全文获取类型
收费全文 | 1071篇 |
免费 | 56篇 |
国内免费 | 1篇 |
专业分类
1128篇 |
出版年
2022年 | 10篇 |
2021年 | 19篇 |
2020年 | 19篇 |
2019年 | 12篇 |
2018年 | 18篇 |
2017年 | 16篇 |
2016年 | 34篇 |
2015年 | 56篇 |
2014年 | 41篇 |
2013年 | 38篇 |
2012年 | 70篇 |
2011年 | 63篇 |
2010年 | 47篇 |
2009年 | 27篇 |
2008年 | 48篇 |
2007年 | 41篇 |
2006年 | 40篇 |
2005年 | 47篇 |
2004年 | 23篇 |
2003年 | 28篇 |
2002年 | 22篇 |
2001年 | 20篇 |
2000年 | 21篇 |
1999年 | 16篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 13篇 |
1986年 | 13篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1975年 | 8篇 |
1971年 | 9篇 |
1969年 | 6篇 |
1968年 | 8篇 |
排序方式: 共有1128条查询结果,搜索用时 15 毫秒
1.
Gaël Clement 《Palaeontology》2002,45(3):577-593
Remains of two large sarcopterygians are described from Famennian deposits in Belgium. One of them is referred to Eusthenodon wängsjöi Jarvik; it is the first occurrence of this genus in Belgium. The other, much larger one, appears to be a tristichopterid. It has a postspiracular; size and shape of the mandible similar to those of Platycephalichthys skuenicus and P. bischoffi ; snout and cheek patterns close to those of Eusthenodon ; unusual shape of the supratemporal resembling that of Hyneria , Mandageria and Platycephalichthys skuenicus ; and tooth histology quite similar to that of Eusthenodon and Platycephalichthys . 相似文献
2.
The turnover rates and steady-state levels of gamma-aminobutyric acid (GABA) have been determined in 15 brain areas of three sublines of inbred mice differing in their susceptibility to audiogenic seizures: Rb3, which is seizure resistant; Rb2, which develops clonic seizures; and Rb1, which develops tonic-clonic seizures. In the Rb1 subline, GABA steady-state levels are lower than in the Rb3 subline in three of the 15 areas examined (cerebellum, anterior colliculus, and amygdala), whereas in the Rb2 subline, steady-state levels are either higher (posterior colliculus and hippocampus) or lower (amygdala) than in the Rb3 subline. GABA turnover rates differ in three brain areas in Rb1 (amygdala, raphe, and hypothalamus) and in a single area (amygdala) in Rb2 when compared with Rb3. Only one area has similar variations of GABA turnover rate and steady-state levels in the two susceptible sublines: the amygdala. After 2 weeks of repeated auditory stimulations (two times a day, 8,000 Hz, 100 dB), additional alterations in GABA metabolism are observed: mainly large increases in GABA turnover rates (from 40% to three- to fourfold). The Rb2 subline displays a greater number of alterations (increases of turnover rates in pons, cerebellum, anterior and posterior colliculus, amygdala, olfactory bulbs and tubercles, striatum, and frontal cortex) than the Rb1 subline (increases of turnover rates in cerebellum, posterior colliculus, olfactory tubercles, raphe, and frontal cortex and a decrease in hypothalamus). In the Rb3 subline, increases of the turnover rate in amygdala and olfactory tubercles and decreases in olfactory bulbs and hippocampus are observed.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
3.
