全文获取类型
收费全文 | 6779篇 |
免费 | 482篇 |
出版年
2023年 | 51篇 |
2022年 | 60篇 |
2021年 | 215篇 |
2020年 | 145篇 |
2019年 | 167篇 |
2018年 | 234篇 |
2017年 | 180篇 |
2016年 | 279篇 |
2015年 | 427篇 |
2014年 | 455篇 |
2013年 | 528篇 |
2012年 | 624篇 |
2011年 | 582篇 |
2010年 | 362篇 |
2009年 | 295篇 |
2008年 | 405篇 |
2007年 | 377篇 |
2006年 | 323篇 |
2005年 | 270篇 |
2004年 | 261篇 |
2003年 | 243篇 |
2002年 | 219篇 |
2001年 | 75篇 |
2000年 | 59篇 |
1999年 | 50篇 |
1998年 | 35篇 |
1997年 | 28篇 |
1996年 | 33篇 |
1995年 | 19篇 |
1994年 | 28篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 15篇 |
1990年 | 20篇 |
1989年 | 15篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 16篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1976年 | 7篇 |
1975年 | 7篇 |
1974年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有7261条查询结果,搜索用时 31 毫秒
1.
The effects of exogenous fatty acids and hypoxia on cardiac energy metabolism were studied by measuring mitochondrial and cytosolic adenine nucleotides as well as CoA and carnitine esters using a tissue fractionation technique in non-aqueous solvents. During normoxia, the administration of 0.5 mM palmitate caused a considerable increase in acyl-CoA and acylcarnitine, particularly in mitochondria. High-energy phosphates, however, were only slightly altered. A 90 min low-flow hypoxia caused a dramatic increase in mitochondrial acyl esters. The mitochondrial ATP content decreased significantly, while the cytosolic concentration was only slightly diminished, suggesting an inhibition of mitochondrial adenine nucleotide translocation by long-chain acyl-CoA. Addition of palmitate during hypoxia amplified hypoxic damage and reduced adenine nucleotides in both compartments considerably, while fatty acid metabolites were only slightly affected. In presence of an inhibitor of fatty acid oxidation (BM 42.304), the fatty-acid-induced acceleration of cardiac injury was prevented. Since BM 42.304 decreased mitochondrial acylcarnitine and increased the cytosolic concentration significantly, BM 42.304 was presumed to inhibit mitochondrial acylcarnitine translocase. However, a causal relationship between lipid metabolites and ischemic damage seemed unlikely. 相似文献
2.
Here we report that prioritizing sites in order of rarity-weighted richness (RWR) is a simple, reliable way to identify sites that represent all species in the fewest number of sites (minimum set problem) or to identify sites that represent the largest number of species within a given number of sites (maximum coverage problem). We compared the number of species represented in sites prioritized by RWR to numbers of species represented in sites prioritized by the Zonation software package for 11 datasets in which the size of individual planning units (sites) ranged from <1 ha to 2,500 km2. On average, RWR solutions were more efficient than Zonation solutions. Integer programming remains the only guaranteed way find an optimal solution, and heuristic algorithms remain superior for conservation prioritizations that consider compactness and multiple near-optimal solutions in addition to species representation. But because RWR can be implemented easily and quickly in R or a spreadsheet, it is an attractive alternative to integer programming or heuristic algorithms in some conservation prioritization contexts. 相似文献
3.
4.
5.
Santosh R. D'Mello Fabio Aglieco Melanie R. Roberts Kristin Borodezt John W. Haycock 《Journal of neurochemistry》1998,70(5):1809-1818
Abstract: Cultured cerebellar granule neurons undergo apoptosis when switched from a medium containing depolarizing levels of K+ (25 mM KCI) to medium containing lower levels of K+ (5 mM KCI). We used this paradigm to investigate the role of caspases in the death process. Two broad-spectrum caspase inhibitors, tert-butoxycarbonyl-Asp·(O-methyl)·fluoromethyl ketone and benzyloxycarbonyl-Val-Ala-Asp·fluoromethyl ketone, significantly reduced cell death (90 and 60%, respectively) at relatively low concentrations (10–25 µM), suggesting that caspase activation is involved in the apoptotic process. DNA fragmentation, a hallmark of apoptosis, was also reduced by these caspase inhibitors, suggesting that caspase activation occurred upstream of DNA cleavage in the sequence of events leading to cell death. As a step toward identifying the caspase(s) involved, the effects of N-acetyl Tyr-Val-Ala-Asp·chloromethyl ketone (YVAD·cmk), an interleukin-1β converting enzyme-preferring inhibitor, and N-acetyl Asp-Glu-Val-Asp·fluoromethyl ketone (DEVD·fmk), a CPP32-preferring inhibitor, were also evaluated. YVAD·cmk provided only modest (<20%) protection and only at the highest concentration (100 µM) tested, suggesting that interleukin-1β converting enzyme and/or closely related caspases were not involved. In comparison, DEVD·fmk inhibited cell death by up to 50%. Western blot analyses, however, failed to detect an increase in processing/activation of CPP32 or in the proteolysis of a CPP32 substrate, poly(ADP-ribose) polymerase, during the induction of apoptosis in granule neurons. Similarly, the levels of Nedd2, a caspase that is highly expressed in the brain and that is partially inhibited by DEVD·fmk, also remained unaffected in apoptotic neurons undergoing apoptosis. These results suggest that a DEVD-sensitive caspase other than CPP32 or Nedd2 mediates the induction of apoptosis in K+-deprived granule neurons. 相似文献
6.
