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1.
Amines at alkaline pH induce in cells of the halotolerant alga Dunaliella a transient stress that is manifested by a drop in ATP and an increase of cytoplasmic pH. As much as 300 millimolar NH 4+ are taken up by the cells at pH 9. The uptake is not associated with gross changes in volume and is accompanied by K + efflux. Most of the amine is not metabolized, and can be released by external acidification. Recovery of the cells from the amine-induced stress occurs within 30 to 60 minutes and is accompanied by massive swelling of vacuoles and by release of the fluorescent dye atebrin from these vacuoles, suggesting that amines are compartmentalized into acidic vacuoles. The time course of ammonia uptake into Dunaliella cells is biphasic—a rapid influx, associated with cytoplasmic alkalinization, followed by a temperature-dependent slow uptake phase, which is correlated with recovery of cellular ATP and cytoplasmic pH. The dependence of amine uptake on external pH indicates that it diffuses into the cells in the free amine form. Studies with lysed cell preparations, in which vacuoles become exposed but retain their capacity to accumulate amines, indicate that the permeability of the vacuolar membrane to amines is much higher than that of the plasma membrane. The results can be retionalized by assuming that the initial amine accumulation, which leads to rapid vacuolar alkalinization, activates metabolic reactions that further increase the capacity of the vacuoles to sequester most of the amine from the cytoplasm. The results indicate that acidic vacuoles in Dunaliella serve as a high-capacity buffering system for amines, and as a safeguard against cytoplasmic alkalinization and uncoupling of photosynthesis. 相似文献
2.
Ammonium and methylammonium are rapidly taken up by cultures of Azotobacter vinelandii respiring in the presence of succinate. The rate of methylamine uptake increased with external pH from 5.5 to 7.5 but increasing the pH further to 8.5 had little effect on activity, indicating that methylammonium cation rather than uncharged methylamine is the permeant species. The kinetics of methylammonium entry followed the Michaelis-Menten relationship, yielding a Km of 25 μM and a Vmax of 3.8 nmol/min per mg of cell protein. At saturating concentrations ammonium was taken up at rates 30-fold higher than those for methylammonium. Ammonium was a competitive inhibitor of methylammonium uptake and gave an inhibition constant of 1 μM. Ammonium derivatives were inhibitors of methylammonium entry in order of effectiveness: hydrazine > methylhydrazine > formamidine > guanidine > dimethylamine > ethylamine; amides and amino acids did not block uptake. Likewise, metal cations inhibited in the order Tl + > Cs + > Rb +, whereas Na +, K +, and Li + produced no significant effect. Methylammonium uptake was blocked in cells exposed to an uncoupler, p-trifluorome-thoxycarbonyl cyanide-phenyl hydrazone or gramicidin D, but not with dicyclo-hexylcarbodiimide or arsenate. Valinomycin stimulated methylammonium entry into cells in a K +-free medium but prevented entry in the presence of 10 mM K +. Monensin and nigericin had little effect on transport. These results indicate that methylammonium and ammonium ions enter A. vinelandii electrogenically via a specific transporter. 相似文献
3.
The effect of a variety of ions and other solutes on the accumulation of the β-amino acid, taurine, was examined in rat renal brush-border membrane vesicles. Initial taurine uptake (15 and 30 s) is sodium-dependent with a typical overshoot. This Na + effect was confirmed by exchange diffusion and gramicidin inhibition of taurine uptake. External K + or Li + do not increase taurine accumulation more than Na +-free mannitol, except that the combination of external K + and Na 1 in the presence of nigericin enhances uptake. Of all anions tested, including more permeant (SCN − and NO 3−) or less permeant (SO 42−), chloride supported taurine accumulation to a significantly greater degree. Preloading vesicles with choline chloride reduced taurine uptake, suggesting that external Cl − stimulates uptake. Since this choline effect could be related to volume change, due to the slow diffusion of choline into vesicles, brush-border membrane vesicles were pre-incubated with LiCl, LiNO 3 and LiSO 4. Internal LiCl, regardless of the final Na + anion mixture, reduced initial rate (15 and 60 s) and peak (360 s) taurine uptake. Internal LiNO 3 or LiSO 4 with external NaCl resulted in similar or higher values of uptake at 15, 60 and 360 s, indicating a role for external Cl − in taurine uptake in addition to Na + effect. Although uptake by vesicles is greatest at pH 8.0 and inhibited at acidic pH values (pH less than 7.0), an externally directed H + gradient does not influence uptake. Similarly, amiloride, an inhibitor of the Na +/H + antiporter, had no influence on taurine accumulation over a wide variety of concentrations or at low Na + concentrations. Taurine uptake is blocked only by other β-amino acids and in a competitive fashion. d-glucose and p-aminohippurate at high concentrations (> 10 −3 M) reduce taurine uptake, possibly by competing for sodium ions, although gramicidin added in the presence of d-glucose inhibits taurine uptake even further. These studies more clearly define the nature of the renal β-amino acid transport system in brush-border vesicles and indicate a role for external Cl in this uptake system. 相似文献
4.
