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   3867篇
  免费   206篇
  2023年   9篇
  2022年   20篇
  2021年   57篇
  2020年   24篇
  2019年   22篇
  2018年   56篇
  2017年   47篇
  2016年   72篇
  2015年   143篇
  2014年   167篇
  2013年   245篇
  2012年   270篇
  2011年   281篇
  2010年   163篇
  2009年   166篇
  2008年   263篇
  2007年   282篇
  2006年   256篇
  2005年   230篇
  2004年   241篇
  2003年   247篇
  2002年   238篇
  2001年   32篇
  2000年   41篇
  1999年   45篇
  1998年   46篇
  1997年   39篇
  1996年   33篇
  1995年   41篇
  1994年   24篇
  1993年   24篇
  1992年   29篇
  1991年   22篇
  1990年   38篇
  1989年   26篇
  1988年   15篇
  1987年   11篇
  1986年   10篇
  1985年   16篇
  1984年   16篇
  1983年   10篇
  1982年   8篇
  1981年   14篇
  1980年   5篇
  1979年   6篇
  1975年   3篇
  1974年   5篇
  1973年   2篇
  1972年   2篇
  1958年   2篇
排序方式: 共有4073条查询结果,搜索用时 31 毫秒
61.
Effects of taurine or γ-aminobutyric acid (GABA) on akinesia and analgesia induced by D-Ala2-Met-enkephalinamide were investigated in rats. Administration of taurine (dose range: 2.375×10?2 M–9.5×10?2 M/10 μl) into the left lateral ventricle 10 min prior to the injection of D-Ala2-Met-enkephalinamide (50 μg/10 μl) produced a dose-dependent reduction in the duration of akinesia and to some extent of analgesia, as estimated at 30 min and 60 min following the enkephalinamide injection; at the first estimation-time (10 min), taurine did not alter the duration of akinesia or that of analgesia. The median effective dose (ED50) for akinesia determined at 60 min after D-Ala2-Met-enkephalinamide was 5 times greater and that for analgesia assessed at the same time was 1.7 times greater in taurine-treated rats than the respective doses in control animals. Administration of GABA under similar experimental conditions produced a dose-dependent reduction in the duration of analgesia from the initial estimation time (10 min) following the injection of D-Ala2-Met-enkephalinamide. The ED50 for analgesia determined at 30 min after D-Ala2-Met-enkephalinamide was 3 times greater in GABA-treated rats than in control animals. Unlike the effects of taurine, GABA did not alter the duration of akinesia. Neither the duration of akinesia nor that of analgesia was modified by taurine or GABA alone in rats tested 9 min after the injection of each amino acid. These findings suggest that taurine may promote a recovery from both akinesia and analgesia, while GABA decreases only the analgesia induced by D-Ala2-Met-enkephalinamide.  相似文献   
62.
Coloration and sex change were studied in a temperate wrasseHalichoeres poecilopterus in the central part of the Seto Inland Sea, Japan. 1,270 examples, 45–179 mm SL, were collected from May to December both in 1983 and 1984. The species is a diandric, protogynous hermaphrodite, and has three color patterns: pale color type (A), brilliant color type (B) and intermediate color type (AB). A-fish were less than 142 mm SL and consisted of primary males (42.6%), females (55.4%), secondary males (0.3%) and fish with transitional gonads (1.7%). A-females changed their color to B, through AB, in the size range 101–131 mm SL. A-primary males changed their color to B, through AB, in the size range 103–134 mm SL. B-fish consisted of primary males (38.6%), secondary males (54.6%) and fish with transitional gonads (6.8%). The majority of females changed their sex to male in the size range 98–131 mm SL.  相似文献   
63.
Verbascoside was found to be produced in all calli derived from eleven species that contained the compound in their leaves. Cell suspension cultures were also established in three species, i.e., Leucosceptrum japonicum f. barbinerve, Syringa josikaea, and Sy. vulgaris, all of which were found to produce verbascoside at more than 1 g/l. Of the three species, suspension cultures of L. japonicum f. barbinerve showed rapid growth and the highest yield of verbascoside (1.89 g/l). In these cultures, the effects of major salt concentration in B5 medium on cell growth and verbascoside production were examined. Maximum cell growth and maximum verbascoside production were both achieved by reducing the major salt concentration to half that of the original medium.  相似文献   
64.
