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1.
Nectar and pollen within flowers are usually the primary attractants to floral visitors. Chemical analysis of almond nectar
and pollen in this study revealed that they contain the secondary compound amygdalin. Floral display often reflects pollinator
characters, and almond flowers are accordingly designated as “bee flowers”. A previous study in Israel showed that when almonds
bloom early in the season they attract honeybees, but later in the season the bees shift toward other species that start blooming.
In this study, we offered honeybees sugar solutions containing various concentrations of amygdalin. These preference experiments
revealed that in mid-summer bees were not selective, whereas early in the summer they were more discriminating, and consumed
faster the sugar solutions with the lower amygdalin concentrations. Possible roles of amygdalin in almond nectar and pollen
are discussed.
Received September 10, 2002; accepted January 17, 2003
Published online: June 2, 2003 相似文献
2.
Walter W. Ferguson 《Primates; journal of primatology》1992,33(2):273-279
In response to a critique byFerguson (1989),Leonard (1991) reiterates most of his original arguments for supporting “Australopithecus afarensis”Johanson, White, andCoppens, 1978 as a single species. He disregards the principle of morphological equivalence by comparing the dental metrics and morphology
of a hominid with those of species of the Pongidae, which do not correspond with the degree of variation in hominids, instead
of with those of species of the Hominidae. He fails to refute clear evidence that the range of variation of dental metrics
and morphology in “A. afarensis” exceeds that seen in species of the Hominidae. On the basis of extreme variation, “A. afarensis” is, therefore, interpreted as representing a composite species. 相似文献
3.
In previous studies, we showed that lacrimal gland acini express three isoforms of protein kinase C (PKC): PKCα,-δ, and -ε.
In the present study, we report the identification of two other PKC isoforms, namely PKCμ and -ι/λ. Using immunofluorescence
techniques, we showed that these isoforms are differentially located. PKCα and -μ showed the most prominent membrane localization,
whereas PKCδ, -ε and -ι/λ were mainly cytosolic. Using cell fractionation and western blotting techniques, we showed that
the phorbol ester, phorbol 12, 13-dibutyrate (PdBu, 10−6
m), translocated all PKC isoforms, except PKCι/λ, from the soluble fraction into the particulate fraction. The effect was maximum
at 5 min and persisted at 10 min. PKCε was the most responsive to PdBu reaching almost maximal translocation at a PdBu concentration
as low as 10−9
m. The cholinergic agonist, carbachol (10−5 and 10−3
m), induced translocation which was transient for PKCδ, and -μ, but persisted for 10 min for PKCε. Carbachol did not translocate
PKCα and, like PdBu, did not translocate PKCι/λ. We concluded that lacrimal gland PKC isoforms are differentially localized
and that they translocate differentially in response to phorbol esters and cholinergic agonists.
Received: 25 June 1996/Revised: 24 December 1996 相似文献
4.
Cell-volume changes induced by terbutaline (a specific β2-agonist) were studied morphometrically in rat fetal distal lung epithelium (FDLE) cells. Cell-volume changes qualitatively
differed with the concentration of terbutaline. Terbutaline of 10−10–10−8
m induced transient cell swelling. Terbutaline of 10−7
m induced transient cell swelling followed by slow cell shrinkage. Terbutaline of 10−6–10−5
m induced rapid cell shrinkage followed by slow cell shrinkage. Terbutaline of 10−3
m induced transient cell shrinkage; then cell volume oscillated during stimulation. Benzamil of 10−6
m suppressed the cell swelling induced by 10−10–10−8
m terbutaline and quinine of 10−3
m inhibited the cell shrinkage induced by 10−6–10−5
m terbutaline. These results suggest that cell swelling would be induced by NaCl influx and the cell shrinkage is by KCl efflux.
