全文获取类型
收费全文 | 3529篇 |
免费 | 244篇 |
出版年
2023年 | 18篇 |
2022年 | 66篇 |
2021年 | 101篇 |
2020年 | 46篇 |
2019年 | 61篇 |
2018年 | 93篇 |
2017年 | 74篇 |
2016年 | 121篇 |
2015年 | 203篇 |
2014年 | 214篇 |
2013年 | 270篇 |
2012年 | 318篇 |
2011年 | 303篇 |
2010年 | 199篇 |
2009年 | 160篇 |
2008年 | 233篇 |
2007年 | 223篇 |
2006年 | 187篇 |
2005年 | 171篇 |
2004年 | 159篇 |
2003年 | 159篇 |
2002年 | 131篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 13篇 |
1998年 | 26篇 |
1997年 | 17篇 |
1996年 | 18篇 |
1995年 | 6篇 |
1994年 | 12篇 |
1993年 | 9篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 15篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 6篇 |
1986年 | 11篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1982年 | 13篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 4篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1966年 | 1篇 |
1964年 | 1篇 |
排序方式: 共有3773条查询结果,搜索用时 218 毫秒
1.
2.
Francesca Passaretta Domenico Bosco Lucia Centurione Maria Antonietta Centurione Fabio Marongiu Roberta Di Pietro 《Journal of cellular and molecular medicine》2020,24(7):4350-4355
Human Amniotic Epithelial Cells (hAEC) isolated from term placenta are a promising source for regenerative medicine. However, it has long been debated whether the hAEC population consists of heterogeneous or homogeneous cells. In a previous study, we investigated the characteristics of hAEC isolated from four different regions of the amniotic membrane finding significant heterogeneity. The aim of this study was to evaluate the hepatic differentiation capability of hAEC isolated from these four regions. Human term placentae were collected after caesarean section and hAEC were isolated from four regions of the amniotic membrane (R1-R4, according to their relative distance from the umbilical cord) and treated in hepatic differentiation conditions for 14 days. hAEC-derived hepatocyte-like cells showed marked differences in the expression of hepatic markers: R4 showed higher levels of Albumin and Hepatocyte Nuclear Factor (HNF) 4α whereas R1 expressed higher Cytochrome P450 enzymes, both at the gene and protein level. These preliminary results suggest that hAEC isolated from R1 and R4 of the amniotic membrane are more prone to hepatic differentiation. Therefore, the use of hAEC from a specific region of the amniotic membrane should be taken into consideration as it could have an impact on the outcome of therapeutic applications. 相似文献
3.
The involvement of calpain in the activation of protein kinase C in neutrophils stimulated by phorbol myristic acid 总被引:27,自引:0,他引:27
E Melloni S Pontremoli M Michetti O Sacco B Sparatore B L Horecker 《The Journal of biological chemistry》1986,261(9):4101-4105
The Ca2+/phospholipid-dependent protein kinase (protein kinase C) of human neutrophils is converted to a proteolytically modified Ca2+/phospholipid-independent form (Inoue, M., Kishimoto, A., Takai, Y.U., and Nishizuka, Y. (1977) J. Biol. Chem. 252, 7610-7616) on incubation with neutrophil membranes in the presence of micromolar concentrations of Ca2+ and an endogenous Ca2+-requiring proteinase (Melloni, E., Pontremoli, S., Michetti, M., Sacco, O., Sparatore, B., Salamino, F., and Horecker, B. L. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 6435-6439). We have now demonstrated the appearance of a similar Ca2+/phospholipid-independent kinase in intact human neutrophils stimulated by phorbol 12-myristate 13-acetate (PMA). The following evidence supports the conclusion that the Ca2+/phospholipid-independent protein kinase recovered from the PMA-treated cells is a proteolytically modified form of the "native" protein kinase C. 1) In cells exposed to PMA, the rate of disappearance of Ca2+/phospholipid-dependent protein kinase C activity is correlated with the rate of appearance of the Ca2+/phospholipid-independent kinase. 2) The chromatographic behavior of the new protein kinase and its molecular size (approximately 65 kDa) are identical to those previously reported for the proteolytically modified form of protein kinase C. 3) The modified protein kinase no longer binds to the cell membrane and is recovered almost entirely in the cytosol fraction. 4) In neutrophils preloaded with inhibitors of the Ca2+-requiring proteinase, stimulation with PMA results in translocation of protein kinase C from the cytosol fraction to the particulate fraction, but the appearance of the soluble, Ca2+/phospholipid-dependent form is prevented. We conclude that binding of protein kinase C to the plasma membrane and its proteolytic conversion are related, but independent, processes both elicited by exposure of neutrophils to the phorbol ester. Proteolytic cleavage of the membrane-bound protein kinase C provides an alternative mechanism for its activation and may account for certain of the cellular responses observed in PMA-stimulated neutrophils. 相似文献
4.
