全文获取类型
收费全文 | 3127篇 |
免费 | 303篇 |
国内免费 | 34篇 |
专业分类
3464篇 |
出版年
2022年 | 33篇 |
2021年 | 81篇 |
2020年 | 40篇 |
2019年 | 59篇 |
2018年 | 57篇 |
2017年 | 39篇 |
2016年 | 66篇 |
2015年 | 105篇 |
2014年 | 153篇 |
2013年 | 158篇 |
2012年 | 201篇 |
2011年 | 211篇 |
2010年 | 131篇 |
2009年 | 113篇 |
2008年 | 164篇 |
2007年 | 172篇 |
2006年 | 133篇 |
2005年 | 122篇 |
2004年 | 121篇 |
2003年 | 139篇 |
2002年 | 104篇 |
2001年 | 33篇 |
2000年 | 26篇 |
1999年 | 37篇 |
1998年 | 34篇 |
1997年 | 26篇 |
1996年 | 32篇 |
1995年 | 35篇 |
1992年 | 17篇 |
1991年 | 20篇 |
1990年 | 19篇 |
1989年 | 17篇 |
1988年 | 23篇 |
1987年 | 26篇 |
1986年 | 32篇 |
1985年 | 34篇 |
1984年 | 30篇 |
1983年 | 23篇 |
1982年 | 33篇 |
1981年 | 40篇 |
1980年 | 30篇 |
1979年 | 23篇 |
1978年 | 26篇 |
1977年 | 20篇 |
1976年 | 28篇 |
1974年 | 27篇 |
1973年 | 17篇 |
1972年 | 17篇 |
1969年 | 20篇 |
1967年 | 18篇 |
排序方式: 共有3464条查询结果,搜索用时 15 毫秒
81.
Maria Dolores Ramirez-Gonzalez Ewa Widy-Tyszkiewicz Theodore L. Sourkes Guillermina Almazan 《Journal of neurochemistry》1980,35(1):193-201
Abstract: In this work we have studied the mechanism for the increase of adrenal ODC (ornithine decarboxylase, EC 4.1.1.17) activity provoked by oxotremorine, a muscarinic agonist. 1. Oxotremorine increased medullary ODC activity maximally at 2 h. Cortical enzyme responded much more slowly. 2. Blockade of peripheral muscarinic receptors with methylatropine partially reduced the response to oxotremorine in the medulla, but not cortex. 3. Hy-pophysectomy abolished the cortical, but not the medullary, responses to oxotremorine. Methylatropine reduced the effect of oxotremorine on medullary ODC in hypophysectomized rats. 4. In unilaterally splanchnicotomized rats oxotremorine caused an increase of ODC activity of the denervated adrenal gland relative to control value; activities in both medulla and cortex were significantly lower than those observed in the innervated gland. Evidence was obtained for a compensatory increase of ODC activity of the adrenal cortex (but not medulla) on the intact side of unilaterally operated rats. 5. Surgical intervention, in the form of a sham operation for transection of the spinal cord, leads to an increase of ODC activity in both parts of the adrenal gland. Transection of the cord attenuates these increases. 6. The additional increase of medullary ODC activity owing to the administration of oxotremorine to sham-operated rats is partially reduced in the adrenal medulla by muscarinic blockade, and completely in the cortex. This effect of methylatropine in regard to cortical ODC activity was not apparent in the other experiments with intact or unilaterally splanchnicotomized (unoperated side) rats. The results with unilaterally splanchnicotomized rats and those with transected spinal cord suggest that oxotremorine-induced modifications of adrenal ODC activity are centrally mediated, above the level of origin of the splanchnic nerves in the spinal cord (T8–10). Experiments with hypophysectomized rats show that the response of the adrenal cortex to oxotremorine is entirely mediated by the hypophysis. 相似文献
82.
