全文获取类型
收费全文 | 6387篇 |
免费 | 686篇 |
国内免费 | 1篇 |
出版年
2021年 | 59篇 |
2018年 | 79篇 |
2017年 | 60篇 |
2016年 | 105篇 |
2015年 | 181篇 |
2014年 | 188篇 |
2013年 | 261篇 |
2012年 | 288篇 |
2011年 | 313篇 |
2010年 | 185篇 |
2009年 | 167篇 |
2008年 | 253篇 |
2007年 | 236篇 |
2006年 | 258篇 |
2005年 | 228篇 |
2004年 | 231篇 |
2003年 | 185篇 |
2002年 | 252篇 |
2001年 | 189篇 |
2000年 | 169篇 |
1999年 | 162篇 |
1998年 | 91篇 |
1997年 | 61篇 |
1996年 | 76篇 |
1995年 | 90篇 |
1994年 | 73篇 |
1993年 | 90篇 |
1992年 | 144篇 |
1991年 | 129篇 |
1990年 | 144篇 |
1989年 | 109篇 |
1988年 | 122篇 |
1987年 | 119篇 |
1986年 | 95篇 |
1985年 | 108篇 |
1984年 | 97篇 |
1983年 | 84篇 |
1982年 | 95篇 |
1981年 | 89篇 |
1980年 | 81篇 |
1979年 | 104篇 |
1978年 | 102篇 |
1977年 | 72篇 |
1976年 | 72篇 |
1975年 | 65篇 |
1973年 | 54篇 |
1972年 | 61篇 |
1971年 | 56篇 |
1968年 | 49篇 |
1967年 | 46篇 |
排序方式: 共有7074条查询结果,搜索用时 15 毫秒
61.
The trk tyrosine protein kinase mediates the mitogenic properties of nerve growth factor and neurotrophin-3. 总被引:45,自引:0,他引:45
C Cordon-Cardo P Tapley S Q Jing V Nanduri E O'Rourke F Lamballe K Kovary R Klein K R Jones L F Reichardt 《Cell》1991,66(1):173-183
The product of the trk proto-oncogene encodes a receptor for nerve growth factor (NGF). Here we show that NGF is a powerful mitogen that can induce resting NIH 3T3 cells to enter S phase, grow in semisolid medium, and become morphologically transformed. These mitogenic effects are absolutely dependent on expression of gp140trk receptors, but do not require the presence of the previously described low affinity NGF receptor. gp140trk also serves as a receptor for the related factor neurotrophin-3 (NT-3), but not for brain-derived neurotrophic factor. Both NGF and NT-3 induce the rapid phosphorylation of gp140trk receptors and the transient expression of c-Fos proteins. However, NT-3 appears to elicit more limited mitogenic responses than NGF. These results indicate that the product of the trk proto-oncogene is sufficient to mediate signal transduction processes induced by NGF and NT-3, at least in proliferating cells. 相似文献
62.
The trkB tyrosine protein kinase is a receptor for brain-derived neurotrophic factor and neurotrophin-3. 总被引:48,自引:0,他引:48
R Klein V Nanduri S A Jing F Lamballe P Tapley S Bryant C Cordon-Cardo K R Jones L F Reichardt M Barbacid 《Cell》1991,66(2):395-403
trkB is a tyrosine protein kinase gene highly related to trk, a proto-oncogene that encodes a receptor for nerve growth factor (NGF) and neurotrophin-3 (NT-3). trkB expression is confined to structures of the central and peripheral nervous systems, suggesting it also encodes a receptor for neurotrophic factors. Here we show that brain-derived neurotrophic factor (BDNF) and NT-3, but not NGF, can induce rapid phosphorylation on tyrosine of gp145trkB, one of the receptors encoded by trkB. BDNF and NT-3 can induce DNA synthesis in quiescent NIH 3T3 cells that express gp145trkB. Cotransfection of plasmids encoding gp145trkB and BDNF or NT-3 leads to transformation of recipient NIH 3T3 cells. In these assays, BDNF elicits a response at least two orders of magnitude higher than NT-3. Finally, 125I-NT-3 binds to NIH 3T3 cells expressing gp145trkB; binding can be competed by NT-3 and BDNF but not by NGF. These findings indicate that gp145trkB may function as a neurotrophic receptor for BDNF and NT-3. 相似文献
63.
