全文获取类型
收费全文 | 463篇 |
免费 | 107篇 |
出版年
2022年 | 5篇 |
2021年 | 10篇 |
2018年 | 4篇 |
2017年 | 6篇 |
2016年 | 9篇 |
2015年 | 17篇 |
2014年 | 23篇 |
2013年 | 24篇 |
2012年 | 19篇 |
2011年 | 21篇 |
2010年 | 11篇 |
2009年 | 8篇 |
2008年 | 13篇 |
2007年 | 21篇 |
2006年 | 13篇 |
2005年 | 15篇 |
2004年 | 21篇 |
2003年 | 18篇 |
2002年 | 15篇 |
2001年 | 13篇 |
2000年 | 12篇 |
1999年 | 11篇 |
1998年 | 8篇 |
1996年 | 11篇 |
1995年 | 11篇 |
1993年 | 4篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 9篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1979年 | 7篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 11篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1972年 | 9篇 |
1971年 | 11篇 |
1970年 | 9篇 |
1969年 | 6篇 |
1968年 | 8篇 |
排序方式: 共有570条查询结果,搜索用时 62 毫秒
561.
562.
Timothy H. Goldsmith James S. Collins Dan L. Perlman 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1981,143(1):103-110
Summary Free-flying black-chinned hummingbirds (Archilochus alexandri) at a site in southeastern Arizona were attracted to artificial feeders displaying narrow spectral bands of light (7 nm half band width). The birds were taught to discriminate between pairs of wavelengths of approximately equal brightness but with a spectral separation of 10 nm. After training, performance of the birds was not significantly changed by alterations in the relative intensities of the two lights. Moreover, when the spectral composition of the test and training lights was made identical, the birds did not learn to make a discrimination on the basis of intensity differences of 0.5 or 1 log unit. In the learned foraging behavior of these hummingbirds, the salience of brightness is therefore inconsequential relative to hue.Discrimination scores for a constant 10 nm separation of test and training wavelengths were determined between 410 and 650 nm. This measure of the spectral dependence of wavelength discrimination shows a deterioration of performance at the red end of the spectrum but not in the blue and violet. Moreover, the minima at 585 and 555 nm indicate more structure than is present in this region of the spectrum in the human hue discrimination curve, and are similar but not identical to data on pigeon. These results are consistent with a growing body of evidence suggesting that the color space of birds may be more than three dimensional.This work was supported by NIH grants EY03266 and EY00222. We are indebted to Sally and Walter Spofford, who generously allowed us to work at their home, Aguila-Rancho, during May and June of 1980, and without whose kind help these experiments could not have been performed. 相似文献
563.
Bruce T. Liang Karen K. Vaccaro Bernyce A. Perelle Robert L. Perlman 《Journal of neurochemistry》1981,37(5):1164-1169
Abstract— The activity of dihydropteridine reductase (DPR) in pheochromocytoma cells has been studied. The activity of this enzyme in crude extracts of pheochromocytoma cells is approximately 50 nmol/min/mg protein. This activity is very much greater than the activity of tyrosine 3-monooxygenase (TH) in these extracts and the rate of conversion of tyrosine to DOPA in intact pheochromocytoma cells. Incubation of the cells with 56 m m -K+ or with cholera toxin has previously been shown to increase the rate of catecholamine synthesis and to cause a stable activation of TH in the cells. These treatments do not produce a stable activation of DPR, as assayed in vitro. Methotrexate inhibits DPR activity in vitro with an I50 of approximately 20 μ m , but has no effect on the rate of DOPA formation in intact pheochromocytoma cells. Therefore, DPR does not appear to be the rate-limiting enzyme in the pathway of catecholamine synthesis in pheochromocytoma cells. Moreover, the activities of DPR and of TH are not regulated coordinately in these cells. 相似文献
564.
Chemical synthesis of (24R)-24,25-dihydroxy-[26,27-3H]vitamin D3, and its 24-epimer has been devised that allows introduction of 3H at the terminal step of the synthesis. The epimeric mixture is derivatized as the tris(trimethylsilyl) ethers and resolved by high-performance liquid chromatography. The product has a specific activity of 178 Ci/mmol and is fully active in binding to the rat plasma vitamin D binding protein and in the elevation of serum calcium levels of vitamin D deficient rats. The synthesis begins with the readily available 3 beta-hydroxy-5-cholenic acid methyl ester and involves a Pummerer rearrangement, introduction of the delta 7, irradiation, and isolation of the 26,27-dinor-25-carboxylic acid methyl ester of vitamin D3. This compound is then treated with a Grignard reagent containing 3H (80 +/- 10 Ci/mmol). 相似文献
565.
566.
