全文获取类型
收费全文 | 588206篇 |
免费 | 71876篇 |
国内免费 | 211篇 |
专业分类
660293篇 |
出版年
2018年 | 4828篇 |
2016年 | 6651篇 |
2015年 | 9680篇 |
2014年 | 11509篇 |
2013年 | 16230篇 |
2012年 | 18298篇 |
2011年 | 18480篇 |
2010年 | 12234篇 |
2009年 | 11507篇 |
2008年 | 16590篇 |
2007年 | 17423篇 |
2006年 | 16271篇 |
2005年 | 15919篇 |
2004年 | 15906篇 |
2003年 | 15111篇 |
2002年 | 14767篇 |
2001年 | 23400篇 |
2000年 | 23725篇 |
1999年 | 19347篇 |
1998年 | 7629篇 |
1997年 | 7736篇 |
1996年 | 7505篇 |
1995年 | 7216篇 |
1994年 | 7158篇 |
1993年 | 7062篇 |
1992年 | 16775篇 |
1991年 | 16619篇 |
1990年 | 16272篇 |
1989年 | 15866篇 |
1988年 | 14819篇 |
1987年 | 14243篇 |
1986年 | 13308篇 |
1985年 | 13575篇 |
1984年 | 11617篇 |
1983年 | 10229篇 |
1982年 | 8120篇 |
1981年 | 7689篇 |
1980年 | 7204篇 |
1979年 | 11402篇 |
1978年 | 9004篇 |
1977年 | 8300篇 |
1976年 | 7905篇 |
1975年 | 8598篇 |
1974年 | 8981篇 |
1973年 | 8794篇 |
1972年 | 8188篇 |
1971年 | 7138篇 |
1970年 | 6299篇 |
1969年 | 5956篇 |
1968年 | 5401篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
D R Swinson 《BMJ (Clinical research ed.)》1980,280(6224):1188-1189
112.
113.
This study evaluates dopaminergic regulation of aldosterone secretion in 6 patients with high spinal cord transections. Administration of the dopamine antagonist metoclopramide resulted in a marked rise in plasma aldosterone and 18-hydroxycorticosterone levels in 12 normal individuals, but no change in plasma levels of these zona glomerulosa corticosteroid products in spinal cord patients. Spinal cord transected patients also did not have the rise in plasma renin activity that was observed in normals following metoclopramide administration. Basal levels of aldosterone, 18 hydroxycorticosterone, corticosterone and renin activity as well as the aldosterone responses to graded dose infusion of adrenocorticotropin were similar in the spinal cord patients and the normals. These data suggest that dopaminergic regulation of adrenal zona glomerulosa corticosteroid and renal renin secretion is absent in patients with high spinal cord transections, suggesting that intact neural pathways from the central nervous system are necessary for metoclopramide stimulation of aldosterone and renin secretion in men. Since basal plasma aldosterone levels were normal in spinal cord transected patients, it appears that the absence of dopaminergic control does not result in elevated secretion. 相似文献
114.
D-2 dopamine receptors in the frontal cortex of rat and human 总被引:2,自引:0,他引:2
D-2 dopamine receptors and serotonin receptors in the frontal cortex of rat and human were labelled with 3H-spiroperidol. The D-2 receptors were then distinguished in 4 ways. Dissociation of spiroperidol was biphasic, indicating two populations of sites. Cinanserin in competition with 3H-spiroperidol exhibited high (75%) and low (25%) affinity sites. Dopamine and LY 141865 in competition with 1.25 nM 3H-spiroperidol exhibited high (20-25%) and low (80-75%) affinity sites in the absence of cinanserin, while in the presence of 300 nM cinanserin only the high affinity sites remained. Lesioning of the dopaminergic meso-cortical pathway increased the number of cinanserin-resistant sites by 26%. Thus 3H-spiroperidol binding in the presence of cinanserin can be used to selectively label D-2 receptors in the frontal cortex. 相似文献
115.
