首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   333篇
  免费   28篇
  2022年   2篇
  2021年   2篇
  2020年   2篇
  2019年   2篇
  2018年   6篇
  2017年   9篇
  2016年   13篇
  2015年   13篇
  2014年   14篇
  2013年   19篇
  2012年   14篇
  2011年   22篇
  2010年   9篇
  2009年   12篇
  2008年   19篇
  2007年   19篇
  2006年   17篇
  2005年   16篇
  2004年   19篇
  2003年   14篇
  2002年   11篇
  2001年   10篇
  2000年   6篇
  1999年   7篇
  1998年   5篇
  1997年   3篇
  1996年   5篇
  1995年   9篇
  1994年   7篇
  1993年   6篇
  1992年   8篇
  1991年   7篇
  1990年   9篇
  1989年   4篇
  1988年   1篇
  1987年   2篇
  1986年   4篇
  1985年   2篇
  1983年   2篇
  1982年   2篇
  1980年   4篇
  1979年   3篇
  1978年   1篇
排序方式: 共有361条查询结果,搜索用时 15 毫秒
1.
The membrane potential (delta psi) and delta pH of the inner mitochondrial membrane were studied in isolated perfused rat hearts using exogenous labelled probes and tissue fractionation in non-aqueous media. The mitochondrial delta psi, measured by means of the subcellular distribution of [3H]triphenylmethylphosphonium (TPMP+), was 125 +/- 7 mV (negative inside) in hearts beating at 5 Hz and 150 +/- 3 mV (negative inside) in hearts beating at 1.5 Hz. The mitochondrial membrane delta pH, measured by means of the subcellular distribution of low concentrations of [1-14C]propionate, was 0.63 +/- 0.06 pH units (alkaline inside) in hearts beating at 5 Hz and 0.53 +/- 0.12 pH units (alkaline inside) in hearts beating at 1.5 Hz. The implication of proton and electron gradients in the regulation of cellular respiration is discussed. In combination with previous evidence on adenylate distribution in the isolated perfused rat heart, the results indicate that the mitochondrial electrogenic adenylate translocator is in near equilibrium with delta psi.  相似文献   
2.
Densities of microtine rodents in two habitat complexes in the tundra of Finnmarks-vidda, Norwegian Lapland, were studied during 1977-89 by means of snap trapping (Small Quadrat Method) Predator populations were studied by mapping breeding raptors and by snow-tracking small mustelids During 1977-85, snow-trackmg was conducted only during peak and decline years, whereas during 1986-89, snow-tracking was conducted every winter (November-December) and live-trapping (in August) was used as an additional method
Lowland vole populations had regular density fluctuations with peaks in 1978-79. 1982-84 and 1987-88 Highland vole populations fluctuated less regularly and at lower over-all densities Highland lemming populations had two outbreaks, in 1978 and 1988, ending in abrupt winter crashes In the lowland, outbreak levels were reached only in 1978 All microtine declines in relatively productive lowland habitats were accompanied by intense activity of small mustelids. whereas avian predators were common only in 1983 Lowland declines also showed clear between-habitat asynchrony they started in areas with an exceptional abundance of productive habitats and then spread to more barren areas These lowland data are consistent with the hypothesis of a mustelid-microtine limit cycle, although also several other hypotheses remain unrefuted The highland lemming data suggest a simple exploiter-victim interaction between lemmings and the vegetation  相似文献   
3.
