全文获取类型
收费全文 | 4686篇 |
免费 | 412篇 |
国内免费 | 281篇 |
专业分类
5379篇 |
出版年
2024年 | 14篇 |
2023年 | 118篇 |
2022年 | 154篇 |
2021年 | 219篇 |
2020年 | 168篇 |
2019年 | 223篇 |
2018年 | 190篇 |
2017年 | 146篇 |
2016年 | 155篇 |
2015年 | 214篇 |
2014年 | 241篇 |
2013年 | 290篇 |
2012年 | 163篇 |
2011年 | 206篇 |
2010年 | 173篇 |
2009年 | 218篇 |
2008年 | 236篇 |
2007年 | 207篇 |
2006年 | 172篇 |
2005年 | 170篇 |
2004年 | 161篇 |
2003年 | 130篇 |
2002年 | 142篇 |
2001年 | 114篇 |
2000年 | 88篇 |
1999年 | 119篇 |
1998年 | 80篇 |
1997年 | 78篇 |
1996年 | 76篇 |
1995年 | 100篇 |
1994年 | 65篇 |
1993年 | 71篇 |
1992年 | 66篇 |
1991年 | 52篇 |
1990年 | 41篇 |
1989年 | 49篇 |
1988年 | 47篇 |
1987年 | 24篇 |
1986年 | 39篇 |
1985年 | 31篇 |
1984年 | 44篇 |
1983年 | 17篇 |
1982年 | 22篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1976年 | 5篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有5379条查询结果,搜索用时 0 毫秒
61.
Pre- and Posttranslational Regulation of β-Endorphin Biosynthesis in the CNS: Effects of Chronic Naltrexone Treatment 总被引:1,自引:1,他引:0
David M. Bronstein Nicola C. Day Howard B. Gutstein Keith A. Trujillo Huda Akil 《Journal of neurochemistry》1993,60(1):40-49
Abstract: There appear to be two anatomically distinct β-endorphin (βE) pathways in the brain, the major one originating in the arcuate nucleus of the hypothalamus and a smaller one in the area of the nucleus tractus solitarius (NTS) of the caudal medulla. Previous studies have shown that these two proopiomelanocortin (POMC) systems may be differentially regulated by chronic morphine treatment, with arcuate cells down-regulated and NTS cells unaffected. In the present experiments, we examined the effects of chronic opiate antagonist treatment on βE biosynthesis across different CNS regions to assess whether the arcuate POMC system would be regulated in the opposite direction to that seen after opiate agonist treatment and to determine whether different βE-containing areas might be differentially regulated. Male adult rats were administered naltrexone (NTX) by various routes for 8 days (subcutaneous pellets, osmotic minipumps, or repeated intraperitoneal injections). Brain and spinal cord regions were assayed for total βE-ir, different molecular weight immunoreactive β-endorphin (βE-ir) peptides, and POMC mRNA. Chronic NTX treatment, regardless of the route of administration, reduced total βE-ir concentrations by 30–40% in diencephalic areas (the arcuate nucleus, the remaining hypothalamus, and the thalamus) and the midbrain, but had no effect on βE-ir in the NTS or any region of the spinal cord. At the same time, NTX pelleting increased POMC mRNA levels in the arcuate to ~ 140% of control values. These data suggest that arcuate POMC neurons are up-regulated after chronic NTX treatment (whereas NTS and spinal cord systems remain unaffected) and that they appear to be under tonic inhibition by endogenous opioids. Chromatographic analyses demonstrated that, after chronic NTX pelleting, the ratio of full length βE1–31 to more processed βE-ir peptides (i.e., βE1–27 and βE1–26) tended to increase in a dose-dependent manner in diencephalic areas. Because βE1–31 is the only POMC product that possesses opioid agonist properties, and βE1–27 has been posited to function as an endogenous anatgonist of βE1–31, the NTX-induced changes in the relative concentrations of βE1–31 and βE1–27/βE1–26 may represent a novel regulatory mechanism of POMC cells to alter the opioid signal in the synapse. 相似文献
62.
Using cytochemical method,microspectrophotometry and image analysis,effects of va-soactive intestinal peptide(VIP)on activities of succinic dehydrogenase(SDH)and alkalinephosphatase(ALP)in rat hepatoma cells were studied in vitro.The results showed that thehepatoma cell expressed potent positive reactions of SDH and ALP,the positive positionswere located at the cell membranes and/or cytoplasm.Having been treated with VIP,ALPdecreased obviously in activity(P<0. 01,compared with hepatoma cells untreated by VIP).The sites of ALP activty were chiefly located at the cell membranes,particularly at the cell-cell contacts.Cultured rat hepatoma cells had intensive SDH activity in their cytoplasm.Compared with untreated eclls,there was no marked difference in the intensity of SDH activ-ity in VIP-treated hepatoma cells(P>0.05). 相似文献
63.
N. M. Jansonius J. H. van Hateren 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1993,172(4):467-471
We recorded from the spiking on-off unit in the first optic chiasm (between lamina and medulla) in the blowfly Calliphora vicina, and investigated its spatial properties. The receptive field extends over (11.4±0.9)° horizontally and (8.7±0.6)° vertically, i.e. about 7 by 5 interommatidial angles. The line spread function of the on-off unit — calculated from its response to moving sinusoidal gratings — has a half-width of (2.3±0.2)°. This half-width is slightly broader than that of the photoreceptor. Lateral inhibition occurs when two different areas of the receptive field are stimulated simultaneously. Fast temporal adaptation (i.e. adaptation to trains of short light pulses) takes place independently in different areas of the receptive field. 相似文献
64.
65.
