全文获取类型
收费全文 | 420103篇 |
免费 | 43762篇 |
国内免费 | 196篇 |
出版年
2016年 | 4519篇 |
2015年 | 5969篇 |
2014年 | 7071篇 |
2013年 | 10629篇 |
2012年 | 11505篇 |
2011年 | 11978篇 |
2010年 | 8155篇 |
2009年 | 7484篇 |
2008年 | 10691篇 |
2007年 | 11345篇 |
2006年 | 10682篇 |
2005年 | 10159篇 |
2004年 | 10257篇 |
2003年 | 10020篇 |
2002年 | 9820篇 |
2001年 | 17350篇 |
2000年 | 17399篇 |
1999年 | 14053篇 |
1998年 | 4979篇 |
1997年 | 5286篇 |
1996年 | 5012篇 |
1995年 | 4728篇 |
1994年 | 4634篇 |
1993年 | 4630篇 |
1992年 | 12270篇 |
1991年 | 12074篇 |
1990年 | 12085篇 |
1989年 | 11861篇 |
1988年 | 11169篇 |
1987年 | 10503篇 |
1986年 | 9822篇 |
1985年 | 10241篇 |
1984年 | 8441篇 |
1983年 | 7275篇 |
1982年 | 5613篇 |
1981年 | 5021篇 |
1980年 | 4694篇 |
1979年 | 8082篇 |
1978年 | 6302篇 |
1977年 | 5940篇 |
1976年 | 5673篇 |
1975年 | 6130篇 |
1974年 | 6698篇 |
1973年 | 6568篇 |
1972年 | 6123篇 |
1971年 | 5535篇 |
1970年 | 4767篇 |
1969年 | 4782篇 |
1968年 | 4453篇 |
1967年 | 3809篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
901.
902.
PCO(2) threshold for CNS oxygen toxicity in rats in the low range of hyperbaric PO(2). 总被引:3,自引:0,他引:3
Central nervous system (CNS) oxygen toxicity, as manifested by the first electrical discharge (FED) in the electroencephalogram, can occur as convulsions and loss of consciousness. CO(2) potentiates this risk by vasodilation and pH reduction. We suggest that CO(2) can produce CNS oxygen toxicity at a PO(2) that does not on its own ultimately cause FED. We searched for the CO(2) threshold that will result in the appearance of FED at a PO(2) between 507 and 253 kPa. Rats were exposed to a PO(2) and an inspired PCO(2) in 1-kPa steps to define the threshold for FED. The results confirmed our assumption that each rat has its own PCO(2) threshold, any PCO(2) above which will cause FED but below which no FED will occur. As PO(2) decreased from 507 to 456, 405, and 355 kPa, the percentage of rats that exhibited FED without the addition of CO(2) (F(0)) dropped from 91 to 62, to 8 and 0%, respectively. The percentage of rats (F) having FED as a function of PCO(2) was sigmoid in shape and displaced toward high PCO(2) with the reduction in PO(2). The following formula is suggested to express risk as a function of PCO(2) and PO(2) [abstract: see text] where P(50) is the PCO(2)for the half response and N is power. A small increase in PCO(2) at a PO(2) that does not cause CNS oxygen toxicity may shift an entire population into the risk zone. Closed-circuit divers who are CO(2)retainers or divers who have elevated inspired CO(2)are at increased risk of CNS oxygen toxicity. 相似文献
903.
904.
J.N. Beresford J.A. Gallagher M. Gowen M. Couch J. Poser D.D. Wood R.G.G. Russell 《Biochimica et Biophysica Acta (BBA)/General Subjects》1984,801(1):58-65
Cultural adherent human mononuclear cells produce factor(s) which stimulate the release of calcium from new-born mouse calvaria in organ culture. This stimulation of bone resorption is accompanied by an inhibition of the incorporation of [3H]proline into collagen which is independent of increased prostaglandin production by the bone. When human osteoblast-like cells are treated with conditioned medium from human mononuclear cells, collagen accounts for a decreased proportion of the protein synthesised. This effect on matrix synthesis is not accompanied by an inhibitory action of the monocyte-conditioned medium preparations on net cell proliferation. In human osteoblast-like cell cultures, partially purified human interleukin 1 also inhibits the production of the bone-specific protein osteocalcin in a dose-dependent fashion. These observations are consistent with the hypothesis that products of human monocytes similar to, or identical with, human interleukin 1 may be important regulators of bone metabolism and may contribute to the bone loss seen in diseases such as chronic rheumatoid arthritis. 相似文献
905.
