全文获取类型
收费全文 | 4270篇 |
免费 | 359篇 |
国内免费 | 1篇 |
出版年
2023年 | 36篇 |
2022年 | 41篇 |
2021年 | 140篇 |
2020年 | 107篇 |
2019年 | 112篇 |
2018年 | 115篇 |
2017年 | 113篇 |
2016年 | 133篇 |
2015年 | 221篇 |
2014年 | 211篇 |
2013年 | 291篇 |
2012年 | 316篇 |
2011年 | 299篇 |
2010年 | 237篇 |
2009年 | 185篇 |
2008年 | 179篇 |
2007年 | 168篇 |
2006年 | 143篇 |
2005年 | 128篇 |
2004年 | 124篇 |
2003年 | 105篇 |
2002年 | 83篇 |
2001年 | 63篇 |
2000年 | 86篇 |
1999年 | 41篇 |
1998年 | 23篇 |
1997年 | 35篇 |
1996年 | 34篇 |
1995年 | 25篇 |
1994年 | 32篇 |
1993年 | 25篇 |
1992年 | 51篇 |
1991年 | 38篇 |
1990年 | 44篇 |
1989年 | 31篇 |
1988年 | 27篇 |
1987年 | 46篇 |
1986年 | 41篇 |
1985年 | 35篇 |
1984年 | 31篇 |
1983年 | 23篇 |
1982年 | 30篇 |
1981年 | 24篇 |
1979年 | 29篇 |
1978年 | 21篇 |
1977年 | 25篇 |
1975年 | 18篇 |
1974年 | 17篇 |
1973年 | 20篇 |
1972年 | 18篇 |
排序方式: 共有4630条查询结果,搜索用时 15 毫秒
991.
David J. Smith Jan A. Redick Dianna E. Van Orden Lucas S. Van Orden III 《The Histochemical journal》1973,5(3):255-264
Synopsis An indirect immunocytochemical technique (Nakane, 1970) was employed to localize the soluble proteins purified from lysates of catecholamine (CA)-containing vesicles of the bovine adrenal medulla. Antiserum to the proteins, produced in rabbits, was used for incubation of sections of bovine adrenal tissue prepared by fixation in glutaraldehyde and embedding in serum albumin (McLean & Singer, 1970). The site of the antigen-antibody complex was visualized by incubating the sections with anti-rabbit -globulin (goat) conjugated to peroxidase, followed by the deposition of electronopaque reaction product generated by the enzyme. The reaction product, also visible at the level of the light microscope, appeared to have a distribution similar to that of the CA-storage vesicles. Electron microscopic examination revealed that nearly all the reaction product was deposited over the electron-opaque core of the vesicles. The intravesicular localization is consistent with the proposal that these proteins exist primarily in the CA-containing granular vesicle and may function to stabilize the CA-storage complex. 相似文献
992.
ONO 453, a synthetic analogue of prostaglandin F2α (PGF2α) is a potent luteolytic agent in cycling ewes when given as a single intramuscular injection. The drug was effective in doses of 2 mg when administered after day 3 of the oestrous cycle. It is well tolerated by ewes, producing no apparent signs of toxicity at 5 mg and only a mild transient increase in the respiratory rate at 10 mg. To synchronise oestrus two dosing regimens were examined; a single i.m. dose of 2 mg administered without reference to the day of the oestrous cycle, and 2 injections of 2 mg administered 7 days apart. With the first method 86.6 per cent of the ewes were in oestrus within 24–50 hours of treatment, with the second, 82 per cent were in oestrus within 30–54 hours. To test the fertility of the oestrus following ONO 453-induced luteolysis, both groups of ewes were run with fertile rams and 86 per cent and 70.8 per cent of those induced by the single or double injection respectively, conceived and lambed. 相似文献
993.
Lymphotoxin (LT) production, interferon (IF) production, and DNA synthesis were investigated after mitogen stimulation and in the mixed lymphocyte culture (MLC) reaction using human tonsil lymphocytes. Both LT and IF were assayed in parallel and from the same lymphocyte supernatants. An analysis of the PHA, PWM, one-way and two-way MLC reactions showed that the amounts of LT and IF produced could not be correlated. Polyriboinosinic: polyribocytidylic acid (poly(I: C)) failed to induce either LT production or [3H]TdR incorporation but did induce IF production. Removal of glass-adherent cells (GAC) had no effect on mitogen induced LT production but their removal reduced LT production in MLC reactions. GAC were necessary for IF production and optimal [3H]TdR incorporation in both mitogen stimulated cultures and in MLC reactions. IF and LT activities were shown to be the result of different molecules by using a Sephadex G-75 column. These results indicate that mitogen stimulation differs from MLC reactions in the cell type or control mechanisms involved for LT production, and that in mitogen stimulated cultures all three of these in vitro phenomena are probably the results of either different cell types or of different cell to cell interactions. 相似文献
994.
