全文获取类型
收费全文 | 3284篇 |
免费 | 225篇 |
国内免费 | 3篇 |
出版年
2023年 | 21篇 |
2022年 | 32篇 |
2021年 | 67篇 |
2020年 | 30篇 |
2019年 | 52篇 |
2018年 | 58篇 |
2017年 | 55篇 |
2016年 | 119篇 |
2015年 | 181篇 |
2014年 | 198篇 |
2013年 | 241篇 |
2012年 | 304篇 |
2011年 | 264篇 |
2010年 | 132篇 |
2009年 | 119篇 |
2008年 | 195篇 |
2007年 | 215篇 |
2006年 | 169篇 |
2005年 | 166篇 |
2004年 | 142篇 |
2003年 | 128篇 |
2002年 | 122篇 |
2001年 | 37篇 |
2000年 | 28篇 |
1999年 | 33篇 |
1998年 | 23篇 |
1997年 | 24篇 |
1996年 | 25篇 |
1995年 | 22篇 |
1994年 | 19篇 |
1993年 | 24篇 |
1992年 | 21篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 16篇 |
1988年 | 17篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 19篇 |
1983年 | 10篇 |
1982年 | 17篇 |
1981年 | 8篇 |
1979年 | 13篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1974年 | 14篇 |
1971年 | 6篇 |
1964年 | 6篇 |
1952年 | 4篇 |
排序方式: 共有3512条查询结果,搜索用时 31 毫秒
841.
Nicolaus G. Adams Vera L. Trainer Gabrielle Rocap Russell P. Herwig Lorenz Hauser 《Journal of phycology》2009,45(5):1037-1045
Several species of the diatom Pseudo‐nitzschia produce the neurotoxin domoic acid (DA). Consumption of fish and shellfish that have accumulated this potent excitotoxin has resulted in severe illness and even death in humans, marine mammals, and seabirds. Pseudo‐nitzschia pungens (Grunow ex Cleve) Hasle is a cosmopolitan diatom commonly occurring in the waters of the Pacific Northwest (PNW) and the eastern North Atlantic, including the North Sea. However, genetic and physiological relationships among populations throughout this large geographic distribution have not been assessed. Population genetic parameters (e.g., Hardy–Weinberg equilibrium, linkage equilibrium, FST) calculated for P. pungens collected from the Juan de Fuca eddy region in the PNW indicated the presence of two distinct groups that were more divergent from each other than either was from a P. pungens sample from the North Sea. Geographic heterogeneity was also detected within each of the two PNW groups. These results suggested that the populations of P. pungens recently mixed in the Juan de Fuca eddy region (a seasonally retentive feature off the coasts of Washington State, USA, and Vancouver Island, Canada) but did not exchange genetic material by sexual reproduction. Alternatively, these two groups may be cryptic (morphologically identical, but reproductively isolated) species. Identifying cryptic diversity in Pseudo‐nitzschia is important for bloom prediction and aiding the identification of molecular markers that can be used for rapid detection assay development. 相似文献
842.
Dorneles AG Menezes C Sperotto RL Duarte MM Morsch VM Schetinger MR Loro VL 《Neurochemical research》2009,34(3):463-469
The meningitis is a disease with high mortality rates capable to cause neurologic sequelae. The adenosine (the final product
of ATP hydrolysis by ectonucleotidases), have a recognized neuroprotective actions in the central nervous system (CNS) in
pathological conditions. The aim of the present study was evaluate the adenine nucleotides hydrolysis for to verify one possible
role of ATP, ADP and AMP hydrolysis in inflammatory process such as meningitis. The hydrolysis was verified in cerebrospinal
fluid (CSF) from human patients with aseptic and bacterial meningitis. Our results showed that the ATP hydrolysis was reduced
12.28% (P < 0.05) in bacterial meningitis and 22% (P < 0.05) in aseptic meningitis. ADP and AMP hydrolysis increased 79.13% (P < 0.05) and 26.37% (P < 0.05) in bacterial meningitis, respectively, and 57.39% (P < 0.05) and 42.64% (P < 0.05) in aseptic meningitis, respectively. This may be an important protective mechanism in order to increase adenosine
production. 相似文献
843.