Dibutyryl cyclic AMP (DBcAMP) also induced similar cell-volume changes over a wide range of concentrations (10−9–10−3
m): a low concentration induced transient cell swelling; a high concentration, rapid and slow cell shrinkage. Forskolin (10−4
m), like terbutaline (10−5
m), induced rapid cell shrinkage followed by slow cell shrinkage, and this decrease in the cell volume was enhanced by the
presence of benzamil. On the other hand, cell shrinkage was induced by ionomycin (even low concentration; 3 × 10−10
m ionomycin), and after that cell volume remained at a plateau level. Removal of extracellular Ca2+ abolished the cell swelling caused by terbutaline of 10−10–10−8
m. With removal of extracellular Ca2+, the initial, rapid cell shrinkage induced by 10−5
m terbutaline became transient, but we still detected slow cell shrinkage similar to that in the presence of extracellular
Ca2+. Overall, at low concentrations (10−10–10−8
m), terbutaline induced benzamil-sensitive cell swelling in FDLE cells, which was cAMP- and Ca2+-dependent; high concentrations (≥−6) induced quinine-sensitive rapid cell shrinkage, which was Ca2+-dependent; high concentrations (≥−7) induced slow cell shrinkage, which was cAMP-dependent. These findings suggest that terbutaline regulates cell volume in
FDLE cells by cytosolic cAMP and Ca2+ through activation of Na+ and K+ channels.
Received: 13 March 1995/Revised: 17 January 1996 相似文献
5.
The functional properties of the transport of lysine across the chicken erythrocyte membrane were investigated. The animal
population studied (male Leghorn chickens, 6–14 weeks old) was found to consist of two groups presenting either low (LT, 19
individuals) or high transport rates (HT, 20 individuals). The rates of influx in the two groups, measured at a concentration
of l-lysine of 1 μm, differed by a factor of 34. The transport activities observed in LT and HT erythrocytes were compatible with the general
features of system y+L, but showed some differences in specificity. The transporter in the LT group was found to bind l-lysine, l-leucine, l-methionine and l-glutamine with high affinity, in the presence of sodium, as described for system y+L in human erythrocytes. The activity present in HT erythrocytes exhibited a much lower affinity for l-leucine, but was able to interact strongly with l-glutamine and l-methionine. The specificity pattern of the HT transporter, has not been described in other cell types. In other respects,
the properties of the two systems were similar. Sodium replacement with potassium, drastically reduced the affinity for l-leucine, without affecting lysine transport. Both transporters function as tightly coupled exchangers, are inactivated by
p-chloromercuribenzene sulfonate and resistant to N-ethylmaleimide. These findings explain previous results obtained in selective
breeding experiments of chicken with high and low amino-acid transport activity.
Received: 12 February 2001/Revised: 11 June 2001 相似文献
6.
H. Ramos E. Valdivieso M. Gamargo F. Dagger B.E. Cohen 《The Journal of membrane biology》1996,152(1):65-75
The polyene antibiotic amphotericin B (AmB) is known to form two types of ionic channels across sterol-containing liposomes,
depending on its concentration and time after mixing (Cohen, 1992). In the present study, it is shown that AmB only kills
unicellular Leishmania promastigotes (LPs) when aqueous pores permeable to small cations and anions are formed. Changes of membrane potential across
ergosterol-containing liposomes and LPs were followed by fluorescence changes of 3,3′ dipropylthiadicarbocyanine (DiSC3(5)). In KCl-loaded liposomes suspended in an iso-osmotic sucrose solution, low AmB concentrations (≤0.1 μm) induced a polarization potential, indicating K+ leakage, but no movement of cations and anions was allowed until AmB concentrations greater than 0.1 μm were added. In agreement with these data, it was found that AmB altered the negative membrane potential held across LPs in
a manner consistent with the differential cation/anion selectivity exhibited by the channels formed in liposomes. Thus, LPs
suspended in an iso-osmotic sucrose solution did not exhibit any AmB-induced membrane depolarization effect brought about
by efflux of anions until 0.1 μm or higher AmB concentrations were added. By contrast, LPs suspended in an iso-osmotic NaCl solution and exposed to 0.05 μm AmB exhibited a nearly total collapse of the negative membrane potential, indicating Na+ entry into the cells.
