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1.
Abstract Fusicoccin (FC)-stimulated K+ (86Rb) uptake and proton extrusion of maize (Zea mays) root apical segments were inhibited by pretreatment of 4-day-old seedlings with the herbicide Chlorsulfuron. In the range of Chlorsulfuron concentrations 0.01-10 mmol m?3, the percentage of inhibition was 15% at 0.01 mmol m?3 and progressively increased with Chlorsulfuron concentration up to 60% at 10 mmol m?3. At the maximum concentration tested (10 mmol m?3), the inhibition was evident after 1.5 h of pre-treatment. The binding of FC to microsomal fractions of root segments from Chlorsulfuron-pretreated seedlings was inhibited by 30%. It is suggested that Chlorsulfuron causes an alteration at the plasmalemma level involving the FC binding sites. The ineffectiveness of Chlorsulfuron in inhibiting FC-stimulaled K+ uptake when administered to excised segments, while inhibiting the enzyme acetolactate synthase, pointed out by Ray (1984) as the site of action of Chlorsulfuron in pea plants, suggests that the observed inhibition of K+ uptake and H+ extrusion is not induced by Chlorsulfuron inhibition of this enzyme. An alternative site of action of Chlorsulfuron is hypothesized in maize plants.  相似文献   

2.
In isolated Elodea densa leaves, the relationships between H+ extrusion (-ΔH+), K+ fluxes and membrane potential (Em) were investigated for two different conditions of activation of the ATP-dependent H+ pump. The ‘basal condition’ (darkness, no pump activator present) was characterized by low values of-ΔH+ and K+ uptake (ΔK+), wide variability of the ?ΔH+/ΔK+ ratio, relatively low membrane polarization and Em values more positive than EK for external K+ concentrations (|K+]o of up to 2mol m?3. A net K+ uptake was seen already at [K+]o below 1 mol m?3, suggesting that K+ influx in this condition was a thermodynamically uphill process involving an active mechanism. When the H+ pump was stimulated by fusicoccin (FC), by cytosol acidification, or by light (the ‘high polarization condition’), K+ influx largely dominated K+ and C? efflux, and the ?ΔH+/ΔK+ ratio approached unity. In the range 50 mmol m?3?5 mol m?3 [K+]0, Em was consistently more negative than EK. The curve of K+ influx at [K+]0 ranging from 50 to 5000mmol m?3 fitted a monophasic, hyperbolic curve, with an apparent half saturation value = 0–2 mol m?3. Increasing |K+]0 progressively depolarized Em, counteracting the strong hyperpolarizing effect of FC. The effects of K+ in depolarizing Em were well correlated with the effects on both K+ influx and ?ΔH+, suggesting a cause-effect chain: K+0 influx → depolarization → activation of H+ extrusion. Cs+ competitively inhibited K+ influx much more strongly in the ‘high polarization’ than in the ‘basal’ condition (50% inhibition at [Cs+]/[K+]0 ratios of 1:14 and 1:2, respectively) thus confirming the involvement of different K+ uptake systems in the two conditions. These results suggest that in E. densa leaves two distinct modes of interactions rule the relationships between H+ pump, membrane polarization and K+ transport. At low membrane polarization, corresponding to a low state of activation of the PM H+-ATPase and to Em values more positive than EK, K+ influx would mainly  相似文献   