Summary Genetically transformed kiwi fruit (Actinidia deliciosa) plants were obtained from hypocotyl and stem segments co-cultured with Agrobacterium tumefaciens strain EHA101 harboring a binary vector, pLAN411 or pLAN421, which contained the neomycin phosphotransferase II (nptII) gene and the -glucuronidase (GUS) gene. After co-culturing with the A. tumefaciens, the hypocotyl or stem segments were cultured on a selection medium containing 25g/ml kanamycin and 500g/ml Claforan. After one month in culture, shoots had regenerated from the cuttings. Green shoots were analyzed for NPTII activity and GUS activity. Eighty-five percent of the green shoots examined expressed the nptII and GUS genes. GUS histochemical assays revealed strong GUS expression in guard cells, mesophyll cells, and trichomes.  相似文献   
65.
A sialoglycoprotein and a fucose sulfate glycoconjugate (FSG) were purified from egg jelly of the sea urchin Hemicentrotus pulcherrimus . Sialoglycoprotein which consisted of sialic acid (90%, w/w) and protein (10%, w/w) did not cause induction of the acrosome reaction and sperm isoagglutination. FSG which contained one mol sulfate/mol fucose possessed 2.0 times protein to fucose by weight. The proteins in intact FSG were separated to two major (258 kDa and 237 kDa) and one minor (120 kDa) proteins by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) in the presence of 2-mercaptoethanol (2-ME) while the proteins could not be separated by HPLC in the presence of 0.1% SDS or SDS-PAGE without 2-ME. However, after carboxymethylation of FSG, two major (260 kDa and 240 kDa) proteins and two minor (140 kDa and 135 kDa) proteins were separated from the fucose sulfate moiety by HPLC in the presence of 0.1% SDS or SDS-PAGE without 2-ME. When FSG was first carboxymethylated with non-radioactive iodoacetic acid and then reduced with 2-ME and finally carboxymethylated with 14C-iodoacetic acid, the most of radioactivity was detected in 140 kDa and 135 kDa proteins. Carboxymethylted-FSG was less potent than intact FSG in induction of the acrosome reaction. Fucoidan, a fucose sulfate polymer, did not induce the acrosome reaction.  相似文献   
66.
Hydrophobic and charge-charge interactions of Salmonella typhimirium and Serratia marcescens were determined and related to their content of fimbriae and lipopolysaccharide (LPS). The cell surface structures were characterized with hydrophobic interaction chromatography (HIC), electrostatic interaction chromatography (ESIC) and particle electrophoresis measurements. The degree of interaction at the air-water interface was tested using a monolayered lipid film applied to an aqueous surface. The cell surface hydrophobicity of S. typhimurium in the presence of fimbriae was less in smooth than in rought bacteria. Examination of a series of rough mutants of S. typhimurium indicates that reduction of the O-side chain and core oligosaccharides was correlated with increased cell hydrophobicity. The enrichment factors at the air-water interface were significantly higher for fimbriated than for non-fimbriated S. typhimurium cells. Fimbriated S. marcescens cells were less hydrophobic and adhered to a lesser degree at the air-water surface than non-fimbriated counterparts. Electrophoretic measurements and adsorption to ion exchangers gives different information about the surface charge of bacteria. The latter technique gives the interaction between localized charged surfaces.Abbreviations HIC hydrophobic interaction chromatography - ESIC electrostatic interaction chromatography - LPS lipopolysaccharide - PBS phosphate buffered saline solution  相似文献   
67.