Following association to the membrane, human erythrocyte procalpain is converted and released as fully active calpain 总被引:2,自引:0,他引:2
S Pontremoli F Salamino B Sparatore M Michetti O Sacco E Melloni 《Biochimica et biophysica acta》1985,831(3):335-339
When exposed to inside-out human erythrocyte vesicles, in the presence of micromolar Ca2+, the 80 kDa catalytic subunit of procalpain is processed through three successive and sequential steps. These include binding to the cytosolic surface of the membrane, followed by a very rapid conversion into the 75 kDa active subunit, and ultimately by spontaneous and complete release of this active proteinase form. Binding to the membranes is competitively inhibited by the endogenous natural inhibitor through the formation of the proteinase-inhibitor complex, in which form the 80 kDa subunit can no longer be associated to the membranes. Calcium ions and the natural endogenous inhibitor appear to be crucially involved in the modulation of this novel membrane-bound mediated activation of human red cell procalpain. 相似文献
5.
S Pontremoli B Sparatore F Salamino M Michetti E Melloni 《Archives of biochemistry and biophysics》1985,239(2):517-522
Mn2+ (50 microM) satisfies the requirement for activity of the purified Ca2+-dependent neutral proteinase from human erythrocytes. Unlike the activation by Ca2+ [E. Melloni et al. (1984) Biochem. Int. 8, 477-489], the effect of Mn2+ is fully reversible and does not involve autodigestion of the native 80-kDa catalytic subunit. However, the native dimeric proenzyme (procalpain), which contains both the 80-kDa subunit and a smaller 30-kDa subunit, is not activated by Mn2+ alone but also requires the presence of micromolar concentrations of Ca2+. Under these conditions, 40% of the maximum activity is expressed without dissociation of the 80- and 30-kDa subunits. Mn2+, but not micromolar Ca2+, can also partially satisfy the metal requirement of the native 80-kDa subunit isolated after dissociation of the heterodimer. This activity is further enhanced by the addition of 5 microM Ca2+, which is ineffective in the absence of Mn2+. After procalpain is converted to active calpain by incubation with Ca2+ and substrate [S. Pontremoli et al. (1984) Biochem. Biophys. Res. Commun. 123, 331-337] full activity is observed with 5 microM Mn2+, which now substitutes completely for Ca2+. Activation of procalpain by Mn2+ represents a new mechanism for modulation of the Ca2+-dependent proteinase activity. 相似文献
6.
Binding to erythrocyte membrane is the physiological mechanism for activation of Ca2+-dependent neutral proteinase 总被引:6,自引:0,他引:6
S Pontremoli E Melloni B Sparatore F Salamino M Michetti O Sacco B L Horecker 《Biochemical and biophysical research communications》1985,128(1):331-338
In the presence of micromolar concentrations of Ca2+ the catalytic 80 kDa subunit of human erythrocyte procalpain binds to the cytosolic surface of the erythrocyte membrane. Binding is rapid, highly specific and is reversed by the removal of Ca2+. In the bound form the 80 kDa catalytic subunit undergoes a rapid conversion to calpain, the active 75 kDa Ca2+-requiring proteinase. The activated proteinase produces extensive degradation of membrane components, particularly of band 4.1 and 2.1 proteins. Binding to membranes may represent an obligatory physiological mechanism for the conversion of procalpain to calpain. 相似文献
7.
