全文获取类型
收费全文 | 11706篇 |
免费 | 1106篇 |
国内免费 | 5篇 |
出版年
2023年 | 60篇 |
2022年 | 164篇 |
2021年 | 328篇 |
2020年 | 163篇 |
2019年 | 235篇 |
2018年 | 265篇 |
2017年 | 231篇 |
2016年 | 334篇 |
2015年 | 644篇 |
2014年 | 636篇 |
2013年 | 727篇 |
2012年 | 983篇 |
2011年 | 962篇 |
2010年 | 568篇 |
2009年 | 528篇 |
2008年 | 686篇 |
2007年 | 663篇 |
2006年 | 555篇 |
2005年 | 516篇 |
2004年 | 517篇 |
2003年 | 529篇 |
2002年 | 463篇 |
2001年 | 106篇 |
2000年 | 77篇 |
1999年 | 100篇 |
1998年 | 128篇 |
1997年 | 72篇 |
1996年 | 75篇 |
1995年 | 77篇 |
1994年 | 72篇 |
1993年 | 62篇 |
1992年 | 71篇 |
1991年 | 72篇 |
1990年 | 52篇 |
1989年 | 53篇 |
1988年 | 47篇 |
1987年 | 41篇 |
1986年 | 48篇 |
1985年 | 73篇 |
1984年 | 52篇 |
1983年 | 59篇 |
1982年 | 48篇 |
1981年 | 48篇 |
1980年 | 47篇 |
1979年 | 47篇 |
1978年 | 51篇 |
1977年 | 44篇 |
1975年 | 38篇 |
1974年 | 43篇 |
1973年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
201.
Philip Nicholas 《BMJ (Clinical research ed.)》1980,281(6248):1145-1146
202.
Collagen-induced platelet aggregation and thromboxane release is inhibited, in a concentration response relationship, by preincubation of gel-filtered platelets with melatonin in the concentration range 430 nM – 4.3 mM. Inhibition of platelet aggregation and thromboxane release also occurs in the presence of indomethacin (4.3 nM – 4.3 mM), a known potent inhibitor of prostaglandin synthesis. Arachidonic acid-induced platelet aggregation and thromboxane release was inhibited in the presence of 4.0 mM melatonin. We therefore propose that inhibition of prostaglandin synthesis maybe the mechanism by which melatonin expresses its activity. Its antigonadotropic activity may result from inhibition of PGE2 synthesis in the hypothalamus and median eminence. 相似文献
203.
Fifty-two patients with axillary abscesses were seen during two years. Staphylococcus aureus was isolated from 34, anaerobic bacteria from 12, and skin flora from five; in one case the pus was sterile. Seven patients with hidradenitis suppurativa had recurrent infection with abscess formation, which was bilateral in three. Anaerobes were isolated in five of these cases and skin flora alone in two. Anaerobes are secondary invaders in this condition, and histological examination shows that the primary abnormality is obstruction of pilosebaceous follicles and apocrine glands, associated with keratin plugging of the follicles. Chemotherapy offers little hope of cure, although metronidazole removes the offensive smell of the discharge. Radical surgery is usually indicated. 相似文献
204.
Warren I. Schaeffer A. H. L. Chuang Nicholas Heintz Edward Bresnick 《In vitro cellular & developmental biology. Plant》1979,15(6):437-440
Summary We have tested the sensitivity of a cloned rat hepatocyte line, RL-PR-C, to aflatoxin B1 and benzo(a)pyrene as a function of population-doubling level. The cells were much more sensitive to the cytotoxic action
of these agents subsequent to 230 population doublings. This sensitivity corresponded to the enhanced inducibility of arylhydrocarbon
hydroxylase activity by 3-methylcholanthrene.
Supported by Grants CA 21258 and CA 12056 from the National Cancer Institute. 相似文献
205.
Glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2) from Anabaena cylindrica was inhibited by alanine, glycine, serine and aspartate. The effects of alanine and serine were uncompetitive with respect to glutamate, while those of glycine and asparatate were uncompetitive with respect to glutamate, while those of glycine and aspartate were non-competitive and mixed type respectively. Different pairs of amino acids and their various combinations caused a cumulative inhibition of the enzyme activity. Glutamine synthetase was also inhibited by ADP and AMP and both nucleotides affected the enzyme competitively with respect to ATP and non-competitively for glutamate. Inorganic pyrophosphate, between 2 and 3 mM, produced a very pronounced inhibiton of enzyme activity. The inhibition by PPi was uncompetitive for ATP. Various combinations of the adenine nucleotides, PPi and Pi exerted a cumulative inhibitory effect on the enzyme activity, as did the amino acids, in different combinations with either adenine nucleotides, PPi or Pi. The effects of the adenine nucleotides and the amino acids were more pronounced at higher concentrations of ammonia. Except for serine similar responses of these effectors were obtained with increasing concentrations of Mg2+. It is proposed that changes in the free concentrations of Mg2+ are important in energy-dependent regulation of the enzyme activity in this alga. 相似文献
206.
