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1.
This paper reports the synthesis of azomethine-modified gold nanoparticles with azomethine (azomethine-AuNPs) in aqueous media, which were characterized by FT-IR spectroscopy, ultraviolet–visible spectroscopy (UV-Vis), dynamic light scattering (DLS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The azomethine-AuNPs were employed as colorimetric for Cr3+ and Co2+ ions at pH 6.2–7.5 and 8.1–9.1, at room temperature in aqueous solution. In the presence of Cr3+ and Co2+, the azomethine-AuNPs induce aggregation of the nanoparticles. Upon aggregation, the surface plasmon absorption band red-shifts so that the nanoparticle solution appears a blue color. The sensitivity of azomethine-AuNPs towards other metal ions, Mg2+, Mn2+, Cr6+, Na+, Ni2+, Ag+, Al3+, Ca2+, Cd2+, Cu2+, Fe2+, Fe3+, Hg2+, Cd2+, K+, Co3+, Ni2+, Pb2+, and Zn2+ are negligible. This highly selective sensor allows a direct quantitative assay of Co2+ and Cr3+ with colorimetric detection limits of 83.22 and 108 nM, respectively. 相似文献
2.
Summary The outer membranes of plant cells contain channels which are highly selective for K +. However, many of their properties and their similarities to K + channels found in animal cells had not previously been established. The channels open when the cells are depolarized in solutions with a high K +/Ca 2+ ratio. In this work, the pharmacology of a previously identified plant K + channel was examined. This survey showed that the channels have many properties which are similar to those of high-conductance Ca 2+-activated K + channels (high G K +(Ca 2+)). K + currents in Chara were reduced by TEA +, Na +, Cs +, Ba 2+, decamethonium and quinine, all inhibitors of, among other things, high G K +(Ca 2+) channels. Tetracaine also inhibited K + currents Chara, but its effect on most types of K + channels in animal tissues is unknown. The currents were not inhibited by 4-aminopyridine (4AP), caffeine, tolbutamide, dendrotoxin, apamin or tubocurarine, which do not inhibit high G K +(Ca 2+) channels, but affect other classes of K + channels. The channels were locked open by 4AP, in a remarkably similar manner to that reported for K +(Ca 2+) channels of a molluscan neuron. No evidence for the role of the inositol cycle in channel behavior was found, but its role in K + channel control in animal cells is obscure. Potassium conductance was slightly decreased upon reduction of cytoplasmic ATP levels by cyanide + salicylhydroxamic acid (SHAM), consistent with channel control by phosphorylation. The anomalously strong voltage dependence of blockade by some ions (e.g. Cs +) is consistent with the channels being multiion pores. However, the channels also demonstrate some differences from the high G K +(Ca 2+) channels found in animal tissues. The venom of the scorption, Leiurus quinquestriatus (LQV), and a protein component, charybdotoxin (CTX), an apparently specific inhibitor of high G K +(Ca 2+) channels in various animal tissues, had no effect on the K + channels in the Chara plasmalemma. Als,, pinacidil, an antihypertensive drug which may increase high G K +(Ca 2+) channel activity had no effect on the channels in Chara. Although the described properties of the Chara K + channels are most similar to those of high conductance K +(Ca 2+) in animal cells, the effects of CTX and pinacidil are notably different; the channels are clearly of a different structure to those found in animal cells, but are possibly related. 相似文献
3.
