全文获取类型
收费全文 | 474502篇 |
免费 | 47837篇 |
国内免费 | 238篇 |
专业分类
522577篇 |
出版年
2018年 | 9305篇 |
2017年 | 9050篇 |
2016年 | 8856篇 |
2015年 | 7112篇 |
2014年 | 8171篇 |
2013年 | 11258篇 |
2012年 | 14868篇 |
2011年 | 19615篇 |
2010年 | 14301篇 |
2009年 | 13565篇 |
2008年 | 17288篇 |
2007年 | 19150篇 |
2006年 | 11896篇 |
2005年 | 12246篇 |
2004年 | 11959篇 |
2003年 | 11495篇 |
2002年 | 11220篇 |
2001年 | 17528篇 |
2000年 | 17596篇 |
1999年 | 13911篇 |
1998年 | 5023篇 |
1997年 | 5282篇 |
1996年 | 4925篇 |
1995年 | 4616篇 |
1994年 | 4486篇 |
1993年 | 4557篇 |
1992年 | 11617篇 |
1991年 | 11556篇 |
1990年 | 11296篇 |
1989年 | 10923篇 |
1988年 | 10487篇 |
1987年 | 10084篇 |
1986年 | 9362篇 |
1985年 | 9231篇 |
1984年 | 7758篇 |
1983年 | 6715篇 |
1982年 | 5173篇 |
1981年 | 4616篇 |
1980年 | 4462篇 |
1979年 | 7419篇 |
1978年 | 5863篇 |
1977年 | 5396篇 |
1976年 | 5201篇 |
1975年 | 5603篇 |
1974年 | 6300篇 |
1973年 | 6166篇 |
1972年 | 6265篇 |
1971年 | 5665篇 |
1970年 | 4637篇 |
1969年 | 4574篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
Yu. N. Litvinov N. T. Erzhanov N. V. Lopatina T. Zh. Abylkhasanov 《Contemporary Problems of Ecology》2010,3(5):593-596
Small mammals were studied in the Kazakh Uplands in the spring and fall of 2008. The trapping studies revealed 10 species.
Abundances of the animals were low in the four main distinct characteristic biotopes of Bayanaul National Park, but those
of biotope dominants were high. In the Kazakh Uplands, rodents and insectivores are clearly restricted to certain biotopes.
Biodiversity indices for small mammal communities are low, indicating that the community structure is disturbed. 相似文献
942.
An experiment on acceleration and retardation of ontogeny with thyroid manipulation has revealed direct changes in definitive
dentition of pharyngeal bones in Abramis brama bream. As development rate accelerates, the number of teeth reduces to the formula 5-4. When development rate is retarded,
this number increases to the formula 6-5. Moreover, an additional minor row of teeth (1.6–5.1, 2.6–5.2) is formed. The observed
changes in tooth numbers exceed the known variability in natural populations of bream. It is assumed that heterochronies lead
to the changes in the number of teeth. 相似文献
943.
944.
The benzodiazepine antagonist properties of CGS8216 were evaluated in rats trained to discriminate between saline and 1.0 mg/kg of diazepam in a two-choice, stimulus-shock termination procedure. CGS8216 (0.3 to 100 mg/kg) administered alone, either s.c., p.o. or i.p., occasioned only saline-appropriate responding. When administered concomitantly with a constant 1.0 mg/kg dose of diazepam, CGS8216 produced dose-related decreases in drug-appropriate responding. CGS8216 was most potent by the i.p. route, and approximately tenfold less potent by the oral route. CGS8216 was dermatotoxic after s.c. administration. CGS8216 i.p. had a long duration of action. A dose of 30 mg/kg completely antagonized the discriminative effects of the 1.0 mg/kg training dose of diazepam when the antagonist was administered 8 hr before the start of the test session. In order to determine the type of antagonism by CGS8216, the dose-effect curve for diazepam was redetermined in the presence of varying doses of CGS8216 (0.3 to 3.0 mg/kg, i.p.). CGS8216 produced a dose-related rightward shift in the diazepam dose-effect curve, but also decreased the slope and appeared to decrease the maximal effect. These results are consistent with the interpretation that CGS8216 antagonizes diazepam in a noncompetitive manner. It may do so because either it interacts with a subpopulation of benzodiazepine receptors, it functions as a pseudo-irreversible antagonist due to its high affinity, or because it is an antagonist with agonist properties. 相似文献
945.
