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171.
盾叶薯蓣地上部分的三个新甾体皂甙 总被引:11,自引:0,他引:11
从盾叶薯蓣Dioscorea zingiberensis Wright地上部分分离鉴定了四个甾体皂甙,经鉴定甙A为约莫皂甙元-3-O-[α-L-鼠李吡喃糖基(1→2)]-β-D-葡萄吡喃糖甙;甙B为24α-羟基约莫皂甙元-3-O-[α-L-鼠李吡喃糖基(1→2)]β-D-葡萄吡喃糖甙;甙C为约莫皂甙元-3-O-[α-L-鼠李吡喃糖基(1→2)][β-D-葡萄吡喃糖基(1→4)]-β-D-葡萄吡喃糖基;甙D为约莫皂甙元-3-O-[α-L-鼠李吡喃糖基(1→2)][β-D-葡萄吡喃糖基(1→3)]-β-D-葡萄吡喃糖甙。前三者为新化合物,分别命名为盾叶皂甙A_1、A_2、A_3(zingiberoside A_1、A_2、A_3),其中盾叶皂甙A_2的甙元为一新甾体皂甙元,命名为盾叶皂甙元(zingiberogenin)。 相似文献
172.
Salil K. Niyogi Thomas S. Soper Robert S. Foote Frank W. Larimer Richard J. Mural Sankar Mitra Eva H. Lee Richard Machanoff Fred C. Hartman 《Journal of biosciences》1987,11(1-4):203-214
Both Lys-166 and His-291 of ribulosebisphosphate carboxylase/oxygenase fromRhodospirillum rubrum have been implicated as the active-site residue that initiates catalysis. To decide between these two candidates, we resorted
to site-directed mutagenesis to replace Lys-166 and His-291 with several amino acids. All 7 of the position-166 mutants tested
are severely deficient in carboxylase activity, whereas the alanine and serine mutants at position 291 are ∼40% and ∼18% as
active as the native carboxylase, essentially ruling out His-291 in theRhodospirillum rubrum carboxylase (and by inference His-298 in the spinach enzyme) as a catalytically essential residue. The ability of some of
the mutant proteins to undergo carbamate formation or to bind either ribulosebisphosphate or a transition-state analogue remains
largely unimpaired. This implies that Lys-166 is not required for substrate binding; rather, the results corroborate the earlier
postulate that Lys-166 functions as an acid-base group in catalysis or in stabilizing a transition state in the reaction pathway. 相似文献
173.
Summary The presence of a Ca2+ channel in the plasmalemma of tonoplast-freeNitellopsis obtusa cells was demonstrated and its characteristics were studied using current- and voltage-clamp techniques. A long-lasting inward membrane current (I
m
), recorded using a step voltage clamp, consisted of a single component without time-dependent inactivation. Increasing either [Ca2+]
o
or [Cl–]
o
1) enhanced the maximum amplitude of inwardI
m
((I
m
)
p
) and 2) shifted the peak voltage ((V
m
)
p
) at(I
m
)
p
to more positive values under ramp-shaped voltage clamping and 3) depolarized the peak value of action potentials. This behavior is consistent with predictions based on the Nernst equation for Ca2+ but not for Cl–. DIDS (4,4-diisothiocyano-2,2-disulfonic acid stilbene) did not suppress(I
m
)
p
in tonoplast-free cells, in contrast with its effect on normal cells. La3+ and nifedipine blocked(I
m
)
p
irreversibly. On the other hand, Ca2+ channel agonist, BAY K 8644 irreversibly enhanced(I
m
)
p
. Both Sr2+ influx and K+ efflux increased upon excitation. The charge carried by Sr2+ influx was compensated for by K+ efflux. It is concluded that only the Ca2+ channel is activated during plasmalemma excitation in tonoplast-free cells. In terms of the magnitude of(I
m
)
p
, Sr2+ could replace Ca2+, but Mn2+, Mg2+ and Ba2+ could not. External pH affected(I
m
)
p
and the membrane conductance (g
m
) at(I
m
)
p
((g
m
)
p
). Increasing the external ionic strength caused increases in both(I
m
)
p
and(g
m
)
p
, and shifted(V
m
)
p
to positive values. At the same time, Sr2+ influx increased. Thus Ca2+ channel activation seems to be enhanced by increasing external ionic strength. The possible involvement of surface potential is discussed. 相似文献
174.
