全文获取类型
收费全文 | 241篇 |
免费 | 29篇 |
专业分类
270篇 |
出版年
2022年 | 2篇 |
2021年 | 4篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2015年 | 7篇 |
2014年 | 10篇 |
2013年 | 5篇 |
2012年 | 6篇 |
2011年 | 13篇 |
2010年 | 5篇 |
2009年 | 8篇 |
2008年 | 6篇 |
2007年 | 6篇 |
2006年 | 13篇 |
2005年 | 7篇 |
2004年 | 8篇 |
2003年 | 8篇 |
2002年 | 10篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 11篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 5篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 4篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 9篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 8篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1966年 | 3篇 |
1965年 | 3篇 |
1948年 | 1篇 |
1947年 | 1篇 |
1921年 | 1篇 |
排序方式: 共有270条查询结果,搜索用时 15 毫秒
1.
Male rodents were studied before and after undergoing one of three treatment conditions for 9 days: 1) cage control (n = 15, CON), 2) horizontal suspension (n = 15, HOZ), and 3) head-down suspension (n = 18, HDT). Testing included measurements of maximal O2 uptake (VO2 max) and select cardiovascular responses to graded treadmill exercise. VO2 max expressed on an absolute basis (ml/min) was significantly decreased after HOZ (-14.1 +/- 2.5%) and HDT (-14.3 +/- 2.0%), while being essentially unchanged in CON (-1.0 +/- 3.3%). Significant reductions in body weight were observed after both HOZ (-10.1 +/- 4.2 g) and HDT (-22.5 +/- 3.3 g), whereas CON animals exhibited a significant increase in weight (10.4 +/- 3.8 g). As a result, when VO2 max was normalized for body weight, all groups exhibited similar significant reductions of 6-7%. Although no differences in heart rate and blood pressure response to graded exercise were observed, the HDT group exhibited greater increases in mesenteric resistance at the same absolute exercise intensity. Furthermore, both suspended groups had higher iliac resistance values during exercise at similar relative exercise conditions, suggesting that muscle blood flow during treadmill running may have been reduced after suspension. In general, the decrements associated with the HOZ and HDT conditions were similar. It was concluded that reduction in exercise capacity and altered cardiovascular responses to exercise observed after 6-9 days of suspension were attributable to a combination of hypokinesia, lack of hindlimb weight bearing, or restraint, rather than to hydrostatic influences associated with HDT. 相似文献
2.
A de novo pathway for L-fucose synthesis has been detected in porcine thyroid tissue. This system uses guanosine diphospho-alpha-D mannose as a precursor and forms guanosine diphospho-beta-L-fucose as product. The system seems similar to those reported by others to exist in microorganisms and plants in that the first step of the pathway involves a 4-keto sugar nucleotide intermediate. The first enzyme of the pathway, guanosine diphospho-alpha-D-mannose oxidoreductase has been purified 57-fold from crude extracts by virtue of its affinity for Blue Sepharose. 相似文献
3.
Molecular and classical genetic analyses of his-3 mutants of Neurospora crassa. I. Tests for allelic complementation and specific revertibility 总被引:1,自引:0,他引:1
A collection of 81 his-3 mutants of Neurospora crassa was analyzed in assays for allelic complementation and specific revertibility. In these studies, the linearity of the complementation map of the his-3 cistron (Webber, 1965) was confirmed and mutants were classified as complementing with non-polarized or polarized complementation patterns, or non-complementing. In the assays for spontaneous or induced revertibility, 89% (71/80) of the mutants reverted either spontaneously or after treatment with the chemical mutagens N-methyl-N'-nitro-N-nitrosoguanidine, ethyl methanesulfonate, 2-methoxy-6-chloro-9-(3-[ethyl-2-chloroethyl]aminopropylamino) acridine dihydrochloride, nitrous acid or hydroxylamine. The frequency of revertible mutants among the non-polarized complementing mutants was 96% (45/47), and 79% (15/19) for the polarized complementing and 79% (11/14) for the non-complementing mutants. The results of these classical genetic assays for allelic complementation and specific revertibility suggest a correlation between complementation pattern and presumptive genetic alterations at the molecular level among his-3 mutants similar to that found with ad-3B mutants induced by nitrous acid (Malling and de Serres, 1967), ethyl methanesulfonate (Malling and de Serres, 1968), or ultraviolet (Kilbey et al., 1971). 相似文献
4.
W R Overton 《Cytometry》1988,9(6):619-626
Analysis of flow cytometry histogram data by the subjective selection of an integration window can be a tedious and time-consuming task and is often inaccurate. A new method for automated calculation of the percent positive from immunofluorescence histograms is presented. This new method is a modification of the currently used method of channel-by-channel histogram subtraction. Its accuracy is compared to that of the channel-by-channel histogram subtraction method and to another currently used automated method, which selects an integration window by finding the channels that contain the most fluorescent 2% of a control histogram. The new histogram subtraction method is objective, easy to use, and is more accurate than other currently used automated analysis methods. PASCAL source code is given for each method of analysis. 相似文献
5.