Summary Crude cell-free extracts of Anabaena cylindrica synthesized adenosine-5-phosphosulphate (AP35S) and 3-phosphoadenosine-5-phosphosulphate (PAP35S) from 35SO4
2- in the presence of Mg2+, ATP and inorganic pyrophosphatase. Maximum AP35S and PAP35S were produced at pH 7.15 and 8.05, respectively. APS kinase was detected in the supernatant of crude cell-free extracts by a spectrophotometric procedure. ATP-Sulphurylase had an absolute requirement for Mg2+ and less than 30% AP35S was formed when Mg2+ was replaced by either Mn2+ or Co2+. Nucleotide triphosphates other than ATP and 2-deoxyATP were ineffective in this reaction. Maximum enzyme activity was observed at equimolar concentrations of Mg2+ and ATP and excess of either of these was inhibitory. Other nucleotide triphosphates, like GTP, UTP, CTP, TTP, ITP, or 2-deoxyATP also inhibited the enzyme activity. Inhibition by GTP was competitive with respect to ATP. ATP-sulphurylase activity was not affected by cysteine, methionine or glutathione.Abbreviations APS
adenosine-5-phosphosulphate
- PAPS
3-phosphoadenosine-5-phosphosulphate 相似文献
207.
Activities of enzymes which mediate the oxidation of thiosulfate to sulfate and the assimilation of sulfate to sulfide were assayed in various cell-free fractions of Thiobacillus ferrooxidans grown autotrophically on either ferrous iron or thiosulfate or heterotrophically on glucose. There was no activity of the thiosulfate-oxidizing enzyme in extracts of bacteria grown with ferrous iron. Comparable activities for ATP-sulfurylase (EC 2.7.7.4), ADP-sulfurylase (EC 2.7.7.5), and adenylate kinase (EC 2.7.4.3) were found in the bacteria grown autotrophically with either Fe2+ or S2O32- or heterotrophically with glucose. 相似文献
208.
Abstract— The uptake of [14 C]GABA, [14 C]taurine, [3 H] β -alanine and [14 C]dopamine was compared in slices of rat cerebral cortex of three different sizes (0.1 × 0.1 × 2 mm, 0.2 × 0.2 × 2 mm and 0.4 × 0.4 × 2 mm prepared with a mechanical tissue chopper). [14 C]Taurine and [3 H] β -alanine uptake increased whereas [14 C]GABA uptake decreased with increasing slice size. [14 C]Dopamine uptake was optimal in 0.2 × 0.2 × 2 mm slices. Increasing slice size was shown to decrease inhibition of [3 H] β -alanine and [14 C]GABA uptake by l -2,4-diaminobutyric acid. Lactate dehydrogenase activity increased with increasing slice size indicating decreased tissue damage or increased cellular integrity. The possibility that varying slice size can be used to distinguish between neuronal and glial uptake is discussed. It is suggested that taurine uptake in the cerebral cortex is predominantly glial. 相似文献
209.
210.
On incubation Days 9, 11, 12, 14, or 15, chick embryos were injected intravenously with 4.0 × 106L. donovani amastigotes. Embryos were incubated at 33 C immediately after infection. Numbers of amastigotes found in the liver 1 hr after injection increased as the age of embryo recipients increased. Most 14- or 15-day infected embryos hatched when allowed to do so, but many younger embryos were unable to survive at 33 C. Numbers of amastigotes in the liver of chicks, hatched after infection as embryos, decreased as the cloacal temperature of the chicks increased. Despite a 31 C incubation temperature, chicks exhibited a mean 38.3 C cloacal temperature 1 day after hatching.Chick fibroblast cultures were initiated as explants of embryo brain and infected with amastigotes from hamster spleen. Only amastigotes were seen in cultures kept at 37 C, but extracellular promastigotes and intracellular amastigotes were present in cultures at 33 C. Although promastigotes increased in number in the medium overlay at 33 C, amastigotes decreased in number at 33 C and 37 C. One intracellular amastigote was seen in a culture which had been incubated at 25 C after inoculation with promastigotes. 相似文献