全文获取类型
收费全文 | 76423篇 |
免费 | 3180篇 |
国内免费 | 3196篇 |
出版年
2023年 | 658篇 |
2022年 | 1111篇 |
2021年 | 2447篇 |
2020年 | 1318篇 |
2019年 | 1875篇 |
2018年 | 1516篇 |
2017年 | 1214篇 |
2016年 | 1849篇 |
2015年 | 3689篇 |
2014年 | 7000篇 |
2013年 | 6824篇 |
2012年 | 5158篇 |
2011年 | 5991篇 |
2010年 | 4338篇 |
2009年 | 3820篇 |
2008年 | 3788篇 |
2007年 | 4156篇 |
2006年 | 2814篇 |
2005年 | 2431篇 |
2004年 | 1592篇 |
2003年 | 1335篇 |
2002年 | 1212篇 |
2001年 | 848篇 |
2000年 | 776篇 |
1999年 | 811篇 |
1998年 | 666篇 |
1997年 | 575篇 |
1996年 | 586篇 |
1995年 | 740篇 |
1994年 | 639篇 |
1993年 | 664篇 |
1992年 | 611篇 |
1991年 | 594篇 |
1990年 | 490篇 |
1989年 | 488篇 |
1988年 | 496篇 |
1987年 | 413篇 |
1986年 | 372篇 |
1985年 | 607篇 |
1984年 | 949篇 |
1983年 | 592篇 |
1982年 | 776篇 |
1981年 | 753篇 |
1980年 | 546篇 |
1979年 | 553篇 |
1978年 | 350篇 |
1977年 | 342篇 |
1976年 | 324篇 |
1974年 | 234篇 |
1973年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 646 毫秒
991.
M. V. Wright N. Elwess J. van Houten 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(4):288-296
Intracellular Ca2+ levels in Paramecium must be tightly controlled, yet little is understood about the mechanisms of control. We describe here indirect evidence that a phosphoenzyme intermediate is the calmodulin-regulated plasma membrane Ca2+ pump and that a Ca2+-ATPase activity in pellicles (the complex of cell body surface membranes) is the enzyme correlate of the plasma membrane pump protein. A change in Ca2+ pump activity has been implicated in the chemoresponse of paramecia to some attractant stimuli. Indirect support for this is demonstrated using mutants with different modifications of calmodulin to correlate defects in chemoresponse with altered Ca2+ homeostasis and pump activity.Abbreviations EGTA
ethyleneglycol tetra-acetate
- ER
endoplasmic reticulum
- IBMX
isobutyl methylxanthine
-
I
che
index of chemokinesis
- Mops
3-[N-morpholino] propanesulfonic acid
- PEI
phosphoenzyme intermediate
- STEN
sucrose, TRIS, EDTA, sodium chloride
- TCA
trichloroacetic acid
- TRIS
tris[hydroxymethyl] aminomethane 相似文献
992.