Insulin-like growth factor I and epidermal growth factor regulate the expression of transferrin receptors at the cell surface by distinct mechanisms 总被引:10,自引:0,他引:10
R J Davis M Faucher L K Racaniello A Carruthers M P Czech 《The Journal of biological chemistry》1987,262(27):13126-13134
The transferrin receptor cycles rapidly between cell surface and endosomal membrane compartments. Treatment of cultured cells with epidermal growth factor (EGF) or insulin-like growth factor I (IGF-I) at 37 degrees C causes a rapid redistribution of transferrin receptors from an intracellular compartment to the cell surface. The effects of EGF and IGF-I on the kinetics of the cycling of the transferrin receptor in A431 human epidermoid carcinoma cells were compared. The primary site of EGF action was found to be an increase in the rate of transferrin receptor exocytosis. The exocytotic rate constant was measured to be 0.11 min-1 in control cells and 0.33 min-1 in EGF-treated cells. In contrast, IGF-I was found to increase the cell surface expression of transferrin receptors by causing a small increase in the rate of exocytosis (from 0.11 to 0.17 min-1) and a decrease in the rate of endocytosis (from 0.33 to 0.24 min-1). It is concluded that the mechanisms for EGF and IGF-I action to increase the cell surface expression of the transferrin receptor are distinct. A kinetic model of the cycling of the transferrin receptor based on experimentally determined rate constants is presented. The model predicts that a consequence of IGF-I action on transferrin receptor cycling is to decrease the apparent Km for the uptake of diferric transferrin by cells. This prediction is confirmed by direct measurement of the accumulation of 59Fe-labeled diferric transferrin by A431 cells. These data demonstrate that the accumulation of iron by cultured cells is a complex function of the rate of cycling of the transferrin receptor and that this process is under acute regulation by growth factors. 相似文献
995.
Photorespiration and Internal CO(2) Accumulation in Chara corallina as Inferred from the Influence of DIC and O(2) on Photosynthesis 下载免费PDF全文
An O2 electrode system with a specially designed chamber for `whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O2 concentrations on photosynthetic O2 evolution. At pH = 5.5 and 20% O2, cells grown in HCO3− medium (low CO2, pH ≥ 9.0) exhibited a higher affinity for external CO2 (K½(CO2) = 40 ± 6 micromolar) than the cells grown for at least 24 hours in high-CO2 medium (pH = 6.5), (K½(CO2) = 94 ± 16 micromolar). With O2 ≤ 2% in contrast, both types of cells showed a high apparent affinity (K½(CO2) = 50 − 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO2 medium (pH = 6.5). The high-pH, HCO3−-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO2 which exceeded the CO2 externally supplied, and the reverse situation has been observed in high-CO2-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO3− (K½[HCO3−]) was 0.6 ± 0.2 millimolar, at 20% O2. But under low O2 concentrations (≤2%), surprisingly, an inhibition of net O2 evolution was elicited, which was maximal at low HCO3− concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO2 medium, (b) Chara cells are able to accumulate CO2 internally by means of a process apparently independent of the plasmalemma HCO3− transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO3−: pseudocyclic electron flow, sustained by O2 photoreduction, may produce the additional ATP needed for the HCO3− transport. 相似文献
996.
The transforming growth factor-beta system, a complex pattern of cross-reactive ligands and receptors 总被引:65,自引:0,他引:65
S Cheifetz J A Weatherbee M L Tsang J K Anderson J E Mole R Lucas J Massagué 《Cell》1987,48(3):409-415
A new homodimer form of transforming growth factor-beta (TGF-beta), TGF-beta 2, has been identified in porcine blood platelets. TGF-beta 2 is homologous to ordinary TGF-beta (TGF-beta 1), which is also present in platelets. TGF-beta 1.2, a heterodimer containing one TGF-beta 1 chain and one TGF-beta 2 chain, has also been isolated. TGF-beta 1 and TGF-beta 2 interact differently with a family of receptors in target cells. A 280 kd receptor displays high affinity for both TGF-beta 1 and TGF-beta 2. Occupancy of this receptor by TGF-beta 1 or TGF-beta 2 correlates with the ability of these TGF-beta s to inhibit cell proliferation. In contrast, 65 kd and 85 kd receptors have high affinity for TGF-beta 1 but lower affinity for TGF-beta 2. The existence of distinct forms of TGF-beta that interact differently with a family of TGF-beta receptors could provide flexibility to the regulation of tissue growth and differentiation by the TGF-beta system. 相似文献
997.