Biagina Chiofalo Vittorio Lo Presti Vincenzo Chiofalo Fabio Gresta 《Animal Feed Science and Technology》2012,171(2-4):230-239
The aim of this study was to investigate the productive traits and fatty acid profiles of eight sweet varieties of Lupinus albus, Lupinus angustifolius and Lupinus luteus cultivated in a Mediterranean environment (Sicily, southern Italy). This is being done to identify the most suitable species, from an agronomical and nutritional point of view, for supporting animal husbandry in this area. Seed yield, oil quantity and oil quality were considerably different depending on the genotype. L. albus showed the highest yield (2.0–2.4 t/ha), 100 seed weight (27.0 g, on average), oil content (70.4–75.5 g/kg), and n?3/n?6 polyunsaturated fatty acid ratio (0.39–0.51), L. angustifolius showed the highest content of saturated fatty acids (21.03–26.66 g/100 g) and L. luteus showed the highest levels of n?6 polyunsaturated fatty acids (47.56–50.85 g/100 g), unsaturated/saturated (4.93–5.62) and polyunsaturated/saturated (3.33–3.99) fatty acid ratios and the best atherogenic (0.06) and thrombogenic (0.09–0.11) indices. The data obtained suggests L. albus as the most interesting lupin species for aiding the crop-livestock food chain and L. luteus as a promising crop due to its high nutritive traits for the Mediterranean environment. 相似文献
7.
Gilberto Sousa Alves Laurence O’Dwyer Alina Jurcoane Viola Oertel-Kn?chel Christian Kn?chel David Prvulovic Felipe Sudo Carlos Eduardo Alves Letice Valente Denise Moreira Fabian Fuβer Tarik Karakaya Johannes Pantel Eliasz Engelhardt Jerson Laks 《PloS one》2012,7(12)
Alzheimeŕs disease (AD) represents the most prevalent neurodegenerative disorder that causes cognitive decline in old age. In its early stages, AD is associated with microstructural abnormalities in white matter (WM). In the current study, multiple indices of diffusion tensor imaging (DTI) and brain volumetric measurements were employed to comprehensively investigate the landscape of AD pathology. The sample comprised 58 individuals including cognitively normal subjects (controls), amnestic mild cognitive impairment (MCI) and AD patients. Relative to controls, both MCI and AD subjects showed widespread changes of anisotropic fraction (FA) in the corpus callosum, cingulate and uncinate fasciculus. Mean diffusivity and radial changes were also observed in AD patients in comparison with controls. After controlling for the gray matter atrophy the number of regions of significantly lower FA in AD patients relative to controls was decreased; nonetheless, unique areas of microstructural damage remained, e.g., the corpus callosum and uncinate fasciculus. Despite sample size limitations, the current results suggest that a combination of secondary and primary degeneration occurrs in MCI and AD, although the secondary degeneration appears to have a more critical role during the stages of disease involving dementia. 相似文献
8.
Francesca Passaretta Domenico Bosco Lucia Centurione Maria Antonietta Centurione Fabio Marongiu Roberta Di Pietro 《Journal of cellular and molecular medicine》2020,24(7):4350-4355
Human Amniotic Epithelial Cells (hAEC) isolated from term placenta are a promising source for regenerative medicine. However, it has long been debated whether the hAEC population consists of heterogeneous or homogeneous cells. In a previous study, we investigated the characteristics of hAEC isolated from four different regions of the amniotic membrane finding significant heterogeneity. The aim of this study was to evaluate the hepatic differentiation capability of hAEC isolated from these four regions. Human term placentae were collected after caesarean section and hAEC were isolated from four regions of the amniotic membrane (R1-R4, according to their relative distance from the umbilical cord) and treated in hepatic differentiation conditions for 14 days. hAEC-derived hepatocyte-like cells showed marked differences in the expression of hepatic markers: R4 showed higher levels of Albumin and Hepatocyte Nuclear Factor (HNF) 4α whereas R1 expressed higher Cytochrome P450 enzymes, both at the gene and protein level. These preliminary results suggest that hAEC isolated from R1 and R4 of the amniotic membrane are more prone to hepatic differentiation. Therefore, the use of hAEC from a specific region of the amniotic membrane should be taken into consideration as it could have an impact on the outcome of therapeutic applications. 相似文献
9.
A COST Action is a consortium of -mainly- European scientists (but open to international cooperation) working on a common research area, with the same subject; COST provides funding to the Actions for networking and dissemination activities, thus the participating scientists must have secured research funding from other national or European sources. COST funding is in the scale of approximately 100 kEuros per year and in this vein, it is often criticized both in that it does not fund research and the core science and in that its funding is ‘limited’. However, COST with its instruments is an integral pillar of the European Research Area, and it is through its mission that a variety of aspects of the research environment, fundamental to the success of the research, are catered for; these include scientific networking, collaboration/exchange/training and dissemination activities. Through fast procedures, proposals are evaluated and approved for funding in less than one year from submission date and Actions become operational immediately, managed on flexible management. In this way, COST contributes to reducing the fragmentation in European research investments, while opening the European Research Area to cooperation worldwide. COST Actions have an excellent record of building the critical mass for follow up activities in the EU FP or other similarly competitive programmes. 相似文献
10.
Margarida A. Ferreira M.Aurea Cruz Costa A.Correia Alves M.Helena Lopes 《Phytochemistry》1973,12(2):433-435
7-MethyljugIone, 8,8′-dihydroxy-4,4′-dimethoxy-6,6′-dimethyl-2,2′-binaphthyl-1,1′-quinone, 2-methylnaphthazarin, mamegakinone and euclein have been isolated from Euclea pseudebenus. Euclein is the 3,6′-dimer of 7-methyljuglone. 相似文献