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   2769篇
  免费   179篇
  国内免费   1篇
  2949篇
  2022年   16篇
  2021年   25篇
  2020年   13篇
  2019年   15篇
  2018年   30篇
  2017年   33篇
  2016年   43篇
  2015年   68篇
  2014年   118篇
  2013年   192篇
  2012年   145篇
  2011年   171篇
  2010年   118篇
  2009年   94篇
  2008年   135篇
  2007年   185篇
  2006年   176篇
  2005年   155篇
  2004年   203篇
  2003年   166篇
  2002年   193篇
  2001年   59篇
  2000年   43篇
  1999年   44篇
  1998年   43篇
  1997年   36篇
  1996年   30篇
  1995年   36篇
  1994年   30篇
  1993年   34篇
  1992年   23篇
  1991年   14篇
  1990年   20篇
  1989年   10篇
  1988年   15篇
  1987年   12篇
  1986年   11篇
  1985年   11篇
  1984年   13篇
  1983年   18篇
  1982年   12篇
  1981年   21篇
  1980年   12篇
  1979年   9篇
  1978年   10篇
  1977年   8篇
  1976年   10篇
  1975年   12篇
  1974年   12篇
  1968年   7篇
排序方式: 共有2949条查询结果,搜索用时 15 毫秒
991.
Trace element disturbance is often observed in hemodialysis patients. While trace element concentrations have been reported in blood samples from hemodialysis patients, they have not been well investigated in scalp hair. In the present study, 22 trace elemental concentrations were measured by inductively coupled plasma-atomic emission spectrometry in the scalp hair of 80 male hemodialysis patients and compared with those of 100 healthy male subjects. In hemodialysis patients, the concentrations of beryllium, arsenic, magnesium, chromium, manganese, iron, selenium, molybdenum, iodine, vanadium, and cobalt were significantly higher than those in healthy subjects, while lead, mercury, copper, germanium, and bromine were significantly lower than those in the former group. No significant differences were observed for lithium, aluminum, cadmium, zinc, boron, or nickel. There were significant positive correlations between the duration of hemodialysis and the magnesium and manganese concentrations. There was a significant negative correlation between cadmium concentration and the duration of hemodialysis. There were significant positive correlations between dialysis efficacy (Kt/V) and magnesium, manganese, zinc, and selenium concentrations. In conclusion, trace element concentrations of the scalp hair are different between hemodialysis patients and healthy subjects. Essential trace elements, such as magnesium, manganese, zinc, and selenium, may be affected by the duration of hemodialysis and Kt/V.  相似文献   
992.
Carassius RFamide (C-RFa) is a peptide, isolated originally from the brain of Japanese crucian carp and sharing homologies with mammalian prolactin-releasing peptides. From the physiological aspect, it is known that C-RFa has contraction-promoting action on fish intestines, but its localization in peripheral tissues is unknown. We observed the localization of C-RFa in teleost guts using an immunohistochemical technique. C-RFa-like immunoreactive (irC-RFa) sites were observed in not only the smooth muscle cells in the longitudinal muscle layer, but also in both Auerbach's and Meissner's nerve plexus in the stomach, pyloric ceca and intestine. In epithelial mucous cells, irC-RFa sites were observed in the surface mucous cells in the stomach in freshwater fish (FW), and in the goblet cells of the apical sites in the villi of the pyloric ceca and intestine in all fish. In the stomach, irC-RFa sites were found in the fundic glands of the body regions in seawater (SW) and brackish water (BW) fish, but not in FW fish. This study confirmed that one of the functions of C-RFa is the smooth muscle contraction of the longitudinal muscle layer in digestive organs. We suggest that C-RFa may have functional roles in both central and peripheral neurotransmission. In addition, it appears that the difference in C-RFa localization of SW, BW, and FW fish reflects the adaptation of the stomach function to different salinity habitats.  相似文献   
993.
Cyclic AMP (cAMP) is a well-known intracellular signaling molecule improving barrier function in vascular endothelial cells. Here, we delineate a novel cAMP-triggered signal that regulates the barrier function. We found that cAMP-elevating reagents, prostacyclin and forskolin, decreased cell permeability and enhanced vascular endothelial (VE) cadherin-dependent cell adhesion. Although the decreased permeability and the increased VE-cadherin-mediated adhesion by prostacyclin and forskolin were insensitive to a specific inhibitor for cAMP-dependent protein kinase, these effects were mimicked by 8-(4-chlorophenylthio)-2'-O-methyladenosine-3', 5'-cyclic monophosphate, a specific activator for Epac, which is a novel cAMP-dependent guanine nucleotide exchange factor for Rap1. Thus, we investigated the effect of Rap1 on permeability and the VE-cadherin-mediated cell adhesion by expressing either constitutive active Rap1 or Rap1GAPII. Activation of Rap1 resulted in a decrease in permeability and enhancement of VE-cadherin-dependent cell adhesion, whereas inactivation of Rap1 had the counter effect. Furthermore, prostacyclin and forskolin induced cortical actin rearrangement in a Rap1-dependent manner. In conclusion, cAMP-Epac-Rap1 signaling promotes decreased cell permeability by enhancing VE-cadherin-mediated adhesion lined by the rearranged cortical actin.  相似文献   
994.