The trk proto-oncogene encodes a receptor for nerve growth factor. 总被引:127,自引:0,他引:127
Two classes of receptors with distinct affinities for nerve growth factor (NGF) have been identified. The low affinity receptor (Kd approximately 10(-9) to 10(-8) M) is a cysteine-rich glycoprotein encoded by the previously characterized LNGFR gene. The structural nature of the high affinity receptor (Kd approximately 10(-11) to 10(-10) M) has yet to be established. In this study we show that the product of the human trk proto-oncogene (gp140trk) binds NGF with high affinity. Moreover, NGF could be chemically cross-linked to the endogenous gp140trk present in rat PC12 pheochromocytoma cells as well as to gp140trk ectopically expressed in mouse fibroblasts and in insect Sf9 cells. High affinity binding of NGF to gp140trk can occur in the absence of low affinity LNGFR receptors, at least in nonneural cells. Addition of NGF to PC12 cells elicits rapid phosphorylation of gp140trk on tyrosine residues and stimulates its tyrosine kinase activity. These results indicate that gp140trk is a functional NGF receptor that mediates at least some of the signal transduction processes initiated by this neurotrophic factor. 相似文献
64.
65.
In vitro vegetative propagation of Chinese cabbage 总被引:1,自引:0,他引:1
Explants from cotyledons, axillary buds, inflorescence stems and flower buds of Brassica campestris ssp. pekinensis (Lour.) Olsson (Chinese cabbage, cv. Wongbok) were cultured on MS medium with growth regulators. Multiple shoots were obtained from cotyledons, axillary buds and flower buds but not from inflorescence stems. Propagation of shoots from cotyledons was more successful than from axillary buds and flower buds. The vegetative propagation rates varied amongst clones derived from cotyledons of the same cultivar and seed lot. The propagation rates of the cotyledon-derived material followed a normal distribution with an average propagation rate of 2.6 shoots per two weeks subculture when cultured on MS media plus 44.4 m benzyladenine (BA) and 14.8 m -indolebutyric acid (IBA). Shoots from three clones were cultured on MS medium with nine different concentrations of BA. The concentration of BA which promoted the highest rate of shoot propagation varied for the three clones and was in the range 44.4 to 177.6 m. 相似文献
66.
67.
M Whetsell R L Mosley L Whetsell F V Schaefer K S Miller J R Klein 《Molecular and cellular biology》1991,11(12):5902-5909
The molecular organization of rearranged T-cell receptor (TCR) gamma genes intraepithelial lymphocytes (IEL) was studied in athymic radiation chimeras and was compared with the organization of gamma gene rearrangements in IEL from thymus-bearing animals by polymerase chain reaction and by sequence analyses of DNA spanning the junction of the variable (V) and joining (J) genes. In both thymus-bearing mice and athymic chimeras, IEL V-J gamma-gene rearrangements occurred for V gamma 1.2, V gamma 2, and V gamma 5 but not for V gamma 3 or V gamma 4. Sequence analyses of cloned V-J polymerase chain reaction-amplified products indicated that in both thymus-bearing mice and athymic chimeras, rearrangement of V gamma 1.2 and V gamma 5 resulted in in-frame as well as out-of-frame genes, whereas nearly all V gamma 2 rearrangements were out of frame from either type of animal. V-segment nucleotide removal occurred in most V gamma 1.2, V gamma 2, and V gamma 5 rearrangements; J-segment nucleotide removal was common in V gamma 1.2 but not in V gamma 2 or V gamma 5 rearrangements. N-segment nucleotide insertions were present in V gamma 1.2, V gamma 2, and V gamma 5 IEL rearrangements in both thymus-bearing mice and athymic chimeras, resulting in a predominant in-frame sequence for V gamma 5 and a predominant out-of-frame sequence for V gamma 2 genes. These findings demonstrate that (i) TCR gamma-gene rearrangement occurs extrathymically in IEL, (ii) rearrangements of TCR gamma genes involve the same V gene regardless of thymus influence; and (iii) the thymus does not determine the degree to which functional or nonfunctional rearrangements occur in IEL. 相似文献
68.