946.
Up regulation of the transforming growth factor-beta 1 (TGF-β1) axis has been recognized as a pathogenic event for progression of glomerulosclerosis in diabetic nephropathy. We demonstrate that glomeruli isolated from diabetic rats accumulate up to sixfold more extracellular adenosine than normal rats. Both decreased nucleoside uptake activity by the equilibrative nucleoside transporter 1 and increased AMP hydrolysis contribute to raise extracellular adenosine. Ex vivo assays indicate that activation of the low affinity adenosine A2B receptor subtype (A2BAR) mediates TGF-β1 release from glomeruli of diabetic rats, a pathogenic event that could support progression of glomerulopathy when the bioavailability of adenosine is increased. 相似文献
947.
948.
E Shistik T Keren-Raifman G H Idelson Y Blumenstein N Dascal T Ivanina 《The Journal of biological chemistry》1999,274(44):31145-31149
The first 46 amino acids (aa) of the N terminus of the rabbit heart (RH) L-type cardiac Ca(2+) channel alpha(1C) subunit are crucial for the stimulating action of protein kinase C (PKC) and also hinder channel gating (Shistik, E., Ivanina, T., Blumenstein, Y., and Dascal, N. (1998) J. Biol. Chem. 273, 17901-17909). The mechanism of PKC action and the location of the PKC target site are not known. Moreover, uncertainties in the genomic sequence of the N-terminal region of alpha(1C) leave open the question of the presence of RH-type N terminus in L-type channels in mammalian tissues. Here, we demonstrate the presence of alpha(1C) protein containing an RH-type initial N-terminal segment in rat heart and brain by using a newly prepared polyclonal antibody. Using deletion mutants of alpha(1C) expressed in Xenopus oocytes, we further narrowed down the part of the N terminus crucial for both inhibitory gating and for PKC effect to the first 20 amino acid residues, and we identify the first 5 aa as an important determinant of PKC action and of N-terminal effect on gating. The absence of serines and threonines in the first 5 aa and the absence of phosphorylation by PKC of a glutathione S-transferase-fusion protein containing the initial segment suggest that the effect of PKC does not arise through a direct phosphorylation of this segment. We propose that PKC acts by attenuating the inhibitory action of the N terminus via phosphorylation of a remote site, in the channel or in an auxiliary protein, that interacts with the initial segment of the N terminus. 相似文献
949.
950.
E R Morey-Holton B P Halloran L P Garetto S B Doty 《Journal of applied physiology》2000,88(4):1303-1309
The rat has been used extensively as an animal model to study the effects of spaceflight on bone metabolism. The results of these studies have been inconsistent. On some missions, bone formation at the periosteal bone surface of weight-bearing bones is impaired and on others it is not, suggesting that experimental conditions may be an important determinant of bone responsiveness to spaceflight. To determine whether animal housing can affect the response of bone to spaceflight, we studied young growing (juvenile) rats group housed in the animal enclosure module and singly housed in the research animal holding facility under otherwise identical flight conditions (Spacelab Life Science 1). Spaceflight reduced periosteal bone formation by 30% (P < 0.001) and bone mass by 7% in single-housed animals but had little or no effect on formation (-6%) or mass (-3%) in group-housed animals. Group housing reduced the response of bone to spaceflight by as much as 80%. The data suggest that housing can dramatically affect the skeletal response of juvenile rats to spaceflight. These observations explain many of the discrepancies in previous flight studies and emphasize the need to study more closely the effects of housing (physical-social interaction) on the response of bone to the weightlessness of spaceflight. 相似文献