首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   620962篇
  免费   65914篇
  国内免费   402篇
  2018年   5848篇
  2016年   7886篇
  2015年   9852篇
  2014年   11764篇
  2013年   16535篇
  2012年   18549篇
  2011年   19449篇
  2010年   13339篇
  2009年   12221篇
  2008年   17454篇
  2007年   18115篇
  2006年   17052篇
  2005年   16310篇
  2004年   16251篇
  2003年   15530篇
  2002年   15235篇
  2001年   26705篇
  2000年   26700篇
  1999年   21064篇
  1998年   7229篇
  1997年   7504篇
  1996年   7134篇
  1995年   6684篇
  1994年   6477篇
  1993年   6431篇
  1992年   17368篇
  1991年   17152篇
  1990年   16930篇
  1989年   16504篇
  1988年   15598篇
  1987年   14945篇
  1986年   13690篇
  1985年   13834篇
  1984年   11415篇
  1983年   9915篇
  1982年   7431篇
  1981年   6726篇
  1980年   6402篇
  1979年   10887篇
  1978年   8475篇
  1977年   7836篇
  1976年   7586篇
  1975年   8356篇
  1974年   9183篇
  1973年   9108篇
  1972年   8326篇
  1971年   7647篇
  1970年   6765篇
  1969年   6497篇
  1968年   6299篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
The results of the use of cefepime (Maxipime) combination with amikacin vs ceftriaxon combination with amikacin in the treatment of 80 patients with different forms of hemoblastosis are presented. Severe infectious complications in the patients were associated with prolonged and deep neutropenia during inductive or antirelapsing chemotherapy. All the patients in the trial were from the group of high risk of infectious complications with the blood neutrophil count under 100 cells/microliter. The duration of neutropenia averaged 12 days (7 to 15). The average period of the treatment with cefepime and amikacin equaled to 13 days (8 to 16). The treatment with cefepime + amikacin was successful in 38 out of 40 patients (95%). The average period of the treatment with ceftriaxon and amikacin equaled to 14 days (7 to 18). The efficacy of the treatment with ceftriaxon + amikacin was 60% (24 patients out of 40).  相似文献   
952.
The generation and characterization of new sheep-hamster cell hybrids is reported from the fusion of sheep white blood cells with six different hamster auxotrophs. Selection from these and previously generated cell hybrids has led to the production of a panel of 30 hybrids covering the complete sheep genome of 28 chromosomes. Over half of the cell hybrids in this panel contain single sheep chromosomes. By complementation, the following new assignments have been made using the panel: phosphoribosyl N-formylglycinamide amidotransferase (PRFGA) to sheep chromosome (chr) 11; adenylosuccinate synthetase (ADSS) to sheep chr 12; adenylosuccinate lyase (ADSL) to sheep chr 3q; 3-hydroxy-3-methylglutaryl-coenzyme A synthase (HMGCS) to sheep chr 16; dihydrofolate reductase (DHFR) to sheep chr 5; and adenine phosphoribosyltransferase (APRT) to sheep chr 14. The gene phosphoribosylaminoinidazole-carboxamide formyltransferase/Inosinicase (PRACFT) has now been regionally assigned to chr 2q. By isozyme analysis, phosphogluconate dehydrogenase (PGD) was assigned to sheep chr 12, anchoring the sheep syntenic group U1 to this chromosome, and mannose phosphate isomerase (MPI) was assigned to sheep chr 18. Furthermore, the chromosomal assignment of 110 microsatellites was confirmed using this cell panel.  相似文献   
953.
954.
The status of the reproductive system is investigated in five species of Teleostei from the Teterev River and the Kiev Reservoir (Ukraine), descendants of F42–F7 generations of fish exposed to radiation from the accident at the Chernobyl nuclear power plant. Twenty years after the accident, a wide range of morphofuinctional anomalies of this system is present in fish. The most significant of them are: asymmetry and anomalous morphology of gonads, proliferation of connective tissue—sterilization, mass destruction of follicular and sex cells of various developmental stages, and hermaphroditism. Among the investigated species, disturbances of reproductive glands were preset to the highest extent in pike Esox lucius and crucian carp Carassius carassius. In the series of investigated generations the maximum frequency of occurrence of gonad anomalies was noted in F4 and F5, due to prolonged mutagenesis (Dubinin, 1986).  相似文献   
955.
Arundo donax L. (Poaceae) is native to Mediterranean Europe and invasive in the Rio Grande Basin of North America. Rhizomes from nine sites in France and Spain infested with a candidate control agent, the armoured scale Rhizaspidiotus donacis (Hemiptera: Diaspididae) weighed 50% less than those from nine sites without scale.  相似文献   
956.