Theodore Dashman 《Life sciences》1980,27(15):1415-1422
The enol-ether amino acid, L-2-amino-4-methoxy--butenoic acid (AMTB) is an inhibitor of porphobilinogen synthase (PBG synthase) when added prior to the addition of the substrate δ-aminolevulinic acid. The inhibition of PBG synthase by several stereoisomers and analogues of AMTB was investigated to determine those structural features of AMTB which may be necessary for inhibition. The D- isomer was also an inhibitor after preincubation, whereas the L- isomer inhibited with or without preincubation. The amino acid analogues, DL-vinylglycine, DL-2-aminobutanoic acid, the reduced form of L-2-amino-4-methoxy--3-butenoic acid, L-2-amino-4-(2-aminoethoxy)--3-butenoic acid and its reduced congener did not inhibit PBG synthase even with preincubation. This structure activity relationship indicates that the double bond and methoxy moiety of L-2-amino-4-methoxy--3-butenoic acid are probably required for inhibition.Heme, when preincubated with PBG synthase, was an inactivator of the enzyme. However, when both L-2-amino-4-methoxy--3-butenoic acid and heme were simulatneously preincubated with PBG synthase, inactivation of the enzyme was greater than with either compound separately. The possibility of multiple catalytic sites was suggested by the use of multiple inhibition kinetics in the presence of heme and L-2-amino-4-methoxy--3-butenoic acid. 相似文献
83.
The substrate specificity of rat skeletal muscle MAO has been studied. By the use of clorgyline as a MAO A inhibitor, it is found that 5-hydroxytryptamine, tryptamine, and kynuramine are deaminated by MAO A whereas benzylamine is a substrate for both forms of MAO. Phenethylamine displays a concentration-dependent preference for the two forms of MAO. These substrate specificies of the two forms of MAO in skeletal muscle are different from those observed in liver and brain but resemble closely that seen with heart. The half-lives of MAO A and MAO B in muscle estimated by rate of recovery from pargyline inhibition are 6.9 and 6.4 days, respectively. 相似文献
84.
Liver tissue grafts between seven H-2 mutants and their parental strains have been studied. Each of these mutants was originally identified by reciprocal mutant—parental strain skin graft rejection. However, liver grafts among mutants and parental standard strains are not uniformly rejected. Liver graft rejection also fails to correlate with mutant—parental stimulation in CML and MLC. In addition, the immune reaction pattern of female mutant animals against grafts of male liver differs from the reaction pattern found in parental standard strains. Several explanations for the differences between immune response to liver and skin grafts are proposed, including different T cell subsets involved in recognition, availability of antigenic sites to immunocompetent cells, and structural differences between mutant and parental H-2 antigens.
Abbreviations used in this paper: bml, 2, 3, 4,14; dml; fm2=mutants of strains C57BL/6, B10.D2 and B10.M respectively; B6=C57BL/6 相似文献
85.
The impact of an industrially contaminated lake on heavy metal levels in its effluent stream 总被引:2,自引:1,他引:1
Levels of cadmium and zinc in various components of Williamson Ditch (an industrially contaminated stream flowing into Palestine Lake), Trimble Creek (a stream draining Palestine Lake) and the Tippecance River (a river receiving Trimble Creek) were determined. Water, sediment, plant, fish and clam samples were analyzed for cadmium and zinc content by atomic absorption spectrophotometry. Unweighted mean metal concentrations in Trimble Creek were the following: water, 51 µg Zn/1 and 4.2 µg Cd/1; sediment, 592 µg Zn/g and 48.8,µg Cd/g; plants, 375 µm Zn/g and 7.91 µg Cd/g; fish, 145 µg Zn/g and 6.02 µg Cd/g. These concentrations were generally lower than those found in Williamson Ditch and higher than those found in the Tippecanoe River or background levels previously reported for other aquatic ecosystems. 相似文献
86.
87.