906.
This minireview is concerned with a re-examination of the locus of action and the possible peripheral mechanisms involved in the gastrointestinal (GI) stimulant effects of metoclopramide. Such a re-evaluation is opportune given the increasing use of this drug in the therapy of certain GI tract disorders. To provide an orientation on this subject the location in the GI tract and function of several relevant receptor types have been reviewed. In the past metoclopramide has been reported to enhance contractions of a variety of GI preparations to electrical stimulation, acetylcholine, carbachol and ganglion stimulants, to inhibit responses to alpha 2-adrenoreceptor agonists and 5-hydroxytryptamine, as well as blocking those to dopamine. Also in such preparations metoclopramide facilitates the release of acetylcholine to transmural stimulation. One important question is whether this effect is mediated via a specific prejunctional receptor. In this respect 2 suggestions have been made. Firstly that the drug may act as a preferential, prejunctional muscarinic antagonist thus inhibiting the negative feedback inhibition of acetylcholine release and secondly that metoclopramide may be a prejunctional agonist (partial) at 5-hydroxy-tryptamine receptors. Although the latter possibility appears most tenable at present, the involvement of a specific receptor remains to be confirmed. The important finding that dopamine receptors are probably not involved in the local stimulant effects of metoclopramide has important implications for future research orientated towards the discovery of a new generation of GI drugs lacking the side effects associated with central dopamine receptor blockade. Several compounds (cinitapride, BRL 20627A and cisapride) are now in the early stages of clinical evaluation. 相似文献
907.
Don Trinh Nguyen Jens Christian G?pfert Nobuhiro Ikezawa Gillian MacNevin Meena Kathiresan Jürgen Conrad Otmar Spring Dae-Kyun Ro 《The Journal of biological chemistry》2010,285(22):16588-16598
Sesquiterpene lactones are characteristic natural products in Asteraceae, which constitutes ∼8% of all plant species. Despite their physiological and pharmaceutical importance, the biochemistry and evolution of sesquiterpene lactones remain unexplored. Here we show that germacrene A oxidase (GAO), evolutionarily conserved in all major subfamilies of Asteraceae, catalyzes three consecutive oxidations of germacrene A to yield germacrene A acid. Furthermore, it is also capable of oxidizing non-natural substrate amorphadiene. Co-expression of lettuce GAO with germacrene synthase in engineered yeast synthesized aberrant products, costic acids and ilicic acid, in an acidic condition. However, cultivation in a neutral condition allowed the de novo synthesis of a single novel compound that was identified as germacrene A acid by gas and liquid chromatography and NMR analyses. To trace the evolutionary lineage of GAO in Asteraceae, homologous genes were further isolated from the representative species of three major subfamilies of Asteraceae (sunflower, chicory, and costus from Asteroideae, Cichorioideae, and Carduoideae, respectively) and also from the phylogenetically basal species, Barnadesia spinosa, from Barnadesioideae. The recombinant GAOs from these genes clearly showed germacrene A oxidase activities, suggesting that GAO activity is widely conserved in Asteraceae including the basal lineage. All GAOs could catalyze the three-step oxidation of non-natural substrate amorphadiene to artemisinic acid, whereas amorphadiene oxidase diverged from GAO displayed negligible activity for germacrene A oxidation. The observed amorphadiene oxidase activity in GAOs suggests that the catalytic plasticity is embedded in ancestral GAO enzymes that may contribute to the chemical and catalytic diversity in nature. 相似文献
908.
Preparation of chloroplast DNA from pea plastids isolated in a medium of high ionic strength 总被引:8,自引:0,他引:8
A simple, rapid, and inexpensive method for the preparation and purification of chloroplast DNA (cpDNA) from pea has been developed. The crucial step is the isolation of chloroplasts in a medium of high ionic strength (I congruent equal to 1.40 M). CpDNA from pea prepared according to this method has successfully been used for restriction enzyme mapping, Southern transfers, and cloning. 相似文献
909.
910.