The concentration dependence of the AmB-induced permeability to different salts was also measured across vesicles derived
from the plasma membrane of leishmanias (LMVs), by using a rapid mixing technique. At concentrations above 0.1 μm, AmB induced the formation of aqueous pores across LMVs with a positive cooperativity, yielding Hill coefficients between
2 to 3. Measured anion selectivity across such aqueous pores followed the sequence: SCN > NO3 > Cl > I > Br > acetate (SO2−
4 being impermeable). Cell killing by AmB was followed by fluorescence changes of the DNA-binding compound ethidium bromide
(EB). At low concentrations (≤0.1 μm), AmB was found to be nonlethal against LPs but, above this concentration, leishmanias were rapidly killed. The rate and
extent of such an effect were found to be dependent on the type of cation and anion present in the external aqueous solution.
For both NH+
4 and Na+ salts, the measured rank order of AmB cell killing followed the same sequence that was determined for AmB-induced salt permeation
across LMVs. Further, replacement of either extracellular Na+ by choline or Cl− by SO2−
4, or its partial substitution by sucrose, in iso-osmotic conditions, led to a complete inhibition of the killing effect exerted
by otherwise lethal AmB concentrations. Finally, it was shown that tetraethylammonium (TEA+), an organic cation that is known to block AmB-induced salt permeation across LMVs was able to retard the time lag observed
for EB incorporation across LPs, indicating that this parameter can be taken to represent the time taken for salt accumulation
inside the parasites. The present results thus indicate clearly that low AmB concentrations (≤0.1 μm) were able to form across LPs, cation channels that collapsed the parasite membrane potential but are not lytic. At high
concentrations (<≥0.1 μm), a salt influx via the aqueous pores formed by the antibiotic was followed by osmotic changes leading to cell lysis. This
last stage is supported by electron microscopy observations of the changes of parasite morphology immediately upon addition
of AmB, which indicated that the typical elongated promastigote cell forms became rounded and the flagella swells and round
up. The present work is the first demonstration of the in vitro sensitivity of Leishmania promastigotes to osmotic lysis by AmB.
Received: 25 September 1995/Revised: 11 March 1996 相似文献
7.
GABAA channels were activated by GABA in outside-out patches from rat cultured hippocampal neurons. They were blocked by bicuculline
and potentiated by diazepam. In 109 of 190 outside-out patches, no channels were active before exposure to GABA (silent patches).
The other 81 patches showed spontaneous channel activity. In patches containing spontaneous channel activity, rapid application
of GABA rapidly activated channels. In 93 of the silent patches, channels could be activated by GABA but only after a delay
that was sometimes as long as 10 minutes. The maximum channel conductance of the channels activated after a delay increased
with GABA concentration from less than 10 pS (0.5 μm GABA) to more than 100 pS (10 mm GABA). Fitting the data with a Hill-type equation gave an EC
50 value of 33 μm and a Hill coefficient of 0.6. The channels showed outward rectification and were chloride selective. In the presence of
1 μm diazepam, the GABA EC
50 decreased to 0.2 μm but the maximum conductance was unchanged. Diazepam decreased the average latency for channel opening. Bicuculline, a GABA
antagonist, caused a concentration-dependent decrease in channel conductance. In channels activated with 100 μm GABA the bicuculline IC
50 was 19 μm. The effect of GABA on channel conductance shows that the role of the ligand in GABAA receptor channel function is more complex than previously thought.
Received: 23 October 2000/Revised: 27 February 2001 相似文献
8.
Previous studies in our laboratory have shown that Na absorption across the porcine endometrium is stimulated by PGF2α and cAMP-dependent activation of a barium-sensitive K channel located in the basolateral membrane of surface epithelial cells.
