全文获取类型
收费全文 | 7951篇 |
免费 | 739篇 |
国内免费 | 6篇 |
出版年
2023年 | 44篇 |
2022年 | 109篇 |
2021年 | 238篇 |
2020年 | 158篇 |
2019年 | 192篇 |
2018年 | 198篇 |
2017年 | 164篇 |
2016年 | 222篇 |
2015年 | 382篇 |
2014年 | 396篇 |
2013年 | 485篇 |
2012年 | 583篇 |
2011年 | 592篇 |
2010年 | 384篇 |
2009年 | 333篇 |
2008年 | 410篇 |
2007年 | 413篇 |
2006年 | 360篇 |
2005年 | 361篇 |
2004年 | 339篇 |
2003年 | 298篇 |
2002年 | 293篇 |
2001年 | 90篇 |
2000年 | 53篇 |
1999年 | 81篇 |
1998年 | 92篇 |
1997年 | 52篇 |
1996年 | 41篇 |
1995年 | 44篇 |
1994年 | 32篇 |
1993年 | 48篇 |
1992年 | 54篇 |
1991年 | 37篇 |
1990年 | 40篇 |
1989年 | 44篇 |
1988年 | 36篇 |
1987年 | 42篇 |
1986年 | 40篇 |
1985年 | 30篇 |
1984年 | 40篇 |
1983年 | 27篇 |
1982年 | 46篇 |
1981年 | 46篇 |
1980年 | 48篇 |
1979年 | 25篇 |
1978年 | 41篇 |
1977年 | 30篇 |
1975年 | 33篇 |
1974年 | 30篇 |
1973年 | 30篇 |
排序方式: 共有8696条查询结果,搜索用时 171 毫秒
141.
Jia-Ling Chou Zhi-Xiang Shen Irene J. Tan Robert L. Stolfi Daniel S. Martin Steven H. Dikman Samuel Waxman 《Experimental cell research》1990,186(2)
Cells from autochthonous mouse mammary carcinomas which display estrogen-independent growth vivo were studied for their hormonal responses in primary culture. A culture system employing insulin-supplemented, serum-free medium and basement membrane Matrigel as a substratum was used to cultivate tumor cells. The cells did not exhibit in vitro estrogenor prolactin-dependent growth. Primary tumors still displayed a constitutional expression of α-, β-, and γ-casein mRNAs. These messages were dramatically reduced during the culture period. However, seven to eightfold increases in α- and β-casein mRNAs were inducible in the 5-day cultures by treatment with prolactin and hydrocortisone. If the hormones were present through a 2-week culture period, the levels of α-, β-, and γ-casein mRNAs in the cells were maintained and displayed in a time-dependent increase with a peak at 10–14 days. The accumulation of β-casein mRNA in vitro did not require DNA synthesis. Administration of prolactin directly into the growing tumors in vivo could also enhance β-casein mRNA levels in the tumor cells. Morphological studies of the cells cultured in the presence of prolactin and hydrocortisone did not reveal visible changes compared with those without hormonal treatment. Transplantation of tumor cells cultured in the presence or absence of hormones resulted in the development of tumors in mice at approximately the same time. The current studies suggest that the autochthonous mammary tumor cells, independent of estrogen for cell growth, were still inducible for casein gene expression in vitro and in vivo by appropriate hormones. The induction and maintenance of casein messages by a single hormonal treatment did not appear to correlate with morphology and DNA synthesis of cells in vitro or with tumor-producing capacities in vivo. 相似文献
142.
We have devised a universal primer which can be used to sequence the 3′-ends of cloned cDNAs containing a polyA tail. The primer consists of an equimolar mixture of three primers: 20 T nucleotides followed by either an A, C, or G nucleotide (5′→3′). With this primer mixture and the dideoxynucleotide chain termination method, we determined the 3′-terminal sequence of human β-actin cDNA in an Okayama-Berg vector, in four parallel sets of reactions containing either a single primer (T20G, T20C, or T20A) or an equimolar mixture of all three primers. Priming with both T20A and the triple mixture gave clearly readable results that agree with the known sequence of the human β-actin gene, and we have applied this method successfully to several other cDNAs in the Okayama-Berg expression vector. Use of this universal primer mixture facilitates determination of sequences at the 3′-ends of cDNAs while by-passing the polyA tail region. 相似文献
143.
Representative plant allelochemicals were tested for toxicity to larvae of the cigarette beetle Lasioderma serricorne (F.) that were either untreated, treated with fungicides, or treated to render them free of symbiotic yeast (aposymbiotic) through surface sterilization of eggs. Insects rendered symbiont-free had higher mortality and/or developmental rates than controls when fed diets containing flavone, resorcinol or tannic acid. Fungicides reduced symbiont populations and/or caused morphological abnormalities. Two hydrolytic enzymes, which made up a significant protion of mycetome hydrolytic activity, were absent when symbionts were not present, as indicated by gel electrophoresis. This information indicates symbionts do contribute to the survival of their host by detoxifying toxins.
Résumé La toxicité relative de substances allélochimiques végétales typiques a été examinée sur des larves de L. serricorne F. traitées aux fongicides et rendues aposymbiotiques par stérilisation superficielle des ufs. La toxicité de la nicotine est relativement inaffectée chez les larves aposymbiotiques; cependant les toxicités des resorcinol, flavone et acide tannique sont significativement augmentées chez les insectes ayant reçu un traitement pour éliminer leurs symbiontes. L'électrophorèse sur gel a montré que quelques enzymes responsables de l'hydrolyse du 1-naphthyl acétate, réaction enzymatique de la détoxification enzymatique, sont absentes chez les insectes aposymbiotiques. L'acide sorbique a réduit significativement l'effectif de symbiontes, tandis que le bénomyl a réduit leur gamme provoquant la multiplication anormale d'autres espèces. Ces résultats montrent que les symbiontes contribuent aux possibilités de détoxification de l'insecte. L'élimination des symbiontes peut être une technique efficace pour lutter contre les insectes qui en contiennent, puisqu'elle les priverait de leur contribution à l'alimentation et à la détoxification. Cette stratégie est vraisemblablement compatible avec des programmes de lutte intégrée, puisque les prédateurs et parasites ne contiennent pas de symbiontes.相似文献
144.