The effects of cadmium and lead on the internal concentrations of Ca 2+ and K +, as well as on the uptake and translocation of K( 86Rb +) were studied in winter wheat ( Triticum aestivum L. a. MV-8) grown hydroponically at 2 levels of K + (100 uM and 10 mM). Cd 2+ and Pb 2+ were applied in the nutrient solution in the range of 0.3 to 1000 u.M. Growth was more severely inhibited by Cd 2+ and in the high-K + plants as compared to Pb z+ and low-K + plants. Ions of both heavy metals accumulated in the roots and shoots, but the K + status influenced their levels. Ca 2+ accumulation was increased by low concentrations of Cd 2+ mainly in low-K + shoots, whereas it was less influenced by Pb 2+. The distribution of Cd 2+ and Ca 2+ in the plant and in the growth media indicated high selectivity for Cd 2+ in the root uptake, while Ca 2+ was preferred in the radial and/or xylem transport. Cd 2+ strongly inhibited net K + accumulation in high-K + plants but caused stimulation at low K + supply. In contrast, the metabolis-dependent influx of K +( 86Rb +) was inhibited in low-K + plants, while the passive influx in high-K + plants was stimulated. Translocation of K + from the roots to the shoots was inhibited by Cd 2+ but less influenced in Pb 2+-treated plants. It is concluded that the effects of heavy metals depend upon the K +-status of the plants. 相似文献
4.
We prepared an aminothiourea‐derived Schiff base (DA) as a fluorescent chemosensor for Hg 2+ ions. Addition of 1 equiv of Hg 2+ ions to the aqueous solution of DA gave rise to an obvious fluorescence enhancement and the subsequent addition of more Hg 2+ induced gradual fluorescence quenching. Other competing ions, including Pb 2+, Cd 2+, Cr 3+, Zn 2+, Fe 2+, Co 3+, Ni 2+, Ca 2+, Mg 2+, K + and Na +, did not induce any distinct fluorescence changes, indicating that DA can selectively detect Hg 2+ ions in aqueous solution. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
5.
Kinetics and inhibition of Na +/K +-ATPase and Mg 2+-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu 2+, Zn 2+, Fe 2+ and.Co 2+) and heavy metals (Hg 2+and Pb 2+) ions were studied. All investigated metals produced a larger maximum inhibition of Na +/K +-ATPase than Mg 2+-ATPase activity. The free concentrations of the key species (inhibitor, MgATP 2 ? , MeATP 2 ? ) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu 2+/Fe 2+ or Hg 2+/Pb 2+caused additive inhibition, while Cu 2+/Zn 2+ or Fe 2+/Zn 2+ inhibited Na +/K +-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu 2+/Fe 2+ or Cu 2+/Zn 2+ inhibited Mg 2+-ATPase activity synergistically, while Hg 2+/Pb 2+ or Fe 2+/Zn 2+ induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na +/K +-ATPase activity by reducing the maximum velocities (V max) rather than the apparent affinity (K m) for substrate MgATP 2-, implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg 2+-ATPase activity by Zn 2+, Fe 2+ and Co 2+ as well as kinetic analysis indicated two distinct Mg 2+-ATPase subtypes activated in the presence of low and high MgATP 2 ? concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na +/K +-ATPase with various potencies. Furthermore, these ligands also reversed Na +/K +-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg 2+-induced inhibition was not obtained. 相似文献
6.
Summary K +, Cl –, and Ca 2+ channels in the vacuolar membrane of tobacco cell suspension cultures have been investigated using the patch-clamp technique. In symmetrical 100mM K +, K + channels opened at positive vacuolar membrane potentials (cytoplasmic side as reference) had different conductances of 57 pS and 24 pS. K + channel opened at negative vacuolar membrane potentials had a conductance of 43 pS. The K + channels showed a significant discrimination against Na + and Cl –. The Cl – channel opened at positive vacuolar membrane potentials for cytoplasmic Cl – influx had a high conductance of 110pS in symmetrical 100mM Cl –. When K + and Cl – channels were excluded from opening, no traces were found of Ca 2+ channel activity for vacuolar Ca 2+ release induced by inositol 1,4,5-trisphosphate or other events. However, we found a 19pS Ca 2+ channel which allowed influx of cytoplasmic Ca 2+ into the vacuole when the Ca 2+ concentration on the cytoplasmic side was high. When Ca 2+ was substituted by Ba 2+, the conductance of the 19 pS channel became 30 pS and the channel showed a selectivity sequence of Ba 2+Sr 2+Ca 2+Mg 2+=10.60.60.21. The reversal potentials of the channel shifted with the change in Ca 2+ concentration on the vacuolar side. The channel could be efficiently blocked from the cytoplasmic side by Cd 2+, but was insensitive to La 3+, Gd 3+, Ni 2+, verapamil, and nifedipine. The related ion channels in freshly isolated vacuoles from red beet root cells were also recorded. The coexistence of the K +, Cl –, and Ca 2+ channels in the vacuolar membrane of tobacco cells might imply a precise classification and cooperation of the channels in the physiological process of plant cells. 相似文献
7.