J. Mogdans H. -U. Schnitzler J. Ostwald 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1993,172(3):309-323
1. | Echolocating bats (Eptesicus fuscus) were trained to discriminate between simulated targets consisting of one or two echo-wavefronts with internal time delays of up to 100 s. Spectral and temporal properties and total signal energy of the targets were evaluated and predictions for performances of bats derived from receiver models were compared with measured performances. |
2. | Eptesicus fuscus was able to discriminate a one-wavefront target from two-wavefront targets with distinct internal time delays (12 s, 32–40 s and 52–100 s). Performance was not affected by changes in total signal energy. Bats also successfully discriminated between two-wavefront targets with different internal time delays. |
3. | Performance predicted from differences in total energy between targets did not match the measured performance, indicating that bats did not rely on total echo energy. This finding is also supported by the behavioral data. Performance predicted from spectral and temporal receiver models both matched the measured performance and, therefore, neither one of these models can be favored over the other. |
4. | The behavioral data suggest that Eptesicus fuscus did not transform echo information into estimates of target range separation and, therefore, did not perceive the two wavefronts of each simulated two-wavefront echo as two separate targets. |
66.
67.
68.
Gerald Platteau Guido Stroehlein James Van Alstine Masayoshi Nagaya 《Biotechnology and bioengineering》2023,120(10):2907-2916
Prepacked chromatography columns and cassette filtration units offer many advantages in bioprocessing. These include reduced labor costs and processing times, ease of storage, and enhanced process flexibility. Rectangular formats are particularly attractive as they can be easily stacked and multiplexed together for continuous processing. Cylindrical chromatography beds have dominated bioprocessing even though their bed support and pressure-flow performance vary with bed dimensions. This work presents the performance of novel, rhombohedral chromatography devices with internally supported beds. They are compatible with existing chromatography workstations and can be packed with any standard commercial resin. The devices offer pressure-flow characteristics independent of container-volume, simple multiplexing, and separation performance comparable to cylindrical columns. Their bi-planar, internal bed support allows mechanically less-rigid resins to be used at up to four times higher maximal linear velocities, and productivities approaching 200 g/L/h for affinity resins, compared to the 20 g/L/h typical of many column-based devices. Three 5 L devices should allow processing of up to 3 kg of monoclonal antibody per hour. 相似文献
69.
Sven Göbel Karim E. Jaén Rita P. Fernandes Manfred Reiter Jennifer Altomonte Udo Reichl Yvonne Genzel 《Biotechnology and bioengineering》2023,120(11):3335-3346
The development of efficient processes for the production of oncolytic viruses (OV) plays a crucial role regarding the clinical success of virotherapy. Although many different OV platforms are currently under investigation, manufacturing of such viruses still mainly relies on static adherent cell cultures, which bear many challenges, particularly for fusogenic OVs. Availability of GMP-compliant continuous cell lines is limited, further complicating the development of commercially viable products. BHK21, AGE1. CR and HEK293 cells were previously identified as possible cell substrates for the recombinant vesicular stomatitis virus (rVSV)-based fusogenic OV, rVSV-NDV. Now, another promising cell substrate was identified, the CCX.E10 cell line, developed by Nuvonis Technologies. This suspension cell line is considered non-GMO as no foreign genes or viral sequences were used for its development. The CCX.E10 cells were thus thoroughly investigated as a potential candidate for OV production. Cell growth in the chemically defined medium in suspension resulted in concentrations up to 8.9 × 106 cells/mL with a doubling time of 26.6 h in batch mode. Cultivation and production of rVSV-NDV, was demonstrated successfully for various cultivation systems (ambr15, shake flask, stirred tank reactor, and orbitally shaken bioreactor) at vessel scales ranging from 15 mL to 10 L. High infectious virus titers of up to 4.2 × 108 TCID50/mL were reached in orbitally shaken bioreactors and stirred tank reactors in batch mode, respectively. Our results suggest that CCX.E10 cells are a very promising option for industrial production of OVs, particularly for fusogenic VSV-based constructs. 相似文献
70.
Sven Göbel Karim E. Jaén Marie Dorn Victoria Neumeyer Ingo Jordan Volker Sandig Udo Reichl Jennifer Altomonte Yvonne Genzel 《Biotechnology and bioengineering》2023,120(9):2639-2657
We present a proof-of-concept study for production of a recombinant vesicular stomatitis virus (rVSV)-based fusogenic oncolytic virus (OV), rVSV-Newcastle disease virus (NDV), at high cell densities (HCD). Based on comprehensive experiments in 1 L stirred tank reactors (STRs) in batch mode, first optimization studies at HCD were carried out in semi-perfusion in small-scale cultivations using shake flasks. Further, a perfusion process was established using an acoustic settler for cell retention. Growth, production yields, and process-related impurities were evaluated for three candidate cell lines (AGE1.CR, BHK-21, HEK293SF)infected at densities ranging from 15 to 30 × 106 cells/mL. The acoustic settler allowed continuous harvesting of rVSV-NDV with high cell retention efficiencies (above 97%) and infectious virus titers (up to 2.4 × 109 TCID50/mL), more than 4–100 times higher than for optimized batch processes. No decrease in cell-specific virus yield (CSVY) was observed at HCD, regardless of the cell substrate. Taking into account the accumulated number of virions both from the harvest and bioreactor, a 15–30 fold increased volumetric virus productivity for AGE1.CR and HEK293SF was obtained compared to batch processes performed at the same scale. In contrast to all previous findings, formation of syncytia was observed at HCD for the suspension cells BHK 21 and HEK293SF. Oncolytic potency was not affected compared to production in batch mode. Overall, our study describes promising options for the establishment of perfusion processes for efficient large-scale manufacturing of fusogenic rVSV-NDV at HCD for all three candidate cell lines. 相似文献