In this study, we identify and characterize this basolateral, barium-sensitive K conductance. Porcine uterine tissues were
mounted in Ussing chambers and bathed with KMeSO4 Ringer solution. Amphotericin B (70 μm) was added to the luminal solution to permeabilize the apical membrane and determine the current-voltage relationship of
the basolateral K conductance after activation by 100 μm CPT-cAMP. An inwardly rectifying current was identified which possessed a reversal potential of −53 mV when standard Ringer
solution was used to bathe the serosal surface. The K:Na selectivity ratio was calculated to be 12:1. Administration of 5
mm barium to the serosal solution completely inhibited the current activated by cAMP under these conditions. In addition to
these experiments, amphotericin-perforated whole cell patch clamp recordings were obtained from primary cultures of porcine
surface endometrial cells. The isolated cells displayed an inwardly rectifying current under basal conditions. This current
was significantly stimulated by CPT-cAMP and blocked by barium. These results together with our previous studies demonstrate
that cAMP increases Na absorption in porcine endometrial epithelial cells by activating an inwardly rectifying K channel present
in the basolateral membrane. Similar patch clamp experiments were conducted using cells from a human endometrial epithelial
cell line, RL95-2. An inwardly rectifying current was also identified in these cells which possessed a reversal potential
of −56 mV when the cells were bathed in standard Ringer solution. This current was blocked by barium as well as cesium. However,
the current from the human cells did not appear to be activated by cAMP, indicating that distinct subtypes of inwardly rectifying
K channels are present in endometrial epithelial cells from different species.
Received: 6 February 1997/Revised: 10 July 1997 相似文献
9.
We have measured ryanodine (caffeine)-sensitive 45Ca2+ release from isolated microsomal vesicles of endoplasmic reticulum prepared from rat parotid acinar cells. After a steady
state of ATP-dependent 45Ca2+ uptake, the addition of caffeine (40 mm), ryanodine (10∼500 μm) or an NAD+ metabolite, cyclic ADP-ribose (cADPR, 4 μm) released about 10% of the 45Ca2+ that had been taken up. The 45Ca2+ release was not inhibited by heparin, an antagonist of IP3 receptor. The effects of caffeine, ryanodine and cADPR on 45Ca2+ release were also tested in the presence of thapsigargin (TG), an inhibitor of microsomal Ca2+-ATPase. When caffeine (10∼40 mm), ryanodine (10 μm) or cADPR (1∼10 μm) was added in the medium with 100 nm TG, a significant 45Ca2+ release was seen, while higher concentrations of ryanodine (>100 μm) did not cause any 45Ca2+ release in the presence of TG. The initial rate of caffeine (40 mm)-induced 45Ca2+ release was increased by a pretreatment with 10 μm ryanodine, whereas the caffeine-induced 45Ca2+ release was strongly inhibited by the presence of a higher concentration (500 μm) of ryanodine. cADPR-induced 45Ca2+ release was also inhibited by 500 μm ryanodine. Caffeine (40 mm)- or cADPR (4 μm)-induced 45Ca2+ release was abolished by a presence of ruthenium red (50∼100 μm). The presence of a low concentration (0.5 μm) of cADPR shifted the dose-response curve of caffeine-induced 45Ca2+ release to the left. These results indicate the presence of a ryanodine sensitive Ca2+ release mechanism in the endoplasmic reticulum of rat parotid acinar cells that is distinct from the IP3-sensitive Ca2+ channel and is activated by caffeine, cADPR and a low concentration (10 μm) of ryanodine, but is inhibited by higher concentrations (>100 μm) of ryanodine and ruthenium red. The properties of the ryanodine-sensitive mechanism are similar to that of the ryanodine
receptor as described in muscle cells.
Received: 11 June 1996/Revised: 14 November 1996 相似文献
10.