The dependence of the light-induced H + gradient in chloroplasts (ΔpH) on external pH was examined using the distribution of aniline, an amine of low p Ka. ΔpH was essentially independent of pH over the range of 7–8. It was previously reported that ΔpH, determined from the distribution of relatively polar amines of high p Ka, decreased as the pH was lowered below 8. It is suggested that, in the case of amines of high p Ka, ΔpH values determined at low external pH values are too low because the permeability of chloroplasts to the amine cation relative to that of the unprotonated form may be significant. 相似文献
5.
We report here on the putative coupling between a high affinity K + uptake system which operates at low external K + concentrations ( Km = 10-20 micromolar), and H + efflux in roots of intact, low-salt-grown maize plants. An experimental approach combining electrophysiological measurements, quantification of unidirectional K +( 86Rb +) influx, and the simultaneous measurement of net K + and H + fluxes associated with individual cells at the root surface with K +- and H +-selective microelectrodes was utilized. A microelectrode system described previously (IA Newman, LV Kochian, MA Grusak, and WJ Lucas [1987] Plant Physiol 84: 1177-1184) was used to quantify net ion fluxes from the measurement of electrochemical potential gradients for K + and H + ions within the unstirred layer at the root surface. No evidence for coupling between K + uptake and H + efflux could be found based on: (a) extremely variable K +:H + flux stoichiometries, with K + uptake often well in excess of H + efflux; (b) dramatic time-dependent variability in H + extrusion when both fluxes were measured at a particular location along the root over time; and (c) a lack of pH sensitivity by the high affinity K + uptake system (to changes in external pH) when net K + uptake, unidirectional K +( 86Rb +) influx, and K +-induced depolarizations of the membrane potential were determined in uptake solutions buffered at pH values from pH 4 to 8. Based on the results presented here, we propose that high affinity active K + absorption into maize root cells is not mediated by a K +/H + exchange mechanism. Instead, it is either due to the operation of a K +-H + cotransport system, as has been hypothesized for Neurospora, or based on the striking lack of sensitivity to changes in extracellular pH, uptake could be mediated by a K +-ATPase as reported for Escherichia coli and Saccharomyces. 相似文献
6.
Photosynthesis, stroma-pH, and internal K + and Cl − concentrations of isolated intact chloroplasts from Spinacia oleracea, as well as ion (K +, H +, Cl −) movements across the envelope, were measured over a wide range of external KCl concentrations (1-100 millimolar). Isolated intact chloroplasts are a Donnan system which accumulates cations (K+ or added Tetraphenylphosphonium+) and excludes anions (Cl−) at low ionic strength of the medium. The internally negative dark potential becomes still more negative in the light as estimated by Tetraphenylphosphonium+ distribution. At 100 millimolar external KCl, potentials both in the light and in the dark and also the light-induced uptake of K+ or Na+ and the release of protons all become very small. Light-induced K+ uptake is not abolished by valinomycin suggesting that the K+ uptake is not primarily active. Intact chloroplasts contain higher K+ concentrations (112-157 millimolar) than chloroplasts isolated in standard media. Photosynthetic activity of intact chloroplasts is higher at 100 millimolar external KCl than at 5 to 25 millimolar. The pH optimum of CO2 fixation at high K+ concentrations is broadened towards low pH values. This can be correlated with the observation that high external KCl concentrations at a constant pH of the suspending medium produce an increase of stroma-pH both in the light and in the dark. These results demonstrate a requirement of high external concentrations of monovalent cations for CO2 fixation in intact chloroplasts. 相似文献
7.