3.
The stimulation of H+ extrusion by hyper-osmotic stress (0.2–0.3 M mannitol) in cultured cells of Arabidopsis thaliana (L.) Heynh. was shown to be associated with an inhibition of Cl? efflux, whereas hypo-osmotic stress, inhibiting H+ extrusion, early and strongly stimulated Cl? efflux. In this paper, we investigate the contribution of other factors [K+ transport and transmembrane electric potential difference (Em)] to the hyper-osmotic-induced activation of the plasma membrane (PM) H+-ATPase. The effects of mannitol (MA) on K+ transport and on Em were compared with those of fusicoccin (FC) since the modes of action of osmotica and of the toxin in stimulating H+-ATPase activity seem to differ at least in some steps. The changes in H+ extrusion induced by hyper- or hypo-osmotic stress were opposite and could be reversed by the application of the respective opposite stress. The effect of MA on H+ extrusion was dependent on the presence of K+ (or Rb+) similarly to that of FC, while Na+ and Li+, which also stimulated the FC effect, were ineffective on that of MA. The MA effect was independent of the anions (Cl?, SO42?, NO3?) accompanying K+. K+ net uptake and K+ influx were stimulated by both MA and FC. Tetraethylammonium (TEA+) and Cs+ inhibited both MA- and FC-induced H+ extrusion, suggesting the involvement of K+ channels. MA (0.2 M) induced a strong hyperpolarization of Em both in the absence and in the presence of K+. The hyperpolarizing effect of MA was also found when the cells were already hyperpolarized by FC, and was rapidly reversed by removing the osmoticum from the medium. In the presence of the lipophilic cation tributylbenzylammonium (TBBA+), MA was no longer able to stimulate H+ extrusion, while FC still stimulated it. In cells pretreated with TBBA+, which strongly depolarized Em, the subsequent addition of FC repolarized it, while the hyperpolarizing effect of MA was lacking. On the contrary, in cells pretreated with Erythrosine B (EB), Em was strongly depolarized and the following addition of FC did not hyperpolarize it, while the hyperpolarizing effect of MA was still observed. These results suggest that the mechanism of MA in activating H+ extrusion and K+ uptake is different from that of FC. The rise in net K+ uptake seems to be driven by the activation of some hyperpolarizing system that does not seem to depend on a direct activation of PM H+-ATPase, but rather on the inhibition of Cl? efflux induced by hyper-osmotic stress.  相似文献   

4.
Previous data in Egeria densa leaves demonstrated a strong inhibitory effect of Cs+ on passive K+ influx and on K+-induced, ATP-dependent electrogenic proton extrusion. In this paper we analyzed, using the same material, the effects of Cs+ on ammonium (NH4+) and methylammonium (CH3NH3+) transport in order to elucidate whether a common transport system for K+ and NH4+ could be demonstrated. The effects of Cs+ on NH4+- and CH3NH3+-induced titratable H+ extrusion (–ΔH+) and on transmembrane electrical potential difference (Em) in E. densa leaves were analyzed in parallel. All experiments were run either in the absence or presence of fusicoccin, corresponding to low or high H+-ATPase activity and membrane hyperpolarization and leading, in this material, to respectively active or passive transport of K+. The results suggest the presence in E. densa leaves of two distinct pathways for NH4+ uptake: one in common with NH4+ and (with lower affinity) CH3NH3+, insensitive to Cs+, and a second system, operating at higher H+-ATPase activity and Em hyperpolarization, strongly inhibited by Cs+ and impermeable to CH3NH3+. In agreement with this hypothesis, Xenopus laevis oocytes injected with the KAT1 RNA of Arabidopsis thaliana were permeable to K+ and NH4+, but not to CH3NH3+.  相似文献   

5.
Abstract: The Na+-glutamate cotransporters are believed to countertransport OH? and K+. Previous evidence that the velocity of glutamate uptake can exceed the acid extrusion capacity of astrocytes raised the question of whether intracellular pH can become rate limiting for glutamate uptake. Cytoplasmic buffering capacity and acid extrusion in astrocytes are partially HCO3? dependent. Also, it was reported recently that raising extracellular [K+] alkalinizes astrocyte cytoplasm by an HCO3?-dependent mechanism. Here, we have compared glutamate uptake in HCO3?-buffered and HCO3?-depleted solutions at varying [K+]. We observed a pronounced stimulation of glutamate uptake by extracellular K+ (3–24 mM) that was substantially HCO3? dependent and affected preferentially the uptake of high concentrations (>25 µM) of glutamate. Stimulation of uptake by low extracellular [K+] (1.5–3 mM) was less dependent on HCO3?. Potassium-induced stimulation of uptake was weaker in rat astrocyte cultures than in mouse. The effects of Ba2+ and amiloride on glutamate uptake, as well as the HCO3?-dependent stimulatory effects of K+ and the species difference, all related consistently to effects on intracellular pH. The effects on uptake, however, were much larger than predicted by the associated changes in electrochemical gradient of OH?. A “bimodal” scheme for glutamate transport can account qualitatively for the observed correlation between intracellular pH and velocity of glutamate uptake.  相似文献   

