全文获取类型
收费全文 | 3149篇 |
免费 | 458篇 |
国内免费 | 3篇 |
出版年
2022年 | 23篇 |
2021年 | 45篇 |
2020年 | 39篇 |
2019年 | 51篇 |
2018年 | 72篇 |
2017年 | 72篇 |
2016年 | 79篇 |
2015年 | 142篇 |
2014年 | 117篇 |
2013年 | 156篇 |
2012年 | 166篇 |
2011年 | 166篇 |
2010年 | 149篇 |
2009年 | 100篇 |
2008年 | 134篇 |
2007年 | 126篇 |
2006年 | 122篇 |
2005年 | 108篇 |
2004年 | 130篇 |
2003年 | 99篇 |
2002年 | 97篇 |
2001年 | 78篇 |
2000年 | 95篇 |
1999年 | 60篇 |
1998年 | 42篇 |
1997年 | 53篇 |
1996年 | 29篇 |
1995年 | 33篇 |
1994年 | 34篇 |
1993年 | 26篇 |
1992年 | 52篇 |
1991年 | 56篇 |
1990年 | 47篇 |
1989年 | 63篇 |
1988年 | 36篇 |
1987年 | 48篇 |
1986年 | 49篇 |
1985年 | 41篇 |
1984年 | 44篇 |
1983年 | 43篇 |
1982年 | 36篇 |
1981年 | 44篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 22篇 |
1977年 | 30篇 |
1976年 | 25篇 |
1975年 | 25篇 |
1974年 | 27篇 |
1973年 | 26篇 |
排序方式: 共有3610条查询结果,搜索用时 15 毫秒
91.
Regulation of GABAB receptors by histamine and neuronal activity in the isolated spinal cord of neonatal opossum in culture. 总被引:1,自引:0,他引:1
D J Zou J M Treherne R R Stewart N R Saunders J G Nicholls 《Proceedings. Biological sciences / The Royal Society》1991,246(1315):77-82
The aim of these experiments has been to analyse the properties of receptors for the transmitter gamma-aminobutyric acid (GABA) in developing mammalian nervous system. Changes in responses of GABAB receptors have been measured after alterations of the chemical environment and the level of electrical activity. We have previously shown that when the central nervous system (CNS) of the new-born opossum, Monodelphis domestica, is cultured for three to five days in the presence of histidine, inhibition by baclofen, a GABAB agonist, disappears (Stewart et al. 1991). We have now investigated whether histidine acts indirectly by way of conversion to histamine. As with histidine, culture with 150 microM histamine for five days virtually abolished the inhibition by baclofen. The effects of histidine, as well as histamine, were blocked by mepyramine, a histamine H1-receptor antagonist, and by ranitidine, an H2-antagonist. Tetrodotoxin (TTX), which blocks all electrical activity, protected preparations from the action of histidine but not histamine. Our results suggest that histidine is converted to histamine, which reduces the efficacy of GABAB agonists. We conclude that, in the developing mammalian CNS, transmitter levels and electrical activity can selectively influence the properties of receptors. 相似文献
92.
Kinesin-related proteins required for structural integrity of the mitotic spindle. 总被引:54,自引:0,他引:54
For S. cerevisiae cells, the assembly of a bipolar mitotic spindle requires the action of either Cin8p or Kip1p, gene products related to the mechanochemical enzyme kinesin. In this paper we demonstrate that the activity of either one of these proteins is also required following spindle assembly. When their function was eliminated, preanaphase bipolar spindles rapidly collapsed, with previously separated poles being drawn together. In contrast, anaphase spindles were apparently resistant to collapse. Deletion of kinesin-related KAR3 partially suppressed the phenotypes associated with loss of Cin8p/Kip1p function. Our findings suggest that the structure of the preanaphase bipolar spindle is maintained by counteracting forces produced by kinesin-related proteins. 相似文献
93.
K. M. Dziegielewska W. M. Brown C. C. Gould N. Matthews J. E. C. Sedgwick N. R. Saunders 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(2):168-171
Summary Fetuin is a plasma protein present in high concentrations during fetal development in animals of the order Artiodactyla. Its role is not known. The human homologue of fetuin — 2HS glycoprotein — has been shown to be a negative acute phase protein in adult plasma. In the present study, the concentration of fetuin was measured in the serum of healthy cattle (Bovis bovis) and in animals with various injuries and inflammatory disorders. The levels were decreased by 30% in pregnancy but increased up to 10-fold in some trauma cases. A significant negative correlation between the concentrations of fetuin and albumin has also been found. Thus, fetuin appears to be a positive acute phase protein in cattle.Abbreviations 2HS
2HS glycoprotein
- AP
acute phase 相似文献
94.
