全文获取类型
收费全文 | 21102篇 |
免费 | 2105篇 |
国内免费 | 16篇 |
出版年
2022年 | 205篇 |
2021年 | 426篇 |
2020年 | 231篇 |
2019年 | 298篇 |
2018年 | 385篇 |
2017年 | 310篇 |
2016年 | 450篇 |
2015年 | 876篇 |
2014年 | 856篇 |
2013年 | 1187篇 |
2012年 | 1496篇 |
2011年 | 1449篇 |
2010年 | 908篇 |
2009年 | 824篇 |
2008年 | 1161篇 |
2007年 | 1183篇 |
2006年 | 1096篇 |
2005年 | 1141篇 |
2004年 | 1080篇 |
2003年 | 1006篇 |
2002年 | 1011篇 |
2001年 | 216篇 |
2000年 | 156篇 |
1999年 | 258篇 |
1998年 | 270篇 |
1997年 | 180篇 |
1996年 | 167篇 |
1995年 | 156篇 |
1994年 | 147篇 |
1993年 | 169篇 |
1992年 | 160篇 |
1991年 | 147篇 |
1990年 | 138篇 |
1989年 | 169篇 |
1988年 | 156篇 |
1987年 | 135篇 |
1986年 | 126篇 |
1985年 | 149篇 |
1984年 | 150篇 |
1983年 | 146篇 |
1982年 | 192篇 |
1981年 | 198篇 |
1980年 | 147篇 |
1979年 | 119篇 |
1978年 | 138篇 |
1977年 | 107篇 |
1976年 | 111篇 |
1975年 | 94篇 |
1974年 | 101篇 |
1973年 | 88篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Adenosine Triphosphatase Activity at the External Surface of Chicken Brain Synaptosomes 总被引:9,自引:6,他引:3
An ATP-hydrolysing activity on the external surface of intact synaptosomes from chicken forebrain has been investigated. The observed ATPase activity was not due to leakage of the intracellular ATPase activities, of artefacts resulting from breakage of the nerve endings during the incubation and isolation periods, or to possible contamination by other subcellular particles. Disruption of the synaptosomes resulted in an approximately 2.5-fold increase of the basal, Mg2+-dependent ATPase activity, suggesting that the plasma membrane was acting as permeability barrier to the substrate. ATP hydrolysis was maximal (0.8 mumol Pi/min/mg protein) at pH 8.2 in a medium containing either Mg2+ or Ca2+ ions. Ouabain (0.2 mM) and oligomycin (2 micrograms/mg protein) had no appreciable effect on this ATPase activity. Kinetic studies of the enzyme revealed an apparent Km value of ATP of approximately 4 x 10(-5) M. These data are consistent with the view that the observed ATP hydrolysis was being catalysed by an ectoenzyme, i.e., an enzyme in the plasma membrane of the nerve endings with its active site facing the external medium. The rapid hydrolysis of the released ATP is a suspected function for this ecto-ATPase. 相似文献
172.
Epinephrine: A Potential Neurotransmitter in Retina 总被引:17,自引:13,他引:4
Abstract: Dopamine (DA), norepinephrine (NE), and epinephrine (EPI) are present in rat retina. DA is the major catecholamine, whereas NE and EPI represent ∼5% of the DA content. DA is contained in a subpopulation of amacrine cells and has been the subject of numerous studies. We investigated the origin and properties of NE and EPI in retina. Following superior cervical ganglionectomy, there was a decrease in NE content, but no decrease in EPI or phenylethanolamine- N -methyltransferase (PNMT) activity. PNMT in retina has many of the substrate-specificity and inhibitor-sensitivity characteristics of other tissues. Enzyme activity is enhanced in newborn rats by treatment with dexamethasone. Exposure to a lighted environment increases retinal EPI in normal and superior cervical ganglionectomized rats. EPI content increased for more than 2 h in a lighted environment. We conclude that most of the NE is contained within the sympathetic neurons that innervate the eye from the superior cervical ganglion, whereas EPI is contained in retinal elements that are responsive to photic stimulation. 相似文献
173.