6.
Abstract: The effect of endothelins (ET-1 and ET-3) on 86Rb+ uptake as a measure of K+ uptake was investigated in cultured rat brain capillary endothelium. ET-1 or ET-3 dose-dependently enhanced K+ uptake (EC50 = 0.60 ± 0.15 and 21.5 ± 4.1 nM, respectively), which was inhibited by the selective ETA receptor antagonist BQ 123 (cyclo-d -Trp-d -Asp-Pro-d -Val-Leu). Neither the selective ETB agonists IRL 1620 [N-succinyl-(Glu9,-Ala11,15)-ET-1] and sarafotoxin S6c, nor the ETB receptor antagonist IRL 1038 [(Cys11,Cys15)-ET-1] had any effect on K+ uptake. Ouabain (inhibitor of Na+,K+-ATPase) and bumetanide (inhibitor of Na+-K+-Cl? cotransport) reduced (up to 40% and up to 70%, respectively) the ET-1-stimulated K+ uptake. Complete inhibition was seen with both agents. Phorbol 12-myristate 13-acetate (PMA), activator of protein kinase C (PKC), stimulated Na+,K+-ATPase and Na+-K+-Cl? cotransport. ET-1-but not PMA-stimulated K+ uptake was inhibited by 5-(N-ethyl-N-isopropyl)amiloride (inhibitor of Na+/H+ exchange system), suggesting a linkage of Na+/H+ exchange with ET-1-stimulated Na+,K+-ATPase and Na+-K+-Cl? cotransport activity that is not mediated by PKC.  相似文献   

7.
In order to identify physiological components that contribute to salinity tolerance, we compared the effects of Na+, Mg2+ and K+ salts (NaCl, Na2SO4, MgCl2, MgSO4, KCl and K2SO4), Ca2+ (CaSO4), mannitol and melibiose on the wild type and the single-gene NaCl-tolerant mutants stl1 and stl2 of Ceratopteris richardii. Compared with gametophytic growth of the wild type, stl2 showed a low level of tolerance that was restricted to Na+ salts and osmotic stress. stl2 exhibited high tolerance to both Na+ and Mg2+ salts, as well as to osmotic stress. In response to short-term exposure (3 d) to NaCl, accumulation of K+ and Na+ was similar in the wild type and stl1. In contrast, stl2 accumulated higher levels of K+ and lower levels of Na+. Ca2+ supplementation (1.0 mol m?3) ameliorated growth inhibition by Na+ and Mg2+ stress in wild type and stll, but not in stl2. In addition, under Na+ stress (175 mol m?3) wild-type, stll and stl2 gametopbytes maintained higher tissue levels of K+ and lower levels of Na+ when supplemented with Ca2+ (1.0 mol m?3). stl2 gametophytes were extremely sensitive to K+ supplementation. Growth of stl2 was greater than or equal to that of the wild type at trace concentrations of K+ but decreased substantially with increasing K+ concentration. Supplementation with K+ from 0 to 1.85 mol m?3 alleviated some of the inhibition by 75 mol m?3 NaCl in the wild type and in stl1. In stl2, growth at 75 mol m?3 NaCl was similar at 0 and 1.85 mol m?3 K+ supplementation. Although K+ supplementation above 1.85 mol m?3 did not alleviate inhibition of growth by Na+ in any genotype, stl2 maintained greater relative tolerance to NaCl at all K+ concentrations tested.  相似文献   

8.
Interactive effects of K+ and N (principally NH4+) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH4+ or NO3? and K+ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m?3 NH4+ and 200mmolm?3 K+ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. 86Rb+(K+) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH4+ during flux measurements. This inhibition was a function of prior N/K provision and the [NH4+]0 present during flux determinations. NH4+ was least inhibitory to 86Rb+(K+) influx in high-N/low-K plants. Pretreatments with K+ failed to stimulate NH4+ uptake, and the presence of K+ in the uptake solutions reduced NH4+ fluxes only in high-N/low-K plants.  相似文献   

9.
In N-starved (?N) fronds of Lemna gibba L. G 1, NH4+ uptake rates were several-fold those of NO3?-supplied (+N) fronds. NO3?, uptake in +N-plants was slow and not inhibited by addition of NH4+. However, in ?N-plants with higher NO3? and still higher NH4+ uptake rates, addition of NH4+ immediately reduced the NO3? uptake rates to about one third until the NH4+ was consumed. The membrane potential (Em) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where Em remained at the level of the K+ diffusion potential (ED) until NH4+ was removed. In N-starved plants NH4+ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH4+ similarly to NO3? uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO3? and H2PO 4? into the negatively charged cells of Lemna is mediated by anion/H+ cotransport, but NH4+ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NH4+ uniport, the linear component diffusion of NH4+ or NH3. Inhibition of anion/H+ cotransport by high NH4+ influx rates may be due to loss of the proton-driving force, Δμ?H+, across the plasmalemma. Reversible inhibition by NH4+ of the H+ extrusion pump may contribute to the finding that Δμ?H+ cannot be reconstituted in the presence of higher NH4+ concentrations.  相似文献   

