全文获取类型
收费全文 | 8439篇 |
免费 | 613篇 |
国内免费 | 5篇 |
专业分类
9057篇 |
出版年
2023年 | 43篇 |
2022年 | 93篇 |
2021年 | 195篇 |
2020年 | 130篇 |
2019年 | 123篇 |
2018年 | 181篇 |
2017年 | 149篇 |
2016年 | 281篇 |
2015年 | 401篇 |
2014年 | 461篇 |
2013年 | 570篇 |
2012年 | 693篇 |
2011年 | 712篇 |
2010年 | 439篇 |
2009年 | 379篇 |
2008年 | 495篇 |
2007年 | 515篇 |
2006年 | 504篇 |
2005年 | 417篇 |
2004年 | 382篇 |
2003年 | 302篇 |
2002年 | 333篇 |
2001年 | 77篇 |
2000年 | 55篇 |
1999年 | 58篇 |
1998年 | 89篇 |
1997年 | 36篇 |
1996年 | 51篇 |
1995年 | 52篇 |
1994年 | 49篇 |
1993年 | 48篇 |
1992年 | 40篇 |
1991年 | 40篇 |
1990年 | 32篇 |
1989年 | 36篇 |
1988年 | 32篇 |
1987年 | 43篇 |
1986年 | 29篇 |
1985年 | 39篇 |
1984年 | 36篇 |
1983年 | 46篇 |
1982年 | 34篇 |
1981年 | 35篇 |
1980年 | 23篇 |
1979年 | 23篇 |
1978年 | 19篇 |
1977年 | 34篇 |
1976年 | 20篇 |
1973年 | 14篇 |
1964年 | 14篇 |
排序方式: 共有9057条查询结果,搜索用时 19 毫秒
91.
92.
93.
The prostaglandin synthesis inhibitors, indomethacin and eicosa-5,8,11, 14-tetraynoic acid (ETA), have been tested on the isolated lamb ductus arteriosus at low and high PO2 levels. Both compounds produced a gradual contraction of the hypoxic vessel, and at equal doses the effect of indomethacin was stronger. The maximal tension output of the hypoxic tissue under indomethacin was equal to that of the oxygen-contracted control. ETA- and indomethacin-treated preparations contracted further upon transfer from a low to a high oxygen environment, and the response under indomethacin exceeded significantly control values. Control preparations were relaxed markedly by PGE2 in low oxygen but showed little or no response in high oxygen. In contrast, preparations pretreated with the inhibitors retained their sensitivity to PGE2 during exposure to high oxygen. The data are consistent with the idea that E-type prostaglandins play a role in the regulation of the intrinsic tone of the ductus arteriosus during foetal life. It is also suggested that the sensitivity of ductal muscle to E-type prostaglandins is controlled by the rate of endogenous prostaglandin formation. 相似文献
94.
95.
96.
Bonacchi A Taddei ML Petrai I Efsen E Defranco R Nosi D Torcia M Rosini P Formigli L Rombouts K Zecchi S Milani S Pinzani M Laffi G Marra F 《Histology and histopathology》2008,23(3):327-340
The liver represents a site of expression of neurotrophins and their receptors. We have characterized the expression and intracellular localization of the nerve growth factor (NGF) receptor, Trk-A, in liver cells in vivo and in vitro. In both normal and fibrotic liver tissue, Trk-A immunostaining was present in different cell types, including parenchymal cells and cells of the inflammatory infiltrate. In hepatocytes and activated stellate cells (HSC), Trk-A showed a predominant nuclear localization, both in the presence and absence of injury. In cultured HSC, Trk-A was found to be functional, because exposure of the cells to recombinant NGF resulted in stimulation of cell migration and activation of intracellular signaling pathways, including Ras-ERK and PI3K/Akt. Remarkably, in cultured HSC, Trk-A staining was found constitutively in the nucleus. In these cells, Trk-A could be stained only by antibodies directed against the intracellular domain but not by those recognizing the extracellular portion of Trk-A suggesting that the intracellular portion of the receptor is the major determinant of nuclear Trk-A staining. In contrast to HSC, freshly isolated hepatocytes did not show any nuclear localization of the intracellular portion of Trk-A. In pheocromocytoma cells, nuclear staining for Trk-A was not present in conditions of serum deprivation, but could be induced by exposure to NGF or to a mixture of soluble mediators. We conclude that nuclear localization of the intracellular domain of Trk-A is observed constitutively in liver cells such as HSC, while in other cell types it could be induced in response to soluble factors. 相似文献
97.
Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil 总被引:4,自引:0,他引:4
Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (Pi) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of Pi is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with Pi for the same sorption sites (ligand exchange) or solubilize Pi via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote Pi release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main Pi solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased Pi solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20–50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble Pi, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher Pi retention. Across a wider soil pH range (pH 3–8), Pi solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing Pi was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing Pi. 相似文献
98.