Our laboratory has recently reported that intestinal bile acid malabsorption in cystic fibrosis (CF) is a primary mucosal cell defect. Others have suggested that elevated intracellular Ca++ levels in other cell types in CF may represent a common primary dysfunction in Ca++ efflux in these cells. We examined the possibility that intestinal bile acid absorption and Ca++ efflux in mucosal cells may be linked physiologically. Brush border membrane vesicles (BBMV) prepared from guinea pig ileum served as the experimental model to test this hypothesis. Ca++ (2.5×10?3M) present in the incubation medium did not alter the uptake of taurocholic acid (TCA) by BBMV. Also, TCA uptake into BBMV preloaded with Ca++ was not significantly different from that in BBMV not previously loaded with Ca++. Furthermore, with TCA present in the incubation medium, Ca++ efflux from preloaded BBMV was not altered. These data suggest that ileal TCA uptake, as measured by BBMV, is not dependent upon either intra- or extravesicular Ca++. Also, Ca++ efflux from BBMV is unaffected by TCA uptake. Although separate lines of evidence suggest that intestinal bile acid malabsorption and reduced plasma membrane Ca++ flux are primary defects in CF, we conclude that in the normal intestine these functions are independent physiological processes. 相似文献
174.
John C. Meeks Carol S. Enderlin Keith L. Wycoff John S. Chapman Cecillia M. Joseph 《Planta》1983,158(5):384-391
The pathways of assimilation of ammonium by pure cultures of symbiont-free Anthoceros punctatus L. and the reconstituted Anthoceros-Nostoc symbiotic association were determined from time-course (5–300 s) and inhibitor experiments using 13NH 4 + . The major product of assimilation after all incubation times was glutamine, whether the tissues were cultured with excess ammonium or no combined nitrogen. The 13N in glutamine was predominantly in the amide-nitrogen position. Formation of glutamine and glutamate by Anthoceros-Nostoc was strongly inhibited by either 1mM methionine sulfoximine (MSX) or 1 mM exogenous ammonium. These data are consistent with the assimilation of 13NH 4 + and formation of glutamate by the glutamine synthetase (EC 6.3.1.2)-glutamate synthase (EC 1.4.7.1) pathway in dinitrogen-grown Anthoceros-Nostoc. However, in symbiont-free Anthoceros, grown with 2.5 mM ammonium, formation of glutamine, but not glutamate, was decreased by either MSX or exogenous ammonium. These results indicate that during short incubation times ammonium is assimilated in nitrogenreplete Anthoceros by the activities of both glutamine synthetase and glutamate dehydrogenase (EC 1.4.1.2). In-vitro activities of glutamine synthetase were similar in nitrogen-replete Anthoceros and Anthoceros-Nostoc, indicating that the differences in the routes of glutamate formation were not based upon regulation of synthesis of the initial enzyme of the glutamine synthetase-glutamate synthase pathway. When symbiont-free Anthoceros was cultured for 2 d in the absence of combined nitrogen, total 13NH 4 + assimilation, and glutamine and glutamate formation in the presence of inhibitors, were similar to dinitrogen-grown Anthoceros-Nostoc. The routes of immediate (within 2 min) glutamate formation and ammonium assimilation in Anthoceros were apparently determined by the intracellular levels of ammonium; at low levels the glutamine synthetase-glutamate synthase pathway was predominant, while at high levels independent activities of both glutamine synthetase and glutamate dehydrogenase were expressed. 相似文献
175.
Summary Seven mutant strains defective for utilization of glycerol, glyceraldehyde or dihydroxyacetone were isolated. One strain was deficient for NAD-linked glycerol-3-phosphate dehydrogenase, two for glycerol kinase, and four had no detected enzymatic deficiency, although one of the latter strains was deficient in glycerol uptake. Glycerol uptake was increased by incubation in glycerol, glycerol-3-phosphate, erythritol, and propanediol, and was protein-mediated below 0.14 mM glycerol, but at higher concentrations free diffusion predominated. Glycerol uptake was decreased by cycloheximide and was more sensitive to sodium azide than to iodoacetate. 相似文献
176.
Jane E. Langridge-Smith Joseph H. Sellin Michael Field 《The Journal of membrane biology》1983,72(1-2):131-139
Summary In intact ileal mucosa, uptake of SO4 across the brush border membrane requires the presence of Na and is saturable, withK1/2=1.3mm at 140mm Na (P.L. Smith, S.A. Orellana & M. Field, 1981.J. Membrane Biol.
