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   10791篇
  免费   779篇
  国内免费   3篇
  2023年   90篇
  2022年   141篇
  2021年   256篇
  2020年   210篇
  2019年   280篇
  2018年   362篇
  2017年   340篇
  2016年   447篇
  2015年   613篇
  2014年   589篇
  2013年   766篇
  2012年   822篇
  2011年   873篇
  2010年   554篇
  2009年   431篇
  2008年   618篇
  2007年   603篇
  2006年   517篇
  2005年   466篇
  2004年   457篇
  2003年   392篇
  2002年   341篇
  2001年   175篇
  2000年   155篇
  1999年   117篇
  1998年   79篇
  1997年   68篇
  1996年   51篇
  1995年   44篇
  1994年   33篇
  1993年   40篇
  1992年   60篇
  1991年   54篇
  1990年   54篇
  1989年   38篇
  1988年   51篇
  1987年   29篇
  1986年   29篇
  1985年   30篇
  1984年   22篇
  1983年   30篇
  1982年   22篇
  1981年   25篇
  1980年   20篇
  1979年   16篇
  1977年   17篇
  1976年   18篇
  1975年   22篇
  1974年   11篇
  1972年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Summary Transthyretin methionine 30 (TTR Met 30), which is associated with familial amyloidotic polyneuropathy, originates in a single base substitution (A for G) in the second exon of the TTR gene. This autosomal dominant disease can be diagnosed by RFLP analysis of NsiI-digested DNA. The amplification of DNA by PCR improves the diagnosis method, making it suitable for prenatal diagnosis. Using PCR-amplified DNA, prenatal diagnosis of two at-risk fetuses was performed. Control Met 30 and normal DNA (either genomic or produced by site directed mutagenesis) were processed in parallel. The diagnosis was made by hybridization with allele-specific oligonucleotide probes, and later confirmed by screening of the mutant protein in the amniotic fluid and, when possible, in the sera from the newborns. TTR Met 30 was detected in the amniotic fluid of a positive fetus whose father was the carrier of the mutation. This indicates that the mutant protein is expressed very early in development.  相似文献   
32.
We have obtained a set ofEscherichia coli K-12 derivatives with transposon-generated Tn10 insertion mutations at thearo genes of their aromatic biosynthetic pathway. Bacteriophage NK561 (Tn10) has been used for transposon mutagenesis ofE. coli, strain BW545. Tetracycline (Tc)-resistant derivatives were screened by their Aro phenotype by growth on a minimal medium with adequate requirements. Sixaro mutant types were mapped; two strains werearoA, twoaroD, onearoB oraroE, and onearoC. A selective medium and ad-cycloserine enrichment in the presence of tetracycline were used to select for Aro, Tc-sensitive derivatives. The reversion index to aromatic-independent colonies of some derivatives was less than 2 × 10–11 per bacterium per generation. P1 transduction experiments transferred an aroA::Tn10 insertion fromE. coli BW545 to an enterotoxigenicE. coli strain from porcine origin. Derivatives of this strain beingaro, Tc-sensitive and not reverting toaro + at a detectable frequency, and many others transduced at will, may prove their usefulness as live vaccines.  相似文献   
33.
The daily and seasonal distribution of airborne fungal particles was recorded in a high altitude tropical zone. Sampling was carried out in the southern part of Mexico City. An Andersen air sampler was used over a period of six months. Ten minutes sampling for each set of plates was done at fixed schedule: 07:30, 14:00 and 19:00 hours. The sampler was placed 10 m above the ground. Daily variation was found to be associated with the season, weather and atmospheric stability. The highest value of mold counts (3195 CFU m−3) was recorded in the evening on October, a transitional month between the rainy and the dry seasons, the lowest (45 CFU m−3) at noon during the rainy season. Mold counts were significantly correlated with temperature, having negative signs both in the morning and at noon, and being positive in the evening. The abundance of only three genera was recorded.Cladosporium, was isolated more frequently, and its abundance at 14:00 h was of 38%;Alternaria represented 4.0%, at 14:00 h, andAspergillus 3.0% at 7:30 h. Fifteen species belonging to the latter genera were identified and most of them are considered as opportunistic molds of clinical significance.  相似文献   
34.
Patulin and griseofulvin production by twelve strains ofPenicillium griseofulvum Dierckx, eleven of which were isolated from pistachio (Pistacia vera) nuts and the other was supplied by the Spanish Collection of Type Culture, was investigated. Six strains of the eleven isolated had ability to produce patulin and griseofulvin in Yes medium. All the strains studied had no ability to produce patulin in Wickerham medium. Griseofulvin production was significant in both media but higher in Wickerham. These metabolites were separated and determined in the chloroform extracts of cultures by high performance liquid chromatography with ultraviolet detection. The best conditions were: acetonitrile — water (45∶55) as mobile phase, a flow rate of 2.0 mL/min and a μ Bondapack C18 column.  相似文献   
35.
Summary A systematic study was made of the action of 4-acetamido-4-isothiocyanostilbene-2,2-disulfonic acid (SITS) and 4,4-diisothiocyanostilbene-2,2-disulfonic acid (DIDS) on active Ca2+ transport of human erythrocytes. Pumping activity was estimated in inside-out vesicles (IOV's) by means of Ca2+-selective electrodes or use of tracer45Ca2+. The stilbenes exhibited an approximately equal inhibitory potency and their action could be overcome by carbonyl cyanidep-trifluoromethoxyphenylhydrazone (FCCP) at low but not at high stilbene concentrations. In the absence of DIDS. Ca2+ transport was not affected upon addition of valinomycin, but it was appreciably reduced when vesicles were preincubated with low DIDS concentrations. Such an effect was strictly dependent on the external K+ concentration and it was abolished when valinomycin was added together with FCCP. Similar results were obtained using IOV's prepared from intact cells which had been previously exposed to the stilbene. The findings clearly demonstrate the presence in human red cells of a partially electrogenic Ca2+ pump, exchanging one Ca2+ ion for one proton.  相似文献   
36.
