全文获取类型
收费全文 | 300569篇 |
免费 | 22005篇 |
国内免费 | 77篇 |
专业分类
322651篇 |
出版年
2018年 | 3920篇 |
2017年 | 3763篇 |
2016年 | 4734篇 |
2015年 | 4654篇 |
2014年 | 6071篇 |
2013年 | 8680篇 |
2012年 | 10013篇 |
2011年 | 10509篇 |
2010年 | 7481篇 |
2009年 | 6481篇 |
2008年 | 9384篇 |
2007年 | 9642篇 |
2006年 | 9191篇 |
2005年 | 8615篇 |
2004年 | 8715篇 |
2003年 | 8389篇 |
2002年 | 8216篇 |
2001年 | 12685篇 |
2000年 | 12271篇 |
1999年 | 9565篇 |
1998年 | 3014篇 |
1997年 | 2881篇 |
1996年 | 2645篇 |
1995年 | 2419篇 |
1992年 | 7423篇 |
1991年 | 7608篇 |
1990年 | 7587篇 |
1989年 | 7370篇 |
1988年 | 6868篇 |
1987年 | 6483篇 |
1986年 | 5989篇 |
1985年 | 6144篇 |
1984年 | 4921篇 |
1983年 | 4124篇 |
1982年 | 2847篇 |
1981年 | 2506篇 |
1980年 | 2424篇 |
1979年 | 4598篇 |
1978年 | 3572篇 |
1977年 | 3255篇 |
1976年 | 3196篇 |
1975年 | 3835篇 |
1974年 | 4223篇 |
1973年 | 4273篇 |
1972年 | 3773篇 |
1971年 | 3532篇 |
1970年 | 3032篇 |
1969年 | 3007篇 |
1968年 | 2783篇 |
1967年 | 2513篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
231.
S F Silverton O A Adebanjo B S Moonga E M Awumey T Malinski M Zaidi 《Biochemical and biophysical research communications》1999,259(1):73-77
Nitric oxide (NO) triggers marked osteoclast retraction which closely resembles that due to Ca2+. The effect of Ca2+ has been attributed to a stimulated release of NO. Here, we show for the first time, by direct measurement with a microsensor, that osteoclasts do indeed produce NO and that this production is enhanced by a high Ca2+. We also show that the Ca2+ ionophore, A23187, mimics the latter. Furthermore, osteoclasts on dentine produce more NO than osteoclasts on glass and NO release from dentine-plated osteoclasts is much less sensitive to stimulation by Ca2+. Finally, the microsomal Ca2+ store-depleting agent, thapsigargin, attenuates NO release only from osteoclasts on glass, suggesting that stored Ca2+ has the dominant effect in modulating NO release from non-resorbing cells. NO is a powerful inhibitor of bone resorption: a direct demonstration of its production is therefore strong evidence for a role in modulating osteoclast function. 相似文献
232.
233.
The classical motion of an electron in the Coulomb field of an ion and in a uniform external electric field is analyzed. A nondimensionalization method that makes it possible to study electron motion in arbitrarily strong electric fields is proposed. The possible electron trajectories in the plane of motion in a static field are classified. It is noted that, from a practical standpoint, the most interesting trajectories are snakelike trajectories, which are absent in the problem with a weak external field. An adiabatic approximation for transverse electron motions in quasistatic (strong) fields is constructed. A one-dimensional equation of motion is derived that accounts for transverse electron oscillations and the increase in the effective electron mass as an electron approaches an ion. An analytic model is used to calculate the spectra of bremsstrahlung generated by individual electrons. The calculated results are shown to agree well with the results of direct numerical integration of the basic equations. It is predicted that, at frequencies higher than the frequency of the incident light, pronounced peaks can appear in the spectrum of the transverse dipole moment of an electron; as a result, an electron is expected to effectively emit radiation at these frequencies in the direction of the external field. 相似文献
234.
V. V. Smirnov L. N. Churkina A. N. Kravets V. I. Perepnikhatka 《Applied Biochemistry and Microbiology》2000,36(3):262-265
Various plasmids carrying transposon Tn5 were used to generate insertion mutants synthesizing batumin, a unique antibiotic
with a selective antistaphylococcal effect. One of the plasmids used provided a sufficient yield of the clones in question.
