全文获取类型
收费全文 | 4380篇 |
免费 | 368篇 |
出版年
2023年 | 22篇 |
2022年 | 31篇 |
2021年 | 64篇 |
2020年 | 51篇 |
2019年 | 54篇 |
2018年 | 55篇 |
2017年 | 63篇 |
2016年 | 106篇 |
2015年 | 192篇 |
2014年 | 224篇 |
2013年 | 211篇 |
2012年 | 327篇 |
2011年 | 315篇 |
2010年 | 243篇 |
2009年 | 188篇 |
2008年 | 271篇 |
2007年 | 236篇 |
2006年 | 246篇 |
2005年 | 223篇 |
2004年 | 218篇 |
2003年 | 212篇 |
2002年 | 236篇 |
2001年 | 43篇 |
2000年 | 40篇 |
1999年 | 50篇 |
1998年 | 63篇 |
1997年 | 48篇 |
1996年 | 44篇 |
1995年 | 45篇 |
1994年 | 39篇 |
1993年 | 43篇 |
1992年 | 35篇 |
1991年 | 38篇 |
1990年 | 20篇 |
1989年 | 40篇 |
1988年 | 37篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1984年 | 28篇 |
1983年 | 16篇 |
1982年 | 29篇 |
1981年 | 20篇 |
1980年 | 19篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1974年 | 15篇 |
1973年 | 19篇 |
1970年 | 12篇 |
排序方式: 共有4748条查询结果,搜索用时 203 毫秒
31.
Recent studies indicate that there may be multiple subtypes of [3H]5-hydroxytryptamine ([3H]5-HT) binding sites. Mianserin and spiperone inhibited the specific binding of [3H]5-HT (2-3 nM) to rat brain cortical membranes with shallow displacement curves. The displacement data for spiperone were best described by the presence of three independent binding sites, for which spiperone had high, medium, and low affinities. The displacement data for mianserin were best fitted by two independent, high- and low-affinity sites. The inclusion of mianserin (250 nM) to inhibit [3H]5-HT binding to the mianserin-sensitive site selectively blocked one of the sites discriminated by spiperone. These results suggest the presence of three binding sites for [3H]5-HT, one blocked by low concentrations of spiperone (5-HT1A), one blocked by low concentrations of mianserin (5-HT1C), and one blocked only by high concentrations of both mianserin and spiperone (5-HT1B). Regional differences in the relative densities of the three sites were observed. The hippocampus was rich in 5-HT1A sites, whereas the striatum contained mainly 5-HT1B and 5-HT1C sites. Selective degeneration of 5-HT-containing nerve terminals induced by the neurotoxin 5,7-dihydroxytryptamine increased binding to all three sites in the cerebral cortex. Binding of [3H]5-HT to the three sites was differentially modulated by CaCl2 and guanylimidodiphosphate. The present data suggest the presence of three independent 5-HT1 binding sites having different affinities for mianserin and spiperone and having different regional distributions. 相似文献
32.
James Varani Jefferey D. Hasday Robert G. Sitrin Pamela G. Brubaker William A. hillegas 《In vitro cellular & developmental biology. Plant》1986,22(10):575-582
Summary Human diploid fibroblasts were cultured on microcarriers made from DEAE-dextran, denatured collagen, DEAE-dextran linked to
denatured collagen, and glass. Cells grown on these four substrates were examined for the production of proteolytic enzymes
and arachidonic acid metabolites. Culture fluids from cells grown on the DEAE-dextran microcarriers contained the highest
amounts of proteolytic enzyme activity. Both plasminogen-independent and plasminogen-dependent fibrinolytic activities were
present and the plasminogen-dependent activity seemed to result from the presence of both urokinase and tissue plasminogen
activator. Culture fluid from the cells grown on the glass microcarriers contained the least amount of protease activity,
and nearly all of the plasminogen-activator activity seemed to be of the urokinase type. Protease activity in the culture
fluids of cells grown on the other two substrates were intermediate. With regard to arachidonic acid metabolites, cells grown
on the DEAE-dextran microcarriers produced the highest amounts of cyclooxygenase products but very low levels of lipoxygenase
metabolites. Cells grown on the other three substrates produced comparable amounts of various cyclooxygenase products (lower
than that produced by cells on the DEAE-dextrans substrate). Cells grown on the glass microcarriers also produced detectable
amounts of two lipoxygenase metabolites—leukotriene B4 and leukotriene C4. Inasmuch as both proteolytic enzymes and arachidonic acid metabolites regulate basic cell properties, the differential amount
of these metabolites observed in the culture fluids on the various substrates may contribute to the biological differences
that exist on these substrates.
