全文获取类型
收费全文 | 5775篇 |
免费 | 524篇 |
国内免费 | 1篇 |
出版年
2023年 | 54篇 |
2022年 | 87篇 |
2021年 | 215篇 |
2020年 | 90篇 |
2019年 | 128篇 |
2018年 | 147篇 |
2017年 | 133篇 |
2016年 | 214篇 |
2015年 | 390篇 |
2014年 | 414篇 |
2013年 | 442篇 |
2012年 | 564篇 |
2011年 | 515篇 |
2010年 | 312篇 |
2009年 | 223篇 |
2008年 | 314篇 |
2007年 | 302篇 |
2006年 | 245篇 |
2005年 | 224篇 |
2004年 | 174篇 |
2003年 | 195篇 |
2002年 | 175篇 |
2001年 | 64篇 |
2000年 | 67篇 |
1999年 | 57篇 |
1998年 | 41篇 |
1997年 | 29篇 |
1996年 | 20篇 |
1995年 | 26篇 |
1994年 | 14篇 |
1993年 | 19篇 |
1992年 | 33篇 |
1991年 | 41篇 |
1990年 | 44篇 |
1989年 | 29篇 |
1988年 | 31篇 |
1987年 | 23篇 |
1986年 | 13篇 |
1985年 | 20篇 |
1984年 | 18篇 |
1983年 | 13篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1979年 | 13篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1973年 | 11篇 |
1972年 | 9篇 |
1971年 | 7篇 |
1969年 | 7篇 |
排序方式: 共有6300条查询结果,搜索用时 15 毫秒
61.
62.
Sven-Åke Persson 《Life sciences》1978,23(5):523-526
We have earlier shown that d-lysergic acid diethylamide, LSD and its 2-bromo derivative, BOL like the dopamine (DA) antagonists haloperidol increased the rate of the tyrosine hydroxylation in the striatum measured as the accumulation of DOPA after decarboxylase inhibition.Now we have found that several agents structurally similar to LSD increase the tyrosine hydroxylation in the striatum. Psilocybin (50 mg/kg i.p.) and N,N-dimethyltryptamine (50 mg/kg i.p.) caused a short-lasting increase of DOPA accumulation, while mescaline (10 – 100 mg/kg i.p.) did not increase the DOPA accumulation. A marked increase of DOPA accumulation was observed after the 5-hydroxytryptamine (5-HT) antagonist cyproheptadine. The effects of LSD and structurally related drugs on the DOPA accumulation in the striatum appear to be mediated via DA antagonism at receptor level. However, these agents may control the DOPA accumulation via other receptors than DA receptors e.g. 5-HT receptors. A control of DOPA accumulation via receptors other than DA receptors appears to be predominant after treatment with N,N-dimethyltryptamine or psilocybin. 相似文献
63.
A simple and selective assay for monomeric and filamentous actin is presented, based on the inhibition of DNAase I by actin. In mixtures of monomeric and filamentous actin, only the monomeric form is measured as DNAase inhibitor. The total amount of actin in a sample can be determined after depolymerization of F actin with guanidine hydrochloride. The assay is rapid enough to detect changes in the polymerization state of actin in vitro over time intervals as short as 3 min. Data characterizing unpolymerized and filamentous actin pools in extracts of human platelets, lymphocytes and HeLa cells are presented. 相似文献
64.
65.
Metal affinity precipitation of proteins carrying genetically attached polyhistidine affinity tails 总被引:1,自引:0,他引:1
In this study, galactose dehydrogenase (EC 1.1.1.48) was chosen as a prototype target protein to investigate the capability of metal affinity precipitation to facilitate the purification of genetically engineered proteins. A DNA fragment encoding five histidine residues was fused to the 3'-terminal end of the galactose dehydrogenase gene from Pseudomonas fluorescens and thereafter expressed in Escherichia coli. The additional five histidines functioned as an affinity tail and the modified enzyme could be purified using metal affinity precipitation when the metal-chelate complex with ethylene glycol-bis-(beta-aminoethyl ether) N,N,N',N'-tetra-acetic acid, EGTA(Zn)2, was added to the protein solution. The affinity tail could also be applied for the purification of the fusion protein utilising immobilised metal affinity chromatography. After purification, the pentahistidine affinity tail could be removed enzymatically by carboxypeptidase A. Furthermore, growth rate experiments demonstrated that the expression of the metal-binding affinity tail in E. coli cells enhanced the tolerance to zinc ions when added to the growth medium. 相似文献
66.
