全文获取类型
收费全文 | 79934篇 |
免费 | 6579篇 |
国内免费 | 6285篇 |
出版年
2024年 | 120篇 |
2023年 | 900篇 |
2022年 | 1303篇 |
2021年 | 3840篇 |
2020年 | 2664篇 |
2019年 | 3383篇 |
2018年 | 3294篇 |
2017年 | 2434篇 |
2016年 | 3407篇 |
2015年 | 5054篇 |
2014年 | 5977篇 |
2013年 | 6336篇 |
2012年 | 7437篇 |
2011年 | 6815篇 |
2010年 | 4209篇 |
2009年 | 3649篇 |
2008年 | 4479篇 |
2007年 | 3963篇 |
2006年 | 3454篇 |
2005年 | 2941篇 |
2004年 | 2494篇 |
2003年 | 2148篇 |
2002年 | 1901篇 |
2001年 | 1432篇 |
2000年 | 1224篇 |
1999年 | 1228篇 |
1998年 | 741篇 |
1997年 | 713篇 |
1996年 | 657篇 |
1995年 | 586篇 |
1994年 | 549篇 |
1993年 | 390篇 |
1992年 | 579篇 |
1991年 | 437篇 |
1990年 | 346篇 |
1989年 | 307篇 |
1988年 | 216篇 |
1987年 | 223篇 |
1986年 | 164篇 |
1985年 | 162篇 |
1984年 | 98篇 |
1983年 | 84篇 |
1982年 | 67篇 |
1981年 | 51篇 |
1980年 | 42篇 |
1979年 | 46篇 |
1978年 | 25篇 |
1977年 | 20篇 |
1975年 | 28篇 |
1974年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
本文报道石刁柏胚性愈伤组织的可溶性蛋白质含量与组分、过氧化物酶和酯酶的活力及同工酶带均比其体细胞胚的要少。而在体细胞胚胎发生过程中,过氧化物酶和酯酶活力、可溶性蛋白质含量均以球形胚为最低,子叶分化期胚为最高而呈递增趋势;可溶性蛋白质组分以子叶分化期胚、成熟胚为最多,球形胚、香蕉形胚为最少;过氧化物酶同工酶带以梨形胚为最多,子叶分化期胚、成熟胚为最少;酯酶同工酶则以子叶分化期胚为最多,成熟胚为最少。胚性愈伤组织与体细胞胚均有其特异性可溶性蛋白质及同工酶带,可作为体细胞胚胎发生的分子标记。 相似文献
994.
995.
We evaluated the hypothesis that the tonic discharge of pulmonary stretch receptors significantly influences the respiratory-modulated activities of cranial nerves. Decerebrate and paralyzed cats were ventilated with a servo-respirator, which produced changes in lung volume in parallel with integrated phrenic activity. Activities of the facial, hypoglossal, and recurrent laryngeal nerves and nerves to the thyroarytenoid muscle and triangularis sterni were recorded. After a stereotyped pattern of lung inflation, tracheal pressure was held at 1, 2, 4, or 6 cmH2O during the subsequent ventilatory cycle. Increases in tracheal pressure caused progressive reductions in both inspiratory and expiratory cranial nerve activities and progressive elevations in triangularis sterni discharge; peak levels of phrenic activity declined modestly. Similar changes were observed in normocapnia and hypercapnia. We conclude that the tonic discharge of pulmonary stretch receptors is an important determinant of the presence and magnitude of respiratory-modulated cranial nerve activity. This reflex mechanism may maintain upper airway patency and also regulate expiratory airflow. 相似文献
996.
