全文获取类型
收费全文 | 4406篇 |
免费 | 435篇 |
国内免费 | 1篇 |
出版年
2022年 | 33篇 |
2021年 | 57篇 |
2020年 | 40篇 |
2019年 | 60篇 |
2018年 | 62篇 |
2017年 | 48篇 |
2016年 | 96篇 |
2015年 | 147篇 |
2014年 | 139篇 |
2013年 | 232篇 |
2012年 | 259篇 |
2011年 | 242篇 |
2010年 | 154篇 |
2009年 | 166篇 |
2008年 | 206篇 |
2007年 | 234篇 |
2006年 | 246篇 |
2005年 | 208篇 |
2004年 | 203篇 |
2003年 | 204篇 |
2002年 | 195篇 |
2001年 | 69篇 |
2000年 | 60篇 |
1999年 | 65篇 |
1998年 | 58篇 |
1997年 | 44篇 |
1996年 | 54篇 |
1995年 | 32篇 |
1994年 | 54篇 |
1993年 | 60篇 |
1992年 | 49篇 |
1991年 | 50篇 |
1990年 | 48篇 |
1989年 | 37篇 |
1988年 | 46篇 |
1987年 | 53篇 |
1986年 | 27篇 |
1985年 | 45篇 |
1984年 | 53篇 |
1983年 | 40篇 |
1982年 | 47篇 |
1981年 | 47篇 |
1980年 | 38篇 |
1979年 | 23篇 |
1978年 | 39篇 |
1977年 | 42篇 |
1976年 | 21篇 |
1975年 | 25篇 |
1974年 | 27篇 |
1972年 | 28篇 |
排序方式: 共有4842条查询结果,搜索用时 31 毫秒
941.
Robin D Allan Helena W Dickenson Graham A.R Johnston Rymantas Kazlauskas Kenneth N Mewett 《Neurochemistry international》1997,30(6):197-591
Structural analogues of ZAPA, Z-3-[(aminoiminomethyl)thio]prop-2-enoic acid, an isothiouronium analogue of GABA, are potent GABAA agonists as seen in the isolated guinea-pig ileum and in the facilitation of [3H]diazepam binding to rat brain membranes. Compounds with guanidino or amidine groups replacing the amino functionality of GABA were also found to be active. The highest activity was displayed by the isothiouronium salts in which the conformational flexibility of the molecule is restricted by a Z-substituted carbon–carbon double bond. A series of bis-isothiouronium compounds was prepared from aliphatic ,ω-bis-thioureas as mixtures of E and Z adducts. Maximum GABAA agonist activity for this series was found with a C6–C8 carbon chain, and the results were consistent with an interaction at the GABAA receptor with only one end of the molecule, rather than the more potent effect expected of a molecule bridging two active sites. GABAA antagonist/partial agonist activity was observed on the guinea-pig isolated ileum for a number of different analogue types, with the most potent being bis-isothiouronium derivatives. None of the substituted derivatives of ZAPA was as active as ZAPA itself, and maximum GABAA activity was found in the n-pentyl and n-hexyl analogues. 相似文献
942.
Makoto Shibutani Eugene Kim Philip Lazarovici Mari Oshima Dr. Gordon Guroff 《Neurochemical research》1996,21(7):801-807
The postmitochondrial fraction (S10) contains the cellular components essential for translation, and a high-salt wash (HSW)
of the ribosomes is enriched in eukaryotic initiation factors. This report describes the preparation of a cell-free translation
system utilizing an S10 extract from PC12 cells. The products synthesized from either firefly luciferase mRNA or PC12 cell
poly(A) RNAs in the PC12-S10 extract were increased by the addition of the HSW from PC12 cells. Increases in the translation
of luciferase mRNA by the addition of PC12-HSW were dose-dependent and also dependent on the time of incubation. The translation
of human epidermal growth factor receptor (hEGFR) mRNA could also be detected in the PC12-S10 extract translation system by
immunoprecipitation.N-linked glycosylation of the translation products also was observed. The efficiency of translation was altered by the addition
of Mg2+ or K+, and optimization of the concentrations of these ions was necessary for each mRNA. The translation system made from PC12
cells, then, is capable of the synthesis of proteins of relatively high molecular weight and should be useful for analyzing
mechanisms of translational control during proliferation and differentiation of cells from a neuronal lineage.
Special issue dedicated to Dr. Hans Thoenen. 相似文献
943.
The scarcity of resources that can be allocated to genetic testing will ultimately limit the number of diseases subjected to molecular analysis. Medical student David Allan, who claimed first prize in CMAJ''s 1995 Logie Medical Ethics Essay Contest, looks at the ethical principles that should guide decisions about genetic testing, and the importance of communicating these principles to patients and their families. 相似文献
944.
945.
OBJECTIVE--To determine whether seasonality of the sudden infant death syndrome persists now that rates have fallen, mostly after widespread adoption of the "face upwards" sleeping position. DESIGN--Monthly data on the sudden infant death syndrome during 1987-9 were compared for seasonality with those of 1991-3; rates were studied as deaths per 1000 live births. SETTING--Australia and Britain (England, Wales, and Scotland). SUBJECTS--Infants under 1 year dying of the syndrome (2401 for Australia and 6630 for Britain). MAIN OUTCOME MEASURE--Extent of seasonal variation (amplitude) was established by cosinor analysis; amplitudes for the earlier and later years were compared. RESULTS--The rate fell in every month, and, though it did so relatively more in winter than summer, seasonality remained a distinctive feature. In the comparison of amplitudes the ratio between the earlier and later years was 1.4 in both Australia and Britain. Some differences between the hemispheres were noted. CONCLUSIONS--Seasonality of the sudden infant death syndrome remains to be explained and continues to be an important aetiological lead. Studies from other countries are needed. 相似文献
946.