63:199–206). The present study examines the substrate specificities and transport stoichiometry of the Na-dependent SO4 uptake process. The effects of variations in medium anion and cation composition on lumen-to-epithelium influx of SO4 (J
me
SO4
) were determined under short-circuit conditions.J
me
SO4
is inhibited by thiosulfate, but not by phosphate, methylsulfate, vanadate or taurocholate. Cl is weakly inhibitory. Uptake of SO4 is poorly supported by Li, and is unaffected by K, indicating a specific dependence on Na. At low SO4 concentration (0.22mm),J
me
SO4
is a hyperbolic function of medium Na concentration; the corresponding Hill plot is linear with a slope of 1.0, suggesting a transport stoichiometry of 1 Na: 1 SO4. At high SO4 concentration (6.7mm), the Na-dependent SO4 velocity curve is sigmoidal and yields a Hill plot which is again linear but has a slope of 1.56, suggesting transport of more than 1 Na per SO4. SO4 uptake in presence of Na exhibits a dependence on medium pH. At 0.22mm SO4 and 140mm Na,J
me
SO4
was doubled by lowering pH from 7.4 to 6.8. However, at 6.7mm SO4 and 140mm Na, changing pH had no effect onJ
me
SO4
over the range 6.8 to 8.5. The pH dependence ofJ
me
SO4
at 6.7mm SO4 was restored when medium Na was lowered to 3mm, suggesting that pH sensitivity is a function of the concentration of preformed NaSO
4
–
ion pair. The results suggest that SO4 influx across the ileal brush border occurs by electroneutral Na+/NaSO
4
–
or Na+/H+/SO
4
2–
cotransport, the former being favored by high concentrations of Na and SO4. 相似文献
177.
Health care attitudes reflect the basic world view and values of a culture, such as how we relate to nature, other people, time, being, society versus community, children versus elders and independence versus dependence. Illness behavior determines who is vulnerable to illness and who agrees to become a patient—since only about one third of the ill will see a physician. Cultural values determine how one will behave as a patient and what it means to be ill and especially to be a hospital patient. They affect decisions about a patient''s treatment and who makes the decisions. Cultural differences create problems in communication, rapport, physical examination and treatment compliance and follow through. The special meaning of medicines and diet requires particular attention. The perception of physical pain and psychologic distress varies from culture to culture and affects the attitudes and effectiveness of care-givers as much as of patients. Religious beliefs and attitudes about death, which have many cultural variations, are especially relevant to hospital-based treatment. Linguistic and cultural interpreters can be essential; they are more available than realized, though there are pitfalls in their use. Finally, one must recognize that individual characteristics may outweigh the ethnic and that a good caring relationship can compensate for many cultural missteps. 相似文献
178.
Copper deficiency was produced in developing rodents to study a possible interaction between copper and the selenoenzyme, glutathione peroxidase (GSH-Px). Dietary copper deficiency was investigated in Sprague-Dawley rats and in three mouse strains (C57BL, C3H/HeJ, C58); genetic copper deficiency was studied in two of the mouse strains, C57BL and C3H/HeJ, using brindled mice. In certain cases it appeared that copper deficiency was associated with depressed liver GSH-Px activity; values from copper-deficient livers were 40–70% of control values. However, the decrease in liver GSH-Px in both rats and mice was only observed when body weight was also depressed and did not necessarily correlate with copper deficiency signs, such as lower serum ceruloplasmin or liver cytochrome oxidase activities. In weanling rats, serum GSH-Px activity was normal despite a 60% reduction in liver activity. Mouse liver GSH-Px activity rose fourfold during the first 3 weeks of life to 75% of the adult level. Rat liver GSH-Px also increased during the suckling period. When perinatal copper deficiency, nutritional or genetic, was severe enough to retard growth, liver GSH-Px activity was depressed. Unlike rats, adult murine liver GSH-Px was equivalent in males and females. 相似文献
179.
The purpose of this study was to investigate the coexistence of gamma-aminobutyric acid (GABA) and leu-enkephalin in single neurons from the corpus striatum. Monolayer cell cultures, started from newborn rat corpus striatum, were grown in serum-free medium and examined using GABA autoradiography and leu-enkephalin immunocytochemistry in a double-label protocol. Examples of cells were found which were positive for one or the other neurotransmitter or for neither transmitter, but not for both. Furthermore, cells which appear similar by morphological criteria alone differed in transmitter specificity. We conclude that the two transmitters tested are not localized within single cells and that morphology alone is inadequate to identify functional cell classes in this area. 相似文献
180.
Lymphocytes obtained from CLL patients exhibited a delayed and reduced response to PHA when cultured in diffusion chambers. DNA synthesis (8–10 hr) and general time (15–19 hr) of the late-developing CLL blasts were consistent with normal values ( T s : 8–10 hr; T c : 14–17 hr). However, the G2 period of CLL blasts seemed more variable, and their mitotic index during the response at 5–6 days was 30–50% of the values determined for normal blasts during their peak response at 2–3 days. 相似文献