145.
A new technique for studying the effect of insulin on protein degradation is reported. The method is based on measuring the parallel release of a reutilizable and a nonreutilizable amino acid from muscle protein. Animals are prelabeled in vivo with [Me-3H]methionine which labels both the nonreutilizable 3-methylhistidine and the reutilizable methionine of tissue protein. The data presented show that insulin has only a trivial effect on the loss of 3-methylhistidine from muscle protein, while it substantially diminishes the efflux of methionine. The analysis of muscle protein confirms the observation that insulin causes the reincorporation of methionine and has a minimum effect on the loss of 3-methylhistidine. This supports the view that the major inhibitory effect of insulin on gluconeogenesis is the diversion of the flow of amino acids away from the gluconeogenesis pathway back toward protein synthesis. 相似文献
146.
Harvey B. Pollard Samuel S. Stopak Christopher J. Pazoles Carl E. Creutz 《Analytical biochemistry》1981,110(2)
Glucose utilization by cells and tissues can be followed by measuring the release of [3H]H2O from added
-[5-3H]glucose, and we have developed a method whereby the whole reaction and assay can be performed in a single scintillation vial. The basic principle behind our new assay is that the released tritiated hydrogen ion in water can be quantitatively exchanged with the hydroxyl proton of simple alcohols such as isoamyl alcohol. The radiolabeled alcohol can then be extracted into an organic solvent to which 2,5-diphenyloxazole and p-bis[2-(5-phenyloxazoyl)]benzene have been previously added. Using this new assay we studied isolated chromaffin cells and found them to utilize glucose at a linear rate for at least 30 min. The assay was precise and reproducible enough to allow detailed analysis of various inhibitors of glycolysis and of oxidative phosphorylation. The new method is simple and rapid, can be done in open test tubes, requires no complex equipment, and is intrinsically highly accurate. 相似文献
147.
148.
Residual radiation injury was demonstrated in long-term primary cultures of mouse bone marrow. Control cultures underwent three phases of hematopoietic activity as distinguished by initial establishment, steady high (plateau) production of granulocytes, and gradual decline. Irradiation with 50, 300, or 550 rads, given at the end of the initial phase, did not prevent any culture flasks from entering the plateau phase. However, actual production levels and the time they were maintained varied inversely with the radiation dose so that the accumulated postradiation cell production corresponded to an exponential dose-response relationship at any time after treatment. The accumulated cell productions were found to be similar in all groups when expressed by the number of stem cell doublings necessary to produce them. The findings cannot be explained by reproductive cell death and are consistent with the notion of a limited division capacity in hematopoietic stem cells. 相似文献
149.
Antibodies raised against synaptosomal plasma membranes of rat hippocampus (anti-HPC IgG) caused inhibition of [3H]noradrenaline, [3H]5-hydroxytryptamine, [3H]GABA and [3H]aspartate uptake into S1 fractions and slices of hippocampus and cerebral cortex, but not those of caudate nucleus and hypothalamus. Similar inhibition was not observed on using antibodies against synaptosomal membranes of rat caudate nucleus. Anti-HPC IgG raised against synaptosomal membranes of hippocampus failed to alter both spontaneous and K+-evoked release of [3H]noradrenaline. They did not interfere with the binding of [3H]desipramine (the potent noradrenaline-uptake inhibitor) and with the binding of [3H]dihydroalprenolol, thus excluding any interaction of the antibodies with drug receptors which are located on either the pre- or postsynaptic membrane. The anti-HPC IgG inhibit the enzymatic activity of [Na+-K+-]ATPase by 30% upon incubation of the antibodies with crude membrane preparations. A comparison of their inhibitory effects with those of the neurotoxin 6-hydroxydopamine suggests that the corresponding hippocampal specific antigens are located at a presynaptic site. 相似文献
150.
Paul S. Papavasiliou Samuel T. Miller Leon J. Thal Linda J. Nerder Geraldine Houlihan Srinivas N. Rao Janet M. Stevens 《Life sciences》1981,28(26):2945-2952
Old mice reared on regular diet show reduced motor activity, decreased basal adenylate cyclase, and increased MAO activities compared to adults. Brain DDC and COMT activities, DA, NE levels and DA-stimulated adenylate cyclase remained unchanged. By contrast, mice fed levodopa for life did not develop decreased motor activity with aging, lived about 50% longer, had slightly elevated whole brain DA and NE levels and failed to develop the expected rise in MAO activity with aging. Levodopa did not alter the number of dopaminergic and muscarinic cholinergic receptors or the adenylate cyclase activity in the striatum during aging. On levodopa, hepatic and renal DA, dopa, and HVA increased but the latter two returned to basal levels by mid life. In liver, DDC was unchanged but MAO tended to be higher in levodopa-fed mice. Thus, motor impairment is an age-related phenomenon in mice associated with selective alterations in brain dopaminergic systems, which may be prevented by dietary levodopa. Extracerebral tissues, through possibly adaptive metabolic mechanisms, play a significant role in regulating brain catecholamines during chronic administration of large doses of levodopa. 相似文献