全文获取类型
收费全文 | 5444篇 |
免费 | 615篇 |
国内免费 | 1篇 |
出版年
2021年 | 55篇 |
2020年 | 44篇 |
2019年 | 58篇 |
2018年 | 73篇 |
2017年 | 50篇 |
2016年 | 81篇 |
2015年 | 153篇 |
2014年 | 173篇 |
2013年 | 224篇 |
2012年 | 278篇 |
2011年 | 275篇 |
2010年 | 180篇 |
2009年 | 160篇 |
2008年 | 228篇 |
2007年 | 215篇 |
2006年 | 199篇 |
2005年 | 215篇 |
2004年 | 191篇 |
2003年 | 192篇 |
2002年 | 213篇 |
2001年 | 195篇 |
2000年 | 176篇 |
1999年 | 149篇 |
1998年 | 77篇 |
1997年 | 62篇 |
1996年 | 60篇 |
1995年 | 45篇 |
1994年 | 56篇 |
1993年 | 50篇 |
1992年 | 111篇 |
1991年 | 107篇 |
1990年 | 84篇 |
1989年 | 110篇 |
1988年 | 91篇 |
1987年 | 96篇 |
1986年 | 74篇 |
1985年 | 92篇 |
1984年 | 59篇 |
1983年 | 65篇 |
1982年 | 53篇 |
1981年 | 48篇 |
1980年 | 54篇 |
1979年 | 68篇 |
1978年 | 49篇 |
1977年 | 70篇 |
1976年 | 50篇 |
1975年 | 50篇 |
1974年 | 42篇 |
1973年 | 42篇 |
1971年 | 39篇 |
排序方式: 共有6060条查询结果,搜索用时 31 毫秒
81.
Cyclic AMP-dependent protein kinases I and II, partially purified from rat liver cytosol, were inhibited 50% by 40 microM hemin and 100 microM hemin, respectively. With the purified catalytic subunit of cyclic AMP-dependent protein kinase, hemin caused non-competitive inhibition with respect to the peptide substrate and mixed inhibition with respect to ATP. Hemin also inhibited purified phosphorylase b kinase, indicating that hemin concentrations above 10 microM markedly inhibit multiple protein kinases. In isolated intact hepatocytes, hemin inhibited the glucagon-dependent activation of cyclic AMP-dependent protein kinases and the activation of glycogen phosphorylase. For both effects, high heme concentrations (40-60 microM) were required for 50% inhibition. Similar high levels of exogenous hemin inhibited total hepatocyte protein synthesis. By contrast, 5 microM hemin or less was sufficient to raise intracellular heme levels, as indicated by the relative heme-saturation of tryptophan oxygenase in hepatocytes. Hemin, 5 microM, completely repressed induction of 5-aminolevulinate synthase by dexamethasone in hepatocyte primary cultures. Such repression is unlikely to be mediated by inhibition of protein kinases. 相似文献
82.
G Harris W A Cramp J C Edwards A M George S A Sabovljev L Hart G R Hughes A M Denman M B Yatvin 《International journal of radiation biology and related studies in physics, chemistry, and medicine》1985,47(6):689-699
The proliferation of peripheral blood lymphocytes, cultured with Con A, can be inhibited by ionizing radiation. Lymphocytes from patients with conditions associated with autoimmunity, such as rheumatoid arthritis, systemic lupus erythematosus and polymyositis, are more radiosensitive than those from healthy volunteers or patients with conditions not associated with autoimmunity. The nuclear material isolated from the lymphocytes of patients with autoimmune diseases is, on average, lighter in density than the nuclear material from most healthy controls. This difference in density is not related to increased sensitivity to ionizing radiation but the degree of post-irradiation change in density (lightening) is proportional to the initial density, i.e. more dense nuclear material always shows a greater upward shift after radiation. The recovery of preirradiation density of nuclear material, 1 h after radiation exposure, taken as an indication of DNA repair, correlates with the radiosensitivity of lymphocyte proliferation (Con A response); failure to return to pre-irradiation density being associated with increased sensitivity of proliferative response. These results require extension but, taken with previously reported studies of the effects of DNA methylating agents, support the idea that DNA damage and its defective repair could be important in the aetio-pathogenesis of autoimmune disease. 相似文献
83.
A case of pulmonary dirofilariasis in a 62-year-old female was diagnosed by fine needle aspiration biopsy. A review of the literature revealed this to be the first reported case diagnosed by this method. The presence of bilateral lesions in this patient is an uncommon finding for this entity. 相似文献
84.
Dictyostelium amebae alter motility differently in response to increasing versus decreasing temporal gradients of cAMP 总被引:5,自引:4,他引:1 下载免费PDF全文
Using a perfusion chamber, we examined the behavior of individual amebae in increasing and decreasing temporal gradients of cAMP. We demonstrated that amebae respond to increasing temporal gradients of cAMP with stimulated motility and to corresponding decreasing temporal gradients with depressed motility. Depressed motility observed in decreasing temporal gradients corresponded to the inhibited levels observed when cAMP was applied at constant concentrations. These results were consistent with a simple model for the motile behavior of amebae in an early aggregation territory in which nondissipating waves of cAMP originate at the aggregation center and travel outward periodically. We conclude that chemotactically responsive amebae can assess whether a temporal gradient of chemoattractant is increasing or decreasing in the absence of a spatial gradient, and can adjust their motility accordingly. 相似文献
85.