This study was supported in part by grants R44 CA 36656 and IK08HL01332-01 from the Public Health Service, U. S. Department
of Health and Human Services and by grant BC-512 from the American Cancer Society. JDH is a research fellow of the American
Lung Association. 相似文献
33.
The drug resistance plasmid pKM101 makes Escherichia coli resistant to the lethal effects of ultraviolet (UV) irradiation and more susceptible to mutagenesis by a variety of agents. The plasmid operon responsible for increasing mutagenesis has been termed mucAB (Mutagenesis, UV and chemical). We have isolated a derivative of pKM101 called pGW1975 which makes cells more sensitive to killing by UV but which retains the ability of pKM101 to increase susceptibility to methyl methanesulfonate (MMS) mutagenesis. pGW1975 increases UV mutagenesis less than pKM101 in a uvrA+ strain but more than pKM101 in a uvrA− strain. muc− point and insertion mutants of pKM101 and pGW1975 complement to restore the plasmid-mediated: (i) ability to reactivate UV-irradiated phage, (ii) resistance to killing by UV, and (iii) level of susceptibility to UV mutagenesis. We have identified a 2.0 kb region of pKM101 which is responsible for the complementation and which maps counterclockwise of mucAB. 相似文献
34.
Douglas M. Carlson William L. Pflieger Linden Trial Pamela S. Haverland 《Environmental Biology of Fishes》1985,14(1):51-59
Synopsis
Scaphirhynchus albus and S. platorynchus were studied in Missouri during 1978–1979 to assess their distribution and abundance, to obtain information on their life
histories, and to identify existing or potential threats to their survival. S. platorynchus was collected in substantial numbers (4355 specimens) at all 12 sampling stations in the Missouri and Mississippi rivers,
while only 11 S. albus were captured from 6 stations. Twelve specimens identified in the field as hybrids between the two species were captured
from 4 stations. Morphometric and meristic comparisons of presumed hybrids with the parent species, using cluster and principal
components analyses, demonstrated intermediacy of most specimens identified in the field as hybrids. Aquatic insects comprised
most of the diet of S. platorynchus and S. albus, but S. albus and the hybrids had consumed considerable quantities of fish. S. albus grew more rapidly than S. platorynchus, while the growth of hybrids was intermediate. Hybridization appears to be a recent phenomenon, resulting from man-caused
changes in the big-river environment. Hybridization may be a threat to survival of S. albus in the study streams. 相似文献
35.
Pamela J. Weathers James F. Danielli Peter M. Bradley Diane M. Hebb Judith E. Miller Rick L. Pesano 《Physiologia plantarum》1984,61(3):441-448
We report the isolation of a cukaryotic green alga ( Chlorella , strain WPI-2) which accumulates large stores of nitrogen (N) during growth in N-free medium and seems to incorporate14 N2 , yet does not reduce acetylene to ethylene. Total N accumulation during growth on N-free medium and in gases free of combined N was measured by three methods: Kjeldahl, oxidative pyrolysis via chemiluminescence (Antek N analyzer), and Dumas (Coleman N analyzer). Increases in N ranging from 22–64%± 1% were observed. Isotope dilution studies using cells labelled with 15 NO 3 - and then shifted to 14 N2 in N-free medium showed dilution of the 15 N isotope by 14 N from 5.67 to 5.32%± 0.05%. Using a variety of conditions, we were unable to demonstrate the reduction of acctylene to ethylene by WPI-2, although diazotrophic cyanobacteria gave positive results. Although the data on WPI-2 are not conclusive in establishing this alga as a diazotroph, the data do suggest that within the Chlorophyceae there may exist a novel form of nitrogen gas metabolism. 相似文献
36.
