全文获取类型
收费全文 | 14689篇 |
免费 | 1434篇 |
国内免费 | 20篇 |
出版年
2023年 | 94篇 |
2022年 | 169篇 |
2021年 | 366篇 |
2020年 | 172篇 |
2019年 | 273篇 |
2018年 | 300篇 |
2017年 | 281篇 |
2016年 | 421篇 |
2015年 | 755篇 |
2014年 | 773篇 |
2013年 | 972篇 |
2012年 | 1163篇 |
2011年 | 1136篇 |
2010年 | 701篇 |
2009年 | 611篇 |
2008年 | 815篇 |
2007年 | 867篇 |
2006年 | 732篇 |
2005年 | 687篇 |
2004年 | 676篇 |
2003年 | 546篇 |
2002年 | 559篇 |
2001年 | 214篇 |
2000年 | 205篇 |
1999年 | 172篇 |
1998年 | 163篇 |
1997年 | 115篇 |
1996年 | 115篇 |
1995年 | 111篇 |
1994年 | 92篇 |
1993年 | 100篇 |
1992年 | 131篇 |
1991年 | 107篇 |
1990年 | 97篇 |
1989年 | 97篇 |
1988年 | 92篇 |
1987年 | 77篇 |
1986年 | 82篇 |
1985年 | 104篇 |
1984年 | 77篇 |
1983年 | 68篇 |
1982年 | 76篇 |
1981年 | 50篇 |
1980年 | 52篇 |
1979年 | 60篇 |
1977年 | 64篇 |
1976年 | 45篇 |
1975年 | 46篇 |
1974年 | 52篇 |
1973年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Alpha adrenergic agonists and antagonists as clonidine, guanfacine, yohimbine, phenylephrine and prazosin inhibited the [3H]-QNB binding to rat brain cortex muscarinic acetylcholine receptor (mAChR, M-1 subtype), heart (M-2 subtype) and parotid gland homogenate (M-3 subtype) in a dose-dependent competitive fashion. Ki values were between 10(-6) and 10(-3) M. Hill coefficients were about 1. No correlation was found between mAChR inhibiting capacity of these drugs and their activity on alpha adrenergic receptors. In contrast, other transmitters, as dopamine, GABA, glutamic acid, histamine, serotonin, isoproterenol and platelet activating factor (PAF) did not affect the QNB binding. 相似文献
132.
The study was conducted to identify the mechanisms of endotoxin/cortisol action in the suppression of preovulatory LH surges in heifers infused with Escherichia coli (E. coli ) endotoxin. The hypotheses tested were that 1) endotoxin stimulates the release of progesterone, possibly from the adrenal leading to the LH blockade; 2) cortisol released in response to endotoxin infusion blocks the synthesis of estradiol at the ovarian level, culminating in a failure of the LH surge. Eight Holstein heifers were given two injections of prostaglandin F(2alpha) (PG), 11 d apart, to synchronize estrus. Starting from 25 h after the second injection of PG (PG-2), the uterus of each heifer was infused either with 5 ml of pyrogen-free water (control, n = 3) or with E. coli endotoxin (5 mug/kg of body weight) in 5 ml of pyrogen-free water (treated, n = 5), once every 6 h for 10 treatments. Blood samples were obtained every 15 min for 1 h before infusion and again 2 h after each infusion, then hourly until 1 h before the next infusion. After the tenth infusion, blood was collected daily until estrus. Serum progesterone concentrations remained at baseline values (< 1 ng/ml) in control and treated heifers. The total amount of progesterone measured starting 24 to 84 h after PG-2 injection was not different between control and treated heifers (P 0.05). In the control heifers, serum estradiol concentrations remained basal (< 10 pg/ml) until 4 h before the LH surge. Serum estradiol concentrations increased to 20 +/- 5.6 pg/ml, 4 h before the LH surge in control heifers (LH surge occurred 60 to 66 h after the PG-2 injection). There were no changes in serum estradiol concentrations in treated heifers during the sampling period, and the concentrations remained < 10 pg/ml. The total amount of estradiol measured in control heifers was higher (P < 0.05) than in treated heifers. The results if this study suggest that increases in cortisol concentrations after the infusion of endotoxin might block the synthesis of estradiol at the ovarian level, resulting in the failure of a preovulatory LH surge to occur. 相似文献
133.
M N Simon T Winckler R Mutzel M Véron J C da Costa Maia 《Biochemical and biophysical research communications》1992,184(3):1142-1151
Extracts from Dictyostelium discoideum contain type 2A and 2C serine/threonine-specific protein phosphatases with properties very similar to those from mammals according to their sensitivity to okadaic acid and to their dependence for divalent cations. In contrast, no type 1 protein phosphatase is found at any time of development, neither in the cytosolic nor in the particulate fraction, using glycogen phosphorylase a, casein, histone or the non-proteinous 4-Methylumbelliferyl phosphate as substrates. Both type 2A and 2C protein phosphatase activities remain constant throughout the development cycle. 相似文献
134.
