全文获取类型
收费全文 | 16232篇 |
免费 | 1317篇 |
国内免费 | 2篇 |
出版年
2023年 | 48篇 |
2022年 | 146篇 |
2021年 | 272篇 |
2020年 | 124篇 |
2019年 | 219篇 |
2018年 | 185篇 |
2017年 | 201篇 |
2016年 | 377篇 |
2015年 | 622篇 |
2014年 | 738篇 |
2013年 | 892篇 |
2012年 | 1232篇 |
2011年 | 1294篇 |
2010年 | 832篇 |
2009年 | 693篇 |
2008年 | 1093篇 |
2007年 | 1136篇 |
2006年 | 957篇 |
2005年 | 959篇 |
2004年 | 931篇 |
2003年 | 946篇 |
2002年 | 844篇 |
2001年 | 140篇 |
2000年 | 103篇 |
1999年 | 147篇 |
1998年 | 203篇 |
1997年 | 150篇 |
1996年 | 123篇 |
1995年 | 119篇 |
1994年 | 116篇 |
1993年 | 126篇 |
1992年 | 119篇 |
1991年 | 104篇 |
1990年 | 90篇 |
1989年 | 77篇 |
1988年 | 76篇 |
1987年 | 68篇 |
1986年 | 66篇 |
1985年 | 91篇 |
1984年 | 102篇 |
1983年 | 84篇 |
1982年 | 84篇 |
1981年 | 101篇 |
1980年 | 90篇 |
1979年 | 54篇 |
1978年 | 55篇 |
1977年 | 63篇 |
1976年 | 49篇 |
1975年 | 32篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
72.
Jeffrey D. Macklis Richard L. Sidman H. David Shine 《In vitro cellular & developmental biology. Plant》1985,21(3):189-194
Summary A new type of collagen surface for use with cultures of peripheral nervous system cells is described. Collagen is derivatized
to plastic culture dishes by a cross-linking reagent, 1-cyclohexyl-3-(2-morpholinoethyl)-carbodiimide-metho-p-toluenesulfonate (carbodiimide), to form a uniform and durable surface for cell attachment and growth that allows dry storage,
long-term culture, and improved microscopy. Surfaces of collagen derivatized to plastic were compared to surfaces of adsorbed
or ammonia-polymerized collagen in terms of collagen binding and detachment, growth by dorsal root ganglion cells, and electron
microscopy appearances. Derivatized collagen surfaces retained more collagen and showed much less evidence of degradation
and cellular damage over periods of many weeks than did conventional adsorbed surfaces. Long-term survival of cells on derivatized
collagen was far superior to that on the other surfaces, with almost 90% of cultures still viable after 10 wk. Transmission
electron microscopy showed an organized layer of single fibrils that supported cell growth well, and scanning electron microscopy
demonstrated an increased uniformity of derivatized collagen surfaces compared to ammoniated collagen surfaces. Applications
for this improved substrate surface are discussed.
This work was supported by the Leopold Schepp Foundation, the Dysautonomia Foundation, National Institutes of Health Grants
NS14768 and NS11237, and Institutional Core Grant HD06276. 相似文献
73.
The crystal structure of ethyl 2,3-dideoxy-alpha-D-erythro-hex-2-enopyranoside, C8H14O4, is orthorhombic, P2(1)2(1)2(1), with cell dimensions at 123 K [293 K] a = 11.220(2) [11.319(1)], b = 18.387(3) [18.458(2)], c = 8.509(2) [8.635(1)] A, Z = 8. There are two symmetry-independent molecules in the asymmetric unit. In both molecules, the conformation is oH5. The alkenic bond is almost exactly planar in one molecule, with C-1--C-2--C-3--C-4 = +0.8 degrees. In the other molecule, this torsion angle is +3.7 degrees. The glycosidic torsion angle, O-5--C-1--O-1--C-7, has normal exoanomeric values of +71 and +64 degrees. The conformation of the ethoxyl group is extended, with C-1--O-1--C-7--C-8 = +162 and +170 degrees. The primary alcohol group has different orientations, g/t on one molecule, g/g on the other. The characteristic glycosidic bond-shortening observed in the pyranosides is modified in this enopyranoside. Both the ring bond, O-5--C-1, and the glycosidic bond, C-1--O-1, are short, with distances ranging from 1.409 to 1.425 A. Solution and solid-state c.p.-m.a.s., 13C-n.m.r. spectra are reported. 相似文献
74.