10.
Sacchi GA  Cocucci M 《Plant physiology》1992,100(4):1962-1967
Elongation of subapical segments of maize (Zea mays) roots was greatly inhibited by 2H2O in the incubation medium. Short-term exposure (30 min) to 2H2O slightly reduced O2 uptake and significantly increased ATP levels. 2H2O inhibited H+ extrusion in the presence of both low (0.05 mm) and high (5 mm) external concentrations of K+ (about 30 and 53%, respectively at 50% [v/v] 2H2O). Experiments on plasma membrane vesicles showed that H+-pumping and ATPase activities were greatly inhibited by 2H2O (about 35% at 50% [v/v] 2H2O); NADH-ferricyanide reductase and 1,3-β-glucan synthase activities were inhibited to a lesser extent (less than 15%). ATPase activities present in both the tonoplast-enriched and submitochondrial particle preparations were not affected by 2H2O. Therefore, the effect of short incubation time and low concentration of 2H2O is not due to a general action on overall cell metabolism but involves a specific inhibition of the plasma membrane H+ -ATPase. K+ uptake was inhibited by 2H2O only when K+ was present at a low (0.05 mm) external concentration where absorption is against its electrochemical potential. The transmembrane electric potential difference (Em) was slightly hyperpolarized by 2H2O at low K+, but was not affected at the higher K+ concentrations. These results suggest a relationship between H+ extrusion and K+ uptake at low K+ external concentration.  相似文献   

11.
The kinetics of the light-driven Cl? uptake pump of Synechococcus R-2 (PCC 7942) were investigated. The kinetics of Cl? uptake were measured in BG-11 medium (pHo, 7·5; [K+]o, 0·35 mol m?3; [Na+]o, 18 mol m?3; [Cl?]o, 0·508 mol m?3) or modified media based on the above. Net36Cl? fluxes (?Cl?o,i) followed Michaelis-Menten kinetics and were stimulated by Na+ [18 mol m?3 Na+ BG-11 ?Cl?max= 3·29±0·60 (49) nmol m?2 s?1 versus Na+-free BG-11 ?Cl?max= 1·02±0·13 (54) nmol m?2 s?1] but the Km was not significantly different in the presence or absence of Na+ at pHo 10; the Km was lower, but not affected by the presence or absence of Na+ [Km = 22·3±3·54 (20) mmol m?3]. Na+ is a non-competitive activator of net ?Cl?o,i. High [K+]o (18 mol m?3) did not stimulate net ?Cl?o,i or change the Km in Na+-free medium. High [K+]o (18 mol m?3) added to Na+ BG-11 medium decreased net ?Cl?o,i [18 mol m?3K+ BG-11; ?Cl?max= 2·50±0·32 (20) nmol m?2 s?1 versus BG-11 medium; ?Cl?max= 3·35±0·56 (20) nmol m?2 s?1] but did not affect the Km 55·8±8·100 (40) mmol m?3]. Na+-stimulation of net ?Cl?o,i followed Michaelis-Menten kinetics up to 2–5 mol m?3 [Na+]o but higher concentrations were inhibitory. The Km for Na+-stimulation of net ?Cl?o,i [K1/2(Na+)] was different at 47 mmol m?3 [Cl?]o (K1/2[Na+] = 123±27 (37) mmol m?3]. Li+ was only about one-third as effective as Na+ in stimulating Cl? uptake but the activation constant was similar [K1/2(Li+) = 88±46 (16) mmol m?3]. Br? was a competitive inhibitor of Cl? uptake. The inhibition constant (Ki) was not significantly different in the presence and absence of Na+. The overall Ki was 297±23 (45) mmol m?3. The discrimination ratio of Cl? over Br? (δCl?/δBr?) was 6·38±0·92 (df = 147). Synechococcus has a single Na+-stimulated Cl? pump because the Km of the Cl? transporter and its discrimination between Cl? and Br? are not significantly different in the presence and absence of Na+. The Cl? pump is probably driven by ATP.  相似文献   

