全文获取类型
收费全文 | 8797篇 |
免费 | 857篇 |
国内免费 | 468篇 |
出版年
2024年 | 22篇 |
2023年 | 91篇 |
2022年 | 200篇 |
2021年 | 375篇 |
2020年 | 232篇 |
2019年 | 274篇 |
2018年 | 278篇 |
2017年 | 195篇 |
2016年 | 299篇 |
2015年 | 495篇 |
2014年 | 583篇 |
2013年 | 640篇 |
2012年 | 783篇 |
2011年 | 696篇 |
2010年 | 429篇 |
2009年 | 367篇 |
2008年 | 471篇 |
2007年 | 391篇 |
2006年 | 375篇 |
2005年 | 353篇 |
2004年 | 323篇 |
2003年 | 277篇 |
2002年 | 264篇 |
2001年 | 197篇 |
2000年 | 191篇 |
1999年 | 180篇 |
1998年 | 103篇 |
1997年 | 74篇 |
1996年 | 59篇 |
1995年 | 62篇 |
1994年 | 62篇 |
1993年 | 59篇 |
1992年 | 76篇 |
1991年 | 68篇 |
1990年 | 52篇 |
1989年 | 60篇 |
1988年 | 54篇 |
1987年 | 35篇 |
1986年 | 43篇 |
1985年 | 33篇 |
1984年 | 52篇 |
1983年 | 30篇 |
1982年 | 30篇 |
1981年 | 25篇 |
1980年 | 15篇 |
1979年 | 28篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1975年 | 16篇 |
1971年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
The effect of 2-chloroadenosine (2CA) on the binding of alpha 1- and alpha 2-adrenoceptor ligands in the rat vas deferens was investigated. In homogenates of vas deferens, 2CA (10(5)M) increased 3H-clonidine maximal binding sites from an undetectable level to 0.71 +/- 0.08 pmol/g. wet weight or 10.1 +/- 1.1 fmol/mg protein (N=12). This effect lasted for at least 5 hours after removal of 2CA. Concurrent addition of 1.25 mM theophylline completely abolished the effect of 2CA. A similar effect of 2CA on 3H-clonidine binding was observed following incubation of intact tissues with 2CA prior to homogenization. The effect of 2CA were similar in potency in the homogenate to that in the intact organ, suggesting that 2CA-sensitive sites are located on the outer surface of the plasma membrane. The binding of 3H-prazosin was not influenced by the presence of 10(-5)M 2CA. Contractions of isolated vasa deferentia induced by norepinephrine and phenylephrine were not changed by 10(-5)M 2CA, but the inhibition by clonidine of contractions induced by electric stimulation was enhanced by preincubation for 30 min with 10(-5)M 2CA. The results suggest that 2CA increases the number of available alpha 2-adrenoceptors and this interactions occurs, at least in part, presynaptically. 相似文献
52.
Lai Yeap Foo 《Phytochemistry》1984,23(12):2915-2918
The flavonoids and condensed tannins of the heartwood of Acacia baileyana var. purpurea are described. In conformity with other Acacia species, the hydroxylation pattern of the flavonoids is of the resorcinol type but, in sharp contrast, the tannins are heterogeneous consisting of a mixture of the resorcinol and phtoroglucinol series. Dimeric proanthocyanidins of the phloroglucinol type were absent and this exception to the general observation that they invariably co-occur with the polymers may be explained by the relative nucleophilicity of the aromatic A-rings. 相似文献
53.
Oxygen- and carbon-centered free radical formation during carbon tetrachloride metabolism. Observation of lipid radicals in vivo and in vitro 总被引:5,自引:0,他引:5
P B McCay E K Lai J L Poyer C M DuBose E G Janzen 《The Journal of biological chemistry》1984,259(4):2135-2143
Free radical reactions involved in the metabolism of carbon tetrachloride by rat liver have been considered to be a cause of at least part of the injury resulting from exposure to this halocarbon. In an earlier study employing electron spin resonance and spin-trapping techniques, we demonstrated that trichloromethyl (13.CCl3) radicals are readily observed in rat liver microsomes metabolizing 13CCl4, and that the same radical could be shown to form in vivo in the liver of intact rats given a single dose of 13CCl4. This report describes the production of lipid dienyl (L.) and oxygen-centered lipid radicals (LO. or LOO., or both) in in vitro systems metabolizing 13CCl4, and also the formation of lipid dienyl radicals (L.) in liver of intact animals exposed to CCl4. The radicals appear to be produced in a sequence of reactions governed among other things by the oxygen tension in the system. The lipid radicals (L.) which form in intact liver of CCl4-treated rats are apparently the result of an attack on lipids of the endoplasmic reticulum by 13.CCl3 radicals formed by reductive cleavage to CCl4 and are the initial intermediates in the process of lipid peroxidation. These investigations demonstrate that while the events occurring in liver microsomes in vitro appear to parallel those which take place in intact liver in vivo, the conditions in vivo make the spin-trapping studies of radicals in intact animals much more selective than it is in vitro for a given spin trap, and requires the use of more than one type of spin-trapping agent to detect different radical species in vivo. 相似文献
54.