An analysis of over 7000 clones allowed us to select the mutant clones with increased and decreased levels of batumin synthesis
and the mutants that lost the ability to synthesize this antibiotic. 相似文献
235.
M. Gopalakrishnan P. Sureshkumar J. Thanusu V. Kanagarajan 《Journal of enzyme inhibition and medicinal chemistry》2013,28(3):347-351
Compound 26 is more potent against Escherichia coli. and 24 is more active against Staphylococcus aureus, β-Heamolytic streptococcus, Vibreo cholerae, Salmonella typhii, and Shigella flexneri than the standard drug ciprofloxacin. Moreover, of all the compounds tested, 26 is more effective against Aspergillus flavus and Mucor, than the standard drug fluconazole. 相似文献
236.
The phenotype of a ouabain-resistant Aedes albopictus cell line has been partially characterized. Treatment of ouabain-sensitive cells with 0.005-1.0 mM ouabain resulted in an 80% reduction in the uptake of 86rubidium (86Rb+), an ion with an affinity for the K+ pump binding site; ouabain-resistant cells showed only a 40% reduction with 1.0 mM ouabain. When ouabain-sensitive cells were incubated in the presence of ouabain (0.1 mM) for one and one-half to three hours, the molar ratio of intracellular Na+/K+ rose from 0.2 to 4.2. In ouabain-resistant cells, a similar treatment had very little effect. Based on [3H] ouabain-binding studies, ouabain-resistant cells were estimated to have 60% fewer binding sites per cell than ouabain-sensitive cells. The spontaneous mutation rate from ouabain sensitivity to ouabain resistance was calculated to be 1-6 x 10(-8) mutations/cell/generation, a value similar to that reported for mammalian cells at the analogous locus. 相似文献
237.
A G Buiavykh O S Medvedev A F Stukanov 《Biulleten' eksperimental'no? biologii i meditsiny》1987,104(7):28-30
The effect of low-frequency magnetic field (MF) on systemic blood pressure has been studied in chronic experiments on 21 spontaneously hypertensive rats. The animals' kidney area was exposed to MF (induction value 30T). Direct blood pressure measurements have revealed an antihypertensive effect. 相似文献
238.
- Urban areas are often considered to be a hostile environment for wildlife as they are highly fragmented and frequently disturbed. However, these same habitats can contain abundant resources, while lacking many common competitors and predators. The urban environment can have a direct impact on the species living there but can also have indirect effects on their parasites and pathogens. To date, relatively few studies have measured how fine‐scale spatial heterogeneity within urban landscapes can affect parasite transmission and persistence.
- Here, we surveyed 237 greenspaces across the urban environment of Edinburgh (UK) to investigate how fine‐scale variation in socio‐economic and ecological variables can affect red fox (Vulpes vulpes) marking behavior, gastrointestinal (GI) parasite prevalence, and parasite community diversity.
- We found that the presence and abundance of red fox fecal markings were nonuniformly distributed across greenspaces and instead were dependent on the ecological characteristics of a site. Specifically, common foraging areas were left largely unmarked, which indicates that suitable resting and denning sites may be limiting factor in urban environments. In addition, the amount of greenspace around each site was positively correlated with overall GI parasite prevalence, species richness, and diversity, highlighting the importance of greenspace (a commonly used measure of landscape connectivity) in determining the composition of the parasite community in urban areas.
- Our results suggest that fine‐scale variation within urban environments can be important for understanding the ecology of infectious diseases in urban wildlife and could have wider implication for the management of urban carnivores.
239.
Experiments in mice have demonstrated that in the presence of influenza infection the suppressing effect of virulent shigellae on the secondary humoral and cell-mediated immune response is enhanced. Under such conditions avirulent shigellae can also show suppressing effect which is usually absent in these organisms. 相似文献
240.
G T Sukhikh I V Bogdashin L V Van'ko 《Biulleten' eksperimental'no? biologii i meditsiny》1985,99(4):399-401
It has been shown that at late periods after immobilization stress there is an essential reduction in the cytostatic activity of CBA mice splenocytes detected by inhibition of mastocytoma P 815 cell proliferation in the test with 3H-uridine incorporation. The cytostatic activity returns to normal 5 to 14 days after stress. The changes in the cytolytic and cytostatic activity of mouse splenocytes following stress are correlated and a possible biological value of changes in the immunoassays for the host are discussed. 相似文献