全文获取类型
收费全文 | 2108篇 |
免费 | 229篇 |
国内免费 | 2篇 |
出版年
2021年 | 16篇 |
2016年 | 32篇 |
2015年 | 46篇 |
2014年 | 56篇 |
2013年 | 74篇 |
2012年 | 91篇 |
2011年 | 88篇 |
2010年 | 40篇 |
2009年 | 28篇 |
2008年 | 54篇 |
2007年 | 59篇 |
2006年 | 77篇 |
2005年 | 45篇 |
2004年 | 63篇 |
2003年 | 55篇 |
2002年 | 51篇 |
2001年 | 55篇 |
2000年 | 51篇 |
1999年 | 47篇 |
1998年 | 19篇 |
1997年 | 22篇 |
1995年 | 15篇 |
1993年 | 17篇 |
1992年 | 61篇 |
1991年 | 56篇 |
1990年 | 62篇 |
1989年 | 70篇 |
1988年 | 62篇 |
1987年 | 57篇 |
1986年 | 51篇 |
1985年 | 47篇 |
1984年 | 29篇 |
1983年 | 40篇 |
1982年 | 24篇 |
1981年 | 37篇 |
1980年 | 44篇 |
1979年 | 55篇 |
1978年 | 57篇 |
1977年 | 38篇 |
1976年 | 30篇 |
1975年 | 29篇 |
1974年 | 53篇 |
1973年 | 43篇 |
1972年 | 19篇 |
1971年 | 30篇 |
1970年 | 28篇 |
1969年 | 29篇 |
1968年 | 20篇 |
1967年 | 29篇 |
1965年 | 16篇 |
排序方式: 共有2339条查询结果,搜索用时 265 毫秒
71.
The toad, Bufo viridis , can live for several months without access to free water, absorbing soil-bound water down a water-potential gradient created, mainly, by accumulating urea in its body fluids. We investigated if the retention of urine was sufficient to account for the rate of accumulation or if an increased rate of urea production was needed in order to do so. The basal rate of urea production in unfed animals in the absence of osmotic stress was estimated by two methods; first, analysis of the bathing medium and, secondly, collection and analysis of urine at two-hourly intervals. This was then repeated with animals fed a weight-maintaining diet. Generally similar results were obtained by either method in both fed and unfed animals, although higher urea production rates were found in the former. Although it had been planned to apply the short interval method to toads with free access to water, the control condition for toads transferred to soil, it proved to be impracticable. Some animals did not bathe for almost a day, during which time minute quantities of urine were obtained. Larger volumes were only produced during or after bathing. Consequently, animals which were partially immersed in water were substituted as controls. Total urea content was determined in these and in toads after a week on soil. The calculated increase was compared to that which could be expected from urine retention. It was found that urea accumulated at more than twice the predicted rate. When rates of accumulation were calculated over longer periods, urine retention alone was sufficient to account for them within three weeks on soil, the usual period required for acclimation. We concluded that B. viridis increased its rate of urea production only for a short period, until a favourable water potential gradient was achieved. 相似文献
72.
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
73.
Manuel Sanchez-Fernandez George M. Katz Guilherme Suarez-Kurtz Gregory J. Kaczorowski John P. Reuben 《The Journal of membrane biology》1993,135(3):273-287
The known action of uridine triphosphate (UTP) to contract some types of vascular smooth muscle, and the present finding that it is more potent than adenosine triphosphate in eliciting an increase in cytosolic Ca2+ concentration in aortic smooth muscle, led us to investigate the mode of action of this nucleotide. With this aim, cultured bovine aorta cells were subjected to patch-clamp methodologies under various conditions. Nucleotide-induced variations in cytosolic Ca2+ were monitored by using single channel recordings of the high conductance Ca2+-activated K+ (Maxi-K) channel within on-cell patches as a reporter, and whole-cell currents were measured following perforation of the patch. In cells bathed in Na+-saline, UTP (>30 nm) induced an inward current, and both Maxi-K channel activity and unitary current amplitude of the Maxi-K channel transiently increased. Repetitive exposures elicited similar responses when 5 to 10 min wash intervals were allowed between challenges of nucleotide. Oscillations in channel activity, but not oscillation in current amplitude were frequently observed with UTP levels > 0.1 m. Cells bathed in K+ saline (150
m) were less sensitive to UTP (5-fold), and did not show an increase in unitary Maxi-K current amplitude. Since the increase in amplitude occurs due to depolarization of the cell membrane, a change in amplitude was not observed in cells previously depolarized with K+ saline. The enhancement of Maxi-K channel activity in the presence of UTP was not diminished by Ca2+ entry blockers or by removal of extracellular Ca2+. However, in the latter case, repetitive responses progressively declined. These observations, as well as data comparing the action of low concentrations of Ca2+ ionophores (<5 m) to that of UTP indicate that both agents elevate cytosolic Ca2+ by mobilization of this ion from intracellular pools. However, the Ca2+ ionophore did not cause membrane depolarization, and thus did not change unitary current amplitude. The effect of UTP on Maxi-K channel activity and current amplitude was blocked by pertussis toxin and by phorbol 12-myristate 13-acetate (PMA), but was not modified by okadaic acid, or by inhibitors of protein kinase C (PKC). Our data support a model in which a pyrimidinergic receptor is coupled to a G protein, and this interaction mediates release of Ca2+ from intracellular pools, presumably via the phosphatidyl inositol pathway. This also results in activation of membrane channels that give rise to an inward current and depolarization. Ultimately, smooth muscle contraction ensues. PKC does not appear to be directly involved, even though the UTP response is blocked by low nm levels of PMA. While the latter data implicate PKC in diminishing the UTP response, agents that inhibit either PKC or phosphatase activity did not prevent abolition of UTP responses by PMA, nor did they modify basal channel activity. 相似文献
74.