This review is aimed at unifying our understanding of cartilage viscoelastic properties in compression, in particular the role of compression-dependent permeability in controlling interstitial fluid flow and its contribution to the observed viscoelastic effects. During the previous decade, it was shown that compression causes the permeability of cartilage to drop in a functional manner described by k = ko exp (epsilon M) where ko and M were defined as intrinsic permeability parameters and epsilon is the dilatation of the solid matrix (epsilon = tr delta u). Since permeability is inversely related to the diffusive drag coefficient of relative fluid motion with respect to the porous solid matrix, the measured load-deformation response of the tissue must therefore also depend on the non-linearly permeable nature of the tissue. We have summarized in this review our understanding of this non-linear phenomenon. This understanding of these flow-dependent viscoelastic effects are put into the historical perspective of a comprehensive literature review of earlier attempts to model the compressive viscoelastic properties of articular cartilage. 相似文献
55.
A method is described to measure directly in rat brain the activity of pyruvate dehydrogenase kinase (PDHa kinase; EC 2.7.1.99), which catalyzes the inactivation of pyruvate dehydrogenase complex (PDHC, EC 1.2.4.1, EC 2.3.1.12, and EC 1.6.4.3). The activity showed the expected dependence on added ATP and divalent cation, and the expected inhibition by dichloroacetate, pyruvate, and thiamin pyrophosphate. These results, and the properties of pyruvate dehydrogenase phosphate phosphatase (EC 3.1.3.43), indicate that the mechanisms of control of phosphorylation of PDHC seem qualitatively similar in brain to those in other tissues. Regionally, PDHa kinase is more active in cerebral cortex and hippocampus, and less active in hypothalamus, pons and medulla, and olfactory bulbs. Indeed, the PDHa kinase activity in olfactory bulbs is uniquely low, and is more sensitive to inhibition by pyruvate and dichloroacetate than that in the cerebral cortex. Thus, there are significant quantitative differences in the enzymatic apparatus for controlling PDHC activity in different parts of the brain. 相似文献
56.
The Yc and Ya subunits of rat liver glutathione S-transferases are the products of separate genes 总被引:7,自引:0,他引:7
57.
To further examine the role that substance P plays in initiating the observed massive postmortem bronchoconstriction in guinea pig lungs and to explore the role of neural reflex in this airway spasm, six groups of animals were employed: control (n = 6), morphine (n = 6), substance P (n = 5), chronic capsaicin pretreatment + substance P (n = 5), tetrodotoxin (TTX) + acute capsaicin (n = 4), and chlorisondamine + acute capsaicin (n = 5). Pressure-volume curves were performed prior to and following the initiation of artificial pulmonary perfusion with 1% bovine serum albumin and 5% dextran in Tyrode's solution. A decrease in inflation volume (the lung volume between transpulmonary pressure of 0 and 30 cmH2O during inflation) was used as an index of bronchoconstriction. In control animals, inflation volume decreased to 20-30% of the base-line value at 15-30 min of perfusion, indicating massive bronchial constriction during this time period. Morphine (an agent inhibiting substance P release) significantly attenuated the spasm, whereas the presence of substance P in the perfusate markedly enhanced the constriction. Depletion of endogenous substance P by chronic capsaicin pretreatment did not affect exogenous substance P-induced spasm. Acute capsaicin-induced bronchoconstriction was significantly attenuated by TTX but was not affected by the ganglionic blocking agent, chlorisondamine. These data suggest that substance P initiates the massive postmortem bronchoconstriction in guinea pig lungs and that substance P is released by local stimulation of sensory nerve endings via axonal reflex. 相似文献
58.