Summary Sodium-calcium exchange has been suggested to play a pivotal role in the regulation of cytosolic free calcium (Ca
f
) by epithelial cells. Using isolated epithelial cells from the toad urinary bladder, Ca
f
has been measured using the intracellular Casensitive fluorescent dyes Fura 2 and Quin. 2. Dye loading did not alter cell viability as assessed by measurements of ATP and ADP content or cell oxygen consumption. When basal Ca
f
was examined over a wide range of cell dye content (from 0.04 to 180 nmol dye/mg protein) an inverse relationship was observed. At low dye content, Ca
f
was 300–380 nM and, as dye content was increased, Ca
f
progressively fell to 60 nM. Using low dye content cells, in which minimal alteration in Ca steady state would be expected, the role for plasma membrane Na–Ca exchange was examined using either medium sodium substitution or ouabain. While medium sodium substitution increased Ca
f
, prolonged treatment with ouabain had no effect on Ca
f
despite a clear increase in cell sodium content. The lack of effect of ouabain suggests that Na–Ca exchange-mediated Ca efflux plays a minimal role in the regulation of basal Ca
f
. However, exchange-mediated Ca efflux may play a role in Ca
f
regulation when cytosolic calcium is elevated. 相似文献
175.
Manabu Sakakibara Carlos Collin Alan Kuzirian Daniel L. Alkon Eliahu Heldman Shigetaka Naito Izja Lederhendler 《Journal of neurochemistry》1987,48(2):405-416
Type B photoreceptors of the nudibranch mollusc Hermissenda crassicornis receive excitatory synaptic potentials (EPSPs) whose frequency is controlled by potential changes of a neighboring cell known as the S optic ganglion cell which is thought to be electrically coupled to the presynaptic source of these EPSPs, the E optic ganglion cell. The frequency of the EPSPs increases when a conditioned stimulus (light) is paired with an unconditioned stimulus (rotation) during acquisition of a Pavlovian conditioned response. The results of the present study are consistent with an adrenergic origin for these EPSPs. Noradrenergic agonists (greater than 100 microM), norepinephrine and clonidine, only slightly depolarize the type B cell but clearly prolong its depolarizing response to light. Serotonin, by contrast, causes hyperpolarization of the type B cell's resting potential as well as after a light step. Clonidine reduces voltage-dependent outward K+ currents (IA, an early current, ICa2+-K+, a late Ca2+-dependent current) that control the type B cell's excitability (and thus its light response and membrane potential). These effects of clonidine are reduced or blocked by the alpha 2-receptor antagonist, yohimbine (0.5 microM), but not the alpha 1-blocker, prazosin. The same yohimbine concentration also blocked depolarizing synaptic excitation of the type B cell in response to depolarization of a simultaneously impaled S optic ganglion cell. Histochemical techniques (both the glyoxylic acid method of de la Torre and Surgeon and the formaldehyde-induced fluorescence or Falck-Hillarp method) demonstrated the presence of a biogenic amine(s) within a single neuron in each optic ganglion as well as three or four cells within the vicinity of previously identified visual interneurons. No serotonergic neurons were found within the optic ganglion or in proximity to visual interneurons. A clonidine-like synaptic effect on type B cells, therefore, could amplify conditioning-specific changes of membrane currents by increasing type B depolarization and possibly, as well, by elevating intracellular second messengers. 相似文献
176.
Yutaka Nagata Masato Ando Mitsuyoshi Iwata Atsushi Hara Tamotsu Taketomi 《Journal of neurochemistry》1987,49(1):201-207
The effects of some gangliosides on active uptake of nonmetabolizable alpha-aminoisobutyric acid (AIB) and Na+, K+-ATPase and Ca2+, Mg2+-ATPase activities in superior cervical ganglia (SCG) and nodose ganglia (NG) excised from adult rats were examined during aerobic incubation at 37 degrees C for 2 h. In NG, amino acid uptake was greatly accelerated with the addition of galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylgluc osyl ceramide (GM1) (85%) and also with N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosylglucosyl ceramide (GM2) or [N-acetylneuraminyl]-galactosyl-N-acetylgalactosaminyl-[N-acetyl- neuraminyl]-galactosylglucosyl ceramide (GD1a) (43% each) compared with a nonaddition control at a 5 nM concentration. Under identical conditions, Na+, K+-ATPase activity was strongly stimulated with GM1 (180%) and GD1a (93%), whereas Ca2+, Mg2+-ATPase activity showed no change. In SCG, on the other hand, AIB uptake was apparently inhibited (-27%) by addition of GM1, with a slight decrease in Na+, K+-ATPase but no change in Ca2+, Mg2+-ATPase activity in the tissue. Both asialo-GM1, in which N-acetylneuraminic acid is deficient, and Forssman glycolipid, which is not present in nervous tissue, failed to produce any significant increase in both SCG and NG not only in amino acid uptake, but also in Na+, K+-ATPase activity. A kinetic study of active AIB uptake showed that GM1 ganglioside produced an increase in Km with no change in Vmax in SCG, whereas it caused a decrease in Km with a slight increase in Vmax in NG. Treatment of NG and SCG with neuraminidase from Vibrio cholerae, an enzyme that split off sialic acid from polysialoganglioside, leaving GM1 intact, caused little inhibition of the amino acid uptake.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
177.