The modulation of the calmodulin-induced inhibition of the calcium release channel (ryanodine receptor) by two sulfhydryl
oxidizing compounds, 4-(chloromercuri)phenyl–sulfonic acid (4-CMPS) and 4,4′-dithiodipyridine (4,4′-DTDP) was determined by
single channel current recordings with the purified and reconstituted calcium release channel from rabbit skeletal muscle
sarcoplasmic reticulum (HSR) and [3H]ryanodine binding to HSR vesicles. 0.1 μm CaM reduced the open probability (P
o
) of the calcium release channel at maximally activating calcium concentrations (50–100 μm) from 0.502 ± 0.02 to 0.137 ± 0.022 (n= 28), with no effect on unitary conductance. 4-CMPS (10–40 μm) and 4,4′-DTDP (0.1–0.3 mm) induced a concentration dependent increase in P
o (> 0.9) and caused the appearance of longer open states. CaM shifted the activation of the calcium release channel by 4-CMPS
or 4,4′-DTDP to higher concentrations in single channel recordings and [3H]ryanodine binding. 40 μm 4-CMPS induced a near maximal (P
o
> 0.9) and 0.3 mm 4,4′-DTDP a submaximal (P
o
= 0.74) channel opening in the presence of CaM, which was reversed by the specific sulfhydryl reducing agent DTT. Neither
4-CMPS nor 4,4′-DTDP affected Ca-[125I]calmodulin binding to HSR. 1 mm MgCl2 reduced P
o
from 0.53 to 0.075 and 20–40 μm 4-CMPS induced a near maximal channel activation (P
o
> 0.9). These results demonstrate that the inhibitory effect of CaM or magnesium in a physiological concentration is diminished
or abolished at high concentrations of 4-CMPS or 4,4′-DTDP through oxidation of activating sulfhydryls on cysteine residues
of the calcium release channel.
Received: 22 July 1999/Revised: 15 November 1999 相似文献
11.
This study examines the diverse maximum wavelength absorption (λmax) found in crayfishes (Decapoda: Cambaridae and Parastacidae) and the associated genetic variation in their opsin locus. We
measured the wavelength absorption in the photoreceptors of six species that inhabit environments of different light intensities
(i.e., burrows, streams, standing waters, and subterranean waters). Our results indicate that there is relatively little variation
in λmax (522–530 nm) among species from different genera and families. The existing variation did not correlate with the habitat
differences of the crayfishes studied. We simultaneously sequenced the rhodopsin gene to identify the amino acid replacements
that affect shifts in maximum wavelength absorption. We then related these to changes that correlated with shifts in λmax by reconstructing ancestral character states using a maximum-likelihood approach. Using amino acid sequences obtained from
five species (all were 301 amino acids in length), we identified a number of candidates for producing shifts of 4 to 8 nm
in λmax. These amino acid replacements occurred in similar regions to those involved in spectral shifts in vertebrates.
Received: 12 March 1997 / Accepted: 3 June 1997 相似文献
12.
The dependence of currents through the cyclic nucleotide-gated (CNG) channels of mammalian olfactory receptor neurons (ORNs)
on the concentration of NaCl was studied in excised inside-out patches from their dendritic knobs using the patch-clamp technique.
With a saturating concentration (100 μm) of adenosine 3′, 5′-cyclic monophosphate (cAMP), the changes in the reversal potential of macroscopic currents were studied
at NaCl concentrations from 25 to 300 mm. In symmetrical NaCl solutions without the addition of divalent cations, the current-voltage relations were almost linear,
reversing close to 0 mV. When the external NaCl concentration was maintained at 150 mm and the internal concentrations were varied, the reversal potentials of the cAMP-activated currents closely followed the
Na+ equilibrium potential indicating that P
Cl/P
Na≈ 0. However, at low external NaCl concentrations (≤100 mm) there was some significant chloride permeability. Our results further indicated that Na+ currents through these channels: (i) did not obey the independence principle; (ii) showed saturation kinetics with K
ms in the range of 100–150 mm and (iii) displayed a lack of voltage dependence of conductance in asymmetric solutions that suggested that ion-binding sites
were situated midway along the channel. Together, these characteristics indicate that the permeation properties of the olfactory
CNG channels are significantly different from those of photoreceptor CNG channels.
Received: 7 November 1996/Revised: 24 March 1997 相似文献
13.
