全文获取类型
收费全文 | 6262篇 |
免费 | 528篇 |
国内免费 | 5篇 |
出版年
2023年 | 39篇 |
2022年 | 95篇 |
2021年 | 175篇 |
2020年 | 90篇 |
2019年 | 96篇 |
2018年 | 138篇 |
2017年 | 128篇 |
2016年 | 221篇 |
2015年 | 402篇 |
2014年 | 440篇 |
2013年 | 430篇 |
2012年 | 622篇 |
2011年 | 567篇 |
2010年 | 334篇 |
2009年 | 291篇 |
2008年 | 401篇 |
2007年 | 398篇 |
2006年 | 385篇 |
2005年 | 292篇 |
2004年 | 311篇 |
2003年 | 212篇 |
2002年 | 218篇 |
2001年 | 43篇 |
2000年 | 27篇 |
1999年 | 46篇 |
1998年 | 42篇 |
1997年 | 45篇 |
1996年 | 29篇 |
1995年 | 20篇 |
1994年 | 35篇 |
1993年 | 19篇 |
1992年 | 22篇 |
1991年 | 19篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 11篇 |
1978年 | 7篇 |
1977年 | 13篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1970年 | 4篇 |
排序方式: 共有6795条查询结果,搜索用时 15 毫秒
821.
Johnson C Jia Y Wang C Lue YH Swerdloff RS Zhang XS Hu ZY Li YC Liu YX Hikim AP 《Biology of reproduction》2008,79(5):806-814
This study investigates the role of caspase 2 in apoptotic signaling of nonhuman primate male germ cells triggered by mild testicular hyperthermia, testosterone (T(e)) implants, or by combined interventions. Mean incidence of germ cell apoptosis increased significantly by Day 3 in the heat (H(e)) alone group and by Day 8 in the Te alone group but peaked at Day 3 in H(e) + T(e) group. We found activation of caspase 2 in both germ cells and Sertoli cells after induction of apoptosis. Most notably, active caspase 2 immunoreactivity was detected only in those germ cells susceptible to apoptosis compared with controls, where little or no such staining is detected. To further explore the role of caspase 2 in regulating male germ cell death, we next evaluated the efficacy of caspase 2 inhibition in preventing or attenuating heat-induced germ cell apoptosis in rats. Caspase 2 inhibition significantly (P < 0.05) prevented such heat-induced germ cell apoptosis. The protection offered by the caspase 2 inhibitor occurred upstream of mitochondria, involving suppression of mitogen-activated protein kinase (MAPK) 14 activation and inducible nitric oxide synthase (NOS2) induction and, in turn, suppression of cytochrome c-mediated death pathway. Together, our results show that caspase 2 is activated in male germ cells undergoing apoptosis in nonhuman primates after heat stress, hormonal deprivation, or after combined interventions. Blockade of caspase 2 activation prevents heat-induced germ cell apoptosis in rats by suppressing the MAPK14- and NO-mediated intrinsic pathway signaling. 相似文献
822.
The activities of acid and alkaline phosphatases along with phosphorus content in leaves of European beech (Fagus sylvatica L.) were studied for a period from April to October. The phosphorus content of beech leaves was highest in April, at the
beginning of the vegetation period; from May to October it was twofold lower than in April. Acid phosphatase activity (per
unit fresh weight) in leaves collected from the middle part of the crown decreased significantly in May and July compared
to the enzyme activity in April. In both the low and middle parts of the crown, the acid phosphatase activity had a peak in
August, and thereafter decreased in September and October. No correlations between acid phosphatase activity and phosphorus
concentrations were found. Alkaline phosphatase activity was very low and in some cases near the detection limit during the
whole observation period. 相似文献
823.
Kolyva C Spaan JA Piek JJ Siebes M 《American journal of physiology. Heart and circulatory physiology》2008,295(2):H482-H490
A novel single-point technique to calculate local arterial wave speed (SPc) has recently been presented and applied in healthy human coronary arteries at baseline flow. We investigated its applicability for conditions commonly encountered in the catheterization laboratory. Intracoronary pressure (P(d)) and Doppler velocity (U) were recorded in 29 patients at rest and during adenosine-induced hyperemia in a distal segment of a normal reference vessel and downstream of a single stenosis before and after revascularization. Conduit vessel tone was minimized with nitroglycerin. Microvascular resistance (MR) and SPc were calculated from P(d) and U. In the reference vessel, SPc decreased from 21.5 m/s (SD 8.0) to 10.5 m/s (SD 4.1) after microvascular dilation (P < 0.0001). SPc was substantially higher in the presence of a proximal stenosis and decreased from 34.4 m/s (SD 18.2) at rest to 27.5 m/s (SD 13.4) during hyperemia (P < 0.0001), with a concomitant reduction in P(d) by 20 mmHg and MR by 55.4%. The stent placement further reduced hyperemic MR by 26% and increased P(d) by 26 mmHg but paradoxically decreased SPc to 13.1 m/s (SD 7.7) (P < 0.0001). Changes in SPc correlated strongly with changes in MR (P < 0.001) but were inversely related to changes in P(d) (P < 0.01). In conclusion, the single-point method yielded erroneous predictions of changes in coronary wave speed induced by a proximal stenosis and distal vasodilation and is therefore not appropriate for estimating local wave speed in coronary vessels. Our findings are well described by a lumped reservoir model reflecting the "windkesselness" of the coronary arteries. 相似文献
824.