Clinical and experimental data suggest that both Captopril and angiotensin II (AII) reduce baroreflex responsiveness, and the main action of this converting enzyme inhibitor (CEI) seems clear to suppress AII synthesis. The aim of this work is to investigate this striking similarity of effects. We have verified that CEI (4 mg/kg) originates tachycardia significantly lower (P less than 0.001) than that produced in response to a similar hypotension elicited by an unspecific vasodilator: sodium nitroprusside (10-45 micrograms/kg min). CEI SQ 20881 has been reported to increase plasma vasopressin concentrations (AVP); this peptide is also known to modify baroreflex responses and has a small direct negative chronotropic effect. However, our determinations of AVP do not show any difference between the control group and the group treated with Captopril (4.78 +/- 0.87 and 5.26 +/- 0.19 pg/ml respectively). On the other hand, although CEI did not modify the rapid responses of heart rate (HR) to changes of mean arterial pressure (MAP), the decrease of MAP induced by nitroprusside was higher in the group treated with Captopril than in control group; it could mean a baroreflex ability decrease to buffer the hypotension. However, AII elicited a strong impairment of both rapid responses of HR and the buffering of hypotension produced by NP, these actions being suggested as centrally mediated. These results could indicate that the suppression of peripheral AII synthesis and therefore, the lack of pre- and postjunctional sympathetic potentiation owing to this hormone, is responsible for the absence of tachycardia under Captopril treatment.  相似文献   
37.
The effects of social isolation and crowding on adrenocortical function and upon behavioral responsiveness to electric shock have been studied in male and female rats. All female experimental groups showed higher corticosterone levels and heavier adrenals than their male counterparts. The major effect of housing condition concerned the corticosterone response to stress, while basal hormone concentration was not modified. Socially housed rats showed a more intense adrenocortical response and also a greater behavioral reactivity to electric shock than the isolates.  相似文献   
38.
In this study, microdialysis was used to investigate functional recovery of central cholinergic neurons in the forebrain of rats with cortical devascularizing lesions. Mature male rats were unilaterally lesioned by disruption of the pia arachnoid vessels and genetically modified fibroblasts secreting nerve growth factor (NGF) were placed at the site of the lesion. One month following surgery, microdialysis probes were installed in the remaining cortex and were perfused with artificial cerebrospinal fluid (csf) containing neostigmine (5 nM) and/or KCl (100 mM). The basal (non-stimulated) release of acetylcholine (ACh) in the cortex was similar in all experimental groups, whereas KCl stimulated release of ACh was significantly augmented (P < 0.05) in the ipsilateral remaining cortex in lesioned animals that have been implanted with fibroblasts secreting NGF. These results suggest that NGF secreted by genetically engineered fibroblasts modulates neuroplasticity in the adult mammalian CNS and may favour recovery of cortical function following injury.  相似文献   
39.
The objective of this work was to characterize lipid metabolism in long-term cultures of adult rat hepatocytes from female rats and explore the potential use of this culture system to study the effect of hormones, drugs and toxic chemicals on it. Hepatocytes, seeded on a feeder layer of 3T3 cells, maintained for 2 weeks their typical morphology. The cultures were able to take up [14C]acetic and [14C]oleic acid from the culture medium and incorporate them into lipids. The synthesis and secretion of lipids by [14C]acetic acid-labeled cultures had a maximum value after 11 and 13 days in culture. Triacylglycerols were the main lipidic species synthesized and secreted by hepatocytes (up to 67% of the total lipids); they also synthesized and secreted phospholipids, cholesterol and cholesterol esters from [14C]acetic acid. Similarly, [14C]oleic acid-labeled cultures synthesized and secreted mostly triacylglycerols (up to 60-70% of the total lipids), but they were also able to incorporate the labeled precursor into both cellular and secreted phospholipids and cholesterol esters. The activity of glycerol-phosphate-dehydrogenase, marker enzyme of glycerolipid synthesis, decreased slightly during the culture time whereas the activity of malic enzyme, marker of fatty acid synthesis, increased. Our results show that long-term cultures of female rat hepatocytes are able to synthesize and secrete several lipids, specially triacylglycerols, from both [14C]acetic and [14C]oleic acid for at least 2 weeks and that they maintain enzyme activities related with the synthetic pathways of glycerolipids and fatty acids.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
40.
Ethanol alters almost all membrane functions, but it behaves essentially like a benzodiazepine-type GABAergic agonist. The mechanism by which ethanol affects the GABA/benzodiazepine complex is not clear. We studied the possible changes in [3H]flunitrazepam binding induced by chronic ethanol treatment, using light microscopic autoradiography, to try to elucidate the controversy underlying this topic. This technique allows us to measure densities of benzodiazepine receptors in different anatomical brain areas—visual cortex and hippocampus—which seem to constitute the anatomical support for the behavioral and physiological responses affected by ethanol. Autoradiographic studies on the visual cortex and hippocampus from rats chronically treated with ethanol do not show statistically significant differences in the binding of, [3H]flunitrazepam with respect to control animals. Furthermore, we did not find either rostro-caudal or medio-lateral differences, in benzodiazepine receptor densities in each layer of the visual cortex.  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