H. McLean S. Caveney 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(4):297-306
It is proposed that the activity of an epidermal cotransport system for Na+ and dicarboxylic amino acids accounts for the small amounts of L-glutamate and L-aspartate in the otherwise amino-acid-rich blood plasma of insects. This Na+-dependent transport system is responsible for more than 95% of the uptake of these amino acids into the larval epidermis of the beetle Tenebrio molitor. Kinetic analysis of uptake showed that the Na+-dependent co-transporter has medium affinity for L-glutamate and L-aspartate. The K
m for L-glutamate uptake was 146 mol·l-1, and the maximum velocity of uptake (V
max) was 12.1 pmol·mm-2 of epidermal sheet per minute. The corresponding values for L-aspartate were 191 mol·l-1 and 8.4 pmol·mm-2·min-1. The Na+/L-glutamate co-transporter has a stoichiometry of at least two Na+ ions for each L-glutamate-ion transported (n=217). The co-transporter has an affinity for Na+ equivalent to a K
m of 21 mmol · l-1 Na+. Na+ is the only external ion apparently required to drive L-glutamate uptake. Li+ substitutes weakly for Na+. Removal of external K+ or addition of ouabain decreases uptake slowly over 1 h, suggesting that these treatments dissipate the Na+/K+ gradient by inhibiting epidermal Na+/K+ ATPase. Several structural analogues of L-glutamate inhibit the medium-affinity uptake of L-glutamate. The order of potency with which these competitive inhibitors block glutamate uptake is L-cysteatethreo-3-hydroxy-Dl-aspartate >
D-aspartateL-aspartate>
L-cysteine sulphinate >
L-homocysteateD-glutamate. L-trans-Pyrrolidine-2,4-dicarboxylate, a potent inhibitor of L-glutamate uptake in mammalian synaptosomes, is a relatively weak blocker of epidermal uptake. The epidermis takes up substantially more L-glutamate by this Na+-dependent system than tissues such as skeletal muscle and ventral nerve cord. The epidermis may be a main site regulating blood L-glutamate levels in insects with high blood [Na+]. Because L-glutamate and L-aspartate stimulate skeletal muscle in insects, a likely role for epidermal L-glutamate/L-aspartate transporter is to keep the level of these excitatory amino acids in the blood below the postsynaptic activation thresholds.Abbreviation ac
acetate
- Ch
choline
- CNS
central nervous system
- cpm
counts per minute
- CDTA
trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetic acids
- HPLC
high performance liquid chromatography
-
K
m
Michaelis constant
-
n
app
apparent number
- NMG
N-methyl-D-glucamine
- Pipes
Piperazine-N,N-bis-[2-ethanesulfonic acid]
- SD
standard deviation
- TEA
tetraethyl-ammonium
-
V
velocity of uptake
-
V
max
maximum velocity of uptake 相似文献
993.
K. Schwalme W. C. Mackay M. T. Clandinin 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(4):277-287
Seasonal changes in the fatty acid composition of neutral and polar lipids were measured in the ovary, liver, white muscle, and adipopancreatic tissue of northern pike. The role of environmental and physiological factors underlying these changes was evaluated. From late summer (August–September) to winter (January–March), the weight percentage of n-3 polyunsaturated fatty acids (especially 22:6n3) declined significantly in the neutral lipids of all somatic tissues examined. However, large quantities of n-3 polyunsaturated fatty acids accumulated in the recrude cing ovaries during fall and the weight percentage of n-3 polyunsaturated fatty acids in ovary polar lipids also increased significantly. Additionally, the n-3 polyunsaturated fatty acid content of somatic polar lipids increased significantly during fall due to increases in the total polar lipid content of the somatic tissues. This suggests that during fall n-3 polyunsaturated fatty acid are diverted away from somatic neutral lipids and thereby conserved for use in ovary construction and for incorporation into tissue polar lipids. The percentage of n-3 polyunsaturated fatty acid in ovary neutral lipids also declined during fall and early winter, perhaps as an adaptation to conserve these fatty acids for storage in oocyte polar lipids and later incorporation into cellular membranes of the developing embryo. Reductions in the n-3 polyunsaturated fatty acids content of somatic and ovarian neutral lipids during fall were compensated for specifically by increases in the percentage of monounsaturated fatty acids rather than saturated fatty acids. This suggests that the ratio of saturated to unsaturated fatty acids in pike neutral lipid, is regulated physiologically, and hence may influence the physiological functioning of these lipids. During fall and early winter the percentage of saturated fatty acids declined significantly in the polar lipids of all tissues examined. This change was consistent with the known effects of cold acclimation on the fatty acid composition of cellular membranes. As the ovaries were recrudescing from September to January, liver polar lipids exhibited significant decreases in the percentage of total polyunsaturated fatty acids and n-3 polyunsaturated fatty acids and increases in monounsaturated fatty acids, and acquired a fatty acid composition very similar to that of ovary polar lipids. Therefore, seasonal changes in the percentage of polyunsaturated and monounsaturated fatty acids in liver polar lipids probably reflect the liver's role in vitellogenesis rather than the effects of temperature on membrane fatty acid composition. At all times of year, the fatty acid compositions of white muscle and adipopancreatic tissue neutral lipids were very similar, which may indicate a close metabolic relationship between these lipid compartments.Abbreviations AP
adipopancreatic
- BHT
butylated hydroxytoluene
- CI
confidence interval
- EFA
essential fatty acids
- MUFA
monounsaturated fatty acids
- NL
neutral lipids
- PL
polar lipids
- PUFA
polyunsaturated fatty acids
- SFA
saturated fatty acids 相似文献
994.