Postexercise reductions in blood pressure at rest have been reported for hypertensive subjects. To determine whether post-exercise hypotension would occur in spontaneously hypertensive rats and to test the hypothesis that any reductions would result because of decreases in regional vascular resistances, hypertensive rats (n = 19) were instrumented with indwelling arterial catheters and Doppler probes to measure regional blood flows from the iliac, superior mesenteric, and renal arteries. Data were collected from animals who performed a 20- and a 40-min treadmill test at between 60 and 70% of their maximum O2 uptake. When the animals ran for 20 min, there was a pre- to postexercise drop in mean arterial pressure (MAP) from 158 +/- 3.6 to 150 +/- 3.6 mmHg (P less than 0.05), which was recorded 30 min after the exercise had ceased. The pre- to postexercise reduction in MAP after 40 min of treadmill running was from 154 +/- 3.1 to 138 +/- 3.0 mmHg (P less than 0.05) as recorded 30 min postexercise. Postexercise heart rate was significantly lower after the 40-min exercise bout, from a preexercise mean of 351 +/- 3 beats/min to 324 +/- 5 beats/min 30 min after the treadmill had stopped. Surprisingly, marked pre- to postexercise reductions in regional vascular resistance were not observed in either the iliac, superior mesenteric, or renal vascular beds. These data demonstrated the existence of postexercise hypotension in genetic hypertensive rats and suggested that reductions in cardiac output were the primary hemodynamic mechanism for this finding. 相似文献
6.
7.
Jane Overton 《The Journal of cell biology》1965,24(2):211-222
Changes at the level of cell fine structure have been studied during lens regeneration in the toad, Xenopus laevis, where cornea gives rise to the new lens. The transformation of these cells may be divided into three phases. (1) In the cornea, flattened cells become cuboidal and rough endoplasmic reticulum increases in amount. (2) In the new lens vesicle, cisternae of the rough ER break down into vesicles, smooth-walled vesicles and free ribosomes increase in number, and mitochondria can become enlarged and irregular, then centrally attenuated. Rudimentary cilia form. (3) As new lens fibers form, ribosomes become very numerous and low density fibrous elements and dense clumps appear in the cytoplasm. These phases are accompanied by marked nucleolar changes. The changes during the 3rd phase are similar to changes in the lens during normal development. The first two phases show an unexpected morphological complexity. 相似文献
8.
Treatment with recombinant IFN-gamma decreases cell surface CD4 levels on peripheral blood monocytes and on myelomonocyte cell lines 总被引:2,自引:0,他引:2
C R Faltynek L R Finch P Miller W R Overton 《Journal of immunology (Baltimore, Md. : 1950)》1989,142(2):500-508
The human cell surface protein CD4 is not only an important accessory molecule in the activation of MHC class-II-restricted T cells, but has also been implicated to be a receptor for the human immunodeficiency virus HIV-I on lymphoid and monocytic cells. We have found that a 24-h treatment of the promonocytic leukemia cell line U937 with rIFN-gamma decreases the expression of the CD4 Ag by 50% as measured by cytofluorographic analysis. The decrease in CD4 expression was dependent on the concentration of rIFN-gamma, with maximal effects occurring at 20 to 200 U/ml. The decrease appeared to be due to actual loss of the CD4 molecule from the cell surface rather than masking of a particular epitope, inasmuch as similar results were obtained with the OKT4 and OKT4A antibodies. The effect of rIFN-gamma to decrease CD4 expression was not due to a general loss of cell surface Ag, because the binding of OKM1 and anti-HLe-1 increased after rIFN-gamma treatment. Treatment of rIFN-gamma also decreased cell surface CD4 expression on the promyelocytic leukemia cell line HL-60, and on the monocytic cell line THP-1, although the extent of the decrease was less than on U937 cells. Freshly isolated normal peripheral blood monocytes treated for 48 h with rIFN-gamma bound much less OKT4 or OKT4A antibody than cells incubated in the absence of rIFN-gamma. Moreover, treatment with rIFN-gamma reduced the percentage of peripheral blood monocytes that were positive for the CD4 Ag. In contrast with the decrease in CD4 levels on rIFN-gamma-treated monocytes, treatment with rIFN-gamma had no effect on CD4 levels on peripheral blood T lymphocytes or T cell lines. 相似文献
9.
Summary Ultrastructural changes during zoospore formation and aggregation into motile, aggregating zoospores were examined in the colonial green algaPediastrum tetras. Developing zoospores are characterized by irregularly shaped nuclei, presence of peripheral networks of rough endoplasmic reticulum, chloroplasts with tightly apposed thylakoids and dictyosome cisternae which are compressed and reduced in size. A single membrane bound organelle with a fine granular matrix of moderate electron density of diameter ranging from 0.2 m to 0.6 m and associated with chloroplasts, mitochondria and endoplasmic reticulum was found only in adult cells. Although this organelle has the morphology of a microbody, it did not stain with 3,3-diaminobenzidine (DAB) at pH 9.6 or pH 7.6, whereas mitochondrial membranes stained. No DAB staining was observed along the cell wall or the plasma membrane of zoospores, or associated with endoplasmic reticulum, plastid membranes or dictyosomes. 相似文献
10.
This paper presents XBAM (the Extended Branch-Arrow Model), a new model of the development of the retino-tectal topographic mapping as observed in frog, toad, and goldfish visual systems. The updating process employed by XBAM is distributed in nature and depends upon interactions between branches of retinal fibers, the branches and the boundaries of the tectum and grafts, and the branches and the tectal surface. Results of computer simulation of the model are related to experimental data obtained from tectal and retinal graft and lesion studies, and comparisons are also made with other models. 相似文献