Nicholas C. Nicolaides Fabio Palombo Kenneth W. Kinzler Bert Vogelstein Josef Jiricny 《Genomics》1996,31(3):395
Defects in mismatch repair genes cause the genetic instability characteristic of hereditary nonpolyposis colorectal cancer and a subset of sporadic colon tumors. The newest member of the mismatch repair gene family,GTBP, has recently been identified as a partial cDNA. Here, we describe the isolation of its 5′ terminus, allowing definition of the entire coding region. Several polymorphisms within the 5′ end were identified and are presented. 相似文献
8.
Gianfranco Tucci Marco Cosimo Simeone Carlo Gregori Fabio Maggini 《Plant Systematics and Evolution》1994,190(3-4):187-193
Intergenic spacers of the rRNA genes of three species of theCynareae tribe:Cynara cardunculus subsp.scolymus (artichoke),Onopordum acanthium, andO. illyricum were cloned in the plasmid pGEM-7zf(+). Detailed restriction mapping and partial sequencing of the IGSs were carried out. The structural analysis showed a clear diversity betweenCynara andOnopordum, while a high degree of homology was found between the twoOnopordum spp. In all three species a fragment of about 450 bp from the 5 end of 18S to the Acc I site with a high sequence homology was present. Nucleotide sequences upstream from the above mentioned Acc I site show a gradual decrease of homology betweenCynara andOnopordum. 相似文献
9.
Davide Malatesta David Simar Yves Dauvilliers Robin Candau Fabio Borrani Christian Prefaut Corinne Caillaud 《Journal of applied physiology》2003,95(6):2248-2256
This study tested whether the lower economy of walking in healthy elderly subjects is due to greater gait instability. We compared the energy cost of walking and gait instability (assessed by stride to stride changes in the stride time) in octogenarians (G80, n = 10), 65-yr-olds (G65, n = 10), and young controls (G25, n = 10) walking on a treadmill at six different speeds. The energy cost of walking was higher for G80 than for G25 across the different walking speeds (P < 0.05). Stride time variability at preferred walking speed was significantly greater in G80 (2.31 +/- 0.68%) and G65 (1.93 +/- 0.39%) compared with G25 (1.40 +/- 0.30%; P < 0.05). There was no significant correlation between gait instability and energy cost of walking at preferred walking speed. These findings demonstrated greater energy expenditure in healthy elderly subjects while walking and increased gait instability. However, no relationship was noted between these two variables. The increase in energy cost is probably multifactorial, and our results suggest that gait instability is probably not the main contributing factor in this population. We thus concluded that other mechanisms, such as the energy expenditure associated with walking movements and related to mechanical work, or neuromuscular factors, are more likely involved in the higher cost of walking in elderly people. 相似文献
10.
Augusto C. Franco Angela Haag-Kerwer Britta Herzog Thorsten E. E. Grams Erika Ball Eduardo A. de Mattos Fabio R. Scarano Suzanne Barreto Maria A. Garcia Andre Mantovani Ulrich Lüttge 《Trees - Structure and Function》1996,10(6):359-365
Sandy plains are characteristic of the coastal region of Brazil. We investigated the diel patterns of changes in organic acid levels, leaf conductance and chlorophylla fluorescence for sun-exposed and shaded plants ofClusia hilariana, one of the dominant woody species in the sandy coastal plains of northern Rio de Janeiro state. Both exposed and shaded plants showed a typical CAM pattern with considerable diel oscillations in organic acid levels. The degradation of both malic and citric acids during the midday stomatal closure period could lead to potential CO2 fixation rates of 28 mol m-2 s-1 in exposed leaves. Moreover, exposed leaves exhibited large increases in total non-photochemical quenching (qN) accompanied by a substantial decrease in effective quantum yield during the course of the day. However, these potential high rates of CO2 fixation and the increases inqn of exposed plants were not enough to maintain the primary electron acceptor of photosystem II (qA) in a low reduction state, similar to that of shaded plants. As a result, there was a moderate increase in the reduction state of qA throughout the day. Most of the decline in photochemical efficiency of exposed leaves ofC. hilariana was reversible, as evidenced by the high levels of pre-dawn potential quantum yields (Fv/Fm) and their rapid recovery after sunset. However, the depletion of the organic acid pool in the afternoon resulted in an accentuated subsequent drop in Fv/Fm, suggesting that prolonged periods of water stress accompanied by high irradiance levels may expose plants ofC. hilariana in unprotected habitats to the danger of photoinhibition. 相似文献