全文获取类型
收费全文 | 2193篇 |
免费 | 90篇 |
国内免费 | 32篇 |
专业分类
2315篇 |
出版年
2023年 | 11篇 |
2022年 | 22篇 |
2021年 | 27篇 |
2020年 | 32篇 |
2019年 | 44篇 |
2018年 | 39篇 |
2017年 | 44篇 |
2016年 | 37篇 |
2015年 | 42篇 |
2014年 | 93篇 |
2013年 | 139篇 |
2012年 | 57篇 |
2011年 | 92篇 |
2010年 | 81篇 |
2009年 | 104篇 |
2008年 | 95篇 |
2007年 | 106篇 |
2006年 | 98篇 |
2005年 | 99篇 |
2004年 | 74篇 |
2003年 | 73篇 |
2002年 | 66篇 |
2001年 | 41篇 |
2000年 | 44篇 |
1999年 | 42篇 |
1998年 | 47篇 |
1997年 | 30篇 |
1996年 | 42篇 |
1995年 | 34篇 |
1994年 | 41篇 |
1993年 | 45篇 |
1992年 | 34篇 |
1991年 | 33篇 |
1990年 | 25篇 |
1989年 | 36篇 |
1988年 | 33篇 |
1987年 | 20篇 |
1986年 | 25篇 |
1985年 | 31篇 |
1984年 | 39篇 |
1983年 | 28篇 |
1982年 | 32篇 |
1981年 | 31篇 |
1980年 | 17篇 |
1979年 | 17篇 |
1978年 | 17篇 |
1977年 | 14篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1973年 | 13篇 |
排序方式: 共有2315条查询结果,搜索用时 15 毫秒
61.
Summary Apical membrane vesicles from human term placenta were isolated using a magnesium precipitation technique, and the purity of the vesicles was assessed morphologically using scanning and transmission electron microscopy, and biochemically, using marker enzymes. The vesicles were found to be morphologically intact and significantly enriched in enzymes associated with apical membranes.
36Cl– uptake into these vesicles was studied in the presence of an outwardly directed Cl– gradient. This uptake was found to be time dependent, with an initial rapid uptake tending to peak between 10 and 20 min and thereafter decline. Uptake was found to be voltage dependent since 5
m valinomycin caused a decrease in uptake. The effects of N-phenylanthranilic acid (NPA) and 4,4-diisothiocyanostilbene-2,2-disulphonic acid (DIDS) and bumetanide on the initial rate of Cl– were examined in the presence and absence of 5
m valinomycin. NPA and DIDS inhibited isotope uptake strongly with IC50 values of 0.83±0.35
m and 3.43±0.37
m, respectively, in the absence of valinomycin. Although valinomycin reduced 36Cl– uptake by about 80% when added before the isotope, DIDS reduced the uptake which remained in a concentration-dependent fashion with an IC50 of 5.6±2.1
m. Under these conditions, NPA was without effect at concentrations below 100
m. Bumetanide was without effect at the concentrations used in the absence of valinomycin. However, following valinomycin pretreatment, bumetanide reduced 36Cl– uptake significantly at 100
m concentration. Vesicle diameter, as assessed by flow cytometry, did not change under the conditions employed.The effects of some fatty acids were also investigated. Arachidonic acid and linoleic acid inhibited Cl– uptake with IC50 values of 37.6±14.9
m and 4.59±0.51
m, respectively. Arachidonyl alcohol and elaidic acid were found to be without effect. These studies show that human placental brush border membrane vesicles possess a chloride conductance channel, the activity of which can be measured in the presence of an outwardly directed Cl– gradient and this channel is sensitive to Cl– channel inhibitors, especially N-phenylanthranilic acid, and can be inhibited by unsaturated fatty acids such as arachidonic acid and linoleic acid.This work was supported in part by the Cystic Fibrosis Association of Ireland and Eolas, The Irish Science and Technology Agency. The technical assistance of Mr. Cormac O' Connell in the preparation of the electron micrographs and of Mr. Roddy Monks in the flow cytometric analysis is gratefully acknowledged. 相似文献
62.
