全文获取类型
收费全文 | 4859篇 |
免费 | 489篇 |
国内免费 | 2篇 |
出版年
2021年 | 57篇 |
2020年 | 34篇 |
2019年 | 50篇 |
2018年 | 65篇 |
2017年 | 73篇 |
2016年 | 88篇 |
2015年 | 170篇 |
2014年 | 159篇 |
2013年 | 202篇 |
2012年 | 270篇 |
2011年 | 285篇 |
2010年 | 168篇 |
2009年 | 139篇 |
2008年 | 189篇 |
2007年 | 214篇 |
2006年 | 202篇 |
2005年 | 205篇 |
2004年 | 190篇 |
2003年 | 211篇 |
2002年 | 172篇 |
2001年 | 178篇 |
2000年 | 153篇 |
1999年 | 144篇 |
1998年 | 62篇 |
1997年 | 78篇 |
1996年 | 62篇 |
1995年 | 64篇 |
1994年 | 59篇 |
1993年 | 71篇 |
1992年 | 99篇 |
1991年 | 102篇 |
1990年 | 63篇 |
1989年 | 86篇 |
1988年 | 80篇 |
1987年 | 69篇 |
1986年 | 51篇 |
1985年 | 63篇 |
1984年 | 46篇 |
1983年 | 53篇 |
1982年 | 55篇 |
1981年 | 48篇 |
1980年 | 31篇 |
1979年 | 42篇 |
1978年 | 46篇 |
1977年 | 32篇 |
1975年 | 28篇 |
1974年 | 42篇 |
1973年 | 33篇 |
1972年 | 30篇 |
1971年 | 31篇 |
排序方式: 共有5350条查询结果,搜索用时 15 毫秒
991.
992.
993.
S. R. Schmolke K. Broeg S. Zander V. Bissinger P. D. Hansen N. Kress B. Herut E. Jantzen G. Krüner A. Sturm W. Körting H. von Westernhagen 《Helgoland Marine Research》1999,53(3-4):257-266
A comprehensive database, containing biological and chemical information, collected in the framework of the bilateral interdisciplinary
MARS project (”biological indicators of natural and man-made changes in marine and coastal waters”) during the years 1995–1997
in the coastal environment of the North Sea, was subjected to a multivariate statistical evaluation. The MARS project was
designated to combine a variety of approaches and to develop a set of methods for the employment of biological indicators
in pollution monitoring and environmental quality assessment. In total, nine ship cruises to four coastal sampling sites were
conducted; 765 fish and 384 mussel samples were analysed for biological and chemical parameters. Additional information on
the chemical background at the sampling sites was derived from sediment samples, collected at each of the four sampling sites.
Based on the available chemical data in sediments and black mussel (Mytilus edulis) a pollution gradient between the selected sites, was established. The chemical body burden of flounder (Platichthys flesus) from these sites, though, did not reflect this gradient equally clear. In contrast, the biological information derived from
measurements in fish samples displayed significant a regional as well as a temporal pattern. A multivariate bioindicator data
matrix was evaluated employing a factor analysis model to identify relations between selected biological indicators, and to
improve the understanding of a regional and temporal component in the parameter response. In a second approach, applying the
k-means algorithm on the data matrix, two significantly different clusters of samples, characterised by the current health
status of the fish, were extracted. Using this classification a temporal, and in the second order, a less pronounced spatial
effect was evident. In particular, during July 1996, a clear sign of deteriorating environmental conditions was extracted
from the biological data matrix.
Received: 20 June 1999 / Received in revised form: 8 November 1999 / Accepted: 8 November 1999 相似文献
994.