This study was carried out to identify unknown allelochemicals released from Myriophyllum spicatum and to investigate their anti-cyanobacterial effects. A series of analyses of culture solutions and methanol extracts of M. spicatum using gas chromatograph equipped with a mass selective detector revealed that M. spicatum released fatty acids, specifically, nonanoic, tetradecanoic, hexadecanoic, octadecanoic, and octadecenoic acids. Nonanoic, cis-6-octadecenoic, and cis-9-octadecenoic acids significantly inhibited growth of Microcystis aeruginosa, whereas tetradecanoic, hexadecanoic, and octadecanoic acids did not show any effect. When the inhibitory effect of nonanoic acid was compared with those of 4 polyphenols and eugeniin, which are anti-cyanobacterial compounds previously reported to be released by M. spicatum, nonanoic acid was found to be the most inhibitory to M. aeruginosa. These results indicate that not only polyphenols and eugeniin but also fatty acids such as nonanoic acid must be studied to reveal how M. spicatum exerts its allelopathic effect on M. aeruginosa.  相似文献   
995.
Seafloor massive sulfides are a potential energy source for the support of chemosynthetic ecosystems in dark, deep‐sea environments; however, little is known about microbial communities in these ecosystems, especially below the seafloor. In the present study, we performed culture‐independent molecular analyses of sub‐seafloor sulfide samples collected in the Southern Mariana Trough by drilling. The depth for the samples ranged from 0.52 m to 2.67 m below the seafloor. A combination of 16S rRNA and functional gene analyses suggested the presence of chemoautotrophs, sulfur‐oxidizers, sulfate‐reducers, iron‐oxidizers and iron‐reducers. In addition, mineralogical and thermodynamic analyses are consistent with chemosynthetic microbial communities sustained by sulfide minerals below the seafloor. Although distinct bacterial community compositions were found among the sub‐seafloor sulfide samples and hydrothermally inactive sulfide chimneys on the seafloor collected from various areas, we also found common bacterial members at species level including the sulfur‐oxidizers and sulfate‐reducers, suggesting that the common members are widely distributed within massive sulfide deposits on and below the seafloor and play a key role in the ecosystem function.  相似文献   
996.
The cold stability of microtubules during seasons of active and dormant cambium was analyzed in the conifers Abies firma, Abies sachalinensis and Larix leptolepis by immunofluorescence microscopy. Samples were fixed at room temperature and at a low temperature of 2–3°C to examine the effects of low temperature on the stability of microtubules. Microtubules were visible in cambium, xylem cells and phloem cells after fixation at room temperature during seasons of active and dormant cambium. By contrast, fixation at low temperature depolymerized microtubules in cambial cells, differentiating tracheids, differentiating xylem ray parenchyma and phloem ray parenchyma cells during the active season. However, similar fixation did not depolymerize microtubules during cambial dormancy in winter. Our results indicate that the stability of microtubules in cambial cells and cambial derivatives at low temperature differs between seasons of active and dormant cambium. Moreover, the change in the stability of microtubules that we observed at low temperature might be closely related to seasonal changes in the cold tolerance of conifers. In addition, low-temperature fixation depolymerized microtubules in cambial cells and differentiating cells that had thin primary cell walls, while such low-temperature fixation did not depolymerize microtubules in differentiating secondary xylem ray parenchyma cells and tracheids that had thick secondary cell walls. The stability of microtubules at low temperature appears to depend on the structure of the cell wall, namely, primary or secondary. Therefore, we propose that the secondary cell wall might be responsible for the cold stability of microtubules in differentiating secondary xylem cells of conifers.  相似文献   
997.
The immediate impact of damming appears most notably at the first filling of water, when the dam blocks the river and a lake suddenly forms. In this review, the changes in meteorology, plant communities, birds and fishes surrounding initial impoundment of Miharu Dam, constructed in an Asian Monsoon region, are summarised based on previous papers and subsequent field research. Although wind and temperature changes were investigated, land and lake wind occur due to the different thermal properties between the land and lake, and this type of wind often occurs at large lakes such as Glen Canyon Dam Reservoir or Lake Biwa. The size of Miharu Dam Reservoir (ponding area 2.9 km2) was insufficient to cause land–lake air differentials. Therefore, wind direction and air temperature were unaffected. Mountain winds weakened at the lake centre and near the dam body. Changes in vegetation were especially diverse at the drawdown zone (the slopes above and below the normal water level). On slopes above this zone, trees died and species composition changed due to submergence. Within the drawdown zone, the pre-existing plant community disappeared, and flood-resistant plants such as Salix subfragilis increased. The natatorial bird population continued to grow for 4 years after dam reservoir emergence and stabilised thereafter. Every year, the majority of natatorial birds utilising the dam reservoir as a resting area were ducks, but populations of diving ducks fluctuated depending on water level and iced area. After impoundment, the fish populations increased. As in most dam reservoirs in Japan, populations of invasive fish species such as Micropterus salmoides and Lepomis macrochirus increased. However, spawning grounds dried up during low-water-level seasons, suggesting that regulating water levels may help reduce invasive species.  相似文献   
998.