The insulin-like growth factor II receptor is phosphorylated by a tyrosine kinase in adipocyte plasma membranes 总被引:5,自引:0,他引:5
S Corvera R E Whitehead C Mottola M P Czech 《The Journal of biological chemistry》1986,261(17):7675-7679
Incorporation of 32P from [gamma-32P]ATP into tyrosine residues of the insulin-like growth factor (IGF)-II receptor was observed in a Triton X-100-insoluble fraction of rat adipocyte plasma membranes. IGF-II receptor phosphorylation proceeded to a stoichiometry of approximately 0.5 mol of phosphate/IGF-II binding site after 10 min of incubation at 4 degrees C. A Km for ATP of 6 microM was calculated for this phosphorylation reaction. Addition of IGF-II caused an approximately 2-fold increase in tyrosine phosphorylation of the IGF-II receptor in this preparation. In contrast, phosphorylation of angiotensin II by the Triton X-100 washed membranes was not stimulated by IGF-II. Incubation of purified receptor immobilized on IGF-II agarose or of receptor-enriched low density microsomal membranes with [gamma-32P]ATP did not result in appreciable incorporation of [32P]phosphate into the IGF-II receptor nor into exogenous substrates. These data suggest that the IGF-II receptor is not a tyrosine protein kinase capable of autophosphorylation but that it is a substrate for a tyrosine protein kinase endogenous to the adipocyte plasma membrane. The stimulatory effect of IGF-II on the tyrosine phosphorylation of its receptor may be due to a conformational change which converts the receptor to a better substrate for this tyrosine kinase. 相似文献
998.
The influence of the type of immunosorbent on rabies antibody EIA; advantages of purified glycoprotein over whole virus 总被引:2,自引:0,他引:2
P Perrin P Versmisse J F Delagneau G Lucas P E Rollin P Sureau 《Journal of biological standardization》1986,14(2):95-102
Two types of in vitro assay (enzyme immunoassay and sero-neutralization test) for the titration of rabies antibodies were used to assay sera from mice and humans immunized with cell culture vaccines or neural tissue vaccines. Enzyme immunoassays (EIA) were performed in plates sensitized with whole virus, purified glycoprotein or purified nucleocapsid. Neutralizing antibody titres were determined by the rapid fluorescent focus inhibition test (REFIT) and by an in vitro seroneutralization test including a rapid enzyme immunotitration of intracellular antigens (REITICA). The results obtained with sera of immunized mice and humans showed that (1) cell culture vaccines mainly induced the synthesis of antiglycoprotein neutralizing antibodies; and (2) neural tissue vaccines induced a high synthesis of antinucleocapsid non-neutralizing antibodies and a more or less important synthesis of antiglycoprotein antibodies depending on the origin of the tissue used for their preparation. Consequently, it was emphasized that when using EIA, the antibody titration must be run in glycoprotein-coated plates rather than in whole virus-coated plates to appreciate correctly the immunizing potency of a rabies vaccine, especially neural tissue vaccine. 相似文献
999.
The theory of ion transport in the vicinity of a vibrating probe is developed. It is shown that the convection loops produced by the probe will not affect the electrical current density, assuming that the action of the probe does not affect the sources of the current in the biological system. However, the convection loops will significantly alter the ion concentration gradients in the unstirred layer near a tissue or cell surface. The concentration gradients within each convection loop will be reduced, while the concentration gradients between the loops and outside of the loops will be increased relative to the gradients existing without the probe. As a consequence, the electrical potential gradients can be changed relative to the potential gradients existing in the absence of the convection caused by the probe. If the mobility of the ion species carrying the electrical current is greater than the average ion mobility in the medium, then a decrease in ion concentration gradient will be accompanied by an increase in electrical potential gradient, while an increase in concentration gradient will be accompanied by a decrease or even a reversal of electrical potential gradient. Thus, the electrical potential gradient measured by the probe will depend on the concentration gradient in the vicinity of the probe, which will depend in turn on the spatial relation of the convection loops to the probe. An example of the effect of the convection loops on ion concentration and electrical potential is obtained from the theory via a numerical computer calculation. Experimental tests of this theory are discussed. 相似文献
1000.
[14C]Formate is incorporated into the C-2 of the pyrimidine moiety of thiamin by Escherichia coli and Salmonella typhimurium. In Saccharomyces cerevisiae, it is incorporated into C-4. Radioactive carbons of [1-14C]glycine and [2-14C]glycine are incorporated by S. typhimurium into the C-4 and C-6 of the pyrimidine, respectively, but not by S. cerevisiae. These facts suggest that procaryotes and eucaryotes have different biosynthetic pathways for pyrimidine. In this study, the procaryotes tested incorporated [14C]formate into the C-2 and the eucaryotes incorporated it into the C-4 of the pyrimidine. 相似文献