The habitat use of small mustelids in a tundra area in Norwegian Lapland was studied chiefly by means of snow-tracking 1986-89 Stoats showed strong peference to a habitat complex immediately beneath the thrust line of the Scandes, with exceptional abundance of luxuriant habitats, whereas weasel activity was more evenly spread over the lowland tundra Mustelid activity on the high tundra above the thrust line was consistently low Within each subarea. stoat activity was concentrated to the most luxuriant habitats Similar preferences were shown by weasels on the lowland but not in the vicinity of the thurst cliff Daily movements of both species varied from local (c 200 m) to extensive (up to 4 km), no consistent interspecific differences in travel distances could be observed
The results largely conform to the hypothesis of patchy exploitation ecosystems (T Oksanen 1990a), according to which predator activity tends to "spill over' from luxuriant habitats, capable of supporting predator populations, to adjacent barren ones, due to despotic behavior within and between species and due to opportunistic predation by transient predators However, predator activity in barren habitats during the crash winter could not be explained by these mechanisms alone A third mechanism - breakdown of habitat preferences of predators during crash phases of a cyclic prey population - was thus introduced  相似文献   
4.
Summary A high prevalence of the lysosomal storage disease aspartylglycosaminuria was found in a study of four birth cohorts of 12882 children in eastern Finland. Using school achievement tests and registers of mentally retarded individuals, 178 mentally retarded children were identified. Randomized urine samples from 151 of the 178 retarded children and from 101 healthy children were analyzed quantitatively for aspartylglucosamine by high-performance liquid chromatography. The results identified three affected individuals in the retarded group indicating an exceptionally high prevalence of aspartylglycosaminuria (1:3643) in the study population, consistent with a carrier frequency of 1:30. The 95% confidence limits for the prevalence are 1:4 352-1:16389. This is the highest prevalence described for any glycoproteinosis in any population and comparable to the incidence figures of the most common lysosomal storage diseases, Gaucher disease type I and Tay-Sachs disease among Ashkenazi Jews. In the study group, aspartylglycosaminuria was, after trisomy 21 (n = 19) and the fragile X syndrome (n = 6), the most common genetic cause for mental retardation.  相似文献   
5.
Transgenic plant cell cultures have a potential for production and secretion of important proteins and peptides. To assess the possibilities of using a stable barley suspension culture for secretion of heterologous proteins in active form, we expressed the cDNA of the thermostable-glucanase (EGI) ofTrichoderma reesei in barley suspension cells. The cDNA coding for EGI and its signal sequence was placed under the control of the CaMV 35S promoter and the construction was transferred to the cells by particle bombardment. Stably transformed lines were obtained by selecting for a cotransformed antibiotic resistance marker. The expression of EGI cDNA led to accumulation of EGI in the culture medium, as shown by analysis with EGI-specific antibodies. Enzymatic assays confirmed that the EGI secreted by the suspension cells retained its activity and thermostable character. Furthermore, it was shown that the enzyme produced by the transgenic suspension culture could be used for degradation of soluble-glucans during mashing.  相似文献   
6.
Fertile transgenic barley by particle bombardment of immature embryos   总被引:5,自引:0,他引:5  
Transgenic, fertile barley (Hordeum vulgare L.) from the Finnish elite cultivar Kymppi was obtained by particle bombardment of immature embryos. Immature embryos were bombarded to the embryonic axis side and grown to plants without selection. Neomycin phosphotransferase II (NPTII) activity was screened in small plantlets. One out of a total of 227 plants expressed the transferred nptII gene. This plant has until now produced 98 fertile spikes (T0), and four of the 90 T0 spikes analyzed to date contained the nptII gene. These shoots were further analyzed and they expressed the transferred gene. From green grains, embryos were isolated and grown to plantlets (T1). The four transgenic shoots of Toivo (the T0 plant) produced 25 plantlets as T1 progeny. Altogether fifteen of these T1 plants carried the transferred nptII gene as detected with the PCR technique, fourteen of which expressed the nptII gene. The integration and inheritance of the transferred nptII gene was confirmed by Southern blot hybridization. Although present as several copies, the transferred gene was inherited as a single Mendelian locus into the T2 progeny.  相似文献   
7.