Objective

The primary purpose of this study was to clarify the influence of the early response to surgery on brain structure and cognitive function in patients with breast cancer. It was hypothesized that the structure of the thalamus would change during the early response after surgery due to the effects of anesthesia and would represent one aspect of an intermediate phenotype of postoperative cognitive dysfunction (POCD).

Methods

We examined 32 postmenopausal females with breast cancer and 20 age-matched controls. We assessed their cognitive function (attention, memory, and executive function), and performed brain structural MRI 1.5 ± 0.5 days before and 5.6 ± 1.2 days after surgery.

Results

We found a significant interaction between regional grey matter volume (rGMV) in the thalamus (P < 0.05, familywise error (FWE), small volume correction (SVC)) and one attention domain subtest (P = 0.001, Bonferroni correction) after surgery in the patient group compared with the control group. Furthermore, the changes in attention were significantly associated with sevoflurane anesthetic dose (r 2 = 0.247, β = ‒0.471, P = 0.032) and marginally associated with rGMV changes in the thalamus (P = 0.07, FWE, SVC) in the Pt group.

Conclusion

Our findings suggest that alterations in brain structure, particularly in the thalamus, may occur shortly after surgery and may be associated with attentional dysfunction. This early postoperative response to anesthesia may represent an intermediate phenotype of POCD. It was assumed that patients experiencing other risk factors of POCD, such as the severity of surgery, the occurrence of complications, and pre-existing cognitive impairments, would develop clinical POCD with broad and multiple types of cognitive dysfunction.  相似文献   
999.
The effect of T-5224, a selective inhibitor of c-Fos/activator protein (AP)-1, on lipopolysaccharide (LPS) induced liver injury was examined in mice. Administration of LPS (10?mg?kg?1, i.p.) markedly increased serum levels of tumor necrosis factor-alpha (TNFα), high mobility group box 1 (HMGB1), alanine aminotransferase/aspartate aminotransferase (ALT/AST), liver tissue levels of macrophage-inflammatory protein-1 alpha (MIP-1α) and monocyte chemoattractant protein-1 (MCP-1), as well as hepatic necrosis and inflammation, leading to 67?% lethality. Administration of T-5224 (300?mg?kg?1, p.o.) after intraperitoneal injection of LPS imparted appreciable protection against acute elevations in serum levels of TNFα, HMGB1, ALT/AST as well as in liver tissue levels of MIP-1α and MCP-1, and reduced the lethality (27?%). These data indicate that T-5224 ameliorates liver injury and improves survival through decreasing production of proinflammatory cytokines and chemokines in endotoxemic mice.  相似文献   
1000.
A heterologous prime-boost strategy with priming plasmid DNA followed by recombinant virus expressing relevant antigens is known to stimulate protective immunity against intracellular parasites. In this study, we have evaluated a heterologous prime-boost strategy for immunizing mice against Toxoplasma gondii infection. Our results revealed that the prime-boost strategy using both plasmid DNA and adenoviral vector encoding TgAMA1 may stimulate both humoral and Th1/Th2 cellular immune responses specific for TgAMA1. Moreover, C57BL/6 mice immunized with the pAMA1/Ad5Null, pNull/Ad5AMA1, and pAMA1/Ad5AMA1 constructs showed survival rates of 12.5%, 37.5%, and 50%, respectively. In contrast, all the pNull/Ad5Null immunized mice died after infection with the PLK-GFP strain of T. gondii. Brain cyst burden was reduced by 23% in mice immunized with pAMA1/Ad5AMA1 compared with the pNull/Ad5AMA1 immunized mice. These results demonstrate that the heterologous DNA priming and recombinant adenovirus boost strategy may provide protective immunity against T. gondii infection.  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