全文获取类型
收费全文 | 9590篇 |
免费 | 1061篇 |
国内免费 | 7篇 |
专业分类
10658篇 |
出版年
2022年 | 69篇 |
2021年 | 124篇 |
2020年 | 74篇 |
2019年 | 96篇 |
2018年 | 115篇 |
2017年 | 111篇 |
2016年 | 172篇 |
2015年 | 261篇 |
2014年 | 298篇 |
2013年 | 376篇 |
2012年 | 465篇 |
2011年 | 494篇 |
2010年 | 309篇 |
2009年 | 256篇 |
2008年 | 394篇 |
2007年 | 432篇 |
2006年 | 350篇 |
2005年 | 360篇 |
2004年 | 395篇 |
2003年 | 378篇 |
2002年 | 336篇 |
2001年 | 265篇 |
2000年 | 285篇 |
1999年 | 244篇 |
1998年 | 131篇 |
1997年 | 107篇 |
1996年 | 99篇 |
1995年 | 78篇 |
1994年 | 99篇 |
1993年 | 79篇 |
1992年 | 177篇 |
1991年 | 167篇 |
1990年 | 140篇 |
1989年 | 145篇 |
1988年 | 126篇 |
1987年 | 132篇 |
1986年 | 105篇 |
1985年 | 145篇 |
1984年 | 121篇 |
1983年 | 103篇 |
1982年 | 94篇 |
1981年 | 125篇 |
1980年 | 111篇 |
1979年 | 115篇 |
1978年 | 95篇 |
1977年 | 103篇 |
1976年 | 86篇 |
1975年 | 113篇 |
1974年 | 105篇 |
1973年 | 69篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Thiophosphorylation causes Ca2+-independent norepinephrine secretion from permeabilized PC12 cells 总被引:2,自引:0,他引:2
Adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) was used to examine the role of phosphorylation in the regulation of norepinephrine secretion by digitonin-permeabilized PC12 cells. While most kinases will use ATP gamma S to thiophosphorylate proteins, thiophosphorylated proteins are relatively resistant to dethiophosphorylation by protein phosphatases. Norepinephrine secretion by permeabilized PC12 cells was ATP- and Ca2+-dependent but resistant to calmodulin antagonists. Half-maximum secretion was obtained in 0.75 microM Ca2+. Permeabilized PC12 cells were incubated with ATP gamma S in the absence of Ca2+, the ATP gamma S was removed, and norepinephrine secretion was determined. Preincubation with ATP gamma S increased the amount of norepinephrine secreted in the absence of Ca2+, but it had no effect on the amount released in the presence of Ca2+. After a 15-min preincubation in 1 mM ATP gamma S, there was almost as much secretion in the absence of Ca2+ as in its presence. Inclusion of ATP in the preincubation inhibited the effect of ATP gamma S. Ca2+ stimulated the rate of modification by ATP gamma S as brief preincubations with ATP gamma S in the presence of Ca2+ resulted in higher levels of Ca2+-independent secretion than did preincubations with ATP gamma S in the absence of Ca2+. Similarly, brief preincubations of permeabilized cells with ATP in the presence of Ca2+ resulted in elevated levels of Ca2+-independent secretion. Secretion of norepinephrine from ATP gamma S-treated cells was ATP-dependent. These results suggest that norepinephrine secretion by PC12 cells is regulated by a Ca2+-dependent phosphorylation. Once this phosphorylation has occurred, secretion is still ATP-dependent, but it no longer requires Ca2+. 相似文献
102.
Apparent affinities (Ki) of (E)- and (Z)-N-(iodoallyl)spiperone [E)- and (Z)-NIASP) for dopamine D2 and serotonin 5-HT2 receptors were determined in competition binding assays. (Z)-NIASP (Ki 0.35 nM, D2; Ki 1.75 nM, 5-HT2) proved slightly more potent and selective for D2 sites in vitro than (E)-NIASP (Ki 0.72 nM, D2; Ki 1.14 nM, 5-HT2). In vivo, radioiodinated (E)- and (Z)-[125I]-NIASP showed regional distributions in mouse brain which are consonant with prolonged binding to dopamine D2 receptors accompanied by a minor serotonergic component of shorter duration. Stereoselective, dose-dependent blockade of (E)-[125I]-NIASP uptake was found for drugs binding to dopamine D2 sites, while drugs selective for serotonin 5-HT2, alpha 1-adrenergic and dopamine D1 receptors did not inhibit radioligand binding 2 hr postinjection. Specific binding in striatal tissue was essentially irreversible over the time course of the study, and (E)-[125I]-NIASP gave a striatal to cerebellar tissue radioactivity concentration of 16.9 to 1 at 6 hr postinjection. Thus, (E)-[125I]-NIASP binds with high selectivity and specificity to dopamine D2 sites in vivo. 相似文献
103.