Mouse heart valve structure and function: echocardiographic and morphometric analyses from the fetus through the aged adult 总被引:1,自引:0,他引:1
Hinton RB Alfieri CM Witt SA Glascock BJ Khoury PR Benson DW Yutzey KE 《American journal of physiology. Heart and circulatory physiology》2008,294(6):H2480-H2488
The purpose of this study is to provide standard echocardiographic and morphometric data for normal mouse valve structure and function from late fetal to aged adult stages. Cross-sectional, two-dimensional and Doppler transthoracic echocardiography was performed in C57BL6 mice anesthetized with 1% to 2% isoflurane at embryonic day 18.5 (late fetal), 10 days (neonate), 1 mo (juvenile), 2 mo (young adult), 9 mo (old adult), and 16 mo (aged adult). Normal annulus dimensions indexed to age or weight, and selected flow velocities, were established by echocardiography. After echocardiographic imaging, hearts were harvested and histological and morphometric analyses were performed. Morphometric analysis demonstrated a progressive valve thinning and elongation during the fetal and juvenile stages that plateaued during adult stages (ANOVA, P < 0.01); however, there was increased thickening of the hinge of the aortic valve with advanced age, reminiscent of human aortic valve sclerosis. There was no age-related calcification. The results of this study provide comprehensive echocardiographic and morphometric data for normal mouse valve structure and function from late fetal to aged adult stages and should prove useful as a reference standard for future studies using mouse models of progressive valve disease. 相似文献
825.
We construct and analyze a model network of the pyloric rhythm of the crustacean stomatogastric ganglion consisting of an oscillator neuron that inhibits two reciprocally inhibitory follower neurons. We derive analytic expressions that determine the phase of firing of the follower neurons with respect to the oscillator. An important aspect of the model is the inclusion of synapses that exhibit short-term synaptic depression. We show that these type of synapses allow there to be a complicated relationship between the intrinsic properties of the neurons and the synapses between them in determining phase relationships. Our analysis reveals the circumstances and ranges of cycle periods under which these properties work in concert with or independently from one another. In particular, we show that phase maintenance over a range of oscillator periods can be enhanced through the interplay of the two follower neurons if the synapses between these neurons are depressing. Since our model represents the core of the oscillatory pyloric network, the results of our analysis can be compared to experimental data and used to make predictions about the biological network. 相似文献
826.
Sakkas GK Karatzaferi C Zintzaras E Giannaki CD Liakopoulos V Lavdas E Damani E Liakos N Fezoulidis I Koutedakis Y Stefanidis I 《American journal of physiology. Regulatory, integrative and comparative physiology》2008,295(6):R1721-R1729
Hemodialysis patients exhibit insulin resistance (IR) in target organs such as liver, muscles, and adipose tissue. The aim of this study was to identify contributors to IR and to develop a model for predicting glucose intolerance in nondiabetic hemodialysis patients. After a 2-h, 75-g oral glucose tolerance test (OGTT), 34 hemodialysis patients were divided into groups with normal (NGT) and impaired glucose tolerance (IGT). Indices of insulin sensitivity were derived from OGTT data. Measurements included liver and muscle fat infiltration and central adiposity by computed tomography scans, body composition by dual energy X-ray absorptiometer, sleep quality by full polysomnography, and functional capacity and quality of life (QoL) by a battery of exercise tests and questionnaires. Cut-off points, as well as sensitivity and specificity calculations were based on IR (insulin sensitivity index by Matsuda) using a receiver operator characteristics (ROC) curve analysis. Fifteen patients were assigned to the IGT, and 19 subjects to the NGT group. Intrahepatic fat content and visceral adiposity were significantly higher in the IGT group. IR indices strongly correlated with sleep disturbances, visceral adiposity, functional capacity, and QoL. Visceral adiposity, O2 desaturation during sleep, intrahepatic fat content, and QoL score fitted into the model for predicting glucose intolerance. A ROC curve analysis identified an intrahepatic fat content of > 3.97% (sensitivity, 100; specificity, 35.7) as the best cutoff point for predicting IR. Visceral and intrahepatic fat content, as well as QoL and sleep seemed to be involved at some point in the development of glucose intolerance in hemodialysis patients. Means of reducing fat depots in the liver and splachnic area might prove promising in combating IR and cardiovascular risk in hemodialysis patients. 相似文献
827.
de Melo Reis RA Ventura AL Schitine CS de Mello MC de Mello FG 《Neurochemical research》2008,33(8):1466-1474
Müller cells represent the main type of glia present in the retina interacting with most, if not all neurons in this tissue.
Müller cells have been claimed to function as optic fibers in the retina delivering light to photoreceptors with minimal distortion
and low loss [Franze et al (2007) Proc Natl Acad Sci 104:8287–8292]. Most of the mediators found in the brain are also detected
in the retinal tissue, and glia cells are active players in the synthesis, release, signaling and uptake of major mediators
of synaptic function. Müller glia trophic factors may regulate many different aspects of neuronal circuitry during synaptogenesis,
differentiation, neuroprotection and survival of photoreceptors, Retinal Ganglion Cells (RGCs) and other targets in the retina.
Here we review the role of several transmitters and trophic factors that participate in the neuron-glia loop in the retina.
Special issue article in honor of Dr. Ricardo Tapia. 相似文献
828.
829.
Kuduk SD Chang RK Dipardo RM Di Marco CN Murphy KL Ransom RW Reiss DR Tang C Prueksaritanont T Pettibone DJ Bock MG 《Bioorganic & medicinal chemistry letters》2008,18(18):5107-5110
A series of carbo- and heterocyclic alpha-hydroxy amide-derived bradykinin B1 antagonists was prepared and evaluated. A 4,4-difluorocyclohexyl alpha-hydroxy amide was incorporated along with a 2-methyl tetrazole in lieu of an oxadiazole to afford a suitable compound with good pharmacokinetic properties, CNS penetration, and clearance by multiple metabolic pathways. 相似文献
830.