Doris Ribitsch Sonja Heumann Eva Trotscha Enrique Herrero Acero Katrin Greimel Regina Leber Ruth Birner‐Gruenberger Sigrid Deller Inge Eiteljoerg Peter Remler Thomas Weber Petra Siegert Karl‐Heinz Maurer Ilaria Donelli Giuliano Freddi Helmut Schwab Georg M. Guebitz 《Biotechnology progress》2011,27(4):951-960
From a screening on agar plates with bis(benzoyloxyethyl) terephthalate (3PET), a Bacillus subtilis p‐nitrobenzylesterase (BsEstB) was isolated and demonstrated to hydrolyze polyethyleneterephthalate (PET). PET‐hydrolase active strains produced clearing zones and led to the release of the 3PET hydrolysis products terephthalic acid (TA), benzoic acid (BA), 2‐hydroxyethyl benzoate (HEB), and mono‐(2‐hydroxyethyl) terephthalate (MHET) in 3PET supplemented liquid cultures. The 3PET‐hydrolase was isolated from non‐denaturating polyacrylamide gels using fluorescein diacetate (FDA) and identified as BsEstB by LC‐MS/MS analysis. BsEstB was expressed in Escherichia coli with C‐terminally fused StrepTag II for purification. The tagged enzyme had a molecular mass of 55.2 kDa and a specific activity of 77 U/mg on p‐nitrophenyl acetate and 108 U/mg on p‐nitrophenyl butyrate. BsEstB was most active at 40°C and pH 7.0 and stable for several days at pH 7.0 and 37°C while the half‐life times decreased to 3 days at 40°C and only 6 h at 45°C. From 3PET, BsEstB released TA, MHET, and BA, but neither bis(2‐hydroxyethyl) terephthalate (BHET) nor hydroxyethylbenzoate (HEB). The kcat values decreased with increasing complexity of the substrate from 6 and 8 (s?1) for p‐nitrophenyl‐acetate (4NPA) and p‐nitrophenyl‐butyrate (4NPB), respectively, to 0.14 (s?1) for bis(2‐hydroxyethyl) terephthalate (BHET). The enzyme hydrolyzed PET films releasing TA and MHET with a concomitant decrease of the water‐contact angle (WCA) from 68.2° ± 1.7° to 62.6° ± 1.1° due to formation of novel hydroxyl and carboxyl groups. These data correlated with a fluorescence emission intensity increase seen for the enzyme treated sample after derivatization with 2‐(bromomethyl)naphthalene. © 2011 American Institute of Chemical Engineers Biotechnol. Prog., 2011 相似文献
99.
Schroeder CP Goeldner EM Schulze-Forster K Eickhoff CA Holtermann P Heidecke H 《Biological trace element research》2004,99(1-3):17-25
Selenite is frequently used in combination with cancer chemotherapeutic agents to reduce side effects. However, the cytoprotective
activity of selenite may also reduce the efficacy of chemotherapeutic drugs on tumor cells. This study was designed to examine
the effects of selenite combined with cytotoxic agents used in clinical protocols [e.g., doxorubicine, docetaxel, 5-fluorouracil
(5-FU), methotrexate (MTX), mafosphamide, mitomycin C, gemcitabine, etoposide, cisplatin, irinotecan, and oxaliplatin] on
the proliferation of various carcinoma cell types. The data demonstrated that selenite had no marked effects on the antiproliferative
activity of docetaxel, doxorubicine, 5-FU, MTX, and mafosphamide in MDA-MB-231 breast cancer cells. Likewise, no consistent
changes were observed in A549 lung cancer cell proliferation when selenite was combined with cisplatin, etoposide, gemcitabine,
or mitomycin C. On the other hand, selenite potentiated the cytotoxicity of 5-FU, oxaliplatin, and irinotecan in HCT116 colon
cancer cells by approx 1.1-fold, 2.7-fold, and 2.6-fold, respectively. In SW620 colon cancer cells, selenite induced a 1.5-fold
and 4.3-fold increase of the antiproliferative activity of 5-FU and oxaliplatin, respectively. Whereas irinotecan showed no
effects on SW620 cell growth, a combination with selenite resulted in 23% inhibition. Our results indicate that selenite did
not reduce the antiproliferative activity of chemotherapeutic agents in vitro. In addition, selenite was able to increase
the inhibitory activity of docetaxel in A549 lung cancer cells, and of 5-FU, oxaliplatin, and irinotecan in HCT116 and SW620
colon cancer cells implying selenite is potentially useful as an adjuvant chemotherapeutic agent. 相似文献