Susan Schenck Theodore Chase Jr. W. D. Rosenzweig David Pramer 《Applied microbiology》1980,40(3):567-570
A number of species of nematode-trapping fungi, which capture and digest nematodes having keratin and collagen in their cuticles, were tested for the ability to produce extracellular collagenase and keratinase. Collagenase, which is active on ichthyocol, earthworm collagen, and procollagen from chicken embryo fibroblasts, was found in the growth medium of all tested species; keratinase was not found. The enzyme from Arthrobotrys amerospora was concentrated by precipitation with (NH4)2SO4 and further purified by adsorption on collagen at 0°C. The collagenase was active over a pH range of 2.5 to 10.0. It was not inactivated by dialysis against ethylenediaminetetraacetic acid for 48 h or by the sulfhydryl group inhibitors N-ethylmaleimide and p-chloromercuribenzoate. The production of collagenase may aid the fungus to penetrate the cuticle of its prey. 相似文献
88.
Mortimer M. Civan Theodore A. Hall Brij L. Gupta 《The Journal of membrane biology》1980,55(3):187-202
Summary The bulk of the intracellular potassium in mucosal epithelial cells from toad urinary bladder has been previously reported to exchange very slowly with the serosal medium, with a half-time of some 9 hr. This observation, based on chemical analyses of mucosal cell scrapings, has been reexamined with simultaneous diffractive and energy dispersive electron probe X-ray microanalysis. Fifty-three intracellular sites in hydrated sections and 286 sites in dehydrated sections were studied in bladders from eight toads under baseline conditions and after removal of serosal K+ for 83–133 min, with or without 10–2
m ouabain. The baseline data confirm and extend previous examinations of the intracellular ionic composition, and provide the most direct measure of intracellular water thus far available for this tissue. Removal of serosal K+ reduced the intracellular K+ content by 20%, increased intracellular Na+ content threefold, and slightly reduced the intracellular Cl– and water contents, qualitatively consistent with published chemical analyses. The intracellular Na+ content of mucosal origin, measured by radioactive tracers and chemical analyses of cell scrapings, has been reported to be unchanged under these conditions Simultaneous addition of ouabain and removal of external K+ produced a dramatic fall in intracellular K+ of more than 80% in a third of the cells and reduced the mean intracellular K+ content by 60%; 20% of the cells appeared to retain K+ more effectively than the bulk of the epithelial cell population. We conclude that: (i) the low rate of net exchange of intracellular K+ with the serosal bulk solution primarily reflects recycling of K+ across the basolateral membranes, (ii) radioactive tracer and chemical measurements of the intracellular Na+ pool of mucosal origin substantially underestimate the total intracellular Na+ content under certain experimental conditions, and (iii) the epithelial cells display a functional heterogeneity of response to the effects of adding ouabain and withdrawing external K+. 相似文献
89.
90.
Herbert M. Kagan Kathleen A. Sullivan Theodore A. Olsson III Anne L. Cronlund 《The Biochemical journal》1979,177(1):203-214
Lysyl oxidase of bovine aorta was resolved into four enzymically active species by elution from DEAE-cellulose with a salt gradient in 6m-urea, consistent with purification results obtained with enzyme of other tissues [Stassen (1976) Biochim. Biophys. Acta438, 49-60]. In the present study, each of the four peaks of activity was purified to apparent homogeneity by subsequent chromatography on gel-filtration media in 6m-urea. Each enzyme is eluted as a species with mol.wt. approx. 30000 under these conditions, although lysyl oxidase polymerizes to a series of multimers with molecular weights ranging up to 1000000 in the absence of urea. The apparent subunit molecular weight of each enzyme species determined by electrophoresis in sodium dodecyl sulphate and 8m-urea is approx. 32000-33000. The amino acid compositions of the purified forms of lysyl oxidase are similar to each other, although sufficient differences exist to conclude that each is a unique molecular species. Incorporation of alpha-toluenesulphonyl fluoride into the purification scheme does not alter the resolution of enzyme into four species, suggesting that proteolysis during isolation is not the basis of the heterogeneity. The similar sensitivities of each form of enzyme to chelating agents and to semicarbazide and isoniazid indicate that each requires the participation of a metal ion, presumably Cu(2+), and of a carbonyl compound for enzyme function. The present study describes a method for the purification of multiple species of lysyl oxidase and reveals that significant chemical differences exist between the different enzyme forms. 相似文献