全文获取类型
收费全文 | 49002篇 |
免费 | 4179篇 |
国内免费 | 8篇 |
专业分类
53189篇 |
出版年
2023年 | 244篇 |
2021年 | 406篇 |
2020年 | 342篇 |
2019年 | 341篇 |
2018年 | 944篇 |
2017年 | 968篇 |
2016年 | 1095篇 |
2015年 | 1008篇 |
2014年 | 1181篇 |
2013年 | 2015篇 |
2012年 | 3222篇 |
2011年 | 3549篇 |
2010年 | 1854篇 |
2009年 | 1236篇 |
2008年 | 2916篇 |
2007年 | 2965篇 |
2006年 | 2757篇 |
2005年 | 2530篇 |
2004年 | 2422篇 |
2003年 | 2267篇 |
2002年 | 2249篇 |
2001年 | 1596篇 |
2000年 | 1793篇 |
1999年 | 1038篇 |
1998年 | 499篇 |
1997年 | 408篇 |
1996年 | 469篇 |
1995年 | 406篇 |
1994年 | 432篇 |
1993年 | 382篇 |
1992年 | 524篇 |
1991年 | 443篇 |
1990年 | 426篇 |
1989年 | 400篇 |
1988年 | 374篇 |
1987年 | 379篇 |
1986年 | 337篇 |
1985年 | 407篇 |
1984年 | 455篇 |
1983年 | 402篇 |
1982年 | 367篇 |
1981年 | 338篇 |
1980年 | 303篇 |
1979年 | 315篇 |
1978年 | 304篇 |
1977年 | 285篇 |
1976年 | 279篇 |
1975年 | 336篇 |
1974年 | 273篇 |
1973年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
981.
Dieter Kabelitz Thomas H. Tötterman Magnus Gidlund Kenneth Nilsson Hans Wigzell 《Cellular immunology》1982,70(2):277-286
Stimulatory effects of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) on human T lymphocytes have been investigated. TPA was found to stimulate highly purified T cells (obtained by a three-step isolation procedure involving plastic adherence, nylon wool passage and Ig-anti-Ig column passage) in the absence of accessory cells (stimulation index of 5 to 10), whereas phytohemaglutinin (PHA) and concanavalin A (Con A) did not. This response was, however, increased by the addition of autologous adherent cells. Addition of TPA, but not adherent cells, induced T-cell proliferation in response to the nonmitogenic lectin, wheat germ agglutinin (WGA), while both adherent cells and TPA restored T-cell proliferation to mitogenic lectins such as PHA and Con A. Furthermore, TPA greatly increased the mixed-lymphocyte response of purified T cells to otherwise nonstimulating allogeneic cells such as T lymphocytes or tumor cells from some patients with chronic lymphocytic leukemia. These results suggest that TPA can directly act on human T cells to render them reactive to a variety of stimuli. 相似文献
982.
Induction and repair of DNA strand breaks in cultured human fibroblasts exposed to various phenols and dihydrodiols of benzo[a]pyrene 总被引:1,自引:0,他引:1
Cultured human fibroblasts from healthy donors were incubated for 30 min with nine different benzo[a]pyrene (BP) derivatives in the presence or absence of liver microsomes from 3-methylcholanthrene treated rats. The induction and repair of DNA strand breaks were analysed by alkaline unwinding and separation of double and single stranded DNA (SS-DNA) by hydroxylapatite chromatography immediately after the incubation or at various times after the treatment. In the absence of microsomes DNA stand breaks were detected in fibroblasts exposed to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP) and the three BP dihydrodiols (BP-4,5-, BP-7,8- or BP-9,10-dihydrodiol). After removal of the BP derivatives from the medium the DNA strand breaks disappeared within 24 h. alpha-Naphthoflavone (alpha-NF) caused a decrease in the induction of strand breaks by 1-, 3- and 9-OH-BP but did not affect the induction of strand breaks in cells exposed to BP-7,8-dihydrodiol. In the presence of microsomes DNA strand breaks were found after exposure to 30 microM of each of the six BP phenols (1-, 2-, 3-, 7-, 9- or 11-OH-BP), as well as BP-7,8- and 9,10-dihydrodiol. In contrast BP-4,5-dihydrodiol did not induce strand breaks under these conditions. The induction of strand breaks by BP-7,8-dihydrodiol was enhanced in the presence of cytosine-1-beta-D-arabinofuranoside (AraC). In all cases the DNA strand breaks had disappeared 24 h after removal of the BP derivatives and microsomes except after treatment with BP-7,8-dihydrodiol. 相似文献
983.