The antioxidative effect of α-tocopherol incorporated into lecithin liposomes was studied. Lipid peroxidation of liposome membranes, assayed as malondialdehyde production, was catalyzed by ascorbic acid and Fe2+. The peroxidation reaction, which did not involve the formation of singlet oxygen, superoxide, hydrogen peroxide, or a hydroxyl radical, was inhibited by α-tocopherol and a model compound of α-tocopherol, 2,2,5,7,8-pentamethyl-6-hydroxy-chroman (TMC), but not by phytol, α-tocopherylquinone, or α-tocopheryl acetate. One mole of α-tocopherol completely prevented peroxidation of about 100 moles of polyunsaturated fatty acid. Decrease in membrane fluidity by lipid peroxidation, estimated as increase of fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the membrane, was also inhibited by α-tocopherol and TMC, reflecting their antioxidant functions. Cholesterol did not act as an antioxidant, even when incorporated in large amount into the liposome membranes, but it increased the antioxidative efficiency of α-tocopherol. When a mixture of liposomes with and without α-tocopherol was incubated with Fe2+ and ascorbic acid, α-tocopherol did not protect the liposomes not containing α-tocopherol from peroxidation. However, preincubation of the mixture, or addition of Triton X-100 allowed the α-tocopherol to prevent peroxidation of the liposomes not containing α-tocopherol. In contrast, in similar experiments, liposomes containing TMC prevented peroxidation of those without TMC without preincubation. Tocopherol in an amount so small as to exhibit only a slight antioxidative effect was oxidized when incorporated in egg lecithin liposomes, but it mostly remained unoxidized when incorporated in dipalmitoyllecithin liposomes, indicating that oxygen activated by ascorbic acid-Fe2+ does not oxidize α-tocopherol directly. Thus, decomposition of α-tocopherol may be caused by its interaction with peroxy and/or alkoxyl radicals generated in the process of lipid peroxidation catalyzed by Fe2+ and ascorbic acid.  相似文献   
68.
Summary Chemical polymerization of adenosine-5-phosphorimidazolide was conducted in the presence of oligouridylate templates. Oligo U with chain length more than eight served as a template and facilitated oligoadenylate formation. No template activity was observed when oligo U up to a hexamer was used. These results correlate with thermal transition temperatures of oligo U-pA complexes.  相似文献   
69.
Abstract: This paper describes a new, sensitive assay for dopamine-β-hydroxylase (DBH) activity in human cerebrospinal fluid (CSF), serum and brain tissues by high performance liquid chromatography (HPLC) with electrochemical detection (ED). Dopamine (DA) was used as a substrate and was incubated under optimal conditions. Norepinephrine (NE) formed enzymatically from DA was isolated by a double-column procedure, the first column of Dowex-50-H+ and the second column of aluminum oxide. NE was adsorbed on the second aluminum oxide column and then eluted with 0.5 M-hydrochloric acid and assayed by HPLC-ED. Epinephrine (EN) was added to each incubation mixture as an internal standard, and this assay was therefore highly reproducible. The peak height in HPLC was linear from 500 fmol to 100 pmol of NE and EN. The lower limit of detection for NE formed enzymatically was about 30 pmol, which indicated that the sensitivity of this procedure was comparable to that of radioassay procedures. We applied the method to measurement of the activity of and examination of some of the characteristics of DBH in human CSF. DBH activity in CSF of Parkinsonian patients was lower than that of control patients. The properties of DBH in human CSF were similar to those in serum and adrenal medulla.  相似文献   
70.
Previously, we showed that fetal bovine cartilage contains a polypeptide that stimulates the incorporation of [35S]sulfate into proteoglycans synthesized by rat and rabbit costal chondrocytes in culture. In this paper, we report that the cartilage-derived factor (CDF) increases not only [35S]sulfate incorporation but also [3H]thymidine incorporation into rabbit chondrocytes in monolayer culture. The dose-response curve of CDF stimulation of DNA synthesis was similar in profile to that of CDF stimulation of proteoglycan synthesis. In addition, CDF markedly enhanced [3H]uridine incorporation into rabbit chondrocytes and significantly enhanced [3H]serine incorporation into total protein. These findings indicate that fetal bovine cartilage contains a factor that shows somatomedin-like activity in monolayer cultures of rabbit chondrocytes.  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