全文获取类型
收费全文 | 7495篇 |
免费 | 904篇 |
国内免费 | 4篇 |
专业分类
8403篇 |
出版年
2022年 | 51篇 |
2021年 | 123篇 |
2020年 | 62篇 |
2019年 | 67篇 |
2018年 | 82篇 |
2017年 | 77篇 |
2016年 | 140篇 |
2015年 | 261篇 |
2014年 | 289篇 |
2013年 | 326篇 |
2012年 | 399篇 |
2011年 | 429篇 |
2010年 | 253篇 |
2009年 | 211篇 |
2008年 | 393篇 |
2007年 | 394篇 |
2006年 | 380篇 |
2005年 | 368篇 |
2004年 | 303篇 |
2003年 | 288篇 |
2002年 | 267篇 |
2001年 | 224篇 |
2000年 | 221篇 |
1999年 | 184篇 |
1998年 | 103篇 |
1997年 | 92篇 |
1996年 | 87篇 |
1995年 | 69篇 |
1994年 | 80篇 |
1993年 | 99篇 |
1992年 | 162篇 |
1991年 | 145篇 |
1990年 | 141篇 |
1989年 | 124篇 |
1988年 | 118篇 |
1987年 | 134篇 |
1986年 | 103篇 |
1985年 | 111篇 |
1984年 | 103篇 |
1983年 | 76篇 |
1982年 | 76篇 |
1981年 | 73篇 |
1980年 | 55篇 |
1979年 | 73篇 |
1978年 | 51篇 |
1977年 | 43篇 |
1976年 | 49篇 |
1975年 | 41篇 |
1974年 | 44篇 |
1973年 | 59篇 |
排序方式: 共有8403条查询结果,搜索用时 15 毫秒
111.
112.
113.
114.
White S. R.; Blake J. S.; Murphy T. M.; Mack M. M.; Munoz N. M.; Leff A. R. 《Journal of applied physiology》1989,66(4):1852-1859
We studied the sympathetic neural response on airways to hypotensive stimuli in 19 swine in vivo. The effects of pharmacologically induced hypotension with nitroprusside (NTP) and hypotension elicited by intravenous compound 48/80 (48/80), a mast cell degranulating agent, were compared after equivalent reductions in mean arterial blood pressure (MAP). Reduction of the MAP to 60% of base line with NTP in six swine caused an increase in plasma epinephrine (E) from 60 +/- 28 to 705 +/- 276 pg/ml (P = 0.032) and plasma norepinephrine (NE) from 270 +/- 46 to 796 +/- 131 pg/ml (P = 0.032). Comparable reduction in MAP elicited with 48/80 in six other swine caused a substantially greater increase in both plasma E (9,581 +/- 4,147 pg/ml; P = 0.012 vs. NTP group) and plasma NE (2,239 +/- 637 pg/ml; P = 0.041 vs. NTP group). Catecholamine secretion attenuated mediator-induced changes in lung resistance (RL). In animals receiving 48/80, RL increased from 2.97 +/- 0.31 to 7.44 +/- 0.56 cmH2O.l-1.s. In animals having ganglionic blockade with 7.5 mg/kg iv hexamethonium and beta-adrenergic blockade with propranolol (4.0 mg/kg iv followed by 40 micrograms/kg-1.min-1), comparable doses of 48/80 caused an increase in RL to 18.6 +/- 4.55 cmH2O.l-1.s (P less than 0.04 vs. swine receiving neither hexamethonium nor propranolol).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
115.
We studied the effect of maturation on potassium-induced parasympathetic activation and Ca2+ entry in tracheal smooth muscle (TSM) from fifteen 2-wk-old (2ws) and sixteen 10-wk-old (10ws) male domestic farm swine. Atropine (10(-7) M) caused inhibition of the maximal contraction elicited by potassium to 50.3 +/- 2.6% maximum of control response (P less than 0.001) in TSM from 2ws but had no significant effect in TSM from 10ws (94.6 +/- 4.2% maximum; P = NS vs. control). Verapamil (10(-7) M) plus 10(-7) M atropine reduced contraction elicited by potassium in both 2ws (23.7 +/- 5.8% maximum; P less than 0.001 vs. control) and 10ws (50.6 +/- 6.3% maximum; P less than 0.001 vs. control, P less than 0.05 vs. 2ws); 10(-6)M verapamil caused greater than 95% blockade of contraction caused by potassium in both 2ws and 10ws. In separate studies, atropine-treated strips were equilibrated with extracellular Ca2+ concentrations ([Ca2+]o) ranging from normal (1X [Ca2+]o) to four times normal (4x [Ca2+]o). Increasing [Ca2+]o increased maximal contractile response in atropine-treated TSM strips from 68.7 +/- 3.8% maximum for 1x [Ca2+]o to 100.8 +/- 4.8% maximum for 4x [Ca2+]o (P less than 0.001) in 2ws. Neither atropine nor [Ca2+]o affected maximal responses of TSM in 10ws (103.5 +/- 3.0% maximum for 1x [Ca2+]o; P = NS vs. control). However, in the presence of atropine and verapamil, 4x [Ca2+]o augmented KCl-elicited contraction of TSM from both 2ws (46.9 +/- 6.3% maximum; P less than 0.01 vs. control) and 10ws (78.6 +/- 2.3% maximum; P less than 0.005 vs. control).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
116.