Y L Lai M W Rana 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1986,181(3):371-381
Retinas of 4-, 10-, and 20-year-old monkeys were studied by light microscopy, electron microscopy, and scanning electron microscopy. Sections from the midperipheral region of every retina were selected for comparison. Although no significant differences were found between 4- and 10-year-old retinas, four major changes were found in 20-year-old monkey retinas: (i) increased number of displaced photoreceptor cells (DPC), (ii) increased number of macrophages of different morphology in subretinal space, (iii) increase in pigment granules in retinal pigment epithelium (RPE) cells, and (iv) altered morphology of Muller cells. DPC included both rods and cones. Their location and morphology depended on the stage of their displacement. These cells were usually oval or rounded in shape and were found either among the outer segments of other photoreceptor cells, having stalks extending into the outer nuclear layer, or were located in the subretinal space and had no stalk. A narrow space around the DPC stalks, indicating a change in the intercellular connection between photoreceptor cells and Muller cells, was observed. Furthermore, the Muller cells related to DPC had shortened and markedly reduced microvilli. Two types of macrophages were found in the subretinal space of aged monkey retinas. One type was similar in morphology to RPE cells. Some of these cells were noticed detaching from RPE. Other types of macrophages were nonpigmented. The modifications in RPE were closely related to the changes in the associated neuroretina. The RPE cells in aged retina were devoid of microvilli or had a few thin microvilli. The pleomorphic pigment granules were dispersed throughout the cytoplasm. These cells varied in their size, shape, and surface features. These changes could significantly alter the retinal metabolic equilibrium and may be indicative of age related degenerative processes. 相似文献
59.
Cloning and sequence analysis of a cDNA for a rat liver glutathione S-transferase Yb subunit. 总被引:7,自引:2,他引:5
下载免费PDF全文
![点击此处可从《Nucleic acids research》网站下载免费的PDF全文](/ch/ext_images/free.gif)
We have isolated a Yb-subunit cDNA clone from a GSH S-transferase (GST) cDNA library made from rat liver polysomal poly(A) RNAs. Sequence analysis of one of these cDNA, pGTR200, revealed an open reading frame of 218 amino acids of Mr = 25,915. The deduced sequence is in agreement with the 19 NH2-terminal residues for GST-A. The sequence of pGTR200 differs from another Yb cDNA, pGTA/C44 by four nucleotides and two amino acids in the coding region, thus revealing sequence microheterogeneity. The cDNA insert in pGTR200 also contains 36 nucleotides in the 5' noncoding region and a complete 3' noncoding region. The Yb subunit cDNA shares very limited homology with those of the Ya or Yc cDNAs, but has relatively higher sequence homology to the placental subunit Yp clone pGP5. The mRNA of pGTR200 is not expressed abundantly in rat hearts and seminal vesicles. Therefore, the GST subunit sequence of pGTR200 probably represents a basic Yb subunit. Genomic DNA hybridization patterns showed a complexity consistent with having a multigene family for Yb subunits. Comparison of the amino acid sequences of the Ya, Yb, Yc, and Yp subunits revealed significant conservation of amino acids (approximately 29%) throughout the coding sequences. These results indicate that the rat GSTs are products of at least four different genes that may constitute a supergene family. 相似文献
60.
Inhibition of protein synthesis enhances transformation of primary cells by viral DNA 总被引:2,自引:0,他引:2
Inhibition of protein synthesis by cycloheximide after transfection and subsequent removal of the drug increased the transformation efficiency of primary cells by plasmids containing the left 4.5, 6.7, or 16% of the adenovirus (Ad) genome. The enhancement factor ranged from 2 to as much as 70 depending on the size of the viral DNA fragments used. Addition of cycloheximide before or at the time of transfection inhibited transformation, suggesting that viral protein synthesis is important during the early phase of transformation. Transient expression assays showed that cells treated with cycloheximide post-transfection contained as much as three times the amount of viral RNA transcribed from regions E1A and E1B. Conversion of a rat cell line lacking thymidine kinase activity (TK-) to the TK+ phenotype by a plasmid containing the herpes TK gene was severely inhibited by the drug treatment, suggesting that the enhancement effects of cycloheximide on transformation may be specific for Ad DNA. Cycloheximide treatment also increased the number of transformants induced by a transformation defective E1B mutant of Ad12 (cyt mutant). Plasmid containing only the E1A region of Ad12 transformed primary rat kidney cells with very low efficiency. The inclusion of E1B in the transfecting DNA fragments increased the transformation frequency by more than 400-fold, much higher than that achieved by cycloheximide. Thus, cycloheximide cannot replace E1B functions in transformation efficiency. 相似文献