The major endogenous inhibitor of neutrophil elastase in the plasma, alpha 1-protease inhibitor (alpha 1-PI), has a single cysteine residue which has been shown to form mixed disulfides with a number of thiols in vitro. Under normal physiological conditions, the plasma concentrations of reduced and oxidized thiols are such that a major fraction of alpha 1-PI in the circulation in vivo is in the form of mixed disulfides [Laurell, C.-B. (1979) in The Chemistry and Physiology of Human Plasma Proteins (Bing, D. H., Ed.) pp 329-341, Pergamon, New York]. We show here that the mixed disulfide between glutathione or cysteine and alpha 1-PI (alpha 1-PI-SSG or alpha 1-PI-SScys) has an intrinsic fluorescence which distinguishes it from the reduced form of alpha 1-PI. By employing the fluorescence difference, we have measured the ratio of alpha 1-PI-SH to mixed disulfide alpha 1-PI in redox buffers of different ratios of reduced to oxidized glutathione (GSH to GSSG) or reduced to oxidized cysteine (cys to cysSScys) and have calculated an equilibrium constant and redox potential of 0.74 +/- 0.08 and 8 +/- 2 mV, respectively, for the alpha 1-PI-SH/alpha 1-PI-SSG couple and of 0.32 +/- 0.02 and 29 +/- 2 mV, respectively, for the alpha 1-PI-SH/alpha 1-PI-SScys couple. We are unable to detect any change in Trp fluorescence in the complex of alpha 1-PI and elastase when the preformed complex is added to the same GSH/GSSG or cys/cysSScys redox buffers.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
135.
136.
Perturbation of nifT expression in Klebsiella pneumoniae has limited effect on nitrogen fixation. 总被引:1,自引:1,他引:0 下载免费PDF全文
In the nitrogenase system of Klebsiella pneumoniae, nifT is located between nifDK, the structural genes for dinitrogenase, and nifY, whose product is involved in nitrogenase maturation. It is, therefore, a reasonable hypothesis that the NifT protein might also have a role in the maturation of nitrogenase. However, the phenotypic characterization of nifT and nifT-overexpressing strains for effects on the regulation, maturation, and activity of nitrogenase identified no properties that were distinct from those of the wild type. We conclude that the K. pneumoniae NifT protein is not essential for nitrogen fixation under the conditions examined. 相似文献
137.
Elizabeth S. Woo Yukihiro Kondo Simon C. Watkins Dale G. Hoyt John S. Lazo 《Experimental cell research》1996,224(2):365
Metallothionein (MT), a major zinc-binding intracellular protein thiol, has been associated with cytoprotection from heavy metals, antineoplastic drugs, mutagens, and cellular oxidants. Despite its small mass (7 kDa), nuclear partitioning of MT has been observed in both normal and malignant tissues. The factors controlling MT sequestration are unknown. Thus, we examined the regulation of MT subcellular distribution in human cancer cell lines that exhibit prominent nuclear MT. The nuclear disposition of MT was unaltered during cell cycle passage in synchronized cells. MT redistributed to the cytoplasm when cells were exposed to reduced temperature. Cytoplasmic redistribution was also seen in DU-145 and HPC36M prostatic cancer cells after ATP depletion, but not in PC3-MA2 and SCC25/CP cells. Pretreatment with 10 μMCdCl2did not significantly alter MT distribution but did render all cells sensitive to cytoplasmic redistribution after either reduced temperature or ATP depletion. Thus, nuclear retention of MT is energy requiring and this ability of MT to accumulate in subcellular compartments against its concentration gradient may be important in the capacity of MT to supply Zn or other metals to target sites within the cell. 相似文献
138.
Harry Adams Neil A. Bailey Philippe Bertrand Simon R. Collinson David E. Fenton Simon J. Kitchen 《Inorganica chimica acta》1996,250(1-2):139-148
The cyclocondensation of 2,5-diformylthiophene and the amines N,N-bis-(2-aminoethyl)-2-phenylethylamine, N,N-bis-(2aminoethyl)-t-butyl-amine and N,N-bis-(2-aminoethyl)-t-butyl-amine in the presence of silver(I) salts yields homodinuclear bibracchial tetraimine Schiff base macrocyclic complexes. The structures of two such complexes are also reported. The complex Ag2L4(NO3)(PF6) (2) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 12.834(6), B = 13.183(6), C = 14.588(7) Å, = 64.86(4), β = 79.77(4), γ = 69.44(3)° with Z = 2; there is a monodentate and singly bridging nitrate anion present and the Ag---Ag separation is 4.161 Å. The complex Ag2L4(CH3CN)2(BF4)2·CH3CN (9) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 9.297(4), B = 12.985(3), C = 21.770(5) Å, = 91.570(10), β = 92.33(3), γ = 97.92(3) ° with Z = 2; there is a strongly bonded acetonitrile molecule coordinated to each silver atom and the Ag---Ag separation is 4.920 Å. 相似文献
139.
The arsenical resistance operon of IncN plasmid R46 总被引:3,自引:0,他引:3
Debby F. Bruhn Jiaxin Li Simon Silver Francisco Roberta Barry P. Rosen 《FEMS microbiology letters》1996,139(2-3):149-153
140.
James K. Friel Claude Mercer Wayne L. Andrews Brian R. Simmons Simon E. Jackson 《Biological trace element research》1996,54(2):135-142
Contamination in a trace element laboratory can come from a variety of sources, including laboratory gloves. Therefore, vinyl
and latex gloves were obtained from as many manufacturers as would supply gloves. These gloves were either prepared for acid-washing
and subsequent soaking in an acid solution, or immersed in an acid solution for a duration of either 1 min or 1 h. Incubation
washes were analyzed for a variety of trace elements by flame atomic abosrption spectroscopy (AAS) or inductively coupled
mass spectrometry (ICP-MS). Results indicated that only three brands of vinyl gloves were acceptable for use in a trace element
laboratory, whereas others had contamination of different elements. Latex gloves contained such high levels of biologically
important elements that they were not considered suitable for routine trace element work. Vinyl gloves of choice should be
routinely acid-washed before use in a trace element laboratory. 相似文献