H +-pumping ATPases were detected in microsomal vesicles of oat ( Avena sativa L. var Lang) roots using [ 14C]methylamine distribution or quinacrine fluorescent quenching. Methylamine (MeA) accumulation into vesicles and quinacrine quench were specifically dependent on Mg,ATP. Both activities reflected formation of a proton gradient (ΔpH) (acid inside) as carbonyl cyanide m-chlorophenylhydrazone, nigericin (in the presence of K +), or gramicidin decreased MeA uptake or increased quinacrine fluorescence. The properties of H + pumping as measured by MeA uptake were characterized. The Kmapp for ATP was about 0.1 millimolar. Mg,GTP and Mg, pyrophosphate were 19% and 30% as effective as Mg,ATP. MeA uptake was inhibited by N,N′-dicyclohexylcarbodiimide and was mostly insensitive to oligomycin, vanadate, or copper. ATP-dependent MeA was stimulated by anions with decreasing order of potency of Cl − > Br − > NO 3− > SO 42−, iminodiacetate, benzene sulfonate. Anion stimulation of H + pumping was caused in part by the ability of permeant anions to dissipate the electrical potential and in part by a specific requirement of Cl − by a H + -pumping ATPase. A pH gradient, probably caused by a Donnan potential, could be dissipated by K + in the presence or absence of ATP. MeA uptake was enriched in vesicles of relatively low density and showed a parallel distribution with vanadate-insensitive ATPase activity on a continuous dextran gradient. ΔpH as measured by quinacrine quench was partially vanadate-sensitive. These results show that plant membranes have at least two types of H + -pumping ATPases. One is vanadate-sensitive and probably enriched in the plasma membrane. One is vanadate-resistant, anion-sensitive and has many properties characteristic of a vacuolar ATPase. These results are consistent with the presence of electrogenic H + pumps at the plasma membrane and tonoplast of higher plant cells. 相似文献
8.
Green thallus cells of the aquatic liverwort, Riccia fluitans, are rapidly depolarized in the presence of 1–20 μM NH 4Cl and 5–100 μM CH 3NH 3Cl, respectively. Simultaneously, the membrane conductance is increased from 0.41 to 1.2 S · m ?2. Uptake of [ 14C]methylamine is stimulated by increasing [K +] o and inhibited by increasing [Na +] o or [H +] o, is highly voltage sensitive, and saturates at low amine concentrations.Double-reciprocal plots of (a) maximal membrane depolarization and (b) methylamine uptake vs. external amine concentration give apparent Km values of 2 ± 1 μM ammonia and 25–50 μM methylamine; Km values for changes in conductance and membrane current are greater and voltage dependent. Whereas the amine transport into the cell is strongly inhibited by CN ?, the amine efflux is stimulated.The current-voltage characteristics of the ammonia transport are represented by a sigmoid curve with an equilibrium potential of ?60 mV, and this is understood as a typical carrier curve with a saturation current of about 70 mA · m ?2. It is further concluded that the evidently carrier-mediated transport is competitive for the two amines tested, and that ammonia and methylamine are transported in the protonated form as NH 4+ and CH 3NH 3+ into the cytoplasm. 相似文献
9.
Mg 2+-ATP-dependent H +-translocation has been studied in membrane vesicles derived from the roots of Gossypium hirsutum L. var. Acala San Jose 2. Establishment of a positive membrane potential was followed by measuring SCN − accumulation; establishment of ΔpH across the vesicle membranes by measuring quinacrine fluorescence quenching. High specificity for ATP was shown, and H +-translocation was oligomycin stable. The pH profile for H +-translocation showed an optimum at 5.5. The relationship between SCN − accumulation and ATP concentration was approximately Michaelian; the apparent Km was 0.7 millimolar. K-2-( N-morpholino)ethanesulfonic acid strongly promoted ATP-dependent SCN − uptake (up to 180% stimulation). The effect was not given by Na-Mes. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone totally inhibited SCN − accumulation, both in the presence and absence of K-2( N-morpholino)ethanesulfonic acid. Vanadate at 200 micromolar inhibited SCN − uptake by about 10 to 40% in the absence of K +, but more strongly in its presence (about 60%). NO 3− at 100 millimolar inhibited initial rate of quinacrine quenching by about 25%. The NO 3− insensitive fraction was activated by K +; and inhibited by 200 micromolar vanadate to about 40%, provided K + was present. Saline conditions during the growth of the plants had no appreciable effect on the observed characteristics of H +-translocation. 相似文献
10.