H D Gresham L T Clement J E Volanakis E J Brown 《Journal of immunology (Baltimore, Md. : 1950)》1987,139(12):4159-4166
Data presented in this paper indicate that polymorphonuclear leukocyte (PMN) Fc receptor-mediated phagocytosis can be markedly augmented and that this augmentation is under regulatory control. Stimulation of PMN with either a low m.w., heat-labile cytokine(s) (the culture supernatant effluent from a YM-10 Centricon unit, YM-10E), phorbol esters (phorbol dibutyrate), or the polyene antibiotic, amphotericin B, enhances Fc-mediated ingestion in a dose-dependent manner. YM-10 effluent- and amphotericin B-stimulated ingestion is completely abrogated by treating the PMN with either pertussis toxin (PT), cholera toxin (CT), or a monoclonal antibody (mAb), 1C2. However, neither toxin nor mAb 1C2 affects nonstimulated ingestion or phagocytosis stimulated by phorbol esters or synthetic diacylglycerol. Increasing intracellular cyclic adenosine monophosphate levels by stimulation with prostaglandin E1 and the phosphodiesterase inhibitor, isobutylmethylxanthine, does not mimic the effect of either toxin or mAb 1C2. In addition, toxin-mediated inhibition is not due to loss of either the Fc receptor recognized by mAb 3G8 or the antigen recognized by mAb 1C2. These data indicate that both CT and PT regulate the phagocytic response of PMN, in a manner like mAb 1C2, probably by affecting a guanosine 5'-triphosphate-binding protein distinct from those that regulate adenylate cyclase. Since phorbol ester-stimulated ingestion is not inhibited by either PT, CT, or mAb 1C2 and phorbol esters activate protein kinase C directly, phagocytosis amplification regulated by PT, CT, and mAb 1C2 may involve protein kinase C activation. 相似文献
4.
N. Arfman E. M. Watling W. Clement R. J. van Oosterwijk G. E. de Vries W. Harder M. M. Attwood L. Dijkhuizen 《Archives of microbiology》1989,152(3):280-288
The enzymology of methanol utilization in thermotolerant methylotrophic Bacillus strains was investigated. In all strains an immunologically related NAD-dependent methanol dehydrogenase was involved in the initial oxidation of methanol. In cells of Bacillus sp. C1 grown under methanol-limiting conditions this enzyme constituted a high percentage of total soluble protein. The methanol dehydrogenase from this organism was purified to homogeneity and characterized. In cell-free extracts the enzyme displayed biphasic kinetics towards methanol, with apparent K
m values of 3.8 and 166 mM. Carbon assimilation was by way of the fructose-1,6-bisphosphate aldolase cleavage and transketolase/transaldolase rearrangement variant of the RuMP cycle of formaldehyde fixation. The key enzymes of the RuMP cycle, hexulose-6-phosphate synthase (HPS) and hexulose-6-phosphate isomerase (HPI), were present at very high levels of activity. Failure of whole cells to oxidize formate, and the absence of formaldehyde-and formate dehydrogenases indicated the operation of a non-linear oxidation sequence for formaldehyde via HPS. A comparison of the levels of methanol dehydrogenase and HPS in cells of Bacillus sp. C1 grown on methanol and glucose suggested that the synthesis of these enzymes is not under coordinate control.Abbreviations RuMP
ribulose monophosphate
- HPS
hexulose-6-phosphate synthase
- HPI
hexulose-6-phosphate isomerase
- MDH
methanol dehydrogenase
- ADH
acohol dehydrogenase
- PQQ
pyrroloquinoline, quinone
- DTT
dithiothreitol
- NBT
nitrobluetetrazolium
- PMS
phenazine methosulphate
- DCPIP
dichlorophenol indophenol 相似文献
5.
Heterotrophic activity and bacterial productivity in assemblages of microbes from sea ice in the high Arctic 总被引:1,自引:1,他引:0
Summary Heterotrophic activity in the bottom few cm of annual sea ice in the Canadian Arctic was measured throughout the spring bloom of ice algae, using tritium-labelled thymidine and glucose. Experiments with chloramphenicol and cyclohexamide indicated that thymidine assimilation was due to procaryotic microbes but that about half of the glucose assimilation was due to eucaryotic organisms. Glucose and thymidine assimilation rates increased with salinity, from 10 ppt to 30 ppt. Thymidine assimilation rates increased from 1.16 to 4.94·10–21mol·cell–1·h–1 during the latter half of the algal bloom, while the exponential growth rate of the in situ populations decreased from 0.058 to 0.025 d–1. Bacterial production and specific growth rates calculated from thymidine assimilation were 149mgC·m–2 and 0.25 d–1 or less respectively over the 50 day observation period, compared with net primary production of 5,500 mgC·m–2. Thymidine assimilation rates suggested that about half of the bacterial production may be consumed or lost from the ice during the bloom. 相似文献
6.
