a spatial resolution up to 50 μm in the case of our device.  相似文献   
7.
Rib bone biopsy samples are often used to estimate changes in skeletal mineral reserves in cattle but differences in sampling procedures and the bone measurements reported often make interpretation and comparisons among experiments difficult. ‘Full-core’ rib bone biopsy samples, which included the external cortical bone, internal cortical bone and trabecular bone (CBext, CBint and Trab, respectively), were obtained from cattle known to be in phosphorus (P) adequate (Padeq) or severely P-deficient (Pdefic) status. Experiments 1 and 2 examined growing steers and Experiment 3 mature breeder cows. The thickness of cortical bone, specific gravity (SG), and the amount and concentration of ash and P per unit fresh bone volume, differed among CBext, CBint and Trab bone. P concentration (mg/cc) was closely correlated with both SG and ash concentrations (pooled data, r=0.99). Thickness of external cortical bone (CBText) was correlated with full-core P concentration (FC-Pconc) (pooled data, r=0.87). However, an index, the amount of P in CBext per unit surface area of CBext (PSACB; mg P/mm2), was more closely correlated with the FC-Pconc (pooled data, FC-Pconc=37.0+146×PSACB; n=42, r=0.94, RSD=7.7). Results for measured or estimated FC-Pconc in 10 published studies with cattle in various physiological states and expected to be Padeq or in various degrees of Pdefic status were collated and the ranges of FC-Pconc indicative of P adequacy and P deficiency for various classes of cattle were evaluated. FC-Pconc was generally in the range 130 to 170 and 100 to 120 mg/cc fresh bone in Padeq mature cows and young growing cattle, respectively. In conclusion, the FC-Pconc could be estimated accurately from biopsy samples of CBext. This allows comparisons between studies where full-core or only CBext biopsy samples of rib bone have been obtained to estimate changes in the skeletal P status of cattle and facilitates evaluation of the P status of cattle.  相似文献   
8.
Piglets can often suffer impaired antioxidant status and poor immune response during post-weaning, especially when chronic inflammation takes place, leading to lower growth rates than expected. Oral administration of dietary antioxidant compounds during this period could be a feasible way to balance oxidation processes and increase health and growth performance. The aim of the trial was to study the effects of an antioxidant feed supplement (melon pulp concentrate) that contains high concentration of the antioxidant superoxide dismutase (SOD) on inflammation, antioxidant status and growth performance of lipopolysaccharide (LPS) challenged weaned piglets. In total, 48 weaned piglets were individually allocated to four experimental groups in a 2×2 factorial design for 29 days. Two different dietary treatments were adopted: (a) control (CTR), fed a basal diet, (b) treatment (MPC), fed the basal diet plus 30 g/ton of melon pulp concentrate. On days 19, 21, 23 and 25 half of the animals within CTR and MPC groups were subjected to a challenge with intramuscular injections of an increasing dosage of LPS from Escherichia coli (serotype 0.55:B5) (+) or were injected with an equal amount of PBS solution (). Blood samples were collected at the beginning of the trial and under the challenge period for interleukin 1β, interleukin 6, tumour necrosis factor α, haptoglobin, plasma SOD activity, total antioxidant capacity, reactive oxygen species, red blood cells and plasma resistance to haemolysis, and 8-oxo-7, 8-dihydro-2’-deoxyguanosine. Growth performance was evaluated weekly. A positive effect of melon pulp concentrate was evidenced on total antioxidant capacity, half-haemolysis time of red blood cells, average daily gain (ADG) and feed intake, while LPS challenge increased pro-inflammatory cytokines and haptoglobin serum concentrations, with a reduced feed intake and gain : feed (G : F). The obtained results show that oral SOD supplementation with melon pulp concentrate ameliorates the total antioxidant capacity and the half-haemolysis time in red blood cell of post-weaning piglets, with positive results on growing performance.  相似文献   
9.
Liver plasma membranes prepared from genetically diabetic (db/db) mice expressed levels of Gi α-2, Gi α-3 and G-protein β-subunits that were reduced by some 75, 63 and 73% compared with levels seen in membranes from lean animals. In contrast, there were no significant differences in the expression of the 42 and 45 kDa forms of Gs α-subunits. Pertussis toxin-catalysed ADP-ribosylation of membranes from lean animals identified a single 41 kDa band whose labelling was reduced by some 86% in membranes from diabetic animals. Cholera toxin-catalysed ADP-ribosylation identified two forms of Gs α-subunits whose labelling was about 4-fold greater in membranes from diabetic animals compared with those from lean animals. Maximal stimulations of adenylyl cyclase activity by forskolin (100 μM), GTP (100 μM), p[NH]ppG (100 μM), NaF (10 mM) and glucagon (10 μM) were similar in membranes from lean and diabetic animals, whereas stimulation by isoprenaline (100 μM) was lower by about 22%. Lower concentrations (EC50-60 nM) of p[NH]ppG were needed to activate adenylyl cyclase in membranes from diabetic animals compared to those from lean animals (EC50-158 nM). As well as causing activation, p[NH]ppG was capable of eliciting a pertussis toxin-sensitive inhibitory effect upon forskolin-stimulated adenylyl cyclase activity in membranes from both lean and diabetic animals. However, maximal inhibition of adenylyl cyclase activity in membranes from diabetic animals was reduced to around 60% of that found using membranes from lean animals. Pertussis toxin-treatment in vivo enhanced maximal stimulation of adenylyl cyclase by glucagon, isoprenaline and p[NH]ppG through a process suggested to be mediated by the abolition of functional Gi activity. The lower levels of expression of G-protein β-subunits, in membranes from diabetic compared with lean animals, is suggested to perturb the equilibria between holomeric and dissociated G-protein subunits. We suggest that this may explain both the enhanced sensitivity of adenylyl cyclase to stimulation by p[NH]ppG in membranes from diabetic animals and the altered ability of pertussis and cholera toxins to catalyse the ADP-ribosylation of G-proteins in membranes from these two animals.  相似文献   
10.
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