全文获取类型
收费全文 | 766篇 |
免费 | 60篇 |
国内免费 | 1篇 |
出版年
2023年 | 5篇 |
2022年 | 8篇 |
2021年 | 15篇 |
2020年 | 6篇 |
2019年 | 7篇 |
2018年 | 29篇 |
2017年 | 23篇 |
2016年 | 38篇 |
2015年 | 39篇 |
2014年 | 30篇 |
2013年 | 48篇 |
2012年 | 57篇 |
2011年 | 55篇 |
2010年 | 36篇 |
2009年 | 27篇 |
2008年 | 49篇 |
2007年 | 39篇 |
2006年 | 40篇 |
2005年 | 16篇 |
2004年 | 24篇 |
2003年 | 20篇 |
2002年 | 21篇 |
2001年 | 9篇 |
2000年 | 26篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1996年 | 3篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1972年 | 5篇 |
1971年 | 5篇 |
1968年 | 3篇 |
1967年 | 4篇 |
排序方式: 共有827条查询结果,搜索用时 15 毫秒
11.
The aim of this study was to characterize the electropharmacological effects of prostacyclin (PGI2) in human atrial fibers and cardiomyocytes. Atrial tissues obtained from the hearts of 28 patients undergoing corrective cardiac surgery were used. Transmembrane action potentials were recorded using a conventional microelectrode technique, and twitch force by a transducer. Effects of PGI2 (1 nM–10 µM) on action potential characteristics and contraction of atrial fibers were evaluated in normal [K]o (4 mM) and high [K]o (27 mM) in the absence and presence of cardiotonic agents. In addition, atrial and ventricular myocytes were isolated enzymatically from atrial tissues and hearts of 4 patients undergoing cardiac transplant. The effects of PGI2 on Na- and Ca-dependent inward currents (INa and ICa) of cardiomyocytes were tested. In 9 human atrial fibers showing fast-response action potentials (mean dV/dtmax = 101 ± 15 Vs–1) in 4 mM [K]o, PGI2 did not influence dV/dtmax of phase 0 depolarization even at 1 µM. However, at a concentration as low as 10 nM, PGI2 depressed spontaneous rhythms or slow-response action potentials in high-K-depolarized fibers. PGI2 also depressed delayed afterdepolarizations and aftercontractions induced by cardiotonic agents. In isolated cardiomyocytes, PGI2 reduced ICa but not INa. The present findings show that, in human atrial fibers and cardiomyocytes, PGI2 induces greater depressant effects on the slow-response action potential, ICa and triggered activity than on the fast-response action potential. It is suggested that PGI2 may act through a selective reduction of transmembrane Ca influx. 相似文献
12.
13.
Jan Tumajer Krešimir Begović Vojtěch Čada Michal Jenicek Jelena Lange Jiří Mašek Ryszard J. Kaczka Miloš Rydval Miroslav Svoboda Lukáš Vlček Václav Treml 《Global Change Biology》2023,29(2):462-476
Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate–growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate–growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate–growth response for temperate forests. 相似文献
14.
15.
A major difference between the divergence patterns within the lines-1 families in mice and voles 总被引:3,自引:0,他引:3
Vanlerberghe F; Bonhomme F; Hutchison CA d; Edgell MH 《Molecular biology and evolution》1993,10(4):719-731
L1 retroposons are represented in mice by subfamilies of interspersed
sequences of varied abundance. Previous analyses have indicated that
subfamilies are generated by duplicative transposition of a small number of
members of the L1 family, the progeny of which then become a major
component of the murine L1 population, and are not due to any active
processes generating homology within preexisting groups of elements in a
particular species. In mice, more than a third of the L1 elements belong to
a clade that became active approximately 5 Mya and whose elements are >
or = 95% identical. We have collected sequence information from 13 L1
elements isolated from two species of voles (Rodentia: Microtinae: Microtus
and Arvicola) and have found that divergence within the vole L1 population
is quite different from that in mice, in that there is no abundant
subfamily of homologous elements. Individual L1 elements from voles are
very divergent from one another and belong to a clade that began a period
of elevated duplicative transposition approximately 13 Mya. Sequence
analyses of portions of these divergent L1 elements (approximately 250 bp
each) gave no evidence for concerted evolution having acted on the vole L1
elements since the split of the two vole lineages approximately 3.5 Mya;
that is, the observed interspecific divergence (6.7%-24.7%) is not larger
than the intraspecific divergence (7.9%-27.2%), and phylogenetic analyses
showed no clustering into Arvicola and Microtus clades.