Jeffery B. Press James J. McNally Pauline J. Sanfilippo Michael F. Addo Deborah Loughney Edward Giardino Laurence B. Katz Robert Falotico Barbara J. Haertlein 《Bioorganic & medicinal chemistry》1993,1(6):423-435
The syntheses and antihypertensive activity of the thieno[3,4-b]pyran and thieno[2,3-b]pyran isosteres of the potassium channel opener (PCO) RWJ 26629 (± 2a) are reported. While the unsubstituted thiophene derivatives were active at 20 mg/kg, introduction of a strong electron withdrawing group in the 2-position of the thieno[3,2-b] series increased potency. Similar substitution on the thieno[3,4-b] series significantly lowered potency. Compounds 26 and 30 are approximately 5-fold more potent than the prototypic PCO cromakalim (± 1). 相似文献
75.
An absorbable catheter for use in regional anticoagulation in microvascular and peripheral vascular surgery was studied in 20 sites in 10 adult beagle dogs to answer three questions: (1) Could the polyglycolic acid and trimethylene carbonate catheter withstand intraarterial pressures of infusion and completely absorb over a predictable time interval? (2) Could the catheter be filled with heparin and maintain patency for reuse after a 24-hour interval? (3) Could the catheter be placed in a side branch of a major artery and, after catheter dissolution, maintain long-term patency of the primary feeding artery? The catheters were completely absorbed from 24 to 34 weeks following implantation. The catheters were able to withstand intraarterial pressures, and no evidence of significant thrombosis of the primary feeding artery was seen in any animal studied. No complications of catheter leak, hematoma formation within the catheter placement sites, or sepsis were noted in any of the 20 catheter sites studied. 相似文献
76.
P Zhong S D Pratt R P Edalji K A Walter T F Holzman A G Shivakumar L Katz 《Journal of bacteriology》1995,177(15):4327-4332
ErmC' is a methyltransferase that confers resistance to the macrolide-lincosamide-streptogramin B group of antibiotics by catalyzing the methylation of 23S rRNA at a specific adenine residue (A-2085 in Bacillus subtilis; A-2058 in Escherichia coli). The gene for ErmC' was cloned and expressed to a high level in E. coli, and the protein was purified to virtual homogeneity. Studies of substrate requirements of ErmC' have shown that a 262-nucleotide RNA fragment within domain V of B. subtilis 23S rRNA can be utilized efficiently as a substrate for methylation at A-2085. Kinetic studies of the monomethylation reaction showed that the apparent Km of this 262-nucleotide RNA oligonucleotide was 26-fold greater than the value determined for full-size and domain V 23S rRNA. In addition, the Vmax for this fragment also rose sevenfold. A model of RNA-ErmC' interaction involving multiple binding sites is proposed from the kinetic data presented. 相似文献
77.
The regulation of the voltage-activated chloride current conductance (G
Cl
) in toad skin was investigated by the use of the SH reagents N-ethylmaleimide (NEM) and p-chloro-mercuricbenzenesulfonic
acid PCMBS. This anion pathway is controlled by a voltage-sensitive gating regulator. Mucosal application of NEM decreased
the voltage-activation in a time and concentration dependent manner, half-maximal inhibition being exerted at a concentration
of 30 μm within 20 min. At concentrations higher than 100 μm, the voltage-activated G
Cl
was near-completely and irreversibly inhibited in less than 10 min. Resting, deactivated conductance was essentially unaffected.