R. R. C. Edwards 《Journal of fish biology》1985,26(1):1-4
Growth rates of three commercial species of Lutjanidae were estimated from sections of vertebrae cross-checked with scales. The von Bertalanffy growth constants for Lutjanus malabaricus were K =0.168, L∞ =70.7 cm standard length (S.L.). For Pristipomoides multidens these were K = 0.219, L ∞ = 59.1cm, and for Pristipomoides typus K =0.254, L ∞ = 51.5 cm. Length/weight was analysed in the form W= aLb where W was fresh weight (g) and L = S.L. (cm). The constants were: L. malabaricus, a = 0.041, b = 2.842; P. multidens, a =0.032, b = 2.897; P. typus, a = 0.038, b = 2.822. The growth rates were considered low in comparison with demersal fish counterparts from temperate seas. 相似文献
86.
Crystallization of a Fe,Zn superoxide dismutase from the archaebacterium Thermoplasma acidophilium 总被引:1,自引:0,他引:1
The novel Fe,Zn superoxide dismutase from the archaebacterium Thermoplasma acidophilium has been crystallized in space groups P1, P2(1) and P2(1)2(1)2, with 2,4 and 1/2 of an 84,000 Mr tetramer, respectively, estimated to be in the asymmetric unit of the unit cell. The orthorhombic crystals, which have unit cell dimensions a = 84.2 A, b = 72.7 A, c = 67.8 A, diffract X-rays to at least 2.0 A and are suitable for a determination of the three-dimensional structure of the Fe,Zn superoxide dismutase. 相似文献
87.
A previously described direct radioimmunoassay for plasma aldosterone has been modified to enable direct measurement of the steroid in saliva. The specificity of the method has been demonstrated by assay after high pressure liquid chromatographic purification of saliva extracts. Assay of matched plasma and saliva samples taken from normal subjects during unrestricted and controlled sodium intakes, either under basal conditions or while undergoing ACTH stimulation or dexamethasone suppression, confirms that salivary aldosterone values provide a good reflection of levels in plasma. Mean salivary aldosterone values are approximately one-third of those in plasma. Sampling immediately upon waking appears to provide reliable values for salivary aldosterone, and the potential application of this technique to the screening of hypertensive patients is discussed. 相似文献
88.
Histoplasma capsulatum yeast cells have been studied by immunoelectron microscopy using rabbit polyclonal antisera and a biotin-avidin-peroxidase detection system. An antigenic surface layer has been visualized in the cell wall of immunostained organisms. This layer was not seen in samples prepared by standard electron microscopic methods or in negative controls used with the immunocytochemical technique. Without immunostaining the cell wall ofHistoplasma appeared almost transparent. In contrast, after immunoperoxidase staining the cell wall was conspicuous, bounded by the darkly stained outer layer. This electron dense layer, appeared to be a reservoir of surface antigens that were recognized by anti-Histoplasma antibodies.Abbreviations CHHA
Cystine-heart-hemoglobin agar
- PBS
phosphate buffered saline
- Ig
immunoglobulin
- TBS
Tris buffered saline
- DAB
3,3-diaminobenzidine tetrachloride
- FITC
fluorescein isothiocyanate
- M199
tissue culture medium 199, according to Morgan et al. (1950) 相似文献
89.
C N Hebert S Edwards S Bushnell P C Jones C T Perry 《Journal of biological standardization》1985,13(3):243-253
The critical statistical parameters of an enzyme-linked immunosorbent assay were determined to enable quantitation of antibody responses in cattle affected with infectious bovine rhinotracheitis. A system of controlling well-to-well variations in optical density reading across a microtitre plate was evolved and dose--response assays were carried out to determine the dilution of serum which gave the greatest discrimination between acute and convalescent sera from an infected animal. Use of a standard serum was studied in further assays. An increase in optical density value of 0.15 was set as a diagnostic criterion for a significantly rising antibody response. This compared well with the conventional criterion of a fourfold rise in virus neutralizing antibody titre. 相似文献
90.
Human glyceraldehyde-3-phosphate dehydrogenase: mRNA levels and enzyme activity in developing muscle. 总被引:3,自引:1,他引:2
Analysis of human glyceraldehyde-3-phosphate dehydrogenase mRNA revealed that levels in adult skeletal muscle are 12-fold greater per microgram of polyadenylated RNA than in fetal skeletal muscle, whereas in cardiac muscle RNA levels were about equal in fetal and adult tissue. The mRNA levels correlate well with glyceraldehyde 3-phosphate dehydrogenase enzyme activities. There was no evidence for fetus- or tissue-specific forms. 相似文献