相似文献
16.
Summary The presence of 1% agar in the fixation and substrate solutions for the histochemical demonstration of thiamine pyrophosphatase (4.4 mM TPP; 3.6 mM Pb2+; 0.025 Tris-maleate buffer, pH 7.2) clearly facilitates the localization of the enzyme in Golgi apparatus in cold microtome sections prepared from unfixed specimens. 相似文献
17.
J M Luk S M Lind R S Tsang A A Lindberg 《The Journal of biological chemistry》1991,266(34):23215-23225
Four murine monoclonal antibodies reactive with distinctive regions of the hexose core domain of Salmonella lipopolysaccharide (LPS) were generated and their epitope specificities were delineated. MAST 56 (IgG1) and MAST 50 (IgG3) antibodies elicited by immunizations with Salmonella typhimurium Rb1 and Rb2 mutants, reacted selectively in enzyme immunoassay with the LPS from rough mutants. In contrast, MATy 1 (IgM) and MATy 2 (IgG2b) antibodies raised by an attenuated Salmonella typhi 620 Ty strain were reactive with LPS from both smooth and rough Salmonellae. Immunoblotting analysis showed that MATy 1 distinguished only the bottom bands (naked LPS core) among the heterogeneous LPS populations, whereas MATy 2 gave a ladder pattern (reactive with both naked and O-chain-substituted LPS cores). Differential binding specificities of MATy 1 and MATy 2 antibodies to the naked and capped LPS cores were further analyzed utilizing S. typhimurium polysaccharide fractions with different O-chain:core ratios which were obtained after separation by Sephacryl S-200 chromatography. Steric effects on the antibody reactivity by the bulky O-polysaccharide chain were detected. The use of chemically defined native and synthetic saccharides as inhibitors, in combination with the conformation of the Salmonella core oligosaccharide, permitted the definition of antigenic determinants carried in the core domain recognized by each antibody: (i) the branches I and VIII are essential for MATy 1 recognition, (ii) the backbone III-IV-V for MATy 2, (iii) the backbone II-III-IV-V for MAST 56, and (iv) the backbone plus the branch III-IV-V-VIII for MAST 50. (formula; see text) 相似文献
18.
19.
Summary The effect of the enzyme collagenase (40–200 units · ml-1) on the spontaneous mechanical activity in vitro and on the fine structure of the taenia coli of the guinea-pig was investigated. Initially, the spontaneous activity of the taenia was enhanced both in the isometric and isotonic recordings; after several minutes the muscles became slack or elongated to up to twice their resting lengths. The structural changes were dramatic but a number of muscle cells remained apparently unaltered even with the highest concentration and the longest incubation time (120 minutes). The large variety of structural changes were tentatively grouped into two separate sequences. One sequence involved swelling of the muscle cell, dispersion of the filaments and breaking up of the cell membrane: the thick myofilaments increased considerably in size and became heterogeneous in size and shape, but were still recognizable after disruption of the cell membrane. The other disruptive sequence involved separation of the superficial part of the muscle cell, which became electron-lucent, from the core of the cell where filaments were very densely packed. Few or no changes were observed in non-muscle cells.Supported by grants from the Medical Research Council and the Central Research Fund of the University of LondonFinancial support from the F.W.G.O. (Grant n 20.487) is gratefully acknowledged 相似文献
20.
Summary The behaviour of DNA from several strains ofB. subtilis andB. brevis on the dropping mercury electrode in a medium of ammonium formate was studied. Native DNA yields in this medium on the oscillogram dE/dt againstE an anodic indentation for which the residues of deoxyguanylic acid are responsible.B. subtilis DNA produces a substantially smaller indentation thanB. brevis DNA does. It was found that the difference is not conditioned by impurities in the DNA samples, nor by the presence of denatured DNA. The difference in the depth of the indentation produced byB. subtilis andB. brevis DNAs almost disappears after denaturation of these DNAs or in an ammonium formate medium of higher concentration. The assumption was advanced that the different oscillopolarographic behaviour of DNAs obtained fromB. subtilis andB. brevis is connected with the different primary structure of these DNAs. 相似文献