Summary Patch-clamp studies of cytoplasmic drops from the charophyte Chara australis have previously revealed K + channels combining high conductance (170 pS) with high selectivity for K +, which are voltage activated. The cation-selectivity sequence of the channel is shown here to be: K +>Rb +>NH
4
+
Na + and Cl –. Divalent cytosolic ions reduce the K + conductance of this channel and alter its K + gating in a voltage-dependent manner. The order of blocking potency is Ba 2+>Sr 2+>Ca 2+>Mg 2+. The channel is activated by micromolar cytosolic Ca 2+, an activation that is found to be only weakly voltage dependent. However, the concentration dependence of calcium activation is quite pronounced, having a Hill coefficient of three, equivalent to three bound Ca 2+ needed to open the channel. The possible role of the Ca 2+-activated K + channel in the tonoplast of Chara is discussed. 相似文献
8.
High-conductance, Ca 2+-activated K + channels from the basolateral membrane of rabbit distal colon epithelial cells were reconstituted into planar phospholipid
bilayers to examine the effect of Mg 2+ on the single-channel properties. Mg 2+ decreases channel current and conductance in a concentration-dependent manner from both the cytoplasmic and the extracellular
side of the channel. In contrast to other K + channels, Mg 2+ does not cause rectification of current through colonic Ca 2+-activated K + channels. In addition, cytoplasmic Mg 2+ decreases the reversal potential of the channel. The Mg 2+-induced decrease in channel conductance is relieved by high K + concentrations, indicating competitive interaction between K + and Mg 2+. The monovalent organic cation choline also decreases channel conductance and reversal potential, suggesting that the effect
is unspecific. The inhibition of channel current by Mg 2+ and choline most likely is a result of electrostatic screening of negative charges located superficially in the channel entrance.
But in addition to charge, other properties appear to be necessary for channel inhibition, as Na + and Ba 2+ are no (or only weak) inhibitors. Mg 2+ and possibly other cations may play a role in the regulation of current through these channels.
Received: 25 August 1995/Revised: 16 November 1995 相似文献
9.
Steady state kinetics were used to examine the influence of Cd 2+ both on K + stimulation of a membrane-bound ATPase from sugar beet roots ( Beta vulgaris L. cv. Monohill) and on K +( 86Rb +) uptake in intact or excised beet roots. The in vitro effect of Cd 2+ was studied both on a 12000–25000 g root fraction of the (Na ++K ++Mg 2+)ATPase and on the ATPase when further purified by an aqueous polymer two-phase system. The observed data can be summarized as follows: 1) Cd 2+ at high concentrations (>100 μ M) inhibits the MgATPase activity in a competitive way, probably by forming a complex with ATP. 2) Cd 2+ at concentrations <100 μ M inhibits the specific K + activation at both high and low affinity sites for K +. The inhibition pattern appears to be the same in the two ATPase preparations of different purity. In the presence of the substrate MgATP, and at K + <5 m M, the inhibition by Cd 2+ with respect to K + is uncompetitive. In the presence of MgATP and K + >10 μ M, the inhibition by Cd 2+ is competitive. 3) At the low concentrations of K +, Cd 2+ also inhibits the 2,4-dinitrophenol(DNP)-sensitive (metabolic) K +( 86Rb +) uptake uncompetitively both in excised roots and in roots of intact plants. 4) The DNP-insensitive (non metabolic) K +( 86Rb +) uptake is little influenced by Cd 2+. As Cd 2+ inhibits the metabolic uptake of K +( 86Rb +) and the K + activation of the ATPase in the same way at low concentrations of K +, the same binding site is probably involved. Therefore, under field conditions, when the concentration of K + is low, the presence of Cd 2+ could be disadvantageous. 相似文献
10.