Ioannis G. Fatouros Allan H. Goldfarb Athanasios Z. Jamurtas Theodore J. Angelopoulos Jiaping Gao 《European journal of applied physiology and occupational physiology》1997,76(3):203-208
β-Endorphin (BE) infusion at rest can influence insulin and glucagon levels and thus may affect glucose availability during
exercise. To clarify the effect of BE on levels of insulin, glucagon and glucose during exercise, 72 untrained male Sprague-Dawley
rats were infused i.v. with either: (1) BE (bolus 0.05 mg · kg−1 +0.05 mg · kg−1 · h−1, n = 24); (2) naloxone (N, bolus 0.8 mg · kg−1 + 0.4 mg · kg−1, n = 24); or (3) volume-matched saline (S, n = 24). Six rats from each group were killed after 0, 60, 90 or 120 min of running at 22 m · min−1, at 0% gradient. BE infusion resulted in higher plasma glucose levels at 60 min [5.93 (0.32) mM] and 90 min [4.16 (0.29) mM]
of exercise compared to S [4.62 (0.27) and 3.41 (0.26 mM] and N [4.97 (0.38) and 3.44 (0.25) mM]. Insulin levels decreased
to a greater extent with BE [21.5 (0.9) and 18.3 (0.6) uIU · ml−1] at 60 and 90 min compared to S [24.5 (0.5) and 20.6 (0.6) uIU · ml−1] and N [24.5 (0.4) and 21.6 (0.7) uIU · ml−1] groups. Plasma C-peptide declined to a greater extent at 60 and 90 min of exercise with BE infusion compared to both S and
N. BE infusion increased glucagon at all times during exercise compared to S and N. These data suggest that BE infusion during
exercise influences plasma glucose by augmenting glucagon levels and attenuating insulin release.
Accepted: 26 February 1997 相似文献
947.
948.
Allan J. Baker Abdul Moeed 《Evolution; international journal of organic evolution》1987,41(3):525-538
Populations of common mynas introduced to Australia, New Zealand, Fiji, Hawaii, and South Africa from India during the last century were compared genetically with the extant native population using isozyme electrophoresis of 39 presumptive loci. Average heterozygosity, mean number of alleles/locus, and the percentage of polymorphic loci are lower in the introduced populations, and the 18% loss of alleles involves only alleles that are rare in the native population. The native population is only weakly subdivided genetically (FST = 0.032) whereas the introduced populations are much more differentiated (FST = 0.123), and the mean genetic distance among them is significantly greater than among native samples. The reduction in mean number of alleles/locus and average heterozygosity is greatest in the South African population, consistent with a very small effective size in the founder population. In the introduced populations, random drift is implicated by the different subsets of polymorphic loci they possess, by their greater variance in allele frequencies, and by shifts either side of the native means. It is concluded that in the evolutionarily short period of 100–120 years, bottlenecks and random drift have promoted genetic shifts equal to those between different subspecies of birds. 相似文献
949.
Measurements of the gas vesicle space in steady-state light or phosphate-limited cultures of Aphanizomenon flos-aquae Ralfs, strain 7905 showed that gas vesicle content decreased as energy-limited growth rate increased hut was the same at several phosphate-limited growth rates. Upon a decrease in growth irradiance, gas vesicle content did increase in phosphate-limited cultures, hut the cultures remained nonbuoyant as long as P was limiting. Buoyant, energy-limited cultures lost their buoyancy in less than 2 h when exposed to higher irradiances. The primary mechanism for buoyancy loss was the accumulation of polysaccharide as ballast. Collapse of gas vesicles by turgor pressure played a minor role in the loss of buoyancy. When cultures were exposed to higher irradiances, cells continued to synthesize gas vesicles at the same rate as before the shift for at least 1 generation time. The amount of ballast required to make individual filaments in the population sink varied 4-fold. This variation appears to be due to differences in gas vesicle content among individual filaments. 相似文献
950.
Gordon G Giesbrecht Tamara L Lockhart Gerald K Bristow Allan M Steinman 《Journal of applied physiology》2005,99(5):1958-1964
Personal floatation devices maintain either a semirecumbent flotation posture with the head and upper chest out of the water or a horizontal flotation posture with the dorsal head and whole body immersed. The contribution of dorsal head and upper chest immersion to core cooling in cold water was isolated when the confounding effect of shivering heat production was inhibited with meperidine (Demerol, 2.5 mg/kg). Six male volunteers were immersed four times for up to 60 min, or until esophageal temperature = 34 degrees C. An insulated hoodless dry suit or two different personal floatation devices were used to create four conditions: 1) body insulated, head out; 2) body insulated, dorsal head immersed; 3) body exposed, head (and upper chest) out; and 4) body exposed, dorsal head (and upper chest) immersed. When the body was insulated, dorsal head immersion did not affect core cooling rate (1.1 degrees C/h) compared with head-out conditions (0.7 degrees C/h). When the body was exposed, however, the rate of core cooling increased by 40% from 3.6 degrees C/h with the head out to 5.0 degrees C/h with the dorsal head and upper chest immersed (P < 0.01). Heat loss from the dorsal head and upper chest was approximately proportional to the extra surface area that was immersed (approximately 10%). The exaggerated core cooling during dorsal head immersion (40% increase) may result from the extra heat loss affecting a smaller thermal core due to intense thermal stimulation of the body and head and resultant peripheral vasoconstriction. Dorsal head and upper chest immersion in cold water increases the rate of core cooling and decreases potential survival time. 相似文献