全文获取类型
收费全文 | 5260篇 |
免费 | 424篇 |
国内免费 | 2篇 |
出版年
2023年 | 19篇 |
2022年 | 45篇 |
2021年 | 85篇 |
2020年 | 47篇 |
2019年 | 84篇 |
2018年 | 89篇 |
2017年 | 79篇 |
2016年 | 114篇 |
2015年 | 220篇 |
2014年 | 240篇 |
2013年 | 299篇 |
2012年 | 411篇 |
2011年 | 419篇 |
2010年 | 273篇 |
2009年 | 235篇 |
2008年 | 338篇 |
2007年 | 333篇 |
2006年 | 283篇 |
2005年 | 288篇 |
2004年 | 266篇 |
2003年 | 269篇 |
2002年 | 263篇 |
2001年 | 84篇 |
2000年 | 78篇 |
1999年 | 81篇 |
1998年 | 54篇 |
1997年 | 39篇 |
1996年 | 31篇 |
1995年 | 41篇 |
1994年 | 39篇 |
1993年 | 27篇 |
1992年 | 37篇 |
1991年 | 40篇 |
1990年 | 42篇 |
1989年 | 33篇 |
1988年 | 23篇 |
1987年 | 29篇 |
1986年 | 16篇 |
1985年 | 25篇 |
1984年 | 21篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 21篇 |
1980年 | 27篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 12篇 |
1976年 | 15篇 |
1975年 | 16篇 |
1974年 | 9篇 |
排序方式: 共有5686条查询结果,搜索用时 15 毫秒
21.
Heparin-like molecules regulate the number of epidermal growth factor receptors on vascular smooth muscle cells 总被引:4,自引:0,他引:4
The effect of heparin on the binding of epidermal growth factor (EGF) to vascular smooth muscle cells (SMC) was examined. Heparin pretreatment of SMC obtained from bovine aortic explant tissue resulted in significant reductions in the amount of EGF bound. Decreases in mitogen binding were observed with both growth arrested as well as exponentially growing cultures. The heparin concentrations (10-100 micrograms/ml) and pretreatment times (48-72 h) necessary for suppression of EGF binding correlated with the concentrations and temporal requirements necessary for growth inhibition. Chondroitin sulfate, which has negligible antiproliferative activity, had no effect on EGF binding. However, a highly inhibitory heparan sulfate species obtained from postconfluent SMC suppressed EGF binding by 45%. Platelet-derived growth factor and insulin-like growth factor-1 binding were unaffected by heparin. Scatchard analysis revealed that heparin induced 50 to 60% reductions in the numbers of high and low affinity EGF receptors without detectable changes in the binding affinity or ratio of high to low receptors. Experiments were also performed with enzymatically dispersed SMC. These cultures were inhibited by heparin in a time dependent manner which was partially reversible in the presence of EGF. Subsequent studies revealed that heparin suppressed EGF binding in these cultures by 20 to 40%. In summary, heparin reduces the number of EGF receptors on both explant and enzyme dispersed SMC by a mechanism which closely parallels the antiproliferative effects of this glycosaminoglycan. 相似文献
22.
Andrew W. Thomas Jay Lewington Steve Hope Andrew W. Topping Andrew J. Weightman J. Howard Slater 《Archives of microbiology》1992,158(3):176-182
Favourable mutations involving the two dehalogenases (DehI and DehII) of Pseudomonas putida PP3 and derivative strains containing the cloned gene for DehI (dehI) occurred in response to specific environmental conditions, namely: starvation conditions; the presence of dehalogenase substrates (halogenated alkanoic acids — HAAs) which were toxic to P. putida; and/or the presence of a potential growth substrate. Fluctuation tests showed that these mutations were environmentally directed by the presence of HAAs. the mutations were associated with complex DNA rearrangements involving the movement of dehI located on a transposon DEH. Some mutations resulted in switching off the expression of either one or both of the dehalogenases, events which were effective in protecting P. putida from toxic compounds in its growth environment. Other mutations partially restored P. putida's dehalogenating capability under conditions where toxic substrates were absent. Restoration of the capability to untilize HAAs was favoured when normal growth substrates were present in the environment. 相似文献
23.