The relationship between Pb 2+ accumulation and cation (K +, Mg 2+, Ca 2+) release in Saccharomyces cerevisiae was extensively investigated. As Pb 2+ accumulation proceeded, the release of cellular metal ions such as K +, Mg 2+ and Ca 2+ was concomitantly released within 24 h, thereafter Pb 2+ penetrated into the inner cellular parts and consequently plasmolysis of the cell was observed by TEM analysis. Pb 2+ accumulation process in S. cerevisiae after 24 h was metabolism-independent because of the absence of cell viability. As the cell storage time was prolonged, the released amount of K + was markedly increased, while the amount of accumulated Pb 2+ was nearly constant regardless of cell storage time and the time required to reach an equilibrium state was shortened. The autoclaved cells had less Pb 2+ accumulation capacity than the untreated cells, and the amounts of released K + and Mg 2+ were very low due to the denaturation of cell surface and cell membrane. 相似文献
11.
Inorganic ions have been used widely to investigate biophysical properties of high voltage-activated calcium channels (HVA:
Ca v1 and Ca v2 families). In contrast, such information regarding low voltage-activated calcium channels (LVA: Ca v3 family) is less documented. We have studied the blocking effect of Cd 2+, Co 2+ and Ni 2+ on T-currents expressed by human Ca v3 channels: Ca v3.1, Ca v3.2, and Ca v3.3. With the use of the whole-cell configuration of the patch-clamp technique, we have recorded Ca 2+ (2 m M) currents from HEK−293 cells stably expressing recombinant T-type channels. Cd 2+ and Co 2+ block was 2- to 3-fold more potent for Ca v3.2 channels ( EC50 = 65 and 122 μM, respectively) than for the other two LVA channel family members. Current-voltage relationships indicate
that Co 2+ and Ni 2+ shift the voltage dependence of Ca v3.1 and Ca v3.3 channels activation to more positive potentials. Interestingly, block of those two Ca v3 channels by Co 2+ and Ni 2+ was drastically increased at extreme negative voltages; in contrast, block due to Cd 2+ was significantly decreased. This unblocking effect was slightly voltage-dependent. Tail-current analysis reveals a differential
effect of Cd 2+ on Ca v3.3 channels, which can not close while the pore is occupied with this metal cation. The results suggest that metal cations
affect differentially T-type channel activity by a mechanism involving the ionic radii of inorganic ions and structural characteristics
of the channels pore. 相似文献
12.
Novel phenanthroline Schiff base fluorescent sensors L1 , L2 , and D1 were designed and synthesized. The sensing abilities of the compounds in the presence of metal cations (Li +, Na +, K +, Ag +, Mg 2+, Ba 2+, Ca 2+, Mn 2+, Pb 2+, Hg 2+, Ni 2+, Zn 2+, Cd 2+, Co 2+, Cu 2+, Cr 3+, Fe 3+, Fe 2+, Al 3+, and Eu 3+) were studied by UV‐vis and fluorescent spectroscopy. The compounds L1 , L2 , and D1 could act as Eu 3+ ion turn‐off fluorescent sensors based on ligand‐to‐metal binding mechanism in DMSO‐H 2O solution (v/v = 1:1, 10 mM Tris, pH = 7.4). Additionally, the L1 –Eu 3+ and D1 –Eu 3+ complexes could be applied as turn‐on enantioselective sensors sensing of malate anion isomers with color changes. Furthermore, biological experiments using living PC‐12 cells demonstrated that L1 and D1 had excellent membrane permeability and could be used as effective fluorescent sensors for detecting Eu 3+ and malate anion in living cells. 相似文献
13.