E. A. Santos R. Keller 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(5):374-379
The effects of glutamate, aspartate, glycine, proline, alanine, taurine, glycerol, glucose and lactate injections on the haemolymph levels of the crustancean hyperglycemic hormone and/or glucose and lactate in the shore crab, Carcinus maenas, were investigated. Only glucose and lactate caused significant changes of hyperglycaemic hormone levels. Glucose injections resulted in a drop of both hormone and lactate, while lactate had an opposite effect, i.e. it raised both crustacean hormone and glucose levels. The results suggest that during increases in glycolytic flux, lactate may cause a release of hormone by a positive feedback mechanism. The hormone would then stimulate glycogenolysis, thus increasing glucose availability. If more glucose is released than is metabolized, excess glucose may leak from the cells and suppress crustancean hyperglycemic hormone release from the X-organ/sinus gland complex by negative feedback.Abbreviations ABTS
2,2-azino-bis (3-ethylbenzthiazoline sulphonic acid)
- ANOVA
one-way analysis of variance
- BSA
bovine serum albumin
- BW
body weight
- CHH
crustacean hyperglycemic hormone
- ELISA
cnzyme-liked immunosorbent assay
- GIH
gonadinhibiting hormone
- IgG
immunoglobin G
- MIH
moult-inhibiting hormone
- MTGXO
medulla terminalis X-organ
- PB
sodium phosphate buffer
- PBS
phosphate buffered saline
- Pi
inorganic phosphate
- XO-SG
X-organ-sinus gland complex 相似文献
995.
P. K. O'Connor B. Reich M. A. Sheridan 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(5):427-431
Rainbow trout were used to characterize the direct influence of growth hormone on hepatic lipid mobilization in vitro. Liver was removed from fish fasted 72 h, sliced into 1-mm3 pieces and incubated in Hanks' medium containing ovine or salmon growth hormone (0.28 ng·ml-1–28 g·ml-1). Lipid mobilization, resulting from triacylglycerol hydrolysis, was evaluated by fatty acid and glycerol release into culture medium. Control rates of fatty acid release and glycerol release were 0.95±0.16 (mean ± SE) and 0.88±0.28 mol·l-1·mg fresh weight, respectively. Both ovine growth hormone (28 ng·ml-1) and salmon growth hormone (28 ng·ml-1) stimulated fatty acid release into culture medium, increasing rates by 112% and 97%, respectively, during the course of a 24-h incubation. Glycerol release was similarly augmented by growth hormone treatment. Growth hormone-stimulated metabolite release became evident within 12 h and persisted for up to 72 h. The presence of amino acids in the culture medium potentiated the lipolytic action of growth hormone. Ovine growth hormone (28 ng·ml-1) in the presence of amino acids stimulated a 53% increase in fatty acid, and a 108% increase in glycerol, release over rates observed in the absence of amino acids. Salmon growth hormone (28 ng·ml-1) in the presence of amino acids stimulated a 53% increase in fatty acid, and a 44% increase in glycerol, release over rates observed in the absence of amino acids. Ovine growth hormone (28 ng·ml-1) also stimulated gluconeogenesis, as indicated by a 75% increase in phosphoenolpyruvate carboxykinase activity in liver pieces incubated in the presence of amino acids. These results indicate that growth hormone directly stimulates lipid breakdown in the liver of trout and that amino acids potentiate growth hormone-stimulated lipolysis.Abbreviations AA
amino acid(s)
- dGDP
deoxy-guanosine diphosphate
-
ED
50
50% effective dose
- FA
fatty acid(s)
- fw
fesh weight
- GH
growth hormone
- Hepes
4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid
- MS-222
tricaine methanesulfonate
- MEM
minimum essential medium
- oGH
ovine growth hormone
- PEPCK
phosphoenolpyruvate carboxykinase
- PKc
protein kinase C
- rpm
revolutions per minute
- sGH
salmon growth hormone
- TG
triacylglycerol
- w/v
weight per volume
This paper was presented in abstract form at the Annual Meeting of the American Society of Zoologists, Dec. 26–30, 1991, Atlanta, GA 相似文献
996.