Studies were conducted to examine the influence of the H+-ATPase inhibitor bafilomycin A1 on cultured rabbit nonpigmented ciliary epithelial cells (NPE). Cytoplasmic pH and sodium concentrations were measured by
digital fluorescence microscopy using BCECF and SBFI respectively. In some experiments, cell sodium content was measured by
atomic absorption spectroscopy. Added alone, bafilomycin A1 (100 nm) failed to change cytoplasmic pH but it caused an increase of cytoplasmic sodium concentration which occurred within 10 min.
It is likely that the rise of cytoplasmic sodium concentration was responsible for the stimulation of active sodium-potassium
transport which occurred in bafilomycin A1-treated cells as judged by a 50% increase of ouabain sensitive potassium (86Rb) uptake. In bafilomycin A1-treated cells, but not in control cells, dimethylamiloride (DMA) inhibited ouabain-sensitive potassium (86Rb) uptake in a dose-dependent manner with an IC50 of ∼2 μm. DMA (10 μm) also prevented the increase of cytoplasmic sodium caused by bafilomycin A1. Added alone, DMA (10 μm) failed to change cytoplasmic sodium content but reduced cytoplasmic pH by ∼0.4 pH units. In cells that first received 10
μm DMA, the subsequent addition of bafilomycin A1 (100 nm) caused a further cytoplasmic pH reduction of ∼0.3 pH units. Taken together, the results suggest H+-ATPase might contribute to the regulation of basal cytoplasmic pH in cultured NPE. In the presence of bafilomycin A1, Na-H exchanger activity appears to be stimulated, so stabilizing cytoplasmic pH but resulting in an increase of cytoplasmic
sodium concentration and consequent stimulation of active sodium-potassium transport.
Received: 19 March 1999/Revised: 20 September 1999 相似文献
14.
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
15.
V.V. Grechko L.V. Fedorova A.N. Fedorov S.Ya. Slobodyanyuk D.M. Ryabinin M.N. Melnikova A.A. Bannikova A.A. Lomov V.A. Sheremet'eva V.A. Gorshkov G.A. Sevostyanova S.K. Semenova A.P. Ryskov B.M. Mednikov I.S. Darevsky 《Journal of molecular evolution》1997,45(3):332-336
Multiple band patterns of DNA repeats in the 20–500-nucleotide range can be detected by digesting genomic DNA with short—cutting
restriction endonucleases, followed by end labeling of the restriction fragments and fractionation in nondenaturing polyacrylamide
gels. We call such band patterns obtained from genomic DNA ``taxonprints' (Fedorov et al. 1992). Here we show that taxonprints
for the taxonomic groups studied (mammals, reptiles, fish, insects—altogether more than 50 species) have the following properties:
(1) All individuals from the same species have identical taxonprints. (2) Taxonprint bands can be subdivided into those specific
for a single species and those specific for groups of closely related species, genera, and even families. (3) Each restriction
endonuclease produces unique band patterns; thus, five to ten restriction enzymes (about 100 bands) may be sufficient for
a statistical treatment of phylogenetic relationships based on polymorphisms of restriction endinuclease sites. We demonstrate
that taxonprint analysis allows one to distinguish closely related species and to establish the degree of similarity among
species and among genera. These characteristics make taxonprint analysis a valuable tool for taxonomic and phylogenetic studies.
Received: 10 February 1997 / Accepted: 10 March 1997 相似文献
16.
We have investigated the interaction of two peptides (ShB — net charge +3 and ShB:E12KD13K — net charge +7) derived from the NH2-terminal domain of the Shaker K+ channel with purified, ryanodine-modified, cardiac Ca2+-release channels (RyR). Both peptides produced well resolved blocking events from the cytosolic face of the channel. At a
holding potential of +60 mV the relationship between the probability of block and peptide concentration was described by a
single-site binding scheme with 50% saturation occurring at 5.92 ± 1.06 μm for ShB and 0.59 ± 0.14 nm for ShB:E12KD13K. The association rates of both peptides varied with concentration (4.0 ± 0.4 sec−1μm
−1 for ShB and 2000 ± 200 sec−1μm
−1 for ShB:E12KD13K); dissociation rates were independent of concentration. The interaction of both peptides was influenced by applied
potential with the bulk of the voltage-dependence residing in Koff. The effectiveness of the inactivation peptides as blockers of RyR is enhanced by an increase in net positive charge. As
is the case with inactivation and block of K+ channels, this is mediated by a large increase in Kon. These observations are consistent with the proposal that the conduction pathway of RyR contains negatively charged sites
which will contribute to the ion handling properties of this channel.