Cadmium (Cd 2+) interferes with the uptake, transport and utilization of several macro‐ and micronutrients, which accounts, at least in part, for Cd 2+ toxicity in plants. However, the mechanisms underlying Cd 2+ interference of ionic homeostasis is not understood. Using biophysical techniques including membrane potential measurements, scanning ion‐selective electrode technique for non‐invasive ion flux assays and patch clamp, we monitored the effect of Cd 2+ on calcium (Ca 2+) and potassium (K +) transport in root hair cells of rice. Our results showed that K + and Ca 2+ contents in both roots and shoots were significantly reduced when treated with exogenous Cd 2+. Further studies revealed that three cellular processes may be affected by Cd 2+, leading to changes in ionic homeostasis. First, Cd 2+‐induced depolarization of the membrane potential was observed in root hair cells, attenuating the driving force for cation uptake. Second, the inward conductance of Ca 2+ and K + was partially blocked by Cd 2+, decreasing uptake of K + and Ca 2+. Third, the outward K + conductance was Cd 2+‐inducible, decreasing the net content of K + in roots. These results provide direct evidence that Cd 2+ impairs uptake of Ca 2+ and K +, thereby disturbing ion homeostasis in plants. 相似文献
14.
To determine if their properties are consistent with a role in regulation of transepithelial transport, Ca 2+-activated K + channels from the basolateral plasma membrane of the surface cells in the distal colon have been characterized by single channel analysis after fusion of vesicles with planar lipid bilayers. A Ca 2+-activated K + channel with a single channel conductance of 275 pS was predominant. The sensitivity to Ca 2+ was strongly dependent on the membrane potential and on the pH. At a neutral pH, the K
0.5 for Ca 2+ was raised from 20 nm at a potential of 0 mV to 300 nm at –40 mV. A decrease in pH at the cytoplasmic face of the K + channel reduced the Ca 2+ sensitivity dramatically. A loss of the high sensitivity to Ca 2+ was also observed after incubation with MgCl 2, possibly a result of dephosphorylation of the channels by endogenous phosphatases. Modification of the channel protein may thus explain the variation in Ca 2+ sensitivity between studies on K + channels from the same tissue. High affinity inhibition ( K
0.5=10 nm) by charybdotoxin of the Ca 2+-activated K + channel from the extracellular face could be lifted by an outward flux of K + through the channel. However, at the ion gradients and potentials found in the intact epithelium, charybdotoxin should be a useful tool for examination of the role of maxi K + channels. The high sensitivity for Ca 2+ and the properties of the activator site are in agreement with an important regulatory role for the high conductance K + channel in the epithelial cells.Dr. E. Moczydlowsky, Yale University School of Medicine, New Haven, CT, and Dr. Per Stampe, Brandeis University, Waltham, MA, are thanked for introduction to the bilayer technique. Tove Soland is thanked for excellent technical assistance. This work was supported by the Novo Nordisk Foundation, the Carlsberg Foundation, the Danish Medical Research Council, and the Austrian Research Council. 相似文献
15.