Monoclonal antibodies directed against nonhistone chromosomal proteins of D. melanogaster were tested for crossreactivity with the homologous antigens of various Drosophila species. — By indirect immunofluorescence it could be shown that three antibodies react only with polytene chromosomes of species of the D. melanogaster subgroup, and only much less with chromosomes of other species of Drosophila. — With chromosomes of various other species of the Sophophora or Drosophila radiations only a reaction at background level could be observed. — The results suggest that the three antibodies react with different antigenic determinants of a single protein whose conformation changed rather fast during evolution of the Drosophilidae. 相似文献
984.
Géza Fejes-Tóth Anikó Náray-Fejes-Tóth Bart Rigter Jürgen C. Frölich 《Prostaglandins & other lipid mediators》1983,25(1):99-103
To study the relationship between urine flow, urinary prostaglandin (PG) and kallikrein excretion in the rat high urine flow was induced in hydropenic Long-Evans rats by either hypotonic volume expansion or with manniitol or with furosemide. PGE, excretion remained unchanged during hypotonic volume expansion (134.5 ± 29.7 before and 153.0 ± 48.9 pg/min after) while it decreased significantly with mannitol (from 166.3 ± 32.4 to 45.2 ± 8.2 pg/min, p<0.01) and with furosemide (from 170.0 ± 20.4 to 29.5 ± 5.3 pg/min, p<0.001). PGF2α excretion rates were slightly reduced following all three interventions. Urinary kallikrein excretion remained unchanged in all three groups of animals. It is concluded that, in contrast to human and dogs in the rat urine flow and urinary PG excretion are not interlinked. 相似文献
985.
The macrobenthic fauna associated withFucus at a station in the Kiel Fjord was investigated from June 1978 until June 1979. The predominant group in number as well as in biomass were gammarids. They formed, together with the isopodsIdotea spp., approximately 95 % of the total average annual biomass. The total dry weight of all macrobenthic animals (excl.Littorina spp.) increased from 1.9 g per kgFucus in May to about 16 g in June–August, and dropped to 8.3 g in September. Winter average dry weight values were only about 6 % of the summer values. 相似文献
986.
J. Beitz M. Panse S. Fischer C. Hora W. Förster 《Prostaglandins & other lipid mediators》1983,26(6):885-892
A hypothesis of Gryglewski et al. explains the correlation between increased level of LDL and development of atherosclerosis by inhibition of PGI2 synthesis by increased peroxide content of LDL. The aim of the present paper was to examine this hypothesis. The major results are: 1) Preparation of LDL in the presence of .02 % butylated hydroxytoluene did not reduce the lipid peroxide content of LDL from men and women and not change the inhibition or stimulation of the in vitro biosynthesis of PGI2 by LDL isolated from blood of men or women, respectively. 2) In the LDL and HDL, respectively, of healthy men we found nearly the same lipid peroxide levels (nmole malondialdehyde (MDA)/mg lipoprotein-cholesterol) as in the lipoproteins of male patients with hyperlipidemia type IIa or IV, but the peroxide concentration is three times higher in HDL as in LDL. 3) LDL isolated from healthy men inhibited in dose dependent fashion the generation of PGI2 from PGH2 by aortic microsomes whereas LDL from premopausal women stimulated PGI2 formation even calculated as LDL lipid peroxides (in nM MDA/ml). The results call into question the hypothesis that diminished PGI2 formation by atherosclerotic vessels is related to inhibition of PGI2 synthetase by lipid peroxides present in LDL in the lesions. A new working hypothesis is presented that also the fatty acid pattern and the lipid class composition in the LDL are important for their influence on the PGI2 formation. 相似文献
987.