The distribution of holly ( Ilex aquifolium ) and its habitat preferences indicate a sensitivity to low temperature, particularly when exposed to high light. Experiments were conducted to determine whether photoinhibition of photosynthesis occurs in holly leaves in the field in United Kingdom during the winter. Photosynthetic efficiency was assessed in holly leaves that were exposed to or shaded from direct sunlight using measurements of photosynthetic oxygen evolution and chlorophyll fluorescence. Field measurements were conducted over 3 weeks during January and February. Correlation of the measurements of photosynthetic efficiency with weather conditions indicated that holly was suffering photoinhibition, particularly in leaves exposed to direct sunlight. Controlled environment studies demonstrated that exposure of leaves to low temperature and high light resulted in reductions in photosynthetic efficiency; however, leaves recovered rapidly when exposed to a higher temperature and reduced light level. 相似文献
24.
Ronald A. Martin Steven P. Lynch Francis J. Schmitz Eva O. Pordesimo Steve Toth Rita Y. Horton 《Phytochemistry》1991,30(12):3935-3939
6′-O-(E-4-hydroxycinnamoyl) Desglucouzarin, the first cardenolide containing a cinnamoyl ester moiety, has been isolated from the ethanolic extract of the milkweed, Asclepias asperula. In addition, five known cardenolides were isolated and identified from A. asperula and A. viridis. 相似文献
25.
Coccidian oöcysts recovered from the faeces of eastern ringneck snakes, Diadophis punctatus arnyi, from Kansas, USA were found to represent a previously unreported eimerian. Oöcysts of Eimeria arnyi n. sp. are subspherical, 16.9×15.1 (15–18.5×13.5–16) m, with a thin, single-layered wall and a shape-index (length/width) of 1.1 (1.1–1.3). A micropyle and öocyst residuum are absent but a large polar granule is present. The sporocysts are elongate, 13.2×6.9 (12–14.5×6.5–7) m, with Stieda and substieda bodies and a shape-index of 1.9 (1.7–2.3). Each sporozoite contains one to two anterior and a single posterior refractile bodies. Sporulation was exogenous and complete within four days at 23°C. 相似文献
26.
Targeting of T7 RNA polymerase to tobacco nuclei mediated by an SV40 nuclear location signal 总被引:9,自引:0,他引:9
Michael W. Lassner Aubrey Jones Steve Daubert Luca Comai 《Plant molecular biology》1991,17(2):229-234
We have expressed two T7 RNA polymerase genes by electroporation into tobacco protoplasts. One of the genes was modified by inserting nucleotides encoding a viral nuclear localization signal (NLS) from the large T antigen of SV40. Both T7 RNA polymerase genes directed synthesis of a ca. 100 kDa protein in the electroporated protoplasts. T7 RNA polymerase activity was detected in extracts of protoplasts electroporated with both genes. Immunofluorescence analysis of these protoplasts indicated that only the polymerase carrying the NLS accumulated in the cell nucleus. These experiments suggest that mechanisms involved in the transport from the cytoplasm to the nucleus are similar in plant and animal cells. This system demonstrates the feasibility of T7 RNA polymerase-based approaches for the high-level expression of introduced genes in plant cells. 相似文献
27.
Michel A. Haring Steve Scofield Marianne J. Teeuwen-de Vroomen Gerjan S. Leuring H. John J. Nijkamp Jacques Hille 《Plant molecular biology》1991,17(5):995-1004
A Tam3 two-element system has been designed by combining an immobilized Tam3 element with a non-autonomous dTam3 element inserted into the HPT gene. The phenotypic assay employed, restored hygromycin resistance, indicated thattrans-activation of the non-autonomous dTam3 element occurred. Molecular analyses of the excision sites revealed that the ends of the dTam3 element remain in the empty donor sites. The predominant consequence of this type of excision appears to be that excised fragments fail to re-integrate into the tobacco genome. Only one case of dTam3 re-integration could be detected. The ends of this element had been degraded upon integration into the tobacco genome. Either the altered structure of the Tam3 derivatives or tobacco host factors are influencing thetrans-activation of a dTam3 element, resulting in aberrant excision. 相似文献
28.