AimRepeated episodes of myocardial stunning may lead to chronic ventricular dysfunction. We attempted to assess the parameters related to post-exercise stunning in patients undergoing gated SPECT.MethodsSix hundred patients undergoing a one-day stress/rest 99mTc-sestamibi gated SPECT were studied. Stress imaging was acquired within 15 minutes after injection. Summed perfusion scores (SSS, SRS and SDS) were calculated using QPS, and LV function assessed using QGS. Stunning was defined as the association of ischemia (SSS ≥ 4 and SDS > 0) and a minimum of 5% decrease in post-stress EF.ResultsIschemia was found in 225 (37.5%) patients. Among these, 67 (30%) showed myocardial stunning. Patients with stunning had a lower rest ESV (47 ± 24 mL vs 65 ± 52 mL, p < 0.0003) and EDV (108 ± 35 mL vs 122 ± 66 mL, p = 0.03), an increased rest LVEF (58 ± 10% vs 52 ± 13%, p < 0.0001) and a decreased post-stress LVEF (49 ± 10% vs 53 ± 13%, p < 0.02) compared to patients with no stunning. The number of myocardial segments showing reversible perfusion defects was increased in patients with stunning (2.7 ± 2.6 vs 1.7 ± 2.1, p < 0.02). On logistic regression, an extent of ischemia greater than two segments and a rest EF greater than 45% were independent predictors of the occurrence of myocardial stunning in patients with ischemia.ConclusionsIn patients with ischemia, exercise-induced stunning was associated with an increased extent of ischemia but also preserved rest ventricular function. 相似文献
844.
Fulvio A. Scorza Roberta M. Cysneiros Vera C. Terra Carla A. Scorza Esper A. Cavalheiro Miriam O. Ribeiro Wagner F. Gattaz 《Prostaglandins, leukotrienes, and essential fatty acids》2009,81(4):241-245
People with schizophrenia show a two- to three-fold increased risk to die prematurely. Mortality is accounted for by a combination of factors (patients’ life style, suicide, premature cardiovascular disease, metabolic syndromes and, not so often mentioned, sudden death). The cause of sudden death in schizophrenia is unknown, but cardiac arrhythmia plays a potential role. Patients with schizophrenia are at high risk for cardiovascular disease, and some antipsychotics may be associated with cardiovascular adverse events (e.g., electrocardiograph QT interval prolongation), suggesting that this could lead to sudden cardiac death. Animal and clinical studies have shown that omega-3 fatty acids could be useful in the prevention and treatment of schizophrenia. As omega-3 fatty acids have been considered a cardioprotector agent, reducing cardiac arrhythmias and hence sudden cardiac deaths and given their relative safety and general health benefits, our update article summarizes the knowledge by the possible positive effects of omega-3 supplementation and fish consumption against sudden cardiac death in patients with schizophrenia. However, fish species should be selected with caution due to contamination with toxic methylmercury. 相似文献
845.
Karin A. Riske Rafael P. Barroso Cíntia C. Vequi-Suplicy Renato Germano Vera B. Henriques M. Teresa Lamy 《生物化学与生物物理学报:生物膜》2009,1788(5):954-13143
We investigate the bilayer pre-transition exhibited by some lipids at temperatures below their main phase transition, and which is generally associated to the formation of periodic ripples in the membrane. Experimentally we focus on the anionic lipid dipalmytoylphosphatidylglycerol (DPPG) at different ionic strengths, and on the neutral lipid dipalmytoylphosphatidylcholine (DPPC). From the analysis of differential scanning calorimetry traces of the two lipids we find that both pre- and main transitions are part of the same melting process. Electron spin resonance of spin labels and excitation generalized polarization of Laurdan reveal the coexistence of gel and fluid domains at temperatures between the pre- and main transitions of both lipids, reinforcing the first finding. Also, the melting process of DPPG at low ionic strength is found to be less cooperative than that of DPPC. From the theoretical side, we introduce a statistical model in which a next-nearest-neighbor competing interaction is added to the usual two-state model. For the first time, modulated phases (ordered and disordered lipids periodically aligned) emerge between the gel and fluid phases as a natural consequence of the competition between lipid-lipid interactions. 相似文献
846.
NAD(P)H oxidoreductases of the outer mitochondrial membrane (OMM) are able to activate various xenobiotics and stimulate the production of reactive oxygen species and the opening of the mitochondrial permeability transition pore. However, the role of these systems in the cell damage by xenobiotics and chemotherapeutic drugs is poorly understood because the methods for the selective assessment of their activity have not been elaborated and specific inhibitors are unknown. Here we propose a method for the semiquantitative assessment of the activity of NAD(P)H oxidoreductases of the OMM in intact and permeabilized cells that is based on the flow cytometry detection of dimethylbiacridene, a fluorescent product of two-electron reduction of lucigenin. The method uses the structural feature of mitochondrial organization: the proximity of the sites of one-electron reduction of lucigenin to cation radical (NAD(P)H oxidoreductases of the OMM) to the sites of its subsequent oxidation (cytochrome c oxidase). The inhibition of cytochrome c oxidase by cyanide selectively activates the dimethylbiacridene formation by oxidoreductases of the OMM but not by other cellular oxidoreductases. The proposed protocol allows one to assess the lucigenin reductase (two-electron) activity of NAD(P)H oxidoreductases of the OMM and to compare it with the activity of other cellular systems that can be used for the analysis of the role of these systems in the cell damage by xenobiotics and antitumor drugs. 相似文献
847.