Alzheimer’s disease (AD) is a devastating neurodegenerative condition with no known cure. While current therapies target late-stage amyloid formation and cholinergic tone, to date, these strategies have proven ineffective at preventing disease progression. The reasons for this may be varied, and could reflect late intervention, or, that earlier pathogenic mechanisms have been overlooked and permitted to accelerate the disease process. One such example would include synaptic pathology, the disease component strongly associated with cognitive impairment. Dysregulated Ca2+ homeostasis may be one of the critical factors driving synaptic dysfunction. One of the earliest pathophysiological indicators in mutant presenilin (PS) AD mice is increased intracellular Ca2+ signaling, predominantly through the ER-localized inositol triphosphate (IP3) and ryanodine receptors (RyR). In particular, the RyR-mediated Ca2+ upregulation within synaptic compartments is associated with altered synaptic homeostasis and network depression at early (presymptomatic) AD stages. Here, we offer an alternative approach to AD therapeutics by stabilizing early pathogenic mechanisms associated with synaptic abnormalities. We targeted the RyR as a means to prevent disease progression, and sub-chronically treated AD mouse models (4-weeks) with a novel formulation of the RyR inhibitor, dantrolene. Using 2-photon Ca2+ imaging and patch clamp recordings, we demonstrate that dantrolene treatment fully normalizes ER Ca2+ signaling within somatic and dendritic compartments in early and later-stage AD mice in hippocampal slices. Additionally, the elevated RyR2 levels in AD mice are restored to control levels with dantrolene treatment, as are synaptic transmission and synaptic plasticity. Aβ deposition within the cortex and hippocampus is also reduced in dantrolene-treated AD mice. In this study, we highlight the pivotal role of Ca2+ aberrations in AD, and propose a novel strategy to preserve synaptic function, and thereby cognitive function, in early AD patients.  相似文献   
957.
In order to develop direct methods for determining the extent of metabolic incorporation of isotopically labeled amino acids into a protein, the determination of deuterated tryptophan in [2H5]tryptophan-bacteriorhodopsin was investigated. The isotopically modified protein was subjected to alkaline hydrolysis. After phenyl isothiocyanate derivatization of the hydrolysate, the mixture was separated by reversed-phase liquid chromatography. Field desorption mass spectrometry and thermospray mass spectrometry were investigated for their ability to determine the ratio between [2H5]tryptophan and total tryptophan in the collected fractions. In order to check the procedure a set of known tryptophan/[2H5]tryptophan mixtures were passed through the same derivatization, HPLC separation, and lyophilization procedure as used for the biological samples.  相似文献   
958.
A method using liquid chromatography - atmospheric pressure chemical ionisation mass spectrometry was evaluated for determining the molecular species composition of phospholipids (phosphatidylcholines from soybean, egg yolk and bovine liver) after conversion to diacylglycerol nicotinate derivatives. The structures could be deduced from pseudo-molecular ions ([MH-123](+)) and three pairs of monoacyl containing fragment ions. All molecular species in mixed peaks were readily identified and many minor components, earlier not encountered in the samples under investigation, were identified. Acyl chain regioisomers were readily distinguished by the ratio of the [MH-RCHCO](+) ions. Molecular species differing only in the position of the double bonds in one polyunsaturated acyl chain were separated on the basis of retention times. A half quantitative estimation of the molecular species composition of complex samples was achieved by a combination of UV detection and, for mixed peaks, the areas of [MH-123](+) ions.  相似文献   
959.
Purification of (Ca2+-Mg2+)-ATPase from rat liver plasma membranes   总被引:1,自引:0,他引:1  
The Ca2+-stimulated, Mg2+-dependent ATPase from rat liver plasma membranes was solubilized using the detergent polyoxyethylene 9 lauryl ether and purified by column chromatography using Polybuffer Exchanger 94, concanavalin A-Sepharose 4B, and Sephadex G-200. The molecular weight of the enzyme, estimated by gel filtration in the presence of the detergent on a Sephadex G-200 column, was 200,000 +/- 15,000. The enzyme was purified at least 300-fold from rat liver plasma membranes and had a specific activity of 19.7 mumol/mg/min. Polyacrylamide gel electrophoresis under nondenaturing conditions of the purified enzyme indicated that the enzymatic activity correlated with the major protein band. In sodium dodecyl sulfate-polyacrylamide gel electrophoresis, one major band in the molecular weight range of 70,000 +/- 5,000 was seen. The isoelectric point of the purified enzyme was 6.9 +/- 0.2 as determined by analytical isoelectric focusing. The enzyme was activated by Ca2+ with an apparent half-saturation constant of 87 +/- 2 nM for Ca2+. Calmodulin and trifluoperazine at the concentration of 1 microgram/ml and 100 microM, respectively, had no effect on the enzymatic activity.  相似文献   
960.
Gradually altered synthetic entities were employed as molecular probes, and arachidonic acid, ADP, human alpha-thrombin and the Ca2+ ionophore A23187 as aggregation-inducing agents, in a comprehensive study on the response profile of human blood platelets with an emphasis on the effects of exogenous and increased intracellular Ca2+. Corroborating further previous conclusions, some representative carbamoylpiperidine derivatives, at concentrations effecting substantial inhibition of ADP-induced aggregation, failed to retain that effect when 5.0 mM Ca2+ was introduced into the otherwise identical test medium; reference compounds chlorpromazine and propranolol registered corresponding inhibitory patterns. At increased concentrations the compounds' inhibitory potency was regenerated even in the presence of 5 mM Ca2+. In fact, in sufficiently high concentrations, the compounds were even capable of inhibiting aggregation elicited by 15 microM of the ionophore A23187; so did chlorpromazine and propranolol. Another set of congeners revealed the striking sensitivity of ionophore A23187-induced human blood platelet aggregation to the surface active potencies of inhibitor molecules. The loss in inhibitory potency was directly related to the lesser hydrophobic character of the molecule.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号