NEM had no effect on active sodium transport (measured as I
sc
) under conditions, which fully dissipated the voltage-activated G
Cl
. After complete inhibition of the voltage-activated G
Cl
with NEM, chloride conductance could still be stimulated by CPT-cAMP as in control tissues. Under these conditions, NEM at
concentrations above 1 mm decreased G
Cl
reversibly. Mucosal application of PCMBS at 500 μm inhibited the activated conductance by 35%, which was slightly reversible. Inhibition of voltage-activated G
Cl
, which was observed after mucosal addition of the membrane-impermeable NEM analogue, eosin-5-maleimide, was completely reversible
after washout. This suggests that the binding site for the maleimide is not accessible from the external face of the apical
membrane. Brief application of NEM at lower concentrations (1–3 min, ≤100 μm) led to partial inhibition of G
Cl
, followed by occasionally complete recovery upon washout of NEM. Recovery of voltage-activated G
Cl
was progressively attenuated and eventually disappeared after subsequent brief applications of NEM. This could reflect recruitment
of permeation/control sites from a finite pool. The data are discussed in the frame of a working model for the voltage-activated
Cl−-pathway, that contains two principle components, i.e., an anion-selective permeation path which is controlled by regulatory
protein(s).
Received: 18 December 1996/Revised: 28 April 1997 相似文献
78.
Shlomit Katz Zvy Dubinsky Chana Rothmann Zvi Malik Michael Friedlander 《Journal of phycology》1997,33(3):425-432
The novel method of Fourier transform multi-pixel spectroscopy was used for the nondestructive analysis of and comparison of pigmentation in different regions of live thalli of the red alga Porphyra linearis. Because the thallus in this alga consists of a monolayer of nonoverlapping cells, we were able to analyze the pigmentation of single cells by combining light absorbance with natural fluorescence data. From the image of each cell in the vegetative male and female reproductive and holdfast regions, more than 4 ± 104 fluorescence and absorbance spectra were obtained. Specific pigments in the different regions were localized by the use of a software program of similarity mapping followed by image construction. The reconstructed images revealed subcellular localization of each pigment according to specific spectroscopic fingerprints. The results showed that the vegetative and female reproductive cell types had a significantly higher content of phycoerythrin than of phycocyanin, and quite similar chlorophyll a levels. Most of the holdfast cells were poorly pigmented, but had more chlorophyll a than phycoerythrin or phycocyanin. The male reproductive cells contained only traces of pigments. Thus, by using Fourier transform multipixel spectroscopy, we were able to characterize the pigmentation of different regions of the thallus and follow the distribution patterns of the different pigments on the subcellular level along the differentiation gradient of the alga. 相似文献
79.
The A34R glycoprotein gene is required for induction of specialized actin-containing microvilli and efficient cell-to-cell transmission of vaccinia virus. 总被引:7,自引:5,他引:2
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The mechanisms allowing vaccinia virus to spread from cell to cell are incompletely understood. The A34R gene of vaccinia virus encodes a glycoprotein that is localized in the outer membranes of extracellular virions. The small-plaque phenotype of an A34R deletion mutant was similar to that of mutants with deletions in other envelope genes that fail to produce extracellular vaccinia virions. Transmission electron microscopy, however, revealed that the A34R mutant produced numerous extracellular particles that were labeled with antibodies to other outer-envelope proteins and with protein A-colloidal gold. Fluorescence and scanning electron microscopy indicated that expression of the A34R protein was necessary for detection of vaccinia virus-induced actin tails, which provide motility to the intracellular enveloped form of vaccinia virus, and of virus-tipped specialized microvilli that project from the cell. The ability of vaccinia virus-infected cells to form syncytia after a brief exposure to a pH below 6, known as fusion from within, failed to occur in the absence of expression of the A34R protein; nevertheless, purified A34R- virions were capable of mediating low-pH-induced fusion from without. The present study provides genetic and microscopic evidence for the involvement of a specific viral protein in the formation or stability of actin-containing microvilli and for a role of these structures in cell-to-cell spread rather than in formation of extracellular virions. 相似文献
80.
Elon Pras Nina Raben Eliahu Golomb Nadir Arber Ivona Aksentijevich Jonathan M. Schapiro Daniela Harel Giora Katz Uri Liberman Mordechai Pras Daniel L. Kastner 《American journal of human genetics》1995,56(6):1297-1303
Cystinuria is an autosomal recessive disease characterized by the development of kidney stones. Guided by the identification of the SLC3A1 amino acid–transport gene on chromosome 2, we recently established genetic linkage of cystinuria to chromosome 2p in 17 families, without evidence for locus heterogeneity. Other authors have independently identified missense mutations in SLC3A1 in cystinuria patients. In this report we describe four additional cystinuria-associated mutations in this gene: a frameshift, a deletion, a transversion inducing a critical amino acid change, and a nonsense mutation. The latter stop codon was found in all of eight Ashkenazi Jewish carrier chromosomes examined. This report brings the number of disease-associated mutations in this gene to 10. We also assess the frequency of these mutations in our 17 cystinuria families. 相似文献