The marine bacterium, Vibrio alginolyticus, regulates the cytoplasmic pH at about 7.8 over the pH range 6.0–9.0. By the addition of diethanolamine (a membrane-permeable amine) at pH 9.0, the internal pH was alkalized and simultaneously the cellular K + was released. Following the K + exit, the internal pH was acidified until 7.8, where the K + exit leveled off. The K + exit was mediated by a K +/H + antiporter that is driven by the outwardly directed K + gradient and ceases to function at the internal pH of 7.8 and below. The Na +-loaded cells assayed in the absence of KCl generated inside acidic ΔpH at alkaline pH due to the function of an Na +/H + antiporter, but the internal pH was not maintained at a constant value. At acidic pH range, the addition of KCl to the external medium was necessary for the alkalization of cell interior. These results suggested that in cooperation with the K + uptake system and H + pumps, the K +/H + antiporter functions as a regulator of cytoplasmic pH to maintain a constant value of 7.8 over the pH range 6.0–9.0. 相似文献
11.
The influence of NO 3− uptake and reduction on ionic balance in barley seedlings ( Hordeum vulgare, cv. Compana) was studied. KNO 3 and KCl treatment solutions were used for comparison of cation and anion uptake. The rate of Cl − uptake was more rapid than the rate of NO 3− uptake during the first 2 to 4 hours of treatment. There was an acceleration in rate of NO 3− uptake after 4 hours resulting in a sustained rate of NO 3− uptake which exceeded the rate of Cl − uptake. The initial (2 to 4 hours) rate of K + uptake appeared to be independent of the rate of anion uptake. After 4 hours the rate of K + uptake was greater with the KNO 3 treatment than with the KCl treatment, and the solution pH, cell sap pH, and organic acid levels with KNO 3 increased, relative to those with the KCl treatment. When absorption experiments were conducted in darkness, K + uptake from KNO 3 did not exceed K + uptake from KCl. We suggest that the greater uptake and accumulation of K + in NO 3−-treated plants resulted from ( a) a more rapid, sustained uptake and transport of NO 3− providing a mobile counteranion for K + transport, and ( b) the synthesis of organic acids in response to NO 3− reduction increasing the capacity for K + accumulation by providing a source of nondiffusible organic anions. 相似文献
12.
Internode disks of tomato ( Lycopersicon esculentum cv. Moneymaker) were shaken in glutamine and sucrose solutions. At low external pH (<±5.5), the uptake of these substances was accompanied with K + efflux, at high pH (>±5.5) with K + influx. Low concentrations of external K + (2 mmol l -1) stimulated the uptake of glutamine, which was strongly inhibited by the supply of high K + concentrations (20 mmol l -1). The effect of K + was particularly pronounced at high pH-values. Addition of CCCP in light reduced the uptake of glutamine to the same level as in the dark, and stopped the K + fluxes which coincided with the uptake. A model is presented wherein the movements of K + across the membrane are related to co-transport, depending on the membrane potential and the Nernst potential of K +.Abbreviation CCCP
carbonylcyanide-m-chlorophenylhydrazone 相似文献
13.
The effect of potential-dependent potassium uptake at 0–120 mM K + on matrix Ca 2+ accumulation in rat brain mitochondria was studied. An increase in oxygen consumption and proton extrusion rates as well as increase in matrix pH with increase in K + content in the medium was observed due to K + uptake into the mitochondria. The accumulation of Ca 2+ was shown to depend on K + concentration in the medium. At K + concentration ?30 mM, Ca 2+ uptake is decreased due to K +-induced membrane depolarization, whereas at higher K + concentrations, up to 120 mM K +, Ca 2+ uptake is increased in spite of membrane depolarization caused by matrix alkalization due to K + uptake. Mitochondrial K ATP + -channel blockers (glibenclamide and 5-hydroxydecanoic acid) diminish K + uptake as well as K +-induced depolarization and matrix alkalization, which results in attenuation of the potassium-induced effects on matrix Ca 2+ uptake, i.e. increase in Ca 2+ uptake at low K + content in the medium due to the smaller membrane depolarization and decrease in Ca 2+ uptake at high potassium concentrations because of restricted rise in matrix pH. The results show the importance of potential-dependent potassium uptake, and especially the K ATP + channel, in the regulation of calcium accumulation in rat brain mitochondria. 相似文献
14.