Vladimir Bresler Vera Bissinger Avigdor Abelson Halim Dizer Armin Sturm Renate Kratke Lev Fishelson Peter-Diedrich Hansen 《Helgoland Marine Research》1999,53(3-4):219-243
The intensive development of industry and urban structures along the seashores of the world, as well as the immense increase
in marine transportation and other activities, has resulted in the deposition of thousands of new chemicals and organic compounds,
endangering the existence of organisms and ecosystems. The conventional single biomarker methods used in ecological assessment
studies cannot provide an adequate base for environmental health assessment, management and sustainability planning. The present
study uses a set of novel biochemical, physiological, cytogenetic and morphological methods to characterize the state of health
of selected molluscs and fish along the shores of the German North Sea, as well as the Israeli Mediterranean and Red Sea.
The methods include measurement of activity of multixenobiotic resistance-mediated transporter (MXRtr) and the system of active
transport of organic anions (SATOA) as indicators of antixenobiotic defence; glutathione S-transferase (GST) activity as an
indicator of biotransformation of xenobiotics; DNA unwinding as a marker of genotoxicity; micronucleus test for clastogenicity;
levels of phagocytosis for immunotoxicity; cholinesterase (ChE) activity and level of catecholamines as indicators of neurotoxicity;
permeability of external epithelia to anionic hydrophilic probe, intralysosomal accumulation of cationic amphiphilic probe
and activity of non-specific esterases as indicators of cell/tissue viability. Complete histopathological examination was
used for diagnostics of environmental pathology. The obtained data show that the activity of the defensive pumps, MXRtr and
SATOA in the studied organisms was significantly higher in the surface epithelia of molluscs from a polluted site than that
of the same species from control, unpolluted stations, providing clear evidence of response to stress. Enhanced frequency
of DNA lesions (alkaline and acidic DNA unwinding) and micronucleus-containing cells was significantly higher in samples from
polluted sites in comparison to those from the clean sites that exhibited genotoxic and clastogenic activity of the pollutants.
In all the studied molluscs a negative correlation was found between the MXRtr levels of activity and the frequency of micronucleus-containing
hemocytes. The expression of this was in accordance with the level of pollution. The complete histopathological examination
demonstrates significantly higher frequencies of pathological alterations in organs of animals from polluted sites. A strong
negative correlation was found between the frequency of these alterations and MXRtr activity in the same specimens. In addition
to these parameters, a decrease in the viability was noted in molluscs from the polluted sites, but ChE activities remained
similar at most sites. The methods applied in our study unmasked numerous early cryptic responses and negative alterations
of health in populations of marine biota sampled from the polluted sites. This demonstrates that genotoxic, clastogenic and
pathogenic xenobiotics are present and act in the studied sites and this knowledge can provide a reliable base for consideration
for sustainable development.
Received: 2 March 1999 / Received in revised form: 2 August 1999 / Accepted: 3 August 1999 相似文献
995.
996.
997.
Blood pressure and plasma catecholamines in acute and prolonged hypoxia: effects of local hypothermia. 总被引:4,自引:0,他引:4
I L Kanstrup T D Poulsen J M Hansen L J Andersen M H Bestle N J Christensen N V Olsen 《Journal of applied physiology》1999,87(6):2053-2058
This study measured the pressor and plasma catecholamine response to local hypothermia during adaptation to hypobaric hypoxia. Eight healthy men were studied at rest and after 10 and 45 min of local cooling of one hand and forearm as well as after 30 min of rewarming at sea level and again 24 h and 5 days after rapid, passive transport to high altitude (4,559 m). Acute mountain sickness scores ranged from 5 to 16 (maximal attainable score: 20) on the first day but were reduced to 0-8 by the fifth day. Systolic blood pressure, heart rate, and plasma epinephrine increased on day 1 at altitude compared with sea level but declined again on day 5, whereas diastolic and mean blood pressures continued to rise in parallel with plasma norepinephrine. With local cooling, an increased vasoactive response was seen on the fifth day at altitude. Very high pressures were obtained, and the pressure elevation was prolonged. Heart rate increased twice as much on day 5 compared with the other two occasions. Thoracic fluid index increased with cooling on day 5, suggesting an increase in pulmonary vascular resistance. In conclusion, prolonged hypoxia seems to elicit an augmented pressor response to local cooling in the systemic and most likely also the pulmonary circulation. 相似文献
998.