Summary Ramets of the understory herb, Aster acuminatus, were transplanted from two source populations into eight understory garden sites that varied in light and soil moisture levels. Ramet growth, clonal growth, flowering and survivorship were monitored for three growing seasons. Large differences among gardens in ramet growth, clonal growth and flowering developed in the first growing season and increased in the next two years. This variation was positively correlated with garden light level but not at all with soil moisture. Mortality rates were low in all gardens and showed that genets from any particular source could survive over a broad range of environmental conditions. There was no conclusive evidence for any source population differences in the capacity to survive or grow in different environments. The rapid, light-induced responses of transplanted ramets resulted in garden populations very similar in appearance to natural populations experiencing similar light regimes. These results combined with those from other studies of A. acuminatus provide strong evidence for the importance of light in explaining population patterns and dynamics in this species. 相似文献
75.
It has been demonstrated previously with the vas deferens of the guinea-pig that the first and second phases of the contractile response to motor nerve stimulation are preferentially antagonized by the P2-purinoceptor antagonist arylazido aminopropionyl ATP (ANAPP3), and the α1-adrenoceptor antagonist prazosin, respectively. We have now investigated the effect of the two antagonists on the biphasic contraction in the vas deferens of two other species; rabbit and rat. ANAPP3, in a concentration which antagonized responses to exogenously applied ATP but not those to exogenous norepinephrine, preferentially reduced the initial phasic response of the rabbit vas deferens to motor nerve stimulation without significantly reducing the secondary, tonic phase of the response. Prazosin had the opposite effect; antagonizing the response to norepinephrine but not to ATP and reducing the tonic response to motor nerve stimulation without significantly reducing the initial phasic response. Results obtained with the rat vas deferens were similar. The present results combined with previous findings suggest that ATP and norepinephrine act as cotransmitters in the vas deferens of several species. 相似文献
76.
The effect of the glutamine synthetase (GS) inhibitor, methionine sulfoximine (MSO), on glutamate levels in, and glutamate release from, rat striatal tissue was examined. Tissue levels of glutamate were unchanged 24 h after an intraventricular injection of MSO, but tissue glutamine levels were decreased 50%. Calcium-dependent, potassium-stimulated glutamate release was diminished in tissue prisms from animals pretreated with MSO compared to controls. The decreased release of glutamate correlated over time with the inhibition of GS following an intraventricular injection of MSO. The maximum diminution of calcium-dependent, potassium-stimulated glutamate release (50%) and the maximum inhibition of GS activity (51%) were observed 24 h after MSO. The addition of 0.5 mM glutamine to the perfusion medium completely reversed the effects of MSO pretreatment on calcium-dependent, potassium-stimulated glutamate release. Since GS is localized in glial cells and the measured glutamate release is presumed to occur from neurons, the data support the contention that astroglial glutamine synthesis is an important contributor to normal neuronal neurotransmitter release. 相似文献
77.
Regulation of Calmodulin- and Dopamine-Stimulated Adenylate Cyclase Activities by Light in Bovine Retina 总被引:1,自引:1,他引:0
Margaret E. Gnegy Nancy Muirhead Jeffrey K. Harrison 《Journal of neurochemistry》1984,42(6):1632-1640
Abstract: Neural retina from most species contains 3,4-dihydroxyphenylethylamine (dopamine) receptors coupled to stimulation of adenylate cyclase activity. It has been demonstrated that release of dopamine from its neurons and subsequent occupation of dopamine receptors is increased by light. In this study, we have shown that adenylate cyclase activity in bovine retina is highly responsive to the endogenous Ca2+-binding protein, cal-modulin, and that calmodulin can increase dopamine-sen-sitive adenylate cyclase activity in bovine retina. We further demonstrate that both dopamine- and calmodulin-stimulated adenylate cyclase activities can be regulated by alterations in light. Bovine retinas were dissected from the eye under a low-intensity red safety light, defined as dark conditions, and incubated for 20 min in an oxygenated Krebs Henseleit buffer under either dark or light conditions. The retinas were then homogenized and adenylate cyclase activity measured in a paniculate fraction washed to deplete it of endogenous Ca2+ and calmodulin. Activation of adenylate cyclase activity by calmodulin, dopamine, and the nonhydrolyzable GTP analog, gua-nosine-5′-(β,γ-imido)triphosphate (GppNHp), was significantly (60%) greater in paniculate fractions from retinas that had been incubated under dark conditions as compared to those incubated under light conditions. Basal, Mn2+-, and GTP-stimulated adenylate cyclase activities were not altered by changes in lighting conditions. Calmodulin could increase the maximum stimulation of adenylate cyclase by dopamine in retinas incubated under either dark or light conditions, but the degree of its effect was greater in retinas incubated under light conditions. Activation of adenylate cyclase by calmodulin, dopamine, and GppNHp in paniculate fractions from retinas incubated under light conditions was indistinguishable from the activation obtained when retinas were incubated in the dark in the presence of exogenous dopamine. These results suggest that an increased release of dopamine occurs in light. The decreased response of adenylate cyclase to exogenous dopamine can then be explained by a subsequent down-regulation of dopamine receptor activity. The down-regulation of dopamine receptor activity can also regulate activation of adenylate cyclase by GppNHp and calmodulin. The results suggest that dopamine, calmodulin, and GppNHp are modulators of a common component of adenylate cyclase activity, and this component is regulated by light. 相似文献
78.