Evidence for WT1 as a Wilms tumor (WT) gene: intragenic germinal deletion in bilateral WT. 总被引:3,自引:0,他引:3
下载免费PDF全文
![点击此处可从《American journal of human genetics》网站下载免费的PDF全文](/ch/ext_images/free.gif)
V Huff H Miwa D A Haber K M Call D Housman L C Strong G F Saunders 《American journal of human genetics》1991,48(5):997-1003
The inactivation of two alleles at a locus on the short arm of chromosome 11 (band 11p13) has been suggested to be critical steps in the development of Wilms tumor (WT), a childhood kidney tumor. Two similar candidate WT cDNA clones (WT33 and LK15) have recently been identified on the basis of both their expression in fetal kidney and their location within the smallest region of overlap of somatic 11p13 deletions in some tumors. These homozygous deletions, however, are large and potentially affect more than one gene. Using a cDNA probe to the candidate gene, we have analyzed DNA from both normal and tumor tissue from WT patients, in an effort to detect rearrangements at this locus. We report here a patient with bilateral WT who is heterozygous for a small (less than 11 kb) germinal deletion within this candidate gene. DNA from both tumors is homozygous for this intragenic deletion allele, which, by RNA-PRC sequence analysis, is predicted to encode a protein truncated by 180 amino acids. These data support the identification of this locus as an 11p13 WT gene (WT1) and provide direct molecular data supporting the two-hit mutational model for WT. 相似文献
95.
Streptomyces marker plasmids for monitoring survival and spread of streptomycetes in soil. 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Plasmid constructs pNW1 through pNW6 containing a controllable xylE gene (for catechol 2,3-dioxygenase) were introduced into Streptomyces lividans strains to provide a selectable marker system. xylE functions in S. lividans under the control of bacteriophage lambda promoters lambda pL and lambda pR. Thermoregulated expression of xylE is provided through the lambda repressor cI857. Catechol 2,3-dioxygenase activity was increased 2.8-fold from plasmid construct pNW2 (lambda pL, xylE, cI857) and 9.5- and 7.4-fold from constructs pNW3 (lambda pR, xylE, cI857) and pNW5 (lambda pR, xylE, cI857), respectively, when the temperature was shifted from 28 degrees C to 37 degrees C. The stability of the constructs varied from 4.7% for pNW2 to 99.4% for pNW4 (lambda pL, xylE) over two rounds of sporulation. Marked S. lividans strains released into soil systems retained the XylE phenotype for more than 80 days, depending on the marker plasmid, when examined by a selective plating method. Furthermore, S. lividans harboring plasmid pNW5 was detectable by nucleic acid hybridization at less than 10 CFU g-1 (dry weight) of soil as mycelium and 10(3) CFU g-1 (dry weight) of soil as spores with the xylE marker DNA extracted from soil and amplified by using the polymerase chain reaction. 相似文献
96.
97.
Information on the commercially utilized marine red alga Furcellaria lumbricalis is summarized from published sources. Biological aspects under discussion include: nomenclature, taxonomy, morphology, development and life history, distribution, phenology, ecophysiology, growth and resource management. A brief history of the utilization of this species, and an account of its chemical constituents including the hydrocolloid furcellaran are also presented. 相似文献
98.
Kim B. Saunders Patricia A. D’Amore 《In vitro cellular & developmental biology. Animal》1992,28(7-8):521-528
Summary Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues.
In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting
a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial
cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition.
However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the
individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane,
permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle
cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8μm maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through
pores of 2.0μm. Vascular cell co-culture across membranes with 0.8-μm pores resulted the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited
endothelial cell growth. The arrangement of the cells in this co-culture system mimics thein vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated
internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact
or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study
of cell-cell interactions in a variety of systems. 相似文献
99.
Diel Nitrogen Fixation by Cyanobacterial Surface Blooms in Sanctuary Lake, Pennsylvania 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Diel nitrogen fixation studies were conducted with assemblages of cyanobacteria sampled from surface blooms on Sanctuary Lake, Pa. The studies were conducted between July and September of 1982 to 1985 by using the acetylene reduction technique. Assemblages with the lowest cell concentrations (0.9 × 109 to 1.0 × 109 cells per liter) exhibited nitrogen fixation activity throughout the day, with maximum fixation rates occurring in mid to late afternoon; fixation proceeded throughout the night at rates equivalent to 23 to 28% of the afternoon maximum. In studies conducted with the highest cell concentrations (3.7 × 109 to 6.7 × 109 cells per liter), fixation rates reached maximum values in mid to late morning. The rates declined rapidly throughout the midday period and subsequently ceased from late afternoon until sunrise on the following day. The afternoon decline and cessation of fixation exhibited by high cell concentrations correlated with photosynthetically induced low total CO2 and supersaturating O2 concentrations. The midday decline could be prevented and partially reversed by experimentally lowering O2 and increasing total CO2 concentrations. Under experimental conditions which simultaneously prevented supersaturating O2 concentrations and maintained high total CO2 availability, nitrogen fixation continued throughout the solar day, with maximum rates occurring at midday. These observations indicate that temporal changes in photosynthetic activity may affect diel fluctuations in nitrogen fixation. 相似文献
100.
Ira S. Kass David A. Larsen Ching C. Wang Antony O.W. Stretton 《Experimental parasitology》1982,54(2):166-174
Avermectin B1a, a novel antiparasitic agent, paralyzes Ascaris suum without causing either flaccid paralysis or a hypercontraction. It reduces the lengthening of the acetylcholine-preconditioned A. suum muscle strip caused by γ-aminobutyric acid. It does not affect the contraction of the isolated muscle strip preparation caused by applying acetylcholine. However, preinjection with Avermectin B1a does significantly reduce the shortening caused by acetylcholine injection without affecting the paralysis of an intact ascarid worm. These results suggest that Avermectin B1a may act on the central nervous system of Ascaris sp. nematodes. 相似文献