The effect of infusion of nerve growth factor (NGF) into the rat testis on the expression of androgen-binding protein (ABP) mRNA was studied. A major 1.7-kb and a minor 3.7-kb ABP mRNA were present at all stages of the seminiferous epithelium with maximal levels at stages VIII-XI and the lowest levels at stages IV-VI. Infusion of 15 ng/h of NGF with a mini-osmotic pump for 14 days resulted in a 2-fold increase of ABP mRNA as revealed by Northern blots, whereas the mRNA level of another Sertoli cell protein, urokinase-type plasminogen activator, remained unchanged. Image analysis of autoradiograms obtained by in situ hybridization of sections from treated testes showed a similar increase in APB mRNA compared to noninfused or PBS-infused testes. However, at the cellular level the labeling intensity for ABP mRNA over Sertoli cells of different stages of the seminiferous epithelium was the same in NGF-infused and control testes. This suggests that the increase of ABP mRNA in NGF-infused testes was caused by prolongation of stages VII-VIII with maximal ABP mRNA expression; the suggestion is supported by an increase of 30 percent in frequency of these stages in histological sections from NGF-infused testes. 相似文献
67.
Disruption of the low affinity receptor-binding site in NGF allows neuronal survival and differentiation by binding to the trk gene product. 总被引:17,自引:0,他引:17
Nerve growth factor (NGF), like many other growth factors and hormones, binds to two different receptor molecules on responsive cells. The product of the proto-oncogene trk, p140trk, is a tyrosine kinase receptor that has been identified as a signal-transducing receptor for NGF, while the role of the low affinity NGF receptor, p75NGFR, in signal transduction is less clear. The crystal structure of NGF has recently been determined, although structures involved in receptor binding and biological activity are unknown. Here we show that Lys-32, Lys-34, and Lys-95 form a positively charged interface involved in binding to p75NGFR. Simultaneous modification of Lys-32 with either of the two other lysines resulted in loss of binding to p75NGFR. Despite the lack of binding to p75NGFR, these mutants retained binding to p140trk and biological activity, demonstrating a functional dissociation between the two NGF receptors. 相似文献
68.
Endogenous nitric oxide is present in the exhaled air of rabbits, guinea pigs and humans. 总被引:33,自引:0,他引:33
L E Gustafsson A M Leone M G Persson N P Wiklund S Moncada 《Biochemical and biophysical research communications》1991,181(2):852-857
The presence of nitric oxide (NO) in the exhaled air of humans and of anaesthetized rabbits and guinea pigs was demonstrated by chemiluminescence, diazotization and mass spectrometry. This NO is endogenously produced in the lung by an NO synthase, since its generation in guinea pigs and rabbits was inhibited by N omega-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine, inhibitors of this enzyme. The effect of the inhibitors was reversed by the precursor of NO synthesis, L-arginine. Since NO is produced by normal vascular endothelium for the physiological regulation of blood flow and pressure and also by activated macrophages to contribute to non-specific immunity, our experiments suggest that NO may play both vascular regulatory and host defence roles in pulmonary physiology and pathophysiology. 相似文献
69.
The rate-controlling enzyme in polyamine synthesis, ornithine decarboxylase (ODC), is subject to feedback regulation by the polyamines at the level of translation. In the present study we used a cell-free translation system to further investigate the mechanism by which this regulation occurs. Lysates of ODC-overproducing cells were capable of synthesizing large amounts of ODC. The degree of initiation was poor in the lysates and the synthesis of ODC was mainly a result of continued elongation of peptide chains on pre-initiated ribosomes. By determining the amount of ODC produced in the lysate, we obtained an estimate of the number of ribosomes that were actively translating ODC mRNA at the moment of lysis. Using this polysomal run-off assay we demonstrated that the polyamine-mediated regulation of ODC synthesis occurs without any change in the number of ribosomes associated with the message. This finding indicates that the polyamines exert a coordinate effect on initiation and elongation. 相似文献
70.