S. E. Gabriel K. J. Racette K. J. Gaspar G. W. Forsyth 《The Journal of membrane biology》1992,129(3):323-328
Summary Antibody raised in mice was used in attempting to identify proteins responsible for the conductive chloride transport that can be measured in porcine ileal brush border membrane vesicles. Ileal brush-border membrane vesicle protein from pig was separated into five different molecular mass fractions by preparative SDS polyacrylamide disc gel electrophoresis. Separated protein fractions were used to immunize mice. Antibody was screened for reactivity with antigen by Western blotting, and for effects on conductive chloride transport in ileal brush border membrane vesicles. Immunization with brush-border protein from fraction I proteins (>110 kDa) produced polyclonal antisera which specifically inhibited the conductive component of chloride uptake by ileal brush border vesicle preparations. Western blotting of the antigen showed the presence of several protein species of molecular mass >100 kDa that were recognized by immune serum. Spleen cells from a mouse producing antiserum that inhibited conductive chloride transport were fused with a myeloma cell line. The resulting hybridoma colonies produced antibody that reacted with at least seven distinct protein bands by Western blot assay and inhibited chloride conductance in brush-border membrane vesicles. 相似文献
63.
Mario Díaz Antonio Lorenzo 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(2):189-196
Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I
sc), as well as in the mucosa-to-serosa (J
m-s
Na
) and net sodium flux (J
net
Na
). In AT tissues, aldosterone did not change net chloride flux (J
net
Cl
) but did so in CT tissues. Amiloride reduced the aldosterone-increased I
sc in AT and CT tissues, inhibited J
net
Na
in AT tissues and abolished it in CT colons. J
net
Cl
was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na+ absorption and totally inhibited Cl- absorption in the AT tissues, but did not change I
sc. However, in CT tissues neither Na+ nor Cl- transport were affected by DIDS. A good relationship between I
sc and J
m-s
Na
was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J
net
Na
and reduced I
sc to untreated control values. Addition of serosal ouabain abolished I
sc and Na+ absorption in AT and CT colons, but Cl- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT
acutely treated
- CT
chronically treated animals
- DIDS
4-4-diisothiocyanatostibene-2-2-disulfonic acid
- DMSO
dimethylsulphoxide
-
G
t
tissue conductance
-
I
sc
short circuit current
- PD
transepithelial potential difference
- SITS
4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid
- UC
untreated controls
Preliminary results of this paper were presented at the X
th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990 相似文献
64.
Isolation of a cloned DNA segment containing a ribosomal protein gene of Drosophila melanogaster 总被引:7,自引:0,他引:7
A Drosophila genomic DNA library in the vector Charon 4 was screened using cDNA derived from the small (6S-12S) poly(A)+ mRNA of 2-6-h-old Drosophila embryos. This fraction of mRNA is enriched for ribosomal protein-coding sequences. The selected recombinants were hybridized to total mRNA under conditions which allowed for isolation of homologous mRNAs. The mRNA from these RNA/DNA hybrids was eluted and translated in vitro. The translation products were analyzed by one- and two-dimensional electrophoresis with authentic ribosomal proteins as standards. One cloned DNA segment was found to contain a ribosomal protein gene, and a sequence which hybridizes strongly to at least 5 other ribosomal protein mRNAs. 相似文献
65.
66.
Summary To assess the mechanism(s) by which intraluminal chloride concentration is raised above equilibrium values, intracellular Cl– activity (
i
Cl
) was studied in the proximal tubule ofNecturus kidney. Paired measurements of cell membrane PD (V
BL) and Cl-selective electrode PD (V
BL
Cl
) were performed in single tubules, during reversible shifts of peritubular or luminal fluid composition. Steadystate
i
Cl
was estimated at 14.6±0.6 mmol/liter, a figure substantially higher than that predicted for passive distribution. To determine the site of the uphill Cl– transport into the cell, an inhibitor of anion transport (SITS) was added to the perfusion fluid. Introduction of SITS in peritubular perfusate decreased
i
Cl
, whereas addition of the drug in luminal fluid slightly increased
i
Cl
; both results are consistent with basolateral membrane uphill Cl– transport from interstitium to the cell. TMA+ for Na+ substitutions in either luminal or peritubular perfusate had no effect on
i
Cl
. Removal of bicarbonate from peritubular fluid, at constant pH (a situation increasing HCO
3
–
outflux), resulted in an increase of
i
Cl
, presumably related to enhanced Cl– cell influx: we infer that Cl– is exchanged against HCO
3
–
at the basolateral membrane. The following mechanism is suggested to account for the rise in luminal Cl– concentration above equilibrium values: intracellular CO2 hydration gives rise to cell HCO
3
–
concentrations above equilibrium. The passive exit of HCO
3
–
at the basolateral membrane energizes an uphill entry of Cl– into the cell. The resulting increase of
i
Cl
, above equilibrium, generates downhill Cl– diffusion from cell to lumen. As a result, luminal Cl– concentration also increases.C.N.R.S. Greco 24. Part of this work was presented at the 12th annual meeting of the American Society of Nephrology, Boston, Mass. (Edelman et al., 1979). 相似文献
67.