Paola Strocchi Victor S. Sapirstein † Charles S. Rubin ‡ Jeffrey M. Gilbert 《Journal of neurochemistry》1984,43(2):466-471
Cyclic-AMP-binding proteins in membrane and soluble fractions from rat forebrain were compared; membrane fractions included smooth and rough microsomes and a plasma membrane fraction enriched in synaptic membranes. Protein fractions were treated with 8-azido-[32P]cyclic AMP and ultraviolet irradiation to covalently tag cyclic-AMP-binding proteins. Labeled proteins were then analyzed by two-dimensional gel electrophoresis (2DGE) and fluorography. The soluble CNS proteins contained two major cyclic-AMP-binding species at 48K (48K 5.5 and 48K 5.45), differing slightly in their isoelectric points. Another protein was seen at 54K (54K 5.3) adjacent to the beta-tubulin subunits in the 2D electrophoretogram. The analysis of the smooth microsome and plasma membrane fractions differed from the soluble fraction in that there were two cyclic-AMP-binding proteins adjacent to the beta-tubulin region (54K 5.3 and 52K 5.3) differing slightly in apparent molecular weight. The membrane fractions also contained a cyclic-AMP-binding protein at 54K 5.8. The 52K 5.3 and 54K 5.8 species were unique to the membrane fractions. The rough microsomes did not contain detectable amounts of cyclic-AMP-binding proteins. Free polysomes were isolated from brain tissue, and translation products were analyzed by cyclic AMP affinity chromatography and immunopurification with antibodies to the brain specific type II regulatory subunit. The translation products that were found to bind cyclic AMP Sepharose are as follows: 48K 5.5, 48K 5.45, 52K 5.3, and 54K 5.8. These species comigrated with proteins that were photoaffinity-labeled in cytosol and membrane fractions.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
79.
David Raeburn Douglas W.P. Hay Roseanna M. Muccitelli Richard D. Dey Jeffrey S. Fedan 《Prostaglandins & other lipid mediators》1987,33(5)
Guinea-pig tracheal strip preparations containing cartilage, placed under an applied load
, develop tone spontaneously. The finding that spontaneous tone is reduced by indomethacin suggests that one or more prostanoids are involved in the development of spontaneous tone in this species. In this study we examined the effects of removing the cartilage component of the preparations on changes in tone induced by indomethacin and isoproterenol. In contrast to preparations containing cartilage, tissues devoid of cartilage, did not develop tone after the application of an initial 1 g resting load. Indomethacin (1 μM) reduced resting tone by 0.62 ± 0.14 g in cartilage-containing tissues but, in contrast, reduced tone by only 0.03 ± 0.01 g in tissues devoid of cartilage. Furthermore, relaxation responses (0.38 ± 0.05 g) to isoproterenol (1 μM) could be produced in cartilage-containing preparations but not in cartilage-free preparations. Radioimmunoassays indicated that the release of PGE2, PGF2α and 6-keto PGF1α, the end-product of PGI2 breakdown, was diminished in preparations lacking cartilage. Thus, in guinea-pig airway preparations cartilage is apparently a source of sufficient prostanoids to induce spontaneous tone 相似文献
80.