The surface pH shift induced by active H + excretion in corn ( Zea mays L.) roots was estimated using acetic acid influx as a pH probe (H Sentenac, C Grignon 1987 Plant Physiol 84: 1367-1372). At constant bulk pH, buffering the medium strongly reduced the magnitude of the surface pH shift. This was used to study the effect of surface pH shift on H + cotransports. In the absence of buffers, the surface pH shift increased with the bulk pH. Buffers decreased 32Pi influx and this effect was stronger at pH 7.2 than at pH 5.8, and stronger in the absence than in the presence of an inhibitor of the proton pump (vanadate). Buffers exerted a similar depressive and pH-dependent effect on net NO 3− uptake. They hyperpolarized the cell membrane, and stimulated 86Rb + influx, K +:H + net exchange, and malate accumulation. These results are consistent with the hypothesis that H + accumulation at the cell surface is effective in driving H + reentry. We concluded that the surface pH shift due to proton pump activity is involved in the energetic coupling of H + cotransports. 相似文献
15.
Summary The uptake of phosphate as influenced by sodium and potassium ions was investigated in the light and in the dark. It was found to be a function of the external phosphate concentration. At a low concentration (up to 10 –5 mol/l) in the presence of Na + phosphate is quickly absorbed and hence phosphate is the limiting factor for further labelling. In the presence of K + phosphate uptake is constant over a long period.The enhancement of phosphate uptake by Na + is also found when the external concentration of P is raised up to 10 –4 mol/l. Then the gross uptake proceeds over six hours, with the greatest Na +-dependent increase occurring in the label of the TCA-insoluble phosphate fraction (P u).The phosphate uptake is strongly dependent on the pH of the reaction mixture. In the presence of Na + it is highest between pH 5.6 and 7. As the uptake in the presence of K + parallels the dissociation curve of the dihydrogen form H 2PO
4
–
, the Na +-enhancement is optimal in the alkaline pH range (pH 8).On the basis of a comparison between the pH-dependence of phosphate uptake and the dependence of the uptake on the external phosphate concentration analysed by a method of enzyme kinetics, it is suggested that Ankistrodesmus metabolically transports H 2PO
4
–
but not HPO
4
=
. Moreover, it is concluded from the absence of light stimulation and the weak inhibition of the uptake by DCMU or CCCP in the presence of K + that at low P-concentrations the diffusion is limiting the uptake. Only at higher concentrations is an active phosphate uptake measured.Furthermore it is concluded that the observed Na +-stimulation of the 32P-labelling of the TCA-soluble and insoluble compounds inside the cell is indirect and depends only on the action of Na + and K + ions at the first transport site in the plasmalemma. 相似文献
17.
The activity of the Escherichia coli K + transport system TrkA was measured as a function of the cytoplasmic pH of the cell. For this purpose, pH in was decreased by the addition of the weak acids acetic acid, benzoic acid or salicylic acid to K +-depleted cells. Under these conditions, the initial rate of K + uptake decreased strongly with pH in, and was almost independent of the acid used. This inhibition was due to a strong decrease in the Vmax for K + uptake, which indicates that low cytoplasmic pH inactivates the TrkA K + uptake system. The relevance of this inhibition for growth and metabolism at low pH in is discussed. 相似文献
18.
Background and aims Salinity is an increasing problem for agricultural production worldwide. Understanding how Na + enters plants is important if reducing Na + influx, a key component of the regulation of Na + accumulation in plants and improving salt tolerance of crop plants, is to be achieved. Our previous work indicated that two distinct low-affinity Na + uptake pathways exist in the halophyte Suaeda maritima. Here, we report the external NaCl concentration at which uptake switches from pathway 1 to pathway 2 and the kinetics of the interaction between external K + concentration and Na + uptake and accumulation in S. maritima in order to determine the roles of K + transporters or channels in low-affinity Na + uptake. Methods Na + influx, Na + and K + accumulations in S. maritima exposed to various concentrations of NaCl (0–200 mM) were analyzed in the absence and presence of the inhibitors TEA and Ba + (5 mM TEA or 3 mM Ba 2+) or KCl (0, 10 or 50 mM). Results Our earlier proposal was confirmed and extended that there are two distinct low-affinity Na + uptake pathways in S. maritima: pathway 1 might be mediated by a HKT-type transporter under low salinity conditions and pathway 2 by an AKT1-type channel or a KUP/HAK/KT type transporter under high salinity conditions. The external NaCl concentration at which two distinct low-affinity Na + uptake switches from pathway 1 to pathway 2, the ‘turning point’, is between 90 and 95 mM. Over a short period (12 h) of Na + and K + treatments, a low concentration of K + (10 mM) facilitated Na + uptake by S. maritima under high salinity (100–200 mM NaCl), whether or not the plants had been subjected to a longer (3 d) period of K + starvation. The kinetics suggests that low concentration of K + (10 mM) might activate AKT1-type channels or KUP/HAK/KT-type transporters under high salinity (100–200 mM NaCl). Conclusions The turning-point of external NaCl concentrations for the two low-affinity Na + uptake pathways in Suaeda maritima is between 90 and 95 mM. A low concentration of K + (10 mM) might activate AKT1 or KUP/HAK/KT and facilitate Na + uptake under high salinity (100–200 mM NaCl). The kinetics of K + on Na + uptake and accumulation in S maritima are also consistent with there being two low-affinity Na + uptake pathways. 相似文献
19.