K. Kikuchi S. Watabe Y. Suzuki K. Aida H. Nakajima 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(5):349-354
Goldfish Carassius auratus were acclimated to either 10 or 30°C for a minimum of 5 weeks. A 65-kDa protein specific to warm-temperature-acclimated fish was extracted from the gel with 70% formic acid after two-dimensional electrophoresis of the muscle cytoplasmic protein fraction. The 65-kDa protein thus prepared to homogeneity was used to raise specific antibodies in rabbit by conventional methods. The antibody produced exhibited specific reaction with a protein having the same molecular weight from brain and liver tissue, suggesting that the 65-kDa protein is a ubiquitous cytosolic component in warm-acclimated goldfish. When water temperature was increased from 20 to 30°C over a 20-h period, a prominent amount of the 65-kDa protein was observed in muscle tissue extracts within 5 days of additional rearing; this was demonstrated by immunoblotting with the specific antibody. The N-terminal amino acid sequence of the 65-kDa protein was determined as Asp-Glu-Pro-Gln-Gly-His-Gln-His (or Asp)-Glu-Leu, differing from that of a family of known heat-shock proteins having about 70 kDa in molecular mass (hsp 70). No interaction between ATP and the 65-kDa protein revealed by ATP-agarose affinity chromatography further confirmed the different properties of the 65-kDa protein from those of hsp 70.Abbreviations ATP
adenosine 5-triphosphate
- hsp
heat-shock protein(s)
- IgG
immunoglobulin G
- mRNA
messenger ribonucleic acid
- PMSF
phenylmethylsulphonyl fluoride
- PVDF
polyvinylidene difluoride
- SDS
sodium dodecyl sulphate
- SDS-PAGE
SDS-polyacrylamide gel electrophoresis 相似文献
997.
U. Katz W. Hanke 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(3):189-195
The acclimation of the clawed toad Xenopus laevis to hyperosmotic solutions of NaCl (balanced solution of sea salt), urea or mannitol was studied. The animals could not be acclimated to salt solutions more concentrated centrated than 400 mosm·l-1. Urea was tolerated till 500 mmol·l-1. Plasma osmolality was always hyperosmotic to the environmental solution, but with diminished osmotic gradient at the highest tolerated solutions. Plasma urea concentration approached 90 mmol·l-1, similar in the three solutions of acclimation. Urine volume was very small under all conditions. Serum aldosterone and corticosterone did not differ significantly, although there was a slight tendency towards lower aldosterone in the NaCl solution. In vivo water uptake in tap water acclimated animals was very small, and was higher in the other groups. Only the salt- and urea-acclimated, but not the tap water and mannitol-acclimated groups responded with a clear increase following injection of oxytocin or theophylline. In vitro urea fluxes were similar and invariable in both directions under all conditions. No significant effect of theophylline was observed. Sodium transport measured by the short-circuit technique in vitro was lower in salt- and mannitol-acclimation conditions, and was stimulated significantly under all conditions in response to serosal oxytocin or theopylline. It is concluded that Xenopus laevis can osmoregulate at a limited range of external solutions. It is limited in the increase of its plasma urea concentration; the transport properties of the skin do not change very much upon acclimation, except for the hydroosmotic response to oxytocin.Abbreviations
I
sc
short circuit current
- PD
potential difference
- SW
balanced sea water
- TW
tap water 相似文献
998.