7.
Specific serotonin binding (5-HT1, 5-HT1A, and 5-HT2 subtypes) and membrane anisotropy were measured at 2 h intervals over a 24 h period in the hippocampus and cortex of Wistar WU rats, housed under a 12 h light-dark cycle, with lights on at 07.00. All experiments were performed both in March and December. In the hippocampus significant circadian rhythms could be ascertained for 5-HT1 binding sites in March and December while for 5-HT1A (subtype of 5-HT1) binding sites the circadian rhythm was only significant in March. The membrane anisotropy also showed significant variations only in March. Circadian rhythms were also found in the cortex for 5-HT1 (December) and 5-HT2 (March and December) binding sites as well as for the membrane anisotropy (December). A correlation was found between membrane anisotropy and 5-HT1 and 5-HT2 binding sites in hippocampus and cortex, respectively. A circadian rhythmicity was also observed for serotonin release as measured by in vivo voltammetry in both brain areas. The results obtained on the diurnal variations of serotonin receptor subtypes and serotonin release and the probable inverse relationship of these two parameters may be relevant in understanding the coupling of pre- and postsynaptic activity. 相似文献
8.
Temporal changes in tissue glutathione in response to chemical form,dose, and duration of selenium treatment 总被引:1,自引:0,他引:1
Selenium has been reported to affect glutathione (GSH) concentrations in short-term animal-feeding experiments. Given the central role that this tripeptide plays in maintaining cellular homeostasis, it was hypothesized that perturbations in glutathione metabolism induced by selenium might account for its cancer chemopreventive activity. In the present study, four experiments were conducted in which the effect of acute, short-, or long-term exposure to selenium was assessed. Selenium was provided as either sodium selenite or D,L-selenomethionine. Selenite was observed to induce a biphasic response in total liver GSH. Injected selenium caused an acute reduction in GSH, whereas short-term feeding (up to 8 wk) increased both total GSH and oxidized glutathione (GSSH), an effect that gradually diminished in magnitude with prolonged feeding. Our data suggest that such changes are unlikely to account for the chemopreventive activity of selenium for the following reasons: Perturbations in glutathione metabolism occurred only at doses of selenite that approached toxicity. These doses are higher than what would be required for producing cancer chemoprevention. The transient nature of these changes also contrasts with the need for a continuous supplementation of selenite in suppression of tumorigenesis. Furthermore, selenomethionine was found to have little activity in altering glutathione metabolism, even though it compares favorably with selenite as a cancer chemopreventive agent. Nonetheless, these findings do not discount the possibility that sulfhydryl compounds, such as glutathione, might be used to modify the toxicity and/or enhance the cancer prophylactic activity of selenium compounds. 相似文献
9.
10.
M G Clement M Albertini M Dimori G Aguggini 《Prostaglandins, leukotrienes, and essential fatty acids》1992,45(2):143-149
In 14 anesthetized, spontaneously breathing pigs we examined the changes in breathing pattern, in respiratory mechanics and in systemic and pulmonary vascular parameters after i.v. PAF administration. In another 3 pigs, the effects of PAF were also examined after bilateral vagotomy. In intact pigs, PAF induces apnea, bronchoconstriction, pulmonary hypertension and systemic hypotension. Our results also show that administration of PAF alters the phasic vagal activity, modifying the slope of VT vs TI and TE vs TI relationships and the TI0/TI ratio. These effects and apnea are vagus-dependent. The central excitatory timing effect of PAF on inspiratory duration (TI0) was correlated with a decrease in passive compliance but not with active compliance. We postulate that the activation by vagal input strengthens the mechanisms that counteract the bronchoconstrictor effect of PAF. 相似文献