全文获取类型
收费全文 | 4732篇 |
免费 | 49篇 |
国内免费 | 49篇 |
专业分类
4830篇 |
出版年
2023年 | 12篇 |
2022年 | 16篇 |
2021年 | 30篇 |
2020年 | 78篇 |
2019年 | 60篇 |
2018年 | 75篇 |
2017年 | 53篇 |
2016年 | 26篇 |
2015年 | 75篇 |
2014年 | 210篇 |
2013年 | 233篇 |
2012年 | 235篇 |
2011年 | 359篇 |
2010年 | 256篇 |
2009年 | 198篇 |
2008年 | 216篇 |
2007年 | 196篇 |
2006年 | 186篇 |
2005年 | 163篇 |
2004年 | 171篇 |
2003年 | 156篇 |
2002年 | 90篇 |
2001年 | 50篇 |
2000年 | 92篇 |
1999年 | 86篇 |
1998年 | 113篇 |
1997年 | 111篇 |
1996年 | 106篇 |
1995年 | 111篇 |
1994年 | 82篇 |
1993年 | 79篇 |
1992年 | 96篇 |
1991年 | 83篇 |
1990年 | 82篇 |
1989年 | 80篇 |
1988年 | 75篇 |
1987年 | 70篇 |
1986年 | 56篇 |
1985年 | 60篇 |
1984年 | 86篇 |
1983年 | 54篇 |
1982年 | 50篇 |
1981年 | 35篇 |
1980年 | 27篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1976年 | 4篇 |
1972年 | 3篇 |
1971年 | 3篇 |
排序方式: 共有4830条查询结果,搜索用时 11 毫秒
91.
Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [Picea abies (L.) Karst.] 总被引:5,自引:0,他引:5
Summary Relationships between root zone temperature, concentrations and uptake rates of NH
4
+
and NO
3
–
were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH
4
+
and NO
3
–
uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (Cmin) range of 100–150 M uptake rates of NH
4
+
were 3–4 times higher than those of NO
3
–
The preference for NH
4
+
uptake was also reflected in the minimum concentration (Cmin) values. Supplying NH4NO3, the rate of NO
3
–
uptake was very low until the NH
4
+
concentrations had fallen below about 100 M. The shift from NH
4
+
to NO
3
–
uptake was correlated with a corresponding shift from net H+ production to net H+ consumption in the external solution. The uptake rates of NH
4
+
were correlated with equimolar net production of H+. With NO
3
–
nutrition net consumption of H+ was approximately twice as high as uptake rates of NO
3
–
In the forest stand the NO
3
–
concentration in the soil solution was more than 10 times higher than the NH
4
+
concentration (<100 M), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO
3
–
uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium. 相似文献
92.
93.
Yanan Li Yu Yan Fang Liu Luguang Wang Fumin Feng Yonghong Xiao 《Journal of cellular biochemistry》2019,120(2):1685-1693
We previously showed that changes in calcium concentrations were related to cell apoptosis in vitro. The endoplasmic reticulum (ER) is the main component of calcium storage and signal transduction, and disrupting the balance of intracellular Ca2+ can cause endoplasmic reticulum stress (ERS). In this process, the ER releases stored Ca 2+ into the cytoplasm and activates calpain-2. To further investigate the effect of calpain in hepatic stellate cells (HSCs), in the current study, we examine the effect of N-acetyl-leu-leu-norleucinal (ALLN) on apoptosis resulting from calcium ionophore A23187–induced ERS. Our findings indicate that calpain inhibition reduces calcium ionophore A23187–induced apoptosis of HSCs and decreases the expression of ER stress proteins that may be related to the calpain/caspase signaling pathway. 相似文献
94.