Steve F. Perry Serge Thomas 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1991,161(5):489-497
Summary An extracorporeal circulation of rainbow trout (Oncorhynchus mykiss) was utilized to continuously monitor the rapid and progressive effects of endogenous or exogenous catecholamines on blood respiratory/acid-base status, and to provide in vivo evidence for adrenergic retention of carbon dioxide (CO2) in fish blood (cf. Wood and Perry 1985). Exposure of fish to severe aquatic hypoxia (final P
wO2=40–60 torr; reached within 10–20 min) elicited an initial respiratory alkalosis resulting from hypoxia-induced hyperventilation. However, at a critical arterial oxygen tension (P
aO2) between 15 and 25 torr, fish became agitated for approximately 5 s and a marked (0.2–0.4 pH unit) but transient arterial blood acidosis ensued. This response is characteristic of abrupt catecholamine mobilization into the circulation and subsequent adrenergic activation of red blood cell (RBC) Na+/H+ exchange (Fievet et al. 1987). Within approximately 1–2 min after the activation of RBC Na+/H+ exchange by endogenous catecholamines, there was a significant rise in arterial PCO2 (P
aCO2) whereas arterial PO2 was unaltered; the elevation of P
aCO2 could not be explained by changes in gill ventilation. Pre-treatment of fish with the -adrenoceptor antagonist phentolamine did not prevent the apparent catecholamine-mediated increase of P
aCO2. Conversely, pre-treatment with the -adrenoceptor antagonist sotalol abolished both the activation of the RBC Na+/H+ antiporter and the associated rise in P
aCO2, suggesting a causal relationship between the stimulation of RBC Na+/H+ exchange and the elevation of P
aCO2. To more clearly establish that elevation of plasma catecholamine levels during severe hypoxia was indeed responsible for causing the elevation of P
aCO2, fish were exposed to moderate hypoxia (final P
wO2=60–80 torr) and then injected intraarterially with a bolus of adrenaline to elicit an estimated circulating level of 400 nmol·l-1 immediately after the injection. This protocol activated RBC Na+/H+ exchange as indicated by abrupt changes in arterial pH (pHa). In all fish examined, P
aCO2 increased after injection of exogenous adrenaline. The effects on P
aO2 were inconsistent, although a reduction in this variable was the most frequent response. Gill ventilation frequency and amplitude were unaffected by exogenous adrenaline. Therefore, it is unlikely that ventilatory changes contributed to the consistently observed rise in P
aCO2. Pretreatment of fish with sotalol did not alter the ventilatory response to adrenaline injection but did prevent the stimulation of RBC Na+/H+ exchange and the accompanying increases and decreases in P
aCO2 and P
aO2, respectively. These results suggest that adrenergic elevation of P
aCO2, in addition to the frequently observed reduction of P
aO2 are linked to activation of RBC Na+/H+ exchange. The physiological significance and the potential mechanisms underlying the changes in blood respiratory status after addition of endogenous or exogenous catecholamines to the circulation of hypoxic rainbow trout are discussed.Abbreviations
P
aCO2
arterial carbon dioxide tension
-
P
aO2
arterial oxygen tension
-
P
da
dorsal aortic pressure
-
pHa
arterial pH
-
P
wO2
water oxygen tension
-
RBC
red blood cell
-
V
f
breathing frequency 相似文献
29.
Molar enthalpy change for hydrolysis of phosphorylcreatine under conditions in muscle cells. 总被引:3,自引:0,他引:3
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The enthalpy change for the hydrolysis of phosphorylcreatine (PCr) by hydrochloric acid or by alkaline phosphatase was observed at 0, 25, and 37 degrees C. The value for delta H is -44 kJ mol-1 under alkaline, Mg2+-free conditions and is almost independent of temperature, ionic strength, and concentration of reactants. In muscle the reaction is accompanied by a transfer of protons from the buffers (largely histidine) to orthophosphate, release of Mg2+ from PCr, and binding of Mg2+ to orthophosphate. Measurements are reported of the heats of these processes. The calculated value of the overall heat of hydrolysis of PCr (including these processes) at pH 7, pMg 3 is -35 kJ mol-1. 相似文献
30.
The potential of a given amount of heparin to inhibit smooth muscle cell (SMC) proliferation can be increased more than 13 fold if quiescent cultures are pretreated with this mucopolysaccharide for 48 h. The large increase in antiproliferative activity was attributable to a 74% inhibition of the first cell cycle traverse of SMC after serum addition. If the mucopolysaccharide was added to SMC coincident with serum, the initial cell cycle traverse was only suppressed by 27%. In both heparin pretreated and nonpretreated SMC cultures, 48 to 72 h elapsed before substantial inhibition was observed. The inhibitory effects of heparin were reversible and inversely proportional to the starting cell density of the cultures. The effects of known heparin binding proteins on the inhibitory capability of heparin were examined. Neither platelet-derived growth factor (PDGF), low density lipoprotein (LDL), nor platelet factor 4 (PF4) were able to reduce the antiproliferative effects. Heparin retained full biological activity in medium containing serum depleted of all heparin binding proteins by heparin-Sepharose chromatography. These results indicate that heparin does not inhibit growth by preventing serum mitogens or nutrients from interacting with SMC. Rather, our data suggest that heparin is slowly internalized by SMC following binding to specific, non-PF4 dissociable sites. Heparin may accumulate intracellularly and block a crucial point in the proliferative machinery of SMC. 相似文献