The action of a wide range of drugs effective on Ca 2+ channels in animal tissues has been measured on Ca 2+ channels open during the action potential of the giant-celled green alga, Chara corallina. Of the organic effectors used, only the 1,4-dihydropyridines were found to inhibit reversibly Ca 2+ influx, including, unexpectedly, Bay K 8644 and both isomers of 202–791. Methoxyverapamil (D-600), diltiazem, and the diphenylbutylpiperidines,
fluspirilene and pimozide were found not to affect the Ca 2+ influx. Conversely, bepridil greatly and irreversibly stimulated Ca 2+ influx, and with time, stopped cytoplasmic streaming (which is sensitive to increases in cytoplasmic Ca 2+). By apparently altering the cytoplasmic Ca 2+ levels with various drugs, it was found that (with the exception of the inorganic cation, La 3+) treatments likely to lead to an increase in cytoplasmic Ca 2+ levels caused an increase in the rate of closure of the K + channels. Similarly, treatments likely to lead to a decrease in cytoplasmic Ca 2+ decreased the rate of K + channel closure. The main effect of bepridil on the K + channels was to increase the rate of voltage-dependent channel closure. The same effect was obtained upon increasing the
external concentration of Ca 2+, but it is likely that this was due to effects on the external face of the K + channel. Addition of any of the 1,4-dihydropyridines had the opposite effect on the K + channels, slowing the rate of channel closure. They sometimes also reduced K + conductance, but this could well be a direct effect on the K + channel; high concentrations (50 to 100 μM) of bepridil also reduced K + conductance. No effect of photon irradiance or of abscisic acid could be consistently shown on the K + channels. These results indicate a control of the gating of K + channels by cytoplasmic Ca 2+, with increased free Ca 2+ levels leading to an increased rate of K +-channel closure. As well as inhibiting Ca 2+ channels, it is suggested that La 3+ acts on a Ca 2+-binding site of the K + channel, mimicking the effect of Ca 2+ and increasing the rate of channel closure. 相似文献
16.
Magnesium-dependent adenosine triphosphatase has been purified from sheep kidney medulla plasma membranes. The purification, which is based on treatment of a kidney plasma membrane fraction with 0.5% digitonin in 3 mm MgCl 2, effectively separates the Mg 2+-ATPase from (Na + + K +)-ATPase present in the same tissue and yields the Mg 2+-ATPase in soluble form. The purified enzyme is activated by a variety of divalent cations and trivalent cations, including Mg 2+, Mn 2+, Ca 2+, Co 2+, Fe 2+, Zn 2+, Eu 3+, Gd 3+, and VO 2+. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme shows two bands with Rf values corresponding to molecular weights of 150,000 and 77,000. The larger peptide is phosphorylated by [γ- 32P]ATP, suggesting that this peptide may contain the active site of the Mg 2+-ATPase. The Mg 2+-ATPase activity is unaffected by the specific (Na + + K +)-ATPase inhibitor ouabain. 相似文献
17.
Two BODIPY derivative sensors for metal ion recognition containing 10-(4-hydroxyphenyl) ( L1) and 10-(3,4-dihydroxyphenyl) ( L2) were synthesized in a one-pot reaction of benzaldehyde derivative and 2,4-dimethylpyrrole in the presence of trifluoroacetic acid as catalyst. The binding abilities between these sensors and 50 equivalents of Na +, K +, Ag +, Ca 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Pb 2+, Al 3+ and Cr 3+ ions were studied using UV–vis and fluorescent spectroscopic methods. Of all the metal ions tested, Al 3+ ion showed the greatest decrease in intensity in the spectra of the sensors, and therefore Al 3+ ion forms the strongest complex. The binding abilities of BODIPY receptors with Na +, Ag +, Ca 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+ and Al 3+ ions were also investigated using density functional theory (DFT) calculations at B3LYP/LanL2DZ theoretical level. The calculated results point to the same conclusion. DFT calculations also provided the HOMO–LUMO energy levels, which can explain the spectrum change upon complexation. Figure Graphical structure, fluorescent spectra, frontier orbital energy diagrams and electron-transfer paths in sensor L1, and after attachment with Al 3+ ion. 相似文献
18.