Yuan Zhang Jiaoqian Ying Dongsheng Jiang Zhigang Chang Hua Li Guoqiang Zhang Shan Gong Xinghong Jiang Jin Tao 《The Journal of biological chemistry》2015,290(13):8644-8655
Recent studies have demonstrated that urotensin-II (U-II) plays important roles in cardiovascular actions including cardiac positive inotropic effects and increasing cardiac output. However, the mechanisms underlying these effects of U-II in cardiomyocytes still remain unknown. We show by electrophysiological studies that U-II dose-dependently potentiates L-type Ca2+ currents (ICa,L) in adult rat ventricular myocytes. This effect was U-II receptor (U-IIR)-dependent and was associated with a depolarizing shift in the voltage dependence of inactivation. Intracellular application of guanosine-5′-O-(2-thiodiphosphate) and pertussis toxin pretreatment both abolished the stimulatory effects of U-II. Dialysis of cells with the QEHA peptide, but not scrambled peptide SKEE, blocked the U-II-induced response. The phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin as well as the class I PI3K antagonist blocked the U-II-induced ICa,L response. Protein kinase C antagonists calphostin C and chelerythrine chloride as well as dialysis of cells with 1,2bis(2aminophenoxy)ethaneN,N,N′,N′-tetraacetic acid abolished the U-II-induced responses, whereas PKCα inhibition or PKA blockade had no effect. Exposure of ventricular myocytes to U-II markedly increased membrane PKCβ1 expression, whereas inhibition of PKCβ1 pharmacologically or by shRNA targeting abolished the U-II-induced ICa,L response. Functionally, we observed a significant increase in the amplitude of sarcomere shortening induced by U-II; blockade of U-IIR as well as PKCβ inhibition abolished this effect, whereas Bay K8644 mimicked the U-II response. Taken together, our results indicate that U-II potentiates ICa,L through the βγ subunits of Gi/o-protein and downstream activation of the class I PI3K-dependent PKCβ1 isoform. This occurred via the activation of U-IIR and contributes to the positive inotropic effect on cardiomyocytes. CH132799相似文献
95.
TRPV4 channels activity in bovine articular chondrocytes: Regulation by obesity-associated mediators
《Cell calcium》2015,57(6):493-503
Turnover of the cartilage extracellular matrix depends exclusively on chondrocytes and varies in response to load and osmolarity fluctuations. Obesity can affect chondrocyte physiology; adipokines, insulin and proinflammatory cytokines levels are all altered in the obese and are related to matrix turnover impairments and thus to osteoarthritis. TRPV4, a mechanosensitive cation channel, is responsible for reacting to hypotonic variations. In this study, the presence and activity of TRPV4 channels in bovine chondrocytes were evaluated using the whole-cell patch-clamp technique and fluorescence measurements to perform characterisations of these channels and to determine intracellular calcium responses. The expression of TRPV4 was determined by RT-PCR. The TRPV4 regulation by hypotonic shock, insulin and adipokines were analysed. Hypoosmolarity induced a Gd3+-, ruthenium red-, and HC-067047-sensitive current, predominantly inward, an intracellular Ca2+ concentration increase and a membrane depolarisation. The current had a reversal potential of +28 ± 4 mV and exhibited preferential permeability to Ca2+; 4αPDD, a specific TRPV4 agonist, evoked the same response. TNFα, IL-1β, insulin, and, to a lesser degree, leptin and resistin attenuated the TRPV4-mediated effects; in contrast, adiponectin did not affect them. These results confirm the function of TRPV4 in bovine articular chondrocytes and its regulation by obesity-associated mediators. 相似文献
96.
The in situ localization of Ca2+ in stage I sporangiophores of the fungus Phycomyces blakesleeanus was achieved with the potassium pyroantimonate technique. Precipitates of calcium-antimonate were present in mitochondria, vacuoles, endoplasmic reticulum and adjacent cytoplasm, Golgi-like bodies, and nuclei but not cell walls. Material treated with the calcium chelator EGTA lacked these precipitates. The preferential localization of Ca2+ in mitochondria, endoplasmic reticulum and vacuoles suggests that these organelles modulate the level of this cation in sporangiophores of P. blakesleeanus.Abbreviations EGTA
ethyleneglycol-bis-(-aminoethyl ether) N,N, tetraacetic acid 相似文献
97.
Although increasing evidence shows the nutritional benefits of calcium fructoborate (CF) on animals and humans, its action
mechanism has not been clearly identified. The present study aims to investigate the possible antioxidant function of CF.
Based on its efficiency in skin wound healing, the authors tested whether CF possesses antioxidant properties on human keratinocytes
cultures, in a complete serum-free medium (KMK-2; Sigma). The cells treated with CF (0–450 nmol/culture medium) were exposed
to exogenous 100 μmol of hydrogen peroxide to mimic the oxidative stress. The changes in general cell oxidant production evaluated
with dihydrorhodamine-123 showed that the intracellular reactive oxygen species (ROS) were markedly reduced by preincubation
with CF. The maximum antioxidant activity was notice at 90 nmol CF. To assess the reactivity of CF on ROS, we analyzed its
ability to inhibit the superoxide-dependent auto-oxidation of pyrogallol. The CF inhibited the pyrogallol auto-oxidation depending
on time and concentration, which suggests its possible role as a superoxide radical scavenger. Taken together, our results
indicate that CF has antioxidant activity, which could have clinical significance in protecting cells from oxidant-induced
injury. A hypothetic mechanism for the antioxidant activity of CF is proposed. 相似文献
98.
