全文获取类型
收费全文 | 21013篇 |
免费 | 1690篇 |
国内免费 | 1729篇 |
专业分类
24432篇 |
出版年
2024年 | 55篇 |
2023年 | 315篇 |
2022年 | 715篇 |
2021年 | 1152篇 |
2020年 | 765篇 |
2019年 | 975篇 |
2018年 | 912篇 |
2017年 | 614篇 |
2016年 | 912篇 |
2015年 | 1340篇 |
2014年 | 1518篇 |
2013年 | 1580篇 |
2012年 | 1937篇 |
2011年 | 1699篇 |
2010年 | 999篇 |
2009年 | 920篇 |
2008年 | 1091篇 |
2007年 | 904篇 |
2006年 | 827篇 |
2005年 | 649篇 |
2004年 | 509篇 |
2003年 | 448篇 |
2002年 | 373篇 |
2001年 | 323篇 |
2000年 | 321篇 |
1999年 | 324篇 |
1998年 | 208篇 |
1997年 | 242篇 |
1996年 | 191篇 |
1995年 | 190篇 |
1994年 | 162篇 |
1993年 | 129篇 |
1992年 | 181篇 |
1991年 | 143篇 |
1990年 | 146篇 |
1989年 | 98篇 |
1988年 | 90篇 |
1987年 | 87篇 |
1986年 | 63篇 |
1985年 | 65篇 |
1984年 | 43篇 |
1983年 | 47篇 |
1982年 | 21篇 |
1981年 | 16篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1969年 | 9篇 |
1968年 | 8篇 |
1965年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Regulation of endothelial permeability by Src kinase signaling: vascular leakage versus transcellular transport of drugs and macromolecules 总被引:4,自引:0,他引:4
An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways have been identified controlling endothelial barrier function. The normally restrictive paracellular pathway, which can become "leaky" during inflammation when gaps are induced between endothelial cells at the level of adherens and tight junctional complexes, and the transcellular pathway, which transports plasma proteins the size of albumin via transcytosis in vesicle carriers originating from cell surface caveolae. During non-inflammatory conditions, caveolae-mediated transport may be the primary mechanism of vascular permeability regulation of fluid phase molecules as well as lipids, hormones, and peptides that bind avidly to albumin. Src family protein tyrosine kinases have been implicated in the upstream signaling pathways that lead to endothelial hyperpermeability through both the paracellular and transcellular pathways. Endothelial barrier dysfunction not only affects vascular homeostasis and cell metabolism, but also governs drug delivery to underlying cells and tissues. In this review of the field, we discuss the current understanding of Src signaling in regulating paracellular and transcellular endothelial permeability pathways and effects on endogenous macromolecule and drug delivery. 相似文献
992.
微生物降解芘过程中的关键细菌 总被引:1,自引:0,他引:1
【目的】探究在渤海沉积物中参与降解芘的关键细菌及他们之间潜在的相互关系。【方法】构建以芘为唯一碳源的微宇宙培养体系驯化来自渤海的表层沉积物,借助Illumina Hiseq 2500获取驯化过程中的细菌群落组成,基于CCLasso算法及相对丰度数据预测细菌之间的相互作用关系以构建微生物生态网络。【结果】30 d后芘的降解率为(67.07±2.37)%,细菌群落结构也发生了明显改变:Alphaproteobacteria、Flavobacteriia、Planctomycetia等的相对丰度明显增加,而Deltaproteobacteria、Anaerolineae及Spirochaetes等则明显减少。本研究获得一个由29个点143条边构成的微生物生态网络。分类已知的属中,Erythrobacter及Planctomyces等拥有较高的点度中心度。较强的互作关系发生在Erythrobacter与Flavobacteriaceae、Alphaproteobacteria中的未知属之间。【结论】在芘的微生物降解过程中,关键细菌之间存在紧密互作。Erythrobacter为关键细菌的代表属。 相似文献
993.
Kraft Z Strouss K Sutton WF Cleveland B Tso FY Polacino P Overbaugh J Hu SL Stamatatos L 《Journal of virology》2008,82(12):5912-5921
The vast majority of studies with candidate immunogens based on the human immunodeficiency virus envelope (Env) have been conducted with Env proteins derived from clade B viruses isolated during chronic infection. Whether non-clade B Env protein immunogens will elicit antibodies with epitope specificities that are similar to those of antibodies elicited by clade B Envs and whether the antibodies elicited by Envs derived from early transmitted viruses will be similar to those elicited by Envs derived from viruses isolated during chronic infection are currently unknown. Here we performed immunizations with four clade A Envs, cloned directly from the peripheral blood of infected individuals during acute infection, which differed in lengths and extents of glycosylation. The antibody responses elicited by these four Envs were compared to each other and to those elicited by a well-characterized clade B Env immunogen derived from the SF162 virus, which was isolated during chronic infection. Only one clade A Env, the one with the fewer glycosylation sites, elicited homologous neutralizing antibodies (NAbs); these did not target the V1, V2, or V3 regions. In contrast, all four clade A Envs elicited anti-V3 NAbs against "easy-to-neutralize" clade B and clade A isolates, irrespective of the variable region length and extent of glycosylation of the Env used as an immunogen. These anti-V3 NAbs did not access their epitopes on homologous and heterologous clade A, or B, neutralization-resistant viruses. The length and extent of glycosylation of the variable regions on the clade A Env immunogens tested did not affect the breadth of the elicited NAbs. Our data also indicate that the development of cross-reactive NAbs against clade A viruses faces similar hurdles to the development of cross-reactive anti-clade B NAbs. 相似文献
994.