117.
Molecular cloning and characterization of a genetic region from Serratia marcescens involved in DNA repair 总被引:3,自引:1,他引:2
We report here the molecular isolation of a DNA fragment which encodes Tag-like activity from the Gram-negative bacterium Serratia marcescens. A recombinant plasmid encoding Tag-like activity was isolated from a S. marcescens plasmid gene library by complementation of an Escherichia coli tag mutant, which is deficient in 3-methyladenine DNA glycosylase I. The clone complements E. coli tag, recA, alkA, but not alkB, mutants for resistance to the DNA-damaging agent methyl methanesulphonate (MMS). The coding region of the Tag activity, initially isolated on a 6.5kb BamHI fragment, was defined to a 1.8kb BglII-SmaI fragment. Labelling of plasmid-encoded proteins using maxicells revealed that the 1.8kb fragment encodes two proteins of molecular weights 42,000 and 16,000. Data presented here suggest that the cloned fragment encodes a DNA repair protein(s) that has similar activity to the 3-methyladenine DNA glycosylase I of E. coli. 相似文献
118.
119.
Submicromolar Ca2+ regulates phosphorylating respiration by normal rat liver and AS-30D hepatoma mitochondria by different mechanisms 总被引:1,自引:0,他引:1
The stimulation of 2-oxoglutarate and NAD(+)-isocitrate dehydrogenase by Ca2+ in mitochondria from normal tissues has been proposed to mediate partially the activation of oxidative energy metabolism elicited by physiological elevations in cytosolic Ca2+. This mode of regulation may also occur in tumor cells in which several aspects of mitochondrial metabolism are known to be altered. This study provides a comparison of the stimulation by submicromolar concentrations of Ca2+ on the rates of ATP-generating (state 3) respiration under physiologically realistic conditions by mitochondria isolated from normal rat liver and from highly malignant rat AS-30D ascites hepatoma cells. The K0.5 for activation of glutamate-dependent state 3 respiration by Ca2+ in the presence of ATP at 37 degrees C was determined to be 0.70 +/- 0.05 (S.E.) microM for hepatoma mitochondria and 0.90 +/- 0.03 microM for rat liver mitochondria. This activation was also reflected by a Ca2(+)-induced shift in the oxidation-reduction state of hepatoma mitochondrial pyridine nucleotides to a more reduced level and Ca2+ stimulation of 14CO2 production from [1-14C]glutamate. Whereas the Ca2+ sensitivity of state 3 respiration by hepatoma mitochondria can be explained by the activation of 2-oxoglutarate and possibly NAD(+)-isocitrate dehydrogenases, the Ca2+ sensitivity of liver mitochondrial respiration appears to be predominantly mediated by activation of electron flow through ubiquinone and Complex III of the electron transport chain, as indicated by the specificity of the effects of Ca2+ on respiration with different oxidizable substrates. Although rat liver and hepatoma mitochondria employ different modes of Ca2(+)-activated ATP generation, these results support the hypothesis that changes in cytosolic Ca2+ play a significant role in the potentiation of energy production in tumor, as well as normal tissue. 相似文献
120.
Marked Amine and Amine Metabolite Changes in Norrie Disease Patients with an X-Chromosomal Deletion Affecting Monoamine Oxidase 总被引:3,自引:0,他引:3
D. L. Murphy K. B. Sims† F. Karoum‡ A. de la Chapelle§ R. Norio E.-M. Sankila§ X. O. Breakefield†# 《Journal of neurochemistry》1990,54(1):242-247
Urinary and plasma amines and amine metabolites were quantified in two individuals with Norrie disease resulting from a deletion in chromosomal region Xp11.3, recently reported to be associated with absence of the gene encoding monoamine oxidase (MAO)-A and nondetectable MAO-A activity in fibroblasts and MAO-B activity in platelets. Marked (four-to 100-fold) elevations in levels of urinary phenylethylamine, o-tyramine, and m-tyramine (which are preferential substrates for MAO-B) and marked reductions (90%) in levels of 3-methoxy-4-hydroxyphenylglycol (a deaminated metabolite of norepinephrine, a preferential substrate for MAO-A) in urine and plasma confirmed the presence of a systemic, functionally significant reduction in the activities of both MAO isozymes. The magnitude of these changes, which are equivalent to those found in subjects taking MAO-inhibiting antidepressants, suggests that early initiation of dietary and drug restrictions may be clinically important in these and other patients with X-chromosomal mutations involving MAO. These findings further support the proposition that the MAOA and MAOB genes are located in close proximity on the X chromosome. Negligible changes in the metabolites of dopamine and serotonin raise the possibility that other metabolic pathways are of importance for their production, that dietary or intestinal bacterial sources contribute substantially to the presence of these amine metabolites in urine, or both. 相似文献