Received: 15 December 1997/Revised: 13 March 1998 相似文献
17.
Hidden treatments in ecological experiments: re-evaluating the ecosystem function of biodiversity 总被引:68,自引:0,他引:68
Michael A. Huston 《Oecologia》1997,110(4):449-460
Interactions between biotic and abiotic processes complicate the design and interpretation of ecological experiments. Separating
causality from simple correlation requires distinguishing among experimental treatments, experimental responses, and the many
processes and properties that are correlated with either the treatments or the responses, or both. When an experimental manipulation
has multiple components, but only one of them is identified as the experimental treatment, erroneous conclusions about cause
and effect relationships are likely because the actual cause of any observed response may be ignored in the interpretation
of the experimental results. This unrecognized cause of an observed response can be considered a “hidden treatment.” Three
types of hidden treatments are potential problems in biodiversity experiments: (1) abiotic conditions, such as resource levels,
or biotic conditions, such as predation, which are intentionally or unintentionally altered in order to create differences
in species numbers for “diversity” treatments; (2) non-random selection of species with particular attributes that produce
treatment differences that exceed those due to “diversity” alone; and (3) the increased statistical probability of including
a species with a dominant negative or positive effect (e.g., dense shade, or nitrogen fixation) in randomly selected groups
of species of increasing number or “diversity.” In each of these cases, treatment responses that are actually the result of
the “hidden treatment” may be inadvertently attributed to variation in species diversity. Case studies re-evaluating three
different types of biodiversity experiments demonstrate that the increases found in such ecosystem properties as productivity,
nutrient use efficiency, and stability (all of which were attributed to higher levels of species diversity) were actually
caused by “hidden treatments” that altered plant biomass and productivity.
Received: 16 December 1996 / Accepted: 2 March 1997 相似文献
18.
Johnson CN 《Journal of the history of biology》2007,40(3):529-556
Almost any modern reader’s first encounter with Darwin’s writing is likely to be the “Historical Sketch,” inserted by Darwin
as a preface to an early edition of the Origin of Species, and having since then appeared as the preface to every edition after the second English edition. The Sketch was intended
by him to serve as a short “history of opinion” on the species question before he presented his own theory in the Origin proper. But the provenance of the “Historical Sketch” is somewhat obscure. Some things are known about its production, such
as when it first appeared and what changes were made to it between its first appearance in 1860 and its final form, for the
fourth English edition, in 1866. But how it evolved in Darwin’s mind, why he wrote it at all, and what he thought he was accomplishing
by prefacing it to the Origin remain questions that have not been carefully addressed in the scholarly literature on Darwin. I attempt to show that Darwin’s
various statements about the “Historical Sketch,” made primarily to several of his correspondents between 1856 and 1860, are
somewhat in conflict with one another, thus making problematic a satisfactory interpretation of how, when, and why the Sketch
came to be. I also suggest some probable resolutions to the several difficulties.
