全文获取类型
收费全文 | 11030篇 |
免费 | 861篇 |
国内免费 | 3篇 |
出版年
2023年 | 51篇 |
2022年 | 76篇 |
2021年 | 274篇 |
2020年 | 155篇 |
2019年 | 213篇 |
2018年 | 258篇 |
2017年 | 236篇 |
2016年 | 341篇 |
2015年 | 579篇 |
2014年 | 634篇 |
2013年 | 805篇 |
2012年 | 1011篇 |
2011年 | 919篇 |
2010年 | 610篇 |
2009年 | 525篇 |
2008年 | 696篇 |
2007年 | 755篇 |
2006年 | 685篇 |
2005年 | 574篇 |
2004年 | 559篇 |
2003年 | 465篇 |
2002年 | 452篇 |
2001年 | 90篇 |
2000年 | 71篇 |
1999年 | 85篇 |
1998年 | 94篇 |
1997年 | 81篇 |
1996年 | 61篇 |
1995年 | 61篇 |
1994年 | 33篇 |
1993年 | 59篇 |
1992年 | 45篇 |
1991年 | 40篇 |
1990年 | 26篇 |
1989年 | 17篇 |
1988年 | 18篇 |
1987年 | 16篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 24篇 |
1983年 | 18篇 |
1982年 | 19篇 |
1981年 | 11篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1977年 | 11篇 |
1976年 | 11篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1966年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The ltk receptor tyrosine kinase is expressed in pre-B lymphocytes and cerebral neurons and uses a non-AUG translational initiator. 总被引:19,自引:6,他引:13 下载免费PDF全文
The recently discovered murine ltk tyrosine kinase has been reported to resemble a transmembrane receptor devoid of an extracellular domain. We report here, however, that in in vitro experiments and in transfected cells, ltk uses an upstream non-AUG translational initiator and resembles a typical glycoprotein receptor, albeit a very small one, with a molecular mass of approximately 69 kd. Ltk is expressed at a very low level in only a few cell lines and tissues and may be the receptor for a pre-B lymphocyte growth or differentiation factor, because 10 tested pre-B lines each contained 2.4 and 2.8 kb mRNAs. By contrast, only one out of seven mature B cell lines expressed ltk and the in vitro maturation of pre-B into B cells was in one case accompanied by the inactivation of ltk expression. Ltk was also expressed in adult, but not in embryonic, mouse brain, and immunostaining detected the protein in essentially all neurons of the cerebral cortex and hippocampus, but not in the cerebellum. 相似文献
42.
Inhibitory action of cyclobutyrol on the secretion of biliary cholesterol and phospholipids. 下载免费PDF全文
A number of organic anions are known to decrease biliary secretion of cholesterol and phospholipid without affecting bile acid secretion. Cyclobutyrol (CB) is a choleretic agent which also inhibits biliary lipid secretion. Using isolated perfused rat liver we have studied this inhibition in relation to possible mechanisms suggested for other anions. Shortly after its administration to the isolated perfused liver, CB decreases biliary outputs of cholesterol and phospholipid, without changes in bile acid secretion, at low (450 nmol/min), high (1350 nmol/min) and nil taurocholate infusion rates. The absolute inhibition does not appear to be decreased by elevated bile acid secretion. There is a differential effect on secretion of cholesterol and phospholipid, more marked at low bile acid secretion rates. Biliary outputs of the canalicular membrane enzymes 5'-nucleotidase and alkaline phosphodiesterase I are also depressed by CB administration, but the anion does not affect the biliary output of bovine serum albumin or the output of rat serum albumin into the perfusion fluid. Since CB does not inhibit intracellular vesicular transport or apparently inhibit intracanalicular events, its effect is different from the effect of several other anions. From these studies it appears that the most likely effect of CB is exerted at the level of the canalicular membrane. 相似文献
43.
Fabrizio Villani Milena Galimberti Elena Monti Francesco Piccinini Enrica Lanza Annalinda Rozza Luigia Favalli Paola Poggi Franco Zunino 《Free radical research》1990,11(1):145-151
The effects of two sulfhydryl compounds, glutathione (GSH) and N-acetylcysteine (NAC), on the cardiotoxicity of doxorubicin (DXR) were tested on in vitro and in vivo models. DXR was administered to rats as 4 weekly i.v. doses of 3mg/kg. GSH (1.5 mmoles/kg), given i.v. 10 min before and 1 hr after DXR, was found to prevent the development of the delayed cardiotoxic effects of DXR, as assessed by electrocardiographic and mechanical parameters, as well as by histological examination of left ventricular preparations. In contrast, equimolar oral doses of NAC (1 hr before and 2hrs after DXR) were found to be ineffective. Both GSH and NAC prevented the negative inotropic effect produced by DXR on isolated rat atria. A good correlation exists between the cardioprotective effects of the two agents and their ability to enhance the non-protein sulfhydryl group content of the myocardium. Differences observed in vivo between GSH and NAC might be accounted for by pharmacokinetic factors. 相似文献
44.
I. S. Novella J. J. Holland S. F. Elena A. Moya E. Domingo 《Molecular & general genetics : MGG》1996,252(6):733-738
The population dynamics of RNA viruses have an important influence on fitness variation and, in consequence, on the adaptative potential and virulence of this ubiquitous group of pathogens. Earlier work with vesicular stomatitis virus showed that large population transfers were reproducibly associated with fitness increases, whereas repeated transfers from plaque to plaque (genetic bottlenecks) lead to losses in fitness. We demonstrate here that repeated five-plaque to five-plaque passage series yield long-term fitness stability, except for occasional stochastic fitness jumps. Repeated five-plaque passages regularly alternating with two consecutive large population transmissions did not cause fitness losses, but did limit the size of fitness gains that would otherwise have occurred. These results underscore the profound effects of bottleneck transmissions in virus evolution. 相似文献
45.
