全文获取类型
收费全文 | 4321篇 |
免费 | 304篇 |
国内免费 | 4篇 |
专业分类
4629篇 |
出版年
2023年 | 30篇 |
2022年 | 54篇 |
2021年 | 110篇 |
2020年 | 71篇 |
2019年 | 94篇 |
2018年 | 131篇 |
2017年 | 102篇 |
2016年 | 152篇 |
2015年 | 232篇 |
2014年 | 205篇 |
2013年 | 286篇 |
2012年 | 291篇 |
2011年 | 295篇 |
2010年 | 209篇 |
2009年 | 171篇 |
2008年 | 209篇 |
2007年 | 203篇 |
2006年 | 203篇 |
2005年 | 164篇 |
2004年 | 165篇 |
2003年 | 140篇 |
2002年 | 130篇 |
2001年 | 92篇 |
2000年 | 80篇 |
1999年 | 77篇 |
1998年 | 34篇 |
1997年 | 32篇 |
1996年 | 28篇 |
1995年 | 43篇 |
1994年 | 24篇 |
1993年 | 27篇 |
1992年 | 38篇 |
1991年 | 46篇 |
1990年 | 33篇 |
1989年 | 38篇 |
1988年 | 33篇 |
1987年 | 33篇 |
1986年 | 38篇 |
1985年 | 28篇 |
1984年 | 15篇 |
1981年 | 17篇 |
1979年 | 20篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 12篇 |
1975年 | 17篇 |
1974年 | 23篇 |
1973年 | 16篇 |
1969年 | 13篇 |
1968年 | 14篇 |
排序方式: 共有4629条查询结果,搜索用时 15 毫秒
21.
Glycerolipid synthesis was studied by determining radioactive incorporation from either [1-14C] acetate or [U-14C] palmitate. Glycerolipid synthesis in adipocytes, mainly from exogenous palmitate, was preferentially directed to the formation of triacylglycerols, whereas in hepatocytes triacylglycerols and phospholipids were synthesized at similar rates. Insulin stimulated glycerolipid synthesis from acetate in both types of cells, being triacylglycerols more significantly increased than phospholipids. The most relevant difference was the finding that in adipocytes insulin strongly stimulated the formation of diglycerides, apparently from phosphatidate, whereas in hepatocytes insulin only slightly increased diglyceride levels. A possible role of diacylglycerol in insulin action in adipocytes, but not in hepatocytes, is also discussed. 相似文献
22.
23.
24.
We have attempted to make quantitative comparisons between the genetic systems provoked by some neo-tropical environments and a European one. These comparisons permit certain clarifications regarding the comprehension of the genetic load and of the genetic norm in reference to particular ecological characteristics. 相似文献
25.
Measurements of intravesical pressure, urinary flow rate, retrograde cystographic residual urine, reflux of contrast medium into the prostatic ducts, and radiological prostate size were made in 45 men with benign prostatic hypertrophy. These factors were all found to be good clinical indices of the degree of urethral resistance to micturition; on the other hand, there was no relation between the latter and the severity of symptoms. 相似文献
26.
Refinement of distance geometry (DG) structures of EETI-II (Heitz et al.: Biochemistry 28:2392-2398, 1989), a member of the squash family trypsin inhibitor, have been carried out by restrained molecular dynamics (RMD) in water. The resulting models show better side chain apolar/polar surface ratio and estimated solvation free energy than structures refined "in vacuo." The consistent lower values of residual NMR constraint violations, apolar/polar surface ratio, and solvation free energy for one of these refined structures allowed prediction of the 3D folding and disulfide connectivity of EETI-II. Except for the few first residues for which no NMR constraints were available, this computer model fully agreed with X-ray structures of CMTI-I (Bode et al.: FEBS Lett. 242:285-292, 1989) and EETI-II complexed with trypsin that appeared after the RMD simulation was completed. Restrained molecular dynamics in water is thus proved to be highly valuable for refinement of DG structures. Also, the successful use of apolar/polar surface ratio and of solvation free energy reinforce the analysis of Novotny et al. (Proteins 4:19-30, 1988) and shows that these criteria are useful indicators of correct versus misfolded models. 相似文献
27.
V Calvo S Rojo D López B Galocha J A López de Castro 《Journal of immunology (Baltimore, Md. : 1950)》1990,144(10):4038-4045
HLA-B27 subtype polymorphism is amenable to differential recognition by CTL. Site-directed mutagenesis was used to construct a series of HLA-B27 mutants reproducing most of the changes occurring in the natural subtypes. The reactivity of 21 anti-HLA-B27 CTL clones was examined with these mutants to address three issues concerning the alloreactive response against HLA-B27: 1) diversity of clonotypic specificities, 2) structural features of the epitopes recognized by these clones, and 3) role of individual positions in the differential recognition of HLA-B27 subtypes. Virtually all CTL clones displayed unique reaction patterns with the mutants, indicating a corresponding diversity of epitopes. However, these share some molecular features, such as certain amino acid residues and related locations. Individual mutations induced complex effects on multiple B27-specific CTL epitopes, revealing some of their very precise stereochemical constrains. An important feature of HLA-B27 subtype polymorphism is that every individual change was relevant, altering recognition by many CTL clones. Although the specific set affected by each mutation was partially different, the global number of clones affected by most changes was very similar. This suggests that the antigenic profile of any given subtype is not dominated by one particular change but is uniquely defined by its corresponding set of changes. An exception was the change at position 152, which totally abrogated recognition by all 20 anti-B*2705 CTL clones. This effect decisively influences the profound differences in T cell recognition between B*2705 and the two subtypes, B*2704 and B*2706, carrying this change. The results are compatible with the idea that HLA-B27 allorecognition may involve multiple peptides bound to the alloantigen on the cell surface. 相似文献
28.