Hans Lindblom U.-B. Axiö-Rredriksson Edward H. Cooper Ron Turner 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》1983,273(1):107-116
Proteins excreted in urine due to renal failure were separated on Mono QTM, a new strong anion exchanger designed for fast high-resolution protein separations. The separation procedure was divided into two steps. The first step involved removal of low-molecular- weight substances by rapid desalting on a Sephadex G-25 Superfine column. In the second step, the total protein fraction (3–6 ml) was loaded onto the Mono Q column with the aid of a superloop. The proteins were adsorbed onto the top of the ion-exchanger column and gradually displaced by a combined pH and salt gradient in 40 min. The choice of ion exchanger and initial operating conditions were based on data obtained from electrophoretic titration curve experiments. Identification of separated proteins was achieved by fused rocket electrophoresis and sodium dodecyl sulphate-polyacrylamide gel electrophoresis, respectively. 相似文献
988.
989.
Forslund Kristina Björkman Camilla Abrahamsson Maivor 《Acta veterinaria Scandinavica》1983,24(2):185-199
Plasma cholinesterase (pChE) levels and erythrocyte acetylcholinesterase (eAChE) levels were studied in 6 cows before, during and after parturition (Group I), their calves (Group II), 38 cows suffering from parturient paresis (Group III) and 14 newly delivered non-paretic cows (Group IV). The mean of the pChE level in Group I was 1.5 μkat/1 ± 0.20 before parturition and decreased significantly (P ≦ 0.05) to 1.2 ukat/1 ± 0.16 after parturition. The eAChE level was before parturition ≅ 140 ukat/1 and decreased to ≅ 130 μkat/1 4–5 weeks after parturition. At birth the pChE level was 12.8 ukat/1 ± 5.9 in Group II. After 4 weeks the level had decreased to 2.3 ukat/1 ±0.3. In the bull calves the pChE level started to increase when they were 6 weeks old and reached a level of 5.7 μkat/1 ± 0.6 before slaughter at 6 months of age. The heifers did not show this increase. They had a level of around 2 μkat/1 throughout the investigation. The eAChE level at birth was 119 μkat/1 and increased slowly to a level of 145 μkat/1 at 6 months. No differences between the sexes were found. The cows suffering from parturient paresis had a pChE level of 1.80 μkat/1 ± 0.30 before treatment with calcium (Ca). The level decreased significantly (P ≦ 0.001) after Ca-infusion to a level of 1.67 ukat/1 ±0.29. Group IV had a pChE level of 1.65 μkat/1 ± 0.42 at parturition. Two to 4 months later the cows that had recovered from milk fever had a level of 1.61 μkat/1 ± 0.31 and the control cows 1.66 ukat/1 ± 0.48. No differences between the groups were found for the eAChE level. The findings show that parturition influences the pChE level in cows and that sex influences the pChE level in calves between 6 weeks to at least 6 months of age. Furthermore the elevated pChE level found in the cows suffering from parturient paresis before Ca infusion may be a further sign of a disturbance in the cholinergic system with a special preference to the neuromuscular junctions. 相似文献
990.
M B Wikstr?m 《Applied and environmental microbiology》1983,45(2):393-400
A screening technique for microbial proteases, the thin-layer enzyme assay cultivation technique, was developed. The inner surface of a polystyrene petri dish was coated with protein and then covered with a culture agar medium. The enzymes, produced during growth of the microorganisms, reach the protein-coated surface by diffusion in the agar. Degradation of the protein was visualized by condensation of water vapor on the surface after removal of the agar medium. The wettability of the enzyme-affected protein-coated polystyrene surface was decreased compared with the unaffected protein surface. Enzyme substrates used were fibrinogen, immunoglobulin G, egg albumin, human serum albumin, bovine serum albumin, hemoglobin, mucin, and gelatin. It was possible to use a variety of culture agar media, nonselective as well as selective, in the assay. The technique provides a sensitive, convenient, and inexpensive method for screening various microbial proteases. In addition, the technique can be used for screening proteolytic enzyme activity of specific microbial species in a mixed microbial sample as well as for studies of factors that influence the cultivation conditions for protease production and activity. 相似文献