Summary
L-Sorbose was converted to 2-keto-L-gulonic acid by mixtures ofGluconobacter melanogenus IFO 3293 andPseudomonas syringae NRRL B-865 entrapped simultaneously in polyacrylamide gel. Since the temperature optima of both enzymatic reactions involved differed, a two-stage process with cells immobilized separately seems to offer a more efficient method to prepare 2-keto-L-gulonic acid.For preceeding paper in this series see Martin & Perlman (1976). 相似文献
567.
Radioactive labeling experiments have shown that radioactivity from L -leucine-U-14C is incorporated into the L-N, β-dimethylleucine residue of the antibiotic etamycin, and that the N-methyl and β-methyl groups of this latter amino acid appear to be derived from L -methionine. 相似文献
568.
Z Hochberg R Perlman A Benderli J M Brandes 《Biochemical and biophysical research communications》1982,108(1):102-109
Human trophoblasts were isolated in a monolayer cell culture and the effect of extra-and intracellular glucopenia on estradiol secretion was studied. Term placentas were dispersed by repeated short term trypsinization and 2×106 cells plated in each dish. The de novo synthesis of estradiol was demonstrated by a 10 fold increase in estradiol secretion by supplementation of androstendione. Incubation with dibutyryl cyclic AMP produced a dose dependent increase in estradiol secretion. Low glucose (10 mg/dl) medium enhanced estradiol secretion when compared to a medium containing 50–100 mg/dl glucose. Intracellular glucopenia by 2-deoxyglucose produced an increase in estradiol secretion. The results indicate negative dependency of estradiol secretion by the trophoblast on intracellular glucose. 相似文献
569.
Abstract— Suspensions of isolated adrenal cells were prepared by digesting hamster adrenal glands with collagenase, and the secretion of catecholamine from these cells was studied. Acetylcholine (ACh) produces a dose-dependent increase in catecholamine secretion; half-maximal secretion is produced by 3 μm -ACh, and maximal secretion by 100 μm -ACh. The cholinergic receptor in these cells appears to be nicotinic, since catecholamine secretion is stimulated by the nicotinic agonists nicotine and dimeth-ylphenylpiperaziniurn, but not by the muscarinic agonists pilocarpine or oxotremorine. ACh-induced catecholamine secretion is inhibited by hexamethonium, tubocurarine, and atropine, but is not inhibited by α-bungarotoxin. ACh-induced catecholamine secretion is dependent upon the presence of extracellular Ca2+, and appears to occur by exocytosis, since the release of catecholamine is accompanied by the release of dopamine β-monooxygenase, but not of lactate dehydrogenase. These biochemical studies complement the morphological evidence for exocytosis in hamster adrenal glands, and indicate that catecholamine secretion from hamster chromaffin cells is similar to that from chromaffin cells of other species. 相似文献
570.
Estimation of the cytoplasmic catecholamine concentrations in pheochromocytoma cells 总被引:1,自引:0,他引:1
Pheochromocytoma cells contain amine oxidase (flavin-containing), and convert dopamine and norepinephrine to deaminated metabolites. Dihydroxyphenylacetic acid is the major dopamine metabolite produced by the cells, whereas dihydroxyphenylglycol is the predominant metabolite of norepinephrine. Cells incubated under control conditions produce deaminated dopamine metabolites at a rate of about 30 pmol/min per mg protein, and dihydroxyphenylglycol at a rate of approx. 10 pmol/min per mg protein. Activation of tyrosine 3-monooxygenase increases the formation of dihydroxyphenylacetic acid, but does not greatly affect the production of dihydroxyphenylglycol. Inhibition of aromatic-L-amino-acid decarboxylase decreases the production of dihydroxyphenylacetic acid, but does not alter the production of dihydroxyphenylglycol. These results are consistent with the idea that newly synthesized dopamine represents the major source of cytoplasmic dopamine, whereas cytoplasmic norepinephrine is derived largely from catecholamine stores in secretory vesicles. The concentrations of dopamine and of norepinephrine in the cytoplasm of pheochromocytoma cells were estimated by measuring the substrate dependence of amine oxidase activity in extracts of these cells. By this method, the cytoplasmic concentrations of dopamine and of norepinephrine were estimated to be in the range of 0.5 to 1 microM. Incubation of the cells with extracellular norepinephrine or with reserpine results in an increase in the production of dihydroxyphenylglycol, and in inhibition of tyrosine 3-monoxygenase activity. Both of these effects are presumably mediated by a rise in the cytoplasmic norepinephrine concentration. Analysis of the relationship between norepinephrine metabolism and tyrosine 3-monooxygenase activity indicates that the apparent Ki of this enzyme for norepinephrine in intact cells is 10-15-times the basal cytoplasmic concentration of norepinephrine, or approx. 10 microM. 相似文献