Abstract— To determine the mechanism of neurotoxicity of kainic acid, striatal slices (350μ) were incubated in oxygenated Krebs buffer with kainic acid and other depolarizing agents; and the alterations in the uptake and retention of 22Na +, 86Rb + (as a measure of K +), 3H zO and the levels of ATP were determined. The excitatory amino acid, L-glutamate (10 mM) increases striatal slice uptake and retention of Na +, K + and H 2O but decreases ATP levels whereas the neuroexcitant, A'-methyl aspartate, increases only Na + and H 2O. Veratridine (100μ M), which opens electrogenic sodium channels, and ouabain (100μM), which inhibits Na +-K + ATPase, both elevate striatal Na + and H 2O but considerably reduce K + and ATP. The effects of these different depolarizing agents on the parameters examined are consistent with their mechanisms of actions and support the validity of this in vitro method. Although 10mM-kainate significantly depresses striatal K + and ATP, lower concentrations of kainate (5mM-0.1μ) elevate striatal uptake of Na + but do not markedly affect H 2O, K + or ATP. Kainate (10mM-lμM) does not exhibit additivity with 10 mM-glutamate with respect to Na + permeability but does significantly potentiate glutamate's ATP depleting effects. Injection of 10 nmol of kainate into the striatum in vivo causes a reduction in striatal ATP 1 h afterward which is comparable to that occurring in vitro with 10mM-kainate alone or with lower concentrations of kainate (≥1/μM) with 10 mM-glutamate. These results suggest that kainate alone is directly neurotoxic at 10mM or neurotoxic at lower concentrations in combination with the high intrasynaptic levels of glutamate on neurons receiving glutamatergic innervation. 相似文献
20.
Although many studies have alluded to a role for boron (B) in membrane function, there is little evidence for a direct effect of B on the plasmalemma of higher plant cells. These studies were conducted to demonstrate, by electrophysiological techniques, a direct effect of B on the membrane potential (E m) of sunflower ( Helianthus annuus [L.], cv Mammoth Grey Stripe) root tip cells and to determine if the response to B occurs rapidly enough to account for the previously observed effects of B on ion uptake. By inserting a glass microelectrode into an individual cell in the root tip, the E m of the cell was determined in basal salt medium (BSM), pH 6.0. The perfusion solution surrounding the root tissue was then changed to BSM + 50 micromolar H 3BO 3, pH 6.0. The exposure to B induced a significant plasmalemma hyperpolarization in sunflower root cells within 20 minutes. After just 3 minutes of exposure to B, the change in E m was already significantly different from the negligible change in E m observed over time in root cells never exposed to B. Membrane hyperpolarization could be caused by a stimulation of the proton pump or by a change in the conductance of one or more permeable ions. Since B has been shown to affect K + uptake by plants, the electrophysiological techniques described above were used to determine if B has an effect on membrane permeability to K +, and could thereby lead to an increased diffusion potential. When sunflower root tips were pretreated in 50 micromolar B for 2 hours, cell membranes exhibited a significantly greater depolarization with each 10-fold increase in external [K +] than minus-B cells. Subsequent studies demonstrated that the depolarization due to increased external [K +] was also significantly greater when tissue was exposed to B at the same time as the 10-fold increase in [K +], indicating that the effect of B on K + permeability was immediate. Analysis of sunflower root tips demonstrated that treatment in 50 micromolar B caused a significantly greater accumulation of K + after 48 hours. The B-induced increase in K + uptake may cause a subsequent stimulation of the H +-ATPase (proton pump) and lead to the observed hyperpolarization of root cell membranes. Alternatively, B may stimulate the proton pump, with the subsequent hyperpolarization resulting in an increased driving force for K + influx. 相似文献
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