Exposure of proliferating cells to specific water solube metal compounds at 0.1 or 1.0 mM concentrations inhibited cell growth and also depressed the induction of ornithine decarboxylase, an enzyme which is tightly coupled to the initiation of cell growth. Salts of Co ++, Ni ++, Cu ++, Cr +6 and Cd ++ significantly reduced incorporation of radiolabeled leucine, thymidine or uridine into trichloroacetic acid insoluble material, inhibited the doubling of Chinese hamster ovary cells, and blocked the the induction of ornithine decarboxylase. The addition of similar concentrations of other metals such as Fe ++, K +, Mg ++, Pb ++, Ca ++ or Sn ++ had no effect on ODC induction and also did not inhibit the other parameters associated with cell proliferation which were measured. These results suggest that ornithine decarboxylase induction can be used as a marker of metal induced growth arrest. 相似文献
19.
Summary Slow muscle fibers were dissected from cruralis muscles of Rana esculenta and Rana pipiens. Isometric contractures were evoked by application of K +-rich Ringer's containing Ca 2+, Ni 2+, Co 2+, Mn 2+ or Mg 2+. High (7.2 mmol/liter) external Ca 2+ concentration raised, 0 Ca 2+ lowered the K + threshold. Replacing Ca 2+ by Ni 2+ or Co 2+ had an effect similar to that of high Ca 2+ Ringer's. In Mg 2+ Ringer's the K + concentration-response curve was flattened. These effects were observed already after short exposure times in both species of slow fibers. When Ca 2+ was removed for long periods of time the slow fibers of R. esculenta lost their contractile response to application of high K + concentrations much more quickly than those of R. pipiens, while the response to caffeine (20 mmol/liter) was maintained. Upon readmission of Ca 2+ contractile ability was quickly restored in the slow fibers of both R. esculenta and R. pipiens, but the effects of Ni 2+ (or Co 2+, Mn 2+ and Mg 2+) were much larger in R. esculenta than in R. pipiens slow fibers. It is concluded that divalent cations have two different sites of action in slow muscle fibers. K + threshold seems to be affected through binding to sites at the membrane surface; these sites bind Ni 2+ and Co 2+ more firmly than Ca 2+. The second site is presumably the voltage sensor in the transverse tubular membrane, which controls force production, and where Ca 2+ is the most effective species of the divalent cations examined.We are grateful to Mrs. S. Pelvay for technical assistance. 相似文献
20.
Ca 2+-activated K + channels play an important role in Ca 2+ signal transduction and may be regulated by mechanisms other than a direct effect of Ca 2+. Inside-out patches of the apical membrane of confluent transformed rabbit cortical collecting duct cells cultured on collagen were subjected to patch clamp analysis. Two types of K + channel, of medium and high conductance, were observed. The latter channel was characterized by a K +/Na + permeability ratio of 10, an inwardly rectified current, a conductance of 80 pS at 0 mV, and an open probability dependent on both voltage and Ca 2+. Guanosine 5-triphosphate (GTP) but not a guanosine 5-diphosphate (GDP) analogue, adenosine 5-triphosphate (ATP), cytidine 5-triphosphate (CTP), or inosine 5-triphosphate (ITP), inhibited the activity of this Ca 2+-activated K + channel. The inhibitory effect of GTP was dose dependent, with a 50% inhibitory concentration of 10 –5
m in the absence of Mg 2+. In the presence of Mg 2+ (1 m m), which is required for the binding of GTP to G proteins, the 50% inhibitory concentration decreased to 3×10 –12
m. Pertussis toxin or cholera toxin (each at 10 ng/ml) did not prevent the inhibitory effect of GTP. After removal of GTP from the medium bathing an inhibited channel, subsequent application of Ca 2+ failed to activate the channel. Ca 2+-activated K + channels of smooth muscle cells and proximal tubule cells did not respond to GTP. Thus, the Ca 2+-activated K + channel in the apical membrane of collecting duct cells is inhibited by GTP, which appears to exert its effect via a G protein that is insensitive to both cholera and pertussis toxins. 相似文献
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