Gerd Wallukat Gyorgy Nemecz Tibor Farkas Hartmut Kuehn Albert Wollenberger 《Molecular and cellular biochemistry》1991,102(1):35-47
Incubation of rocker-cultured neonatal rat heart cells with 3 mM L(+)-lactate led to a sharp increase in the sensitivity of cardiomyocytes to the beta-adrenergic agonist isoprenaline, as measured by their chronotropic response. This effect was accompanied by a reduction in the arachidonic acid content of the total phospholipids. The phospholipase A2-activator melittin as well as free arachidonic acid induced this supersensitivity to the same degree. On the other hand, the L(+)-lactate-evoked supersensitivity could be blocked by the phospholipase A2 inhibitors mepacrine and n-bromophenacyl-bromide, suggesting an involvement of phospholipase A2 in the process of beta-adrenergic sensitization. The sensitizing action of arachidonic acid was blocked by the lipoxygenase inhibitors esculetin and nordihydroguaiaretic acid, but not by the cyclooxygenase inhibitor indomethacin. Supersensitivity was likewise evoked by 15-S-hydroxyeicosatetraenoic acid (15-S-HETE), but not by 5-S-HPETE or 5-S-HETE. These findings suggest that the phospholipase A2-15-lipoxygenase pathway plays a role in the induction of beta-adrenergic supersensitivity in the cultured cardiomyocytes and point to a new physiological role of the lipoxygenase product 15-S-HETE.Abbreviations NDGA
nordihydroguaiaretic acid
- HETE
hydroeicosatetraenoic acid
- HPETE
hydroperoxyeicosatetraenoic acid 相似文献
69.
The moderate halophile Vibrio costicola, growing on a chemically-defined medium, transformed choline into glycine betaine (betaine) by the membrane-bound enzyme choline dehydrogenase and the cytoplasmic enzyme betainal (betaine aldehyde) dehydrogenase. Choline dehydrogenase was strongly induced and betainal dehydrogenase less strongly induced by choline. The formation of these enzymes was also regulated by the NaCl concentration of the growth medium, increasing with increasing NaCl concentrations. Intracellular betaine concentrations also increased with increasing choline and NaCl concentrations in the medium. This increase was almost completely blocked by chloramphenicol, which does not block the increase in salt-tolerant active transport on transfer from a low to a high salt concentration.Choline dehydrogenase was inhibited by chloride salts of Na+, K+, and NH
inf4
su+
, the inhibition being due to the Cl- ions. Betainal dehydrogenase was stimulated by 0.5 M salts and could function in up to 2.0 M salts.Cells grew as well in the presence as in the absence of choline in 0.5 M and 1.0 M NaCl, but formed no intracellular betaine. Choline stimulated growth in 2.0 M NaCl and was essential for growth in 3.0 M NaCl. Thus, while betaine is important for some of the adaptations to high salt concentration by V. costicola, it by no means accounts for all of them.Abbreviations CDMM
chemically-defined minimal medium
- PPT
proteose-peptone tryptone medium
- SDS
sodium dodecyl sulfate
Deceased, 1987 相似文献
70.
Hartmut Böhm Hans-Georg Heinzel Hans Scharstein Gernot Wendler 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,169(6):671-683
The wind-orientated walk of carrion beetles Necrophorus humator F. was analysed under closed-loop conditions with a walking compensator and under openloop conditions with a paired tread wheel (Fig. 1).
相似文献
1. | On the walking compensator an animal runs stable courses with a preferred direction relative to an air current (velocity =; 100 cm/s, Fig. 2B-D). A change in the air-current direction causes a corresponding adjustment of the mean walking direction (Fig. 3). Such course adjustment works best for changes in the air-current direction by an absolute value of 90° (Table 2). |
2. | Under closed-loop conditions the animal shows deviations of less than ± 45° around its preferred direction relative to the wind (Fig. 2B-D). The characteristic curve which describes the animal's angular velocity as a function of the animal's walking direction relative to the air-current stimulus is therefore revealed only in this angular range (Fig. 3, top). |
3. | Under open-loop conditions, however, complete characteristic curves can be obtained because the animal's walking reaction in response to any given angle of air-current stimulus is measurable on the paired tread wheel (Fig. 4). The characteristic curves are approximately sinusoidal functions. They can either show a shift parallel to the ordinale by a superimposed direction-independent constant angular velocity alone or, at the same time, they can independently exhibit an angular shift along the abscissa (Fig. 5). |
4. | The walking tracks straighten with increasing air-current velocity (Fig. 6A, insets), i.e. the animal more rapidly compensates deviations from a preferred course. This corresponds to higher amplitudes of the characterisic curve and steeper slopes at the negative zero-crossing point under open- as well as under closed-loop conditions (Fig. 6). |
5. | Walking in an air-current field can be explained by a model of the course control system using a feedback loop (Fig. 7). This model operates according to a sinusoidal characteristic function on which is superimposed a Gaussian white noise process of angular velocity which is independent of walking direction. The model produces realistic walking tracks in an air-current field (Fig. 8). |