R. W. Wilson E. W. Taylor 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1993,163(3):239-246
Rainbow trout, Oncorhynchus mykiss, acclimated to 33% sea water (12 mg·ml-1 salinity) experienced significant (10 meq·1-1) increases in plasma [Na+] and [Cl-] within 5 h of exposure to 6.3 mol copper·1-1 indicating severe impairment of branchial ionoregulatory capacity. All plasma ion levels subsequently stabilised once the transbranchial [Na+] gradient was reduced to zero. The similar ionic strength of the external medium and their body fluids appeared to protect trout maintained in 33% sea water from further ionoregulatory stress and any secondary physiological disturbances during exposure to copper. Despite three- and fourfold greater transbranchial [Na+] and [Cl-] gradients, trout acclimated to full-strength sea water (35 mg·ml-1 salinity) suffered no major changes in plasma Na+, Cl-, K+, or Ca2+, blood gases or haematology during 24 h exposure to 6.3 mol copper·1-1. This reduction in toxicity in full strength sea water cannot be explained by differences in copper speciation. We suggest that during acute exposure to waterborne copper, active NaCl extrusion is unaffected due to the basolateral location of the gill Na+/K+-ATPase, but that ionoregulatory disturbances can occur due to gill permeability changes secondary to the displacement of surface-bound Ca2+. However, in full strength sea water the three-fold higher ambient [Ca2+] and [Mg2+] appear to be sufficient to prevent any detrimental permeability changes in the presence of 6.3 mol copper·1-1. Plasma [NH
+
4
] and [HCO
-
3
] were both significantly elevated during exposure to copper, indicating that some aspects of gill ion transport (specifically the apical Na+/NH
+
4
and Cl-/HCO
-
3
exchanges involved in acid/base regulation and nitrogenous waste excretion) are vulnerable to inhibition in the presence of waterborne copper.Abbreviations
C
aO2
arterial oxygen content
- Hb
haemoglobin
- Hct
haematocrit
- MABP
mean arterial blood pressure
- MCHC
mean cell haemoglobin content
-
MO2
rate of oxygen consumption
-
P
a
CO2
arterial carbon dioxide tension
-
P
aO2
arterial oxygen partial pressure
- S
salinity
- SW
sea water
-
T
Amm
total ammonia (=NH3+NH
+
4
)
-
T
CO
2
total carbon dioxide
- TEP
transepithelial potential
- TOC
total organic carbon
- %Hb-O2
percentage of haemoglobin saturated with oxygen 相似文献
999.
Sieghart Sopper Susanne Hemm Jürgen Meixensberger Cheik Coulibaly Christiane Stahl-Hennig Gerhard Hunsmann Bernhard Fleckenstein Volker ter Meulen Rüdiger Drries 《Journal of medical primatology》1993,22(2-3):138-146
Paired sera and CSF samples were collected from SIVmac-infected macaques. Animals infected with SIVmac251 maintained low gag and high env-specific antibody levels in plasma. Increasing env-specific antibody titers in CSF were associated in one animal with strong intrathecal synthesis. SIVmac239-infected monkeys revealed high antibody titers of gag and env-specificity, in one animal accompanied by weak intrathecal synthesis of virus-specific antibodies. In all animals, the CD4/CD8 ratio in CSF decreased faster compared to blood. 相似文献
1000.
We examined the effect of soil NH4+ and NO3? content upon the root systems of field-grown tomatoes, and the influence of constant, low concentrations of NH4+ or NO3? upon root growth in solution culture. In two field experiments, few roots were present in soil zones with low extractable NH4+ or NO3?; they increased to a maximum in zones having 2μg-N NO3? g?1 soil and 6 μg-N NO3= g?1 soil, but decreased in zones having higher NH4+ or NO3? levels. Root branching was relatively insensitive to available mineral nitrogen. Plants maintained in solution culture at constant levels of NH4+ or NO3?, had similar shoot biomass, but all root parameters – biomass, length, branching and area – were greater under NH4 nutrition than under NO3?. These results suggest that the size of root system depends on a functional equilibrium between roots and shoots (Brouwer 1967) and on the balance between soil NH4+ and NO3?. 相似文献