12.
The effect of CO2 on potassium transport by Chlorella fusca   总被引:1,自引:1,他引:0  
Abstract. The effect of CO2 on net K+ uptake by Chlorella fusca grown on high CO2 levels was examined by passing 1.5% CO2 through algal suspensions gassed previously with air or CO2-free air Addition of CO2 in the light caused a large net uptake of K+ (initial velocity 4.2–9.2 mmol s?1 m?3 cells) which decreased the concentration of K+ in the supernatant from 0.1–0.2 mol m?3 to 3–10 mmol m?3. In the dark and in the presence of 30 mmol m?3 DCMU, no effects were found. Measurement or the unidirectional K+ fluxes by using 86Rb+ as a label showed that in the presence of 1.5% CO2, influx of K+ was increased by a factor of 2–4 while efflux was inhibited completely. CO2 hyperpolarized the membrane potential (determined through TPP+ uptake) from –120mV to –130 mV which could not explain the more than 15,000-fold K+ accumulations. In the light, CO2 lowered the intracellular pH (determined with DMO) by 0.5 units. In the dark and in the presence of DCMU only, a small acidification of 0.1 units was found. During the first 15 min after addition of CO2 the malate content of the cells increased from 0.7 to 1.5 mol m?3 packed cells. On the basis of these and earlier results, CO2-induced net K+ uptake is interpreted as a stimulation of an electroneutral ATP-dependent K+/H+ exchange at the plasmalemma. This exchange acts as a ‘pHstat’ by reducing the intracellular acidification caused by production of acidic assimilation products.  相似文献   

13.
NH4+ and K+ uptake experiments have been conducted with 3 ectomycorrhizal fungi, originating from Douglas fir (Pseudotsuga menziesii (Mirb.] Franco) stands. At concentrations up to 250 μM, uptake of both NH4+ and K+ follow Michaelis-Menten kinetics. Laccaria bicolor (Maire) P. D. Orton, Lactarius rufus (Scop.) Fr. and Lactarius hepaticus Plowr. ap. Boud. exhibit Km values for NH4+ uptake of 6, 35, and 55 μM, respectively, and Km values for K+ uptake of 24, 18, and 96 μM, respectively. Addition of 100 μM NH4+ raises the Km of K+ uptake by L. bicolor to 35 μM, while the Vmax remains unchanged. It is argued that the increase of Km is possibly caused by depolarization of the plasma membrane. It is not due to a competitive inhibition of K+ by NH4+ since the apparent inhibitor constant is much higher than the Km, for NH4+ uptake. The possibility that NH4+ and K+ are taken up by the same carrier can be excluded. The Km, values for K+ uptake in the two other fungi are not significantly affected by 100 μM NH4+. Except for a direct effect of NH4+ on influx of K+ into the cells, there may also be an indirect effect after prolonged incubation of the cells in the presence of 100 μM NH4+.  相似文献   

14.
The roles of Na+ and K+ (Rb+) uptake were further studied in a NaCl-tolerant strain of Ceratopteris richardii containing the stl2 mutation by direct comparison with the wild-type strain. In addition to Na+ tolerance, stl2 also confers tolerance to Mg2+ and sensitivity to K+. In addition to higher K+ (Rb+) uptake at concentrations commonly associated with low-affinity K+ transport, stl2 maintained higher uptake down to 0·1 mol m–3 Rb+. Up to a 25-fold excess of Na+ had little effect in either genotype on K+ (Rb+) uptake at low concentrations, i.e. 0·2 and 0·5 mol m–3 RbCl. Pretreatment with K+ (20 mol m–3) inhibited uptake of K+ (Rb+) in the wild type, whereas concurrent inclusion of K+ inhibited uptake of Rb+ more in stl2. In the absence of K+, Na+ uptake (0·01–60 mol m–3) was nearly identical in the wild type and stl2. K+ inhibited Na+ uptake more effectively in stl2 than the wild type, especially at 60 mol m–3 Na+. Greater inhibition of K+ uptake in stl2 occurred with MgCl2 or TEA (tetraethylammonium chloride) preincubation or with simultaneous inclusion of Al3+ (Al2SO4). The higher effective velocity of K+ uptake at a wide range of concentrations and the enhanced selectivity for K+ and against Na+