The effect of ventilation strategy on in vivo function of different surfactants was evaluated in preterm rabbits delivered at 27 days gestational age and ventilated with either 0 cmH2O positive end-expiratory pressure (PEEP) at tidal volumes of 10-11 ml/kg or 3 cmH2O PEEP at tidal volumes of 7-8 ml/kg after treatment with one of four different surfactants: sheep surfactant, the lipids of sheep surfactant stripped of protein (LH-20 lipid), Exosurf, and Survanta. The use of 3 cmH2O PEEP decreased pneumothoraces in all groups except for the sheep surfactant group where pneumothoraces increased (P < 0.01). Ventilatory pressures (peak pressures - PEEP) decreased more with the 3 cmH2O PEEP, low-tidal-volume ventilation strategy for Exosurf-, Survanta-, and sheep surfactant-treated rabbits (P < 0.05), whereas ventilation efficiency indexes (VEI) improved only for Survanta- and sheep surfactant-treated rabbits with 3 cmH2O PEEP (P < 0.01). Pressure-volume curves for sheep surfactant-treated rabbits were better than for all other treated groups (P < 0.01), although Exosurf and Survanta increased lung volumes above those in control rabbits (P < 0.05). The recovery of intravascular radiolabeled albumin in the lungs and alveolar washes was used as an indicator of pulmonary edema. Only Survanta and sheep surfactant decreased protein leaks in the absence of PEEP, whereas all treatments decreased labeled albumin recoveries when 3 cmH2O PEEP was used (P < 0.05). These experiments demonstrate that ventilation style will alter a number of measurements of surfactant function, and the effects differ for different surfactants. 相似文献
997.
Expression of the human chorionic gonadotropin (hCG)-alpha gene in placental trophoblasts is markedly stimulated by cAMP, a property preserved in a reporter plasmid containing its cAMP response elements (CREs) linked to the chloramphenicol acetyltransferase coding sequence (CRE alpha CAT). In search of a potential physiologic regulator of hCG gene expression via cAMP, we found that JEG-3 syncytial trophoblast cells have specific binding sites for vasoactive intestinal peptide (VIP) with dissociation constant of 1 nM. VIP maximally increased the transient expression of CRE alpha CAT and the expression of endogenous hCG-alpha mRNA in JEG-3 cells by 4- and 9-fold, respectively. Exposure of JEG-3 cells to 30 nM VIP increased cAMP levels 60-fold after 10-30 min, but cAMP rapidly declined thereafter. As a consequence of this desensitization, the effect of VIP on stimulation of both CRE alpha CAT and endogenous hCG-alpha and hCG-beta mRNA levels more closely resembled that of forskolin or 8-br-cAMP at time points much less than 24 h. Moreover, transient exposure to 8-br-cAMP was much less effective than 24 h of continuous incubation on CRE alpha CAT activity. We conclude that VIP rapidly increases cAMP content and activates hCG-alpha gene expression in JEG-3 cells, but sustained elevations in cAMP are necessary for maximal accumulation of this CRE-regulated gene product. 相似文献
998.
A Q Sun K U Yüksel T M Jacobson R W Gracy 《Archives of biochemistry and biophysics》1990,283(1):120-129
Previously undetected isoforms of human glucose-6-phosphate isomerase (GPI) have been isolated utilizing substrate-induced elution of the enzyme from spherical cross-linked phosphocellulose as an affinity ligand and subjected to a series of physical and chemical studies. The two major isoforms (1, 48%, pI 9.13; 2, 36%, pI 9.00) are homodimers of subunits of 63.2 kDa (Type-A) and are charge isomers, probably representing deamidation of specific Asn-Gly sequences as in other species. Isoform 3 (13%, pI 8.84) is a heterodimer composed of the Type-A subunit and a previously unreported larger subunit of 69.8 kDa (Type-B). Isoform 4 (3%, pI 8.62) is a BB-homodimer. Structural differences in the two types of subunits are also apparent from CNBr fragmentation patterns. Carbohydrate analyses show that, even though potential N- and O-linked glycosylation sites exist, the isoforms are not due to glycosylation. Recently recognized sequence similarities between GPI and the neurotropic lymphokine, neuroleukin (NLK) suggest that GPI and NLK are either derived from the same gene or represent modifications of the same protein. The possibility of NLK-GPI dimers exists, but the new isoforms identified in this study do not appear to represent hybrids of GPI subunits with mature NLK. 相似文献
999.
1000.
Purification of Trypanosoma cruzi surface proteins involved in adhesion to host cells 总被引:1,自引:0,他引:1
We have identified four surface 83 kDa proteins of pI values 6.3, 6.4, 6.5 and 6.6 in T. cruzi trypomastigotes which specifically bind to rat heart myoblasts. These proteins were purified by isoelectric focusing and anion-exchange chromatography in an FPLC system. These 83 kDa proteins inhibit the attachment of trypomastigotes to myoblasts in a concentration-dependent manner, indicating that these trypomastigote proteins mediate the attachment of trypomastigotes to heart myoblasts. 相似文献