Inada NM da Silva AR Jorge RA Borecký J Vercesi AE 《Archives of biochemistry and biophysics》2007,457(2):217-224
The action of irradiated cationic Fe(III)TMPyP and anionic Fe(III)TPPS4 forms of mesoporphyrins on mitochondrial functions was investigated using experimental conditions that caused minimal effects on mitochondria in the dark. Treatment of mitochondria with 1 microM Fe(III)TMPyP for 2 min decreased the respiratory control by 3% in the dark and 28% after irradiation. Fe(III)TPPS4 (1 microM) had no significant effect on respiratory control under any of the above conditions. Both porphyrins increased the mitochondrial production of reactive oxygen species in the presence of Ca2+; however, the effect of Fe(III)TMPyP was significantly stronger. In both cases, this overproduction was associated with membrane lipid peroxidation. It was also observed that the association constant of Fe(III)TMPyP with mitochondria was 11 times higher than that of Fe(III)TPPS4. In conclusion, the damage to isolated mitochondria induced by Fe(III)TMPyP under illumination was larger than by Fe(III)TPPS4, probably because its cationic charge favors association with the mitochondrial membrane. This is supported by the decrease in the association constant of Fe(III)TMPyP with mitochondria in higher salt medium. 相似文献
99.
Ca2+ transport by the sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) is sensitive to monovalent cations. Possible K+ binding sites have been identified in both the cytoplasmic P-domain and the transmembrane transport-domain of the protein. We measured Ca2+ transport into SR vesicles and SERCA ATPase activity in the presence of different monovalent cations. We found that the effects of monovalent cations on Ca2+ transport correlated in most cases with their direct effects on SERCA. Choline+, however, inhibited uptake to a greater extent than could be accounted for by its direct effect on SERCA suggesting a possible effect of choline on compensatory charge movement during Ca2+ transport. Of the monovalent cations tested, only Cs+ significantly affected the Hill coefficient of Ca2+ transport (nH). An increase in nH from ∼2 in K+ to ∼3 in Cs+ was seen in all of the forms of SERCA examined. The effects of Cs+ on the maximum velocity of Ca2+ uptake were also different for different forms of SERCA but these differences could not be attributed to differences in the putative K+ binding sites of the different forms of the protein. 相似文献
100.
Martin D. Bootman Ioannis SmyrniasRüdiger Thul Stephen CoombesH. Llewelyn Roderick 《Biochimica et Biophysica Acta (BBA)/Molecular Cell Research》2011,1813(5):922-934
Whereas Ca2+ signalling in ventricular cardiomyocytes is well described, much less is known regarding the Ca2+ signals within atrial cells. This is surprising given that atrial cardiomyocytes make an important contribution to the refilling of ventricles with blood, which enhances the subsequent ejection of blood from the heart. The dependence of cardiac function on the contribution of atria becomes increasingly important with age and exercise. Disruption of the rhythmic beating of atrial cardiomyocytes can lead to life-threatening conditions such as atrial fibrillation. Atrial and ventricular myocytes have many structural and functional similarities. However, one key structural difference, the lack of transverse tubules (“T-tubules”) in atrial myocytes, make these two cell types display vastly different calcium patterns in response to electrical excitation. The lack of T-tubules in atrial myocytes means that depolarisation provokes calcium signals that originate around the periphery of the cells. Under resting conditions, such Ca2+ signals do not propagate towards the centre of the atrial cells and so do not fully engage the contractile machinery. Consequently, contraction of atrial myocytes under resting conditions is modest. However, when atrial myocytes are stimulated with a positive inotropic agonist, such as isoproterenol, the peripheral Ca2+ signals trigger a global wave of Ca2+ that propagates in a centripetal manner into the cells. Enhanced centripetal movement of Ca2+ in atrial myocytes leads to increased contraction and a more substantial contribution to blood pumping. This article is part of a Special Issue entitled: 11th European Symposium on Calcium. 相似文献