Bodil Kjær Yean-Sung Jung Lian Yu John H. Golbeck Henrik Vibe Scheller 《Photosynthesis research》1994,41(1):105-114
The photosynthetic reaction center complex from the green sulfur bacteriumChlorobium vibrioforme has been isolated under anaerobic conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 30, 18, 15, and 9 kDa. The 80- and 18-kDa polypeptides are identified as the reaction center polypeptide and the secondary donor cytochromec
551 encoded by thepscA andpscC genes, respectively. N-terminal amino acid sequences identify the 40-kDa polypeptide as the bacteriochlorophylla-protein of the baseplate (the Fenna-Matthews-Olson protein) and the 30-kDa polypeptide as the putative 2[4Fe-4S] protein encoded bypscB. Electron paramagnetic resonance (EPR) analysis shows the presence of an iron-sulfur cluster which is irreversibly photoreduced at 9K. Photoaccumulation at higher temperature shows the presence of an additional photoreduced cluster. The EPR spectra of the two iron-sulfur clusters resemble those of FA and FB of Photosystem I, but also show significantly differentg-values, lineshapes, and temperature and power dependencies. We suggest that the two centers are designated Center I (with calculatedg-values of 2.085, 1.898, 1.841), and Center II (with calculatedg-values of 2.083, 1.941, 1.878). The data suggest that Centers I and II are bound to thepscB polypeptide. 相似文献
116.
To define catalytically essential residues of bacteriophage T7 RNA polymerase, we have generated five mutants of the polymerase, D537N, K631M, Y639F, H811Q and D812N, by site-directed mutagenesis and purified them to homogeneity. The choice of specific amino acids for mutagenesis was based upon photoaffinity-labeling studies with 8-azido-ATP and homology comparisons with the Klenow fragment and other DNA/RNA polymerases. Secondary structural analysis by circular dichroism indicates that the protein folding is intact in these mutants. The mutants D537N and D812N are totally inactive. The mutant K631M has 1% activity, confined to short oligonucleotide synthesis. The mutant H811Q has 25% activity for synthesis of both short and long oligonucleotides. The mutant Y639F retains full enzymatic activity although individual kinetic parameters are somewhat different. Kinetic parameters, (kcat)app and (Km)app for the nucleotides, reveal that the mutation of Lys to Met has a much more drastic effect on (kcat)app than on (Km)app, indicating the involvement of K631 primarily in phosphodiester bond formation. The mutation of His to Gln has effects on both (kcat)app and (Km)app; namely, three- to fivefold reduction in (kcat)app and two- to threefold increase in (Km)app, implying that His811 may be involved in both nucleotide binding and phosphodiester bond formation. The ability of the mutant T7 RNA polymerases to bind template has not been greatly impaired. We have shown that amino acids D537 and D812 are essential, that amino acids K631 and H811 play significant roles in catalysis, and that the active site of T7 RNA polymerase is composed of different regions of the polypeptide chain. Possible roles for these catalytically significant residues in the polymerase mechanism are discussed. 相似文献
117.
118.
John L. Graner 《CMAJ》1985,133(9):855-857,880
In 1849 Thomas Addison described the clinical entity now known as pernicious anemia. In 1855 he reported several cases of adrenal insufficiency, or Addison''s disease. Considering the importance of these works, there remains a great deal of confusion about them. Contrary to what many historians have written, a review of Addison''s original publications demonstrates a firm appreciation of the distinction between pernicious anemia and adrenal insufficiency, based particularly on the discoloration of the skin in these conditions. Three major sources of possible confusion for historians who are attempting to understand Addison''s views include Addison''s early attempts to link pernicious anemia with disease of the supra-renal capsules, Addison''s redefinition of pernicious anemia in his monograph on adrenal disease, and several confusing statements made by Wilks and Daldy in the first reprint of Addison''s monograph. 相似文献
119.
120.