Abstract: Cerebral glutamate was monitored in a superfused cerebral cortical preparation by 1H NMR spectroscopy using a semiselective spin-echo sequence N -acetyl aspartate (NAA) as an internal concentration reference. During controlled metabolic conditions, the cerebral 1H NMR-detected glutamate-to-NAA ratio was ∼ 20–30% lower than expected from the ratio of neutralized perchloric acid extracts of the preparations. Inhibition of respiration in the presence of glucose did not change the 1H NMR glutamate-to-NAA ratio in brain slice preparation. In contrast, either complete depletion of ATP during cyanide poisoning together with 0 m M glucose, anoxia in the absence of glucose, or treatment with nigericin or with a protonophore, carbonyl cyanide- m -fluorophenylhydrazone, increased 1H NMR-detected glutamate/NAA in the cerebral preparations without a change in the relative and absolute concentration ratios determined from the tissue acid extracts. Spin-spin relaxation times of glutamate and NAA peaks in anoxic slices were 749 ± 89 and 729 ± 94 ms, respectively, and thus, the portion of glutamate that could not be detected by 1H NMR was quantified in absolute terms. It was calculated that an increase in the glutamate-to-NAA ratio from 0.55 ± 0.02 to 0.67 ± 0.02 during aglycemic anoxia corresponded to some 6 mmol/kg of tissue dry weight of glutamate from the total concentration of 28 mmol/kg dry weight. It is suggested that this 22% of total glutamate pool is present in a noncytoplasmic compartment during controlled metabolic state.  相似文献   
8.
Summary 1. Innervation of the mammalian pineal gland is mainly sympathetic. Pineal synthesis of melatonin and its levels in the circulation are thought to be under strict adrenergic control of serotoninN-acetyltransferase (NAT). In addition, several putative pineal neurotransmitters modulate melatonin synthesis and secretion.2. In this review, we summarize what is currently known on the pineal cholinergic system. Cholinergic signaling in the rat pineal gland is suggested based on the localization of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), as well as muscarinic and nicotinic ACh binding sites in the gland.3. A functional role of ACh may be regulation of pineal synaptic ribbon numbers and modulation of melatonin secretion, events possibly mediated by phosphoinositide (PI) hydrolysis and activation of protein kinase C via muscarinic ACh receptors (mAChRs).4. We also present previously unpublished data obtained using primary cultures of rat pinealocytes in an attempt to get more direct information on the effects of cholinergic stimulus on pinealocyte melatonin secretion. These studies revealed that the cholinergic effects on melatonin release are restricted mainly to intact pineal glands since they were not readily detected in primary pinealocyte cultures.  相似文献   
9.
10.
Ischemic stroke is confounded by conditions such as atherosclerosis, diabetes, and infection, all of which alter peripheral inflammatory processes with concomitant impact on stroke outcome. The majority of the stroke patients are elderly, but the impact of interactions between aging and inflammation on stroke remains unknown. We thus investigated the influence of age on the outcome of stroke in animals predisposed to systemic chronic infection. Th1‐polarized chronic systemic infection was induced in 18–22 month and 4‐month‐old C57BL/6j mice by administration of Trichuris muris (gut parasite). One month after infection, mice underwent permanent middle cerebral artery occlusion and infarct size, brain gliosis, and brain and plasma cytokine profiles were analyzed. Chronic infection increased the infarct size in aged but not in young mice at 24 h. Aged, ischemic mice showed altered plasma and brain cytokine responses, while the lesion size correlated with plasma prestroke levels of RANTES. Moreover, the old, infected mice exhibited significantly increased neutrophil recruitment and upregulation of both plasma interleukin‐17α and tumor necrosis factor‐α levels. Neither age nor infection status alone or in combination altered the ischemia‐induced brain microgliosis. Our results show that chronic peripheral infection in aged animals renders the brain more vulnerable to ischemic insults, possibly by increasing the invasion of neutrophils and altering the inflammation status in the blood and brain. Understanding the interactions between age and infections is crucial for developing a better therapeutic regimen for ischemic stroke and when modeling it as a disease of the elderly.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号