46.
Cristina Elena Canteros Laura Rodero Maria Cristina Rivas Graciela Davel 《Mycopathologia》1996,136(1):21-23
A rapid method to evidence urease activity is described. Urea hydrolysis and consequent production ammonia are detected by a chemical reaction producing a blue phenol compound (indophenol blue). Three hundred and three yeast were tested. Out of 107 urease-positive organisms detected by Christensen's Urea Agar Test (CUAT) 102 were positive by our method. No false negatives were observed by this method when testing 87 Cryptococcus strains. Ths practical screening test for presumptive identification of Cryptococcus neoformans is simple, unaffected by pH changes and requires 15 minutes to be performed. 相似文献
47.
Relationship between the levels of wheat-rye metaphase I chromosomal pairing and recombination revealed by GISH 总被引:3,自引:0,他引:3
The metaphase I and anaphase I stages of meiosis of wheat×rye hybrids carrying the ph1b mutation were analyzed by genomic in situ hybridization. This technique allows distinction between three different types of
wheat-rye associations in metaphase I configurations as well as detection of wheat-rye recombinant chromosomes in anaphase
I cells. The frequency of associations between wheat and rye chromosomes greatly exceeded the level of wheat-rye recombination
found in the three hybrids examined. Extremely distal associations, which account for about 50% of the total wheat-rye metaphase
I chromosomal pairing, can explain such a discrepancy between metaphase I and anaphase I data. It is further discussed whether
these associations reflect very distally located chiasmata or nonchiasmatic pairing. The sizes of the segments exchanged in
wheat-rye recombinant chromosomes provide cytological evidence that wheat-rye recombination is restricted to the distal chromosomal
regions.
Received: 24 August 1995; in revised form: 27 February 1996 / Accepted: 28 March 1996 相似文献
48.
Elena Kosenko Yuri Kaminsky Eugenio Grau María-Dolores Miñana Goizane Marcaida Santiago Grisolía Vicente Felipo 《Journal of neurochemistry》1994,63(6):2172-2178
Abstract: Injection of large doses of ammonia into rats leads to depletion of brain ATP. However, the molecular mechanism leading to ATP depletion is not clear. The aim of the present work was to assess whether ammonium-induced depletion of ATP is mediated by activation of the NMDA receptor. It is shown that injection of MK-801, an antagonist of the NMDA receptor, prevented ammonia-induced ATP depletion but did not prevent changes in glutamine, glutamate, glycogen, glucose, and ketone bodies. Ammonia injection increased Na+ ,K+ -ATPase activity by 76%. This increase was also prevented by previous injection of MK-801. The molecular mechanism leading to activation of the ATPase was further studied. Na+ ,K+ -ATPase activity in samples from ammonia-injected rats was normalized by "in vitro" incubation with phorbol 12-myristate 13-acetate, an activator of protein kinase C. The results obtained suggest that ammonia-induced ATP depletion is mediated by activation of the NMDA receptor, which results in decreased protein kinase C-mediated phosphorylation of Na+ ,K+ -ATPase and, therefore, increased activity of the ATPase and increased consumption of ATP. 相似文献
49.
Induction of Nitric Oxide Synthase in Rat C6 Glioma Cells 总被引:9,自引:1,他引:8
Douglas L. Feinstein Elena Galea Steven Roberts Henrik Berquist Hong Wang Donald J. Reis 《Journal of neurochemistry》1994,62(1):315-321
Abstract: We have examined the induction of nitric oxide syhthase (NOS) activity in the rat astrocyte-derived C6 glioma cell line. In contrast to the previous results with primary astrocyte cultures, incubation of C6 cells with bacterial endotoxin lipopolysaccharide (LPS; 1 μg/ml for 24 h) did not stimulate NO2 production. However, addition of either tumor necrosis factor-a (TNF-α) or interferon-γ (IFN-γ), cytokines that by themselves had no effect on NOS activity, imparted LPS responsiveness onto these cells in a dose-dependent manner (EC50 values of 39 ng/ml of TNF-α and 9.4 U/ml of IFN-γ), and the effect of TNF-α could be further potentiated (twofold) by the presence of interleukin-1β. The simultaneous presence of TNF-α and IFN-γ yielded a greater response than either cytokine alone; however, the respective EC50 values were not affected. A cytoplasmic extract from induced C6 cells catalyzed the Ca2+ -independent conversion of l -arginine to l - citrulline, with an apparent K m of 51.2 n M , and this activity could be blocked by l -arginine analogues in the potency order amino > methyl > nitroarginine. Immunoblot analysis revealed an apparent molecular mass of 125 kDa for the NOS protein induced in C6 cells. These results indicate that the combination of LPS plus cytokines can induce NOS activity in C6 glioma cells with properties similar to those of the enzyme expressed in primary astrocyte cultures. 相似文献
50.
Elena Albertazzi Rosanna Cardillo Stefano Servi Gioia Zucchi 《Biotechnology letters》1994,16(5):491-496
Summary 2-Phenylethanol and 2-phenylethylacetate important aroma components are obtained in substantial amounts whenHansenula anomala CBS 110 andKloeckera saturnus CBS 5761 are grown with L-phenylalanine as sole nitrogen source. Up to 2 g/L of mixtures of alcohol and ester are obtained. While in the first microorganism the alcohol predominates, in the second the acetate is formed almost exclusively. Experiments with 1-13C-d-Glucose show complete incorporation of the label into the methyl group of the acetate. 相似文献