Increased Neostriatal Tyrosine Hydroxylation During Stress: Role of Extracellular Dopamine and Excitatory Amino Acids 总被引:2,自引:0,他引:2
Abstract: We examined the regulation of neostriatal tyrosine hydroxylation during acute stress, testing the hypothesis that excitatory amino acids (EAAs) contribute to the stress-evoked increase in dopamine (DA) synthesis. Dialysis probes implanted into neostriatum permitted delivery of drugs and sampling of extracellular fluid. Rats were exposed to 30 min of intermittent tail shock during infusion of an inhibitor of aromatic amino acid decarboxylase (AAAD), NSD-1015 (100 µM), and DOPA was measured in the dialysate. Tail shock was applied beginning either 15 min after the onset of NSD-1015 treatment (the initial rate of DOPA accumulation) or 75 min after the onset of treatment (when DOPA had approached steady state). Tail shock increased the steady-state levels of extracellular DOPA in neostriatum (+40%). However, there was no change in the initial rate of DOPA accumulation unless animals also received the D2 receptor antagonist eticlopride (50 nM), in which case an increase was observed (+228%). The impact of tail shock on the steady-state level of DOPA was attenuated by the D2 agonist quinpirole (100 µM), or by 2-amino-5-phosphonovalerate (APV) (100 µM) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (100 µM), EAA antagonists acting at NMDA or d ,l -α-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) receptors, respectively. These data suggest that acute stress normally has little effect on tyrosine hydroxylation in neostriatum due to the inhibitory influence of DA in the extracellular fluid. However, when that influence is absent (e.g., during extended inhibition of DOPA decarboxylation or blockade of DA receptors), stress increases tyrosine hydroxylation via EAAs acting on NMDA and AMPA receptors. Thus, EAAs released from corticostriatal projections may stimulate DA synthesis and thereby restore dopaminergic activity under conditions in which the availability of DA for release has been compromised. 相似文献
29.
Two Components of Glutamate Exocytosis Differentially Affected by Presynaptic Modulation 总被引:1,自引:1,他引:0
I. Herrero E. Castro M. T. Miras-Portugal J. Sánchez-Prieto 《Journal of neurochemistry》1996,67(6):2346-2354
Abstract: The total Ca2+ -dependent release of glutamate induced by depolarization of cerebrocortical nerve terminals with KCl was analyzed into a fast and a slow component. The fast component exhibited a decay time of <1 s and accounted for 0.95 ± 0.10 nmol of glutamate, whereas the slow component, which exhibited a decay time of 52 ± 7 s, accounted for the release of 2.48 ± 0.19 nmol of glutamate. These two components were differentially affected by the Ca2+ chelator BAPTA, the divalent cation Sr2+ , or the botulinum neurotoxin A. The adenosine A1 receptor agonist N 6 -cyclohexyladenosine strongly reduced the fast component without altering the slow component. In contrast, the inhibitory effect of arachidonic acid and the facilitatory action of the metabotropic glutamate receptor agonist (1 S ,3 R )-1-aminocyclopentane-1,3-dicarboxylic acid were observed as a decrease and an increase, respectively, in the two components. It is concluded, first, that the fast and slow components correspond to the release of docked and mobilized vesicles, respectively, and second, that presynaptic modulation more significantly alters the fast component of release. 相似文献
30.
Cyclic beta-(1,2)-glucan synthesis in Rhizobiaceae: roles of the 319-kilodalton protein intermediate. 总被引:1,自引:0,他引:1 下载免费PDF全文
Cyclic beta-(1,2)-glucans are synthesized by members of the Rhizobiaceae family through protein-linked oligosaccharides as intermediates. The protein moiety is a large inner membrane molecule of about 319 kDa. In Agrobacterium tumefaciens and in Rhizobium meliloti the protein is termed ChvB and NdvB, respectively. Inner membranes of R. meliloti 102F34 and A. tumefaciens A348 were first incubated with UDP-[14C]Glc and then solubilized with Triton X-100 and analyzed by polyacrylamide gel electrophoresis under native conditions. A radioactive band corresponding to the 319-kDa protein was detected in both bacteria. Triton-solubilized inner membranes of A. tumefaciens were submitted to native electrophoresis and then assayed for oligosaccharide-protein intermediate formation in situ by incubating the gel with UDP-[14C]Glc. A [14C]glucose-labeled protein with an electrophoretic mobility identical to that corresponding to the 319-kDa [14C]glucan protein intermediate was detected. In addition, protein-linked radioactivity was partially chased when the gel was incubated with unlabeled UDP-Glc. A heterogeneous family of cyclic beta-(1,2)-glucans was formed upon incubation of the gel portion containing the 319-kDa protein intermediate with UDP-[14C]Glc. A protein with an electrophoretic behavior similar to the 319-kDa protein intermediate was "in gel" labeled by using Triton-solubilized inner membranes of an A. tumefaciens exoC mutant, which contains a protein intermediate without nascent glucan. These results indicate that initiation (protein glucosylation), elongation, and cyclization were catalyzed in situ. Therefore, the three enzymatic activities detected in situ reside in a unique protein component (i.e., cyclic beta-(1,2)-glucan synthase). It is suggested that the protein component is the 319-kDa protein intermediate, which might catalyze the overall cyclic beta-(1,2)-glucan synthesis. 相似文献