Uncoupling of Rat Cerebral Cortical α2 -Adrenoceptors from GTP-Binding Proteins by N-Ethylmaleimide 总被引:2,自引:0,他引:2
Pretreatment of membranes from rat cerebral cortex with N-ethylmaleimide (NEM) decreased [3H]-clonidine binding in a concentration-dependent manner. The Bmax values of high-affinity sites for [3H]clonidine were reduced by 50 microM NEM treatment. Treatment with 500 microM NEM diminished the sum of Bmax of both high- and low-affinity components. GTP, Na+, and Mn2+ exerted little effect on [3H]clonidine binding in NEM-treated membranes. The addition of purified GTP-binding proteins caused an increase in the binding to the membranes pretreated with 50 microM NEM, but did not increase [3H]-clonidine binding in membranes treated with 500 microM NEM. In contrast, NEM pretreatment inhibited islet activating protein (IAP)-catalyzed ADP ribosylation of membrane-bound (41,000-dalton) and purified (39,000/41,000-dalton) GTP-binding proteins. From these results, it is suggested that two or three categories of essential sulfhydryl groups are involved in the coupling between agonist, alpha 2-adrenoceptor, and GTP-binding protein. One is a highly sensitive site to NEM (a concentration range of 1-50 microM), which is probably a cysteine residue, IAP-catalyzed ADP-ribosylating site on the alpha-subunit of GTP-binding protein. Other sites have low sensitivity to NEM (a concentration range of 0.1-1 mM), and are the binding domain of agonist and/or the coupling domain of GTP-binding protein on the alpha 2-adrenoceptor. In addition, Ki-ras p21 protein may lack the capacity to couple with the alpha 2-adrenoceptor. 相似文献
178.
179.
Beth J. Hoffman Ursula Scheffel† John R. Lever† Michael D. Karpa Paul R. Hartig† 《Journal of neurochemistry》1987,48(1):115-124
Methylation of 2-125I-lysergic acid diethylamide (125I-LSD) at the N1 position produces a new derivative, N1-methyl-2-125I-lysergic acid diethylamide (125I-MIL), with improved selectivity and higher affinity for serotonin 5-HT2 receptors. In rat frontal cortex homogenates, specific binding of 125I-MIL represents 80-90% of total binding, and the apparent dissociation constant (KD) for serotonin 5-HT2 receptors is 0.14 nM (using 2 mg of tissue/ml). 125I-MIL also displays a high affinity for serotonin 5-HT1C receptors, with an apparent dissociation constant of 0.41 nM at this site. 125I-MIL exhibits at least 60-fold higher affinity for serotonin 5-HT2 receptors than for other classes of neurotransmitter receptors, with the dopamine D2 receptor as its most potent secondary binding site. Studies of the association and dissociation kinetics of 125I-MIL reveal a strong temperature dependence, with very slow association and dissociation rates at 0 degree C. Autoradiographic experiments confirm the improved specificity of 125I-MIL. Selective labeling of serotonin receptors was observed in all brain areas examined. In vivo binding studies in mice indicate that 125I-MIL is the best serotonin receptor label yet described, with the highest frontal cortex to cerebellum ratio of any serotonergic radioligand. 125I-MIL is a promising ligand for both in vitro and in vivo labeling of serotonin receptors in the mammalian brain. 相似文献
180.
Stimulatory Effect of the D2 Antagonist Sulpiride on Glucose Utilization in Dopaminergic Regions of Rat Brain 总被引:1,自引:0,他引:1
Gilberto Pizzolato Timothy T. Soncrant Denise M. Larson Stanley I. Rapoport 《Journal of neurochemistry》1987,49(2):631-638
Local cerebral glucose utilization (LCGU) was measured, using the quantitative autoradiographic [14C]2-deoxy-D-glucose method, in 56 brain regions of 3-month-old, awake Fischer-344 rats, after intraperitoneal administration of sulpiride (SULP) 100 mg/kg. SULP, an "atypical" neuroleptic, is a selective antagonist of D2 dopamine receptors. LCGU was reduced in a few nondopaminergic regions at 1 h after drug administration. Thereafter, SULP progressively elevated LCGU in many other regions. At 3 h, LCGU was elevated in 23% of the regions examined, most of which are related to the CNS dopaminergic system (caudate-putamen, nucleus accumbens, olfactory tubercle, lateral habenula, median eminence, paraventricular hypothalamic nucleus). Increases of LCGU were observed also in the suprachiasmatic nucleus, lateral geniculate, and inferior olive. These effects of SULP on LCGU differ from the effects of the "typical" neuroleptic haloperidol, which produces widespread decreases in LCGU in the rat brain. Selective actions on different subpopulations of dopamine receptors may explain the different effects of the two neuroleptics on brain metabolism, which correspond to their different clinical and behavioral actions. 相似文献