Different hormonal requirements for androgen-independent growth of normal and tumor epithelial cells from rat prostate 总被引:4,自引:0,他引:4
Wallace L. McKeehan Pamela S. Adams Danna Fast 《In vitro cellular & developmental biology. Plant》1987,23(2):147-152
Summary The proliferation of isolated normal prostate epithelial cells from rat and man is androgen-independent and requires cholera
toxin, insulin, dexamethasone, epidermal growth factor (EGF) and one or more polypeptide factors that are concentrated in
bovine neural tissue. The active agents in the neural tissue extract are heparin-binding polypeptides (prostatropins), the
predominant form of which has a molecular weight of 17400 and an acetylalanine at the aminoterminus. Prostatropins supported
a half-maximal increase in normal prostate epithelial cell number at 50 picomolar. The proliferation of primary and serially-cultured
epithelial cells from androgen-responsive Dunning R3327 rat prostate tumors was also androgen-independent, but exhibited dramatic
alterations in response to hormones that stimulated normal cell proliferation. At low cell density, androgen-independent growth
of isolated tumor-derived epithelial cells was independent on cholera toxin, was stimulated by dexamethasone, required insulin
andeither EGFor prostatropin. The presence of either EGF or prostatropin masked the response to the other factor. In the absence of EGF,
purified prostatropins supported a half-maximal increase in tumor cell number at 7 picomolar. Endogenous production of EGF-like
and prostatropin-like factors or both was suggested by the reduced requirement for EGF and prostatropin at high prostate tumor
cell density. These results suggest that anti-hormonal therapies against prostate tumor growth should be based on intervention
with the activity of insulin (or insulin-like factors) or simultaneous intervention with both EGF and prostatropin (or their
homologues).
This work was supported by NIH grants CA 37589 and HL 33847, and grant 1718 from the Council for Tobacco Research.
Editor’s Statement This paper is the first report of the comparison of the hormone requirements of primary cultures of normal
and tumor prostate epithelial cells from the same system. 相似文献
37.
Pamela M. Reilly Dr. Palmer Rogers 《Journal of industrial microbiology & biotechnology》1987,1(5):329-334
Summary Conditions that allow regeneration of cells fromClostridium acetobutylicum strain B643 protoplasts were studied. Protoplast formation and stabilization in minimal media with 50 mM CaCl2, 50 mM MgCl2 and 0.3 M sucrose were crucial to subsequent regeneration on soft yeast extract agar containing 25 mM CaCl2 and 25 mM MgCl2. A regeneration frequency of 8–25% was consistently obtained. 相似文献
38.
Exponentially growing cultures ofNitrosomonas europaea were inhibited by addition of 0.5 g nitrapyrin ml–1. This inhibition was increased by simultaneous addition of 0.046 g Cu2+ ml–1 as copper sulfate. This contradicts a previous report that copper relieves inhibition of ammonia oxidation by nitrapyrin, which report has formed the basis for hypotheses regarding the mechanism of action of this inhibitor. 相似文献
39.
40.
Pamela J. Letts Les Pinteric Harry Schachter 《Biochimica et Biophysica Acta (BBA)/General Subjects》1974,372(2):304-320
Golgi fractions prepared from rat testis have been shown to be enriched in the following glycoprotein glycosyltransferases: N-acetylglucosaminyltransferase, 47-fold, galactosyltransferase, 33-fold, and N-acetylglucosaminide fucosyltransferase, 15-fold. Appreciably lower transferase levels were obtained in other subcellular fractions. In the mouse, Golgi fractions were prepared from testis homogenates, testis cell suspensions and partially purified testis germinal cells; these fractions were also enriched in the above glycoprotein glycosyltransferases. Electron microscopic analysis indicated that a major portion of the total transferase activity was located in the Golgi apparatus of both rat and mouse testis although these experiments could not rule out the possible presence of some transferase activity in other organelles. 相似文献