Background
The objective of this study was to establish a culture system and elucidate the unique characteristics of a bovine mammary epithelial cell line in vitro.Methodology
Mammary tissue from a three year old lactating dairy cow (ca. 100 d relative to parturition) was used as a source of the epithelial cell line, which was cultured in collagen-coated tissue culture dishes. Fibroblasts and epithelial cells successively grew and extended from the culturing mammary tissue at the third day. Pure epithelial cells were obtained by passages culture.Principal Findings
The strong positive immunostaining to cytokeratin 18 suggested that the resulting cell line exhibited the specific character of epithelial cells. Epithelial cells cultured in the presence of 10% FBS, supraphysiologic concentrations of insulin, and hydrocortisone maintained a normal diploid chromosome modal number of 2n = 60. Furthermore, they were capable of synthesizing β-casein (CSN2), acetyl-CoA carboxylase-α (ACACA) and butyrophilin (BTN1A1). An important finding was that frozen preservation in a mixture of 90% FBS and 10% DMSO did not influence the growth characteristics, chromosome number, or protein secretion of the isolated epithelial cell line.Conclusions
The obtained mammary epithelial cell line had normal morphology, growth characteristics, cytogenetic and secretory characteristics, thus, it might represent an useful tool for studying the function of Chinese Holstein dairy cows mammary epithelial cell (CMECs). 相似文献995.
Park GY Wang X Hu N Pedchenko TV Blackwell TS Christman JW 《The Journal of biological chemistry》2006,281(27):18684-18690
The exact physiological role of NF-kappaB-inducing kinase (NIK) in the NF-kappaB activation pathway has not been defined, although it is an upstream kinase of IKKalpha. Recent studies have indicated that IKKalpha is a nucleosomal modifier of NF-kappaB signaling. We hypothesized that NIK generates a proximal signal that contributes to IKKalpha modification of nucleosomal structure through phosphorylation of histone H3 and enhancement of target gene expression. By using a chromatin immunoprecipitation assay, our data show that endogenous IKKalpha is recruited to the promoter site of several NF-kappaB-dependent genes in macrophages. Our data show that immunoreactive NIK is rapidly recruited to nuclear compartment in macrophages in response to treatment with endotoxin where it augments phosphorylation of histone H3 by inducing phosphorylation and kinase activity of IKKalpha. A small interfering RNA knockdown of NIK markedly reduces phosphorylation of histone H3 in endotoxin treated macrophages. These data, together, demonstrate a novel role for NIK as a histone H3 modifier, through an accessory pathway from NIK to IKKalpha, that could play an important role in the endotoxin response through modification of nucleosomal structure. 相似文献
996.
We examined the effects of endogenous basic proteins rich in the amino acidL-arginine on neuronal NO synthase activity by monitoring cyclic GMP formation in intact neuron-like neuroblastoma N1E-115 cells. Histone, protamine and myelin basic protein significantly stimulated cyclic GMP formation, both in a time- and concentration-dependent manner. These effects were blocked by hemoglobin and NO synthase inhibitors. Removal of the extracellular/intracellular Ca2+ gradient by a Ca2+ chelator completely abolished the cyclic GMP responses elicited by histone and protamine, suggesting that influex of extracellular Ca2+ might be involved in their activation of NO synthase. The effects of myelin basic protein on cyclic GMP formation, however, appeared to be due to Ca2+ release from intracellular stores. In cytosolic preparations of rat cerebellum, these basic proteins inhibited the metabolism ofL-arginine intoL-citrulline by NO synthase. We conclude from our findings that endogenous basic proteins might be involved in the regulation of neuronal NO synthase activity. Their effects on the enzyme could be either stimulatory or inhibitory, depending on whether the basic proteins exert their effects extracellularly or intracellularly, respectively. 相似文献
997.
AlateratioofMyzuspersicaeSulzerwasgreatlycloserinrelationshiptocertaininclusionintherapecrop-QingyouⅡ,suchastryptophan,ratioofcontentsofsolublesugartoproteinandmeancrowdingnessunderconditionsofvariedmateproportionoffertilizers-N,P,K.Thefollowinginclusionintherapecropwerealsorelatedtotheratioofalatate:asperticacid,troeoninie,serine,glutancicacid,phenylalanine,lysine,histidine,valine,methionine,isoleucine,leucine,solublesugarandwater-solubleprotein. 相似文献
998.