848.
Nataliya Butenko Ofelia Nouri Virtudes Moreno Vera Ribeiro João Costa Pesssoa Isabel Cavaco 《Journal of inorganic biochemistry》2009,103(4):622-632
The DNA cleavage activity of several β-diketonate vanadyl complexes is examined. Vanadyl acetylacetonate, VIVO(acac)2, 1, shows a remarkable activity in degrading plasmid DNA in the absence of any activating agents, air and photoirradiation. The cleaving activity of several related complexes VIVO(hd)2 (2, Hhd = 3,5-heptanedione), VIVO(acac-NH2)2 (3, Hacac-NH2 = acetoacetamide) and VIVO(acac-NMe2)2 (4, Hacac-NMe2 = N,N-dimethylacetoacetamide) is also evaluated. It is shown that 2 exhibits an activity similar to 1, while 3 and 4 are much less efficient cleaving agents. The different activity of the complexes is related to their stability towards hydrolysis in aqueous solution, which follows the order 1∼2 ? 3∼4. The nature of the pH buffer was also found to be determinant in the nuclease activity of 1 and 2. In a phosphate buffered medium DNA cleavage by these agents is much more efficient than in tris, hepes, mes or mops buffers. The reaction seems to take place through a mixed mechanism, involving the formation of reactive oxygen species (ROS), namely OH radicals, and possibly also direct cleavage at phosphodiester linkages induced by the vanadium complexes. 相似文献
849.
Anna Bachmann Claudia Esser Michaela Petter Sabine Predehl Vera von Kalckreuth Stefan Schmiedel Iris Bruchhaus Egbert Tannich 《PloS one》2009,4(10)
Background
To avoid spleen-dependent killing mechanisms parasite-infected erythrocytes (IE) of Plasmodium falciparum malaria patients have the capacity to bind to endothelial receptors. This binding also known as sequestration, is mediated by parasite proteins, which are targeted to the erythrocyte surface. Candidate proteins are those encoded by P. falciparum multicopy gene families, such as var, rif, stevor or PfMC-2TM. However, a direct in vivo proof of IE sequestration and expression of multicopy gene families is still lacking. Here, we report on the analysis of IE from a black African immigrant, who received the diagnosis of a malignant lymphoproliferative disorder and subsequently underwent splenectomy. Three weeks after surgery, the patient experienced clinical falciparum malaria with high parasitemia and circulating developmental parasite stages usually sequestered to the vascular endothelium such as late trophozoites, schizonts or immature gametocytes.Methodology/Principal Findings
Initially, when isolated from the patient, the infected erythrocytes were incapable to bind to various endothelial receptors in vitro. Moreover, the parasites failed to express the multicopy gene families var, A-type rif and stevor but expression of B-type rif and PfMC-2TM genes were detected. In the course of in vitro cultivation, the parasites started to express all investigated multicopy gene families and concomitantly developed the ability to adhere to endothelial receptors such as CD36 and ICAM-1, respectively.Conclusion/Significance
This case strongly supports the hypothesis that parasite surface proteins such as PfEMP1, A-type RIFIN or STEVOR are involved in interactions of infected erythrocytes with endothelial receptors mediating sequestration of mature asexual and immature sexual stages of P. falciparum. In contrast, multicopy gene families coding for B-type RIFIN and PfMC-2TM proteins may not be involved in sequestration, as these genes were transcribed in infected but not sequestered erythrocytes. 相似文献850.
Eva Maria Feny? Alan Heath Stefania Dispinseri Harvey Holmes Paolo Lusso Susan Zolla-Pazner Helen Donners Leo Heyndrickx Jose Alcami Vera Bongertz Christian Jassoy Mauro Malnati David Montefiori Christiane Moog Lynn Morris Saladin Osmanov Victoria Polonis Quentin Sattentau Hanneke Schuitemaker Ruengpung Sutthent Terri Wrin Gabriella Scarlatti 《PloS one》2009,4(2)