How Darwin came to settle on the title “Historical Sketch” for the Preface to the Origin is not certain, but a guess may be ventured. When he first submitted the text to Asa Gray in February 1860 he called it simply
“Preface Contributed by the Author to this American Edition” (Burkhardt et al., eds., vol. 8, 1993, p. 572; the collected
correspondence is hereafter cited as CCD). In fact he had thought of it as being properly called a Preface much earlier, perhaps as early as 1856, as will be seen
in what follows. It came to be called “An Historical Sketch of the Recent Progress of Opinion on the Origin of Species” only
in the third English edition, April 1861. This is the title it retained thereafter, with the exception of an addition to the
title in the sixth English edition, “Previously to the Publication of the First Edition of this Work” (Peckham, 1959, pp. 20, 59). The word “sketch,” on the other hand was one of two words Darwin commonly used in private correspondence to
refer to the book that would later become the Origin, the other word being “Abstract,” and both signifying that Darwin thought of the work as being a resume rather than a full-fledged
study (e.g., letter to J.D. Hooker, May 9 1856, CCD vol. 6 p. 106; letter to Baden Powell January 18 1860, CCD vol. 8 p. 41; letter to Lyell 25 June 1858, CCD v. 7, 1991, pp. 117–8; letter to Lyell May 1856, CCD, v. 6 p. 100). The most likely source of the title “Historical Sketch” for Darwin’s Preface is Charles Lyell’s Principles of Geology in which, beginning with the third edition (1834), Lyell added titles to his chapters, calling chapters 2–4 “Historical Sketch
of the Progress of Geology” (Secord, in Lyell [1997], p. xlvii; for other uses by Lyell of this expression, cf. Porter, 1976, p. 95; idem 1982, p. 38; and Lyell, 1830 [1990], p. 30). Further parallels between Lyell’s Introduction and Darwin’s “Historical Sketch” in terms of content and strategy
are suggested below. 相似文献
19.
How thyroid hormones move across biological or model membranes is a subject of controversy. The passage of the 3,5,3′triiodo
l-thyronine and 3,5,3′,5′ tetraiodo l-thyronine across model membranes was evaluated by the addition of the hormones to liposomes containing 2,4,6-trinitrobenzene
sulfonic acid. Results indicate that hormones can react with an amino-reactive compound pre-encapsulated into phosphatidylcholine
liposomes. The transversal motions of thyroid hormones were characterized by using physiological concentration levels of (125I) 3,5,3′triiodo l-thyronine and (125I) 3,5,3′,5′ tetraiodo l-thyronine. The hormone distribution between the two monolayers was time-dependent and kinetic data were fitted to a single
exponential. Results obtained show that 3,5,3′ triiodo l-thyronine can permeate phospholipid membranes and the diffusion time increases in the gel and liquid-ordered phase. On the
contrary, 3,5,3′, 5′ tetraiodo l-thyronine could not diffuse the liposomal membrane from dimyristoyl and dipalmitoyl phosphatidylcholine in gel phase and
egg yolk phosphatidylcholine:cholesterol in the liquid-ordered phase. Our results in the liquid-ordered phase suggest that
diffusion movement of thyroid hormones across cell membranes depends on the amount of cholesterol in the bilayer.
Received: 1 June 1998/Revised: 14 October 1998 相似文献
20.
T. Miyashita H. Tatsumi H. Furuta N. Mori M. Sokabe 《The Journal of membrane biology》2001,182(2):113-122
We identified a Ca2+-sensitive cation channel in acutely dissociated epithelial cells from the endolymphatic sac (ES) of guinea pigs using the
patch-clamp technique. Single-channel recordings showed that the cation channel had a conductance of 24.0 ± 1.3 pS (n= 8) in our standard solution. The relative ionic permeability of the channel was in the order K+= Na+ > Ca2+≫ Cl−. This channel was weakly voltage-dependent but was strongly activated by Ca2+ on the cytosolic side at a concentration of around 1 mm in inside-out excised patches. With cell-attached patches, however, the channel was activated by much lower Ca2+ concentrations. Treatment of the cells, under cell-attached configuration, with ionomycin (10 μm), carbonyl cyanide 3-chlorophenylhydrazone (CCCP, 20 μm), or ATP (1 mm), which increased intracellular Ca2+ concentration ([Ca2+]i), activated the channel at an estimated [Ca2+]i from 0.6 μm to 10 μm. It is suggested that some activators of the channel were deteriorated or washed out during the formation of excised patches.
Based on this Ca2+ sensitivity, we speculated that the channel contributes to the regulation of ionic balance and volume of the ES by absorbing
Na+ under certain pathological conditions that will increase [Ca2+]i. This is the first report of single-channel recordings in endolymphatic sac epithelial cells.
Received: 24 October 2000/Revised: 10 April 2001 相似文献