Philip L. Smith Stephanie A. Orellana Michael Field M.D. 《The Journal of membrane biology》1981,63(3):199-206
Summary Unidirectional fluxes of35SO4 across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO4 concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J
ms,J
sm) were 0.456 and 0.067 moles hr–1 cm–2, respectively.J
ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO4 transport (J
net) by reducingJ
ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO4 fluxes. Theophylline, a stimulus of Cl secretion, reducedJ
ms without affectingJ
sm, causing a 33% reduction inJ
net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ
net through both a decrease inJ
ms and an increase inJ
sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ
ms by 94%, nearly abolishingJ
net. SITS also decreasedJ
sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J
me) revealed the following: (1)J
me is a saturable function of medium concentration with aV
max of 0.94 moles hr–1 cm–2 and aK
1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ
me; (3) replacing all Cl with gluconate increasedJ
me by 40%; (4) serosal SITS had no effect onJ
me; and (5) stimuli of Cl secretion had no effect onJ
me or increased it slightly. Determination of cell SO4 with35SO4 indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO4 was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr–1 cm–2, neither addition of SO4 to the SO4-free medium nor addition of SITS to SO4-containing medium altered short-circuit current. The results suggest that (1) ileal SO4 absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO4 absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO4 fluxes, the mechanisms for their transcellular transports are under separate regulation. 相似文献
68.
The manipulation of polar head group composition of phospholipids in the wheat Miranovskaja 808 affects frost tolerance 总被引:1,自引:0,他引:1
Caryopses of the frost-resistant cultivar of the wheat Triticum aestivum L., Miranovskaja 808, were germinated and grown in the presence of various concentrations of choline chloride. Changes in the composition of leaf total phospholipids and leaf total fatty acids at two extreme temperatures (25°C and 2°C) as well as changes in frost resistance were followed. A choline chloride concentration-dependent accumulation of phosphatidyl choline was observed in the leaves. Seedlings grown at 2°C accumulated more phosphatidyl choline at each choline chloride concentration than those grown at 25°C. There was an inverse relationship between the contents of phosphatidyl choline and phosphatidic acid in the leaves. Neither the temperature nor choline chloride seemed to affect fatty-acid composition. Modification of polar-head group composition of phospholipids affected frost tolerance: Seedlings grown in the presence of 15 mM choline chloride at 25°C exhibited a freezing resistance equal to that of hardened controls. The data indicate that the polar-head group composition of membrane phospholipids in plants can be easily manipulated and point to the importance of phosphatidyl choline in cold adaptation processes. 相似文献
69.
Summary A cell line resistant to sodium chloride was selected from callus cultures of haploidDatura innoxia by cloning under selective pressure. Cells of the resistant cell line retained their resistance even after subculture in absence of NaCl. Plantlets could be regenerated from resistant cells in the presence as well as absence of NaCl. In contrast, regeneration of plantlets was not possible from normal cells in the presence of NaCl, although regeneration readily occurred in the absence of NaCl.To examine the stability of the resistance in the long-term, callus cultures were initiated in presence of NaCl from stem expiants of the differentiated plantlets. All expiants of plantlets derived from resistant cells showed callus formation. This callus, derived from resistant explants, retained the trait of resistance upon subculture. 相似文献
70.
William H. J. Douglas James A. McAteer Robert T. Dell'orco David Phelps 《In vitro cellular & developmental biology. Plant》1980,16(4):306-312
Summary A one-step vital stain is described for the macroscopic visualization of histotypic cell aggregates in fetal rat lung organotypic
cultures. Organotypic cultures are incubated in 0.05-0.1% 2,3,5′-triphenyl tetrazolium chloride (TTC) in culture medium (37°C).
Living cells reduce the tetrazole to a water-insoluble red colored formazan. Cell aggregates appear as densely stained foci
against the lighter background of the Gelfoam substrate. Stained cultures may be scanned macroscopically to determine the
degree of reaggregation and assess cell viability. Identification of aggregates by TTC staining improves the efficiency of
tissue processing for electron microscopy and does not alter the ultrastructural appearance of the cultured cells.
This work was funded in part by the United Cerebral Palsy Research and Educational Foundation, Inc. and the National Heart,
Lung, and Blood Institute (Grants 1ROHL19513 and 1 R01HL21008). 相似文献