V. T. Wagner C. Dumas H. L. Mogensen 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1990,79(1):72-76
Summary The position of the embryo sac in the spikelet and of the embryo sac's constituent cells within the sporophytic tissues of Zea mays was localized by scanning electron microscopy, serial thick sectioning, and computer three-dimensional reconstruction. Within certain limits, the embryo sac is consistently oriented in the same position inside of the spikelet. This information is a prerequisite for successful microinjections into the in situ female cells of Zea mays. 相似文献
104.
Laboratory reared Ixodes scapularis proved to be an efficient vector of Babesia odocoilei Emerson and Wright between white-tailed deer (Odocoileus virginianus). Transtadial survival of the babesia occurred between nymph and adult stages of the tick, and the adult stage transmitted the babesia. 相似文献
105.
A method for cultivating morphologically undifferentiated embryonic stem cells from porcine blastocysts 总被引:11,自引:0,他引:11
Variable conditions were tested to determine an in-vitro cultivation method for the formation of morphologically undifferentiated embryonic stem cells from the inner cell mass (ICM) derived outgrowth of porcine blastocysts. Although all 16 Day-9 embryos failed to form colonies, 14 such colonies were obtained from a total of 69 Day-10 embryos when they were co-cultivated with porcine uterine fibroblast (PUF) cells over a 6-day period. The best results were obtained in Dulbecco's modified Eagle medium (DMEM) with 10% fetal calf serum and 10% porcine serum supplemented with bovine insulin and beta-mercaptoethanol, in which six out of seven embryos formed adequate ICM-derived colonies. Since murine fibroblasts were not found to be suitable feeder cells in this procedure, an endocrine synergistic interaction, which promotes embryonic attachment and colony formation, between porcine blastocysts and PUF cells is hypothesized. Continued propagation of the ICM-derived cells was not dependent on these factors; a total of seven cell lines were obtained after three to five subsequent passages on murine feeder-layers that resembled morphologically undifferentiated embryonic cells. 相似文献
106.
107.
A C Wagner C Sch?fer J A Williams 《Biochemical and biophysical research communications》1992,189(3):1606-1612
The effects of the phosphatase inhibitors calyculin A and okadaic acid on amylase release from streptolysin-O permeabilized rat pancreatic acini were investigated. Both agents induced similar biphasic effects with moderate potentiation of calcium-stimulated amylase release at medium and strong inhibition at higher concentrations. Calyculin A was thirty times more potent than okadaic acid and at 100 nM totally inhibited calcium-induced amylase release while 3 microM okadaic acid reduced amylase release by 78%. 100nM calyculin A also completely inhibited GTP gamma S-potentiated amylase release and partially inhibited phorbol ester potentiated secretion. The data indicate that inhibition of a serine/threonine phosphatase, probably a type 1 phosphatase, leads to inhibition of calcium-induced amylase release in permeabilized pancreatic acini. 相似文献
108.
S L Wagner R S Siegel T S Vedvick W C Raschke W E Van Nostrand 《Biochemical and biophysical research communications》1992,186(2):1138-1145
The protease inhibitor, protease nexin-2 (PN-2), is the secreted isoform of the Alzheimer's amyloid beta-protein precursor (A beta PP) that contains the Kunitz-type protease inhibitor (KPI) domain. Here we describe the use of the methylotrophic industrial yeast Pichia pastoris as a host system for the large scale production of the KPI domain of PN-2/A beta PP. In addition to the 57 amino acid KPI domain, the expression product contained an additional four amino acid residues at its amino terminus that correspond to amino acids 285-288 of A beta PP (Ponte et al. 1988 Nature 311:525-527). This expression system generated yields of greater than 1.0 gram of KPI domain per liter of fermentation media. The secreted 61 amino acid product was purified to homogeneity and biochemically characterized. Amino acid analysis and sequencing of the entire expressed KPI domain verified its integrity. Similar to native PN-2/A beta PP, the purified KPI domain potently inhibited trypsin, chymotrypsin, and coagulation factor XIa. Although heparin augments the inhibition of factor XIa by native PN-2/A beta PP it had no effect on the inhibition of factor XIa by expressed KPI domain suggesting that heparin binds to regions on native PN-2/A beta PP outside of the protease inhibitory domain. This KPI domain expression product should be useful in studying the physiologic and pathophysiologic functions of PN-2/A beta PP. 相似文献
109.
110.