B Losman M Biller S Olofsson K Sch?nning O S Lund B Svennerholm J E Hansen A Bolmstedt 《FEBS letters》1999,454(1-2):47-52
We have previously shown that an N-glycosylation site of N306 of HIV-1 gp120 is not necessary for the HIV-1 infectivity but protects HIV-1 from neutralising antibodies. In contrast Nakayama et al. [FEBS Lett. (1998) 426, 367-372], using a virus with an identical V3 region, suggested that elimination of this particular glycan reduced the ability of T-tropic HIV to bind to CXCR4 and hence its ability to infect T cell lines. We therefore re-examined the ability of a mutant virus, lacking the N306 glycan, to replicate in various types of cells and found no change in co-receptor usage for mutant virus. The ability of mutant virus to replicate or to induce syncytia in infected cells was similar to that of wild type virus. These results corroborate our original observation, confirming that the induced mutation in the N306 glycosylation site neither impairs nor improves the ability of mutant virus to replicate in permissive cells. 相似文献
999.
The N-acylphosphatidylethanolamine-hydrolysing phospholipase D (NAPE-PLD) generates N-acylethanolamines, including N-arachidonoyl-ethanolamine (anandamide), that may be neuroprotective and analgesic. The properties of NAPE-PLD from rat heart and brain microsomes are investigated and compared to those of other PLDs. NAPE-PLD is inhibited by the fatty acid aminohydrolase inhibitor MAFP in high concentrations (> or = 100 microM) while PMSF in high concentrations (10 mM) tends to stabilise NAPE-PLD activity. Oleate inhibits NAPE-PLD but the enzyme is not affected by PIP2, alpha-synuclein or mastoparan. Furthermore, it is for the first time reported that NAPE-PLD is not capable of catalysing a transphosphatidylation reaction like most other known PLDs. 相似文献
1000.
L L Hansen Y Ikeda G S Olsen A K Busch L Mosthaf 《The Journal of biological chemistry》1999,274(35):25078-25084
Both hyperglycemia and tumor necrosis factor alpha (TNFalpha) were found to induce insulin resistance at the level of the insulin receptor (IR). How this effect is mediated is, however, not understood. We investigated whether oxidative stress and production of hydrogen peroxide could be a common mediator of the inhibitory effect. We report here that micromolar concentrations of H(2)O(2) dramatically inhibit insulin-induced IR tyrosine phosphorylation (pretreatment with 500 microM H(2)O(2) for 5 min inhibits insulin-induced IR tyrosine phosphorylation to 8%), insulin receptor substrate 1 phosphorylation, as well as insulin downstream signaling such as activation of phosphatidylinositol 3-kinase (inhibited to 57%), glucose transport (inhibited to 36%), and mitogen-activated protein kinase activation (inhibited to 7.2%). Both sodium orthovanadate, a selective inhibitor of tyrosine-specific phosphatases, as well as the protein kinase C inhibitor G?6976 reduced the inhibitory effect of hydrogen peroxide on IR tyrosine phosphorylation. To investigate whether H(2)O(2) is involved in hyperglycemia- and/or TNFalpha-induced insulin resistance, we preincubated the cells with the H(2)O(2) scavenger catalase prior to incubation with 25 mM glucose, 25 mM 2-deoxyglucose, 5.7 nM TNFalpha, or 500 microM H(2)O(2), respectively, and subsequent insulin stimulation. Whereas catalase treatment completely abolished the inhibitory effect of H(2)O(2) and TNFalpha on insulin receptor autophosphorylation, it did not reverse the inhibitory effect of hyperglycemia. In conclusion, these results demonstrate that hydrogen peroxide at low concentrations is a potent inhibitor of insulin signaling and may be involved in the development of insulin resistance in response to TNFalpha. 相似文献