Y. X. Jia J. Sun S. R. Guo J. Li X. H. Hu S. P. Wang 《Russian Journal of Plant Physiology》2010,57(5):648-655
The effects of exogenous spermidine (Spd) application to hypoxic nutrient solution on the contents of endogenous polyamines
(PAs) and respiratory metabolism in the roots of cucumber (Cucumis sativus L.) seedlings were investigated. Cucumber seedlings were grown hydroponically in control and hypoxic nutrient solutions with
and without addition of Spd at a concentration of 0.05 mM. The activities of key enzymes involved in the tricarboxylic acid
cycle (TCAC), such as succinate dehydrogenase (SDH) and isocitrate dehydrogenase (IDH), were significantly inhibited under
root-zone hypoxia with dissolved oxygen (DO) at 1 mg/l. In contrast, the activities of enzymes involved in the process of
fermentation, such as pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), and alanine
aminotransferase (AlaAT), were significantly increased. Thus, aerobic respiration was inhibited and fermentation was enhanced
in the roots of cucumber seedlings as a result of decreasing ATP content to inhibit the dry weight of seedlings under hypoxic
stress. Moreover, the contents of free, soluble conjugated, and insoluble bound putrescine (Put), Spd, and spermine (Spm)
in the roots of cucumber seedlings were significantly increased under hypoxia stress. Interestingly, application of Spd to
hypoxic roots markedly suppressed the accumulation of free Put and, in contrast, promoted an increase in free Spd and Spm,
as well as soluble conjugated and insoluble bound Put, Spd, and Spm contents. From these data, we deduced that exogenous Spd
promotes the conversion of free Put into free Spd and Spm, and soluble conjugated and insoluble bound PAs under hypoxia stress.
Furthermore, the activities of LDH, PDC, and ADH were suppressed and, in contrast, the activities of SDH and IDH were enhanced
by application of exogenous Spd to hypoxic roots. As a result, aerobic respiration was enhanced but fermentation metabolism
was inhibited in the roots of cucumber seedlings, leading to an increase in ATP content to alleviate the inhibited dry weight
of seedlings due to hypoxia stress. These results suggest that application of Spd to hypoxic nutrient solution promoted conversion
of free Put into free Spd and Spm as well as soluble conjugated and insoluble bound PAs, further enhanced IDH and SDH activities,
and inhibited ethanol fermentation and lactate fermentation, resulting in increased ATP content and eventually enhanced tolerance
of cucumber plants to root-zone hypoxia. 相似文献
999.
Hu Qiu 《生物化学与生物物理学报:生物膜》2010,1798(3):318-326
Despite sharing overall sequence and structural similarities, water channel aquaporin 0 (AQP0) transports water more slowly than other aquaporins. Using molecular dynamics simulations of AQP0 and AQP1, we find that there is a sudden decrease in the distribution profile of water density along the pore of AQP0 in the region of residue Tyr23, which significantly disrupts the single file water chain by forming hydrogen bond with permeating water molecules. Comparisons of free-energy and interaction-energy profiles for water conduction between AQP0 and AQP1 indicate that this interruption of the water chain causes a huge energy barrier opposing water translocation through AQP0. We further show that a mutation of Tyr23 to phenylalanine leads to a 2- to 4-fold enhancement in water permeability of AQP0, from (0.5 ± 0.2) × 10− 14 cm3s− 1 to (1.9 ± 0.6) × 10− 14 cm3s− 1. Therefore, Tyr23 is a dominate factor leading to the low water permeability in AQP0. 相似文献
1000.
Hu Y Jin Y Han D Zhang G Cao S Xie J Xue J Li Y Meng D Fan X Sun LQ Wang M 《Journal of virology》2012,86(6):3347-3356
Although an important role for mast cells in several viral infections has been demonstrated, its role in the invasion of highly pathogenic H5N1 influenza virus is unknown. In the present study, we demonstrate that mast cells were activated significantly by H5N1 virus (A/chicken/Henan/1/2004) infection both in vivo and in vitro. Mast cells could possibly intensify the lung injury that results from H5N1 infection by releasing proinflammatory mediators, including histamine, tryptase, and gamma interferon (IFN-γ). Lung lesions and apoptosis induced by H5N1 infection were reduced dramatically by treatment with ketotifen, which is a mast cell degranulation inhibitor. A combination of ketotifen and the neuraminidase inhibitor oseltamivir protected 100% of the mice from death postinfection. In conclusion, our data suggest that mast cells play a crucial role in the early stages of H5N1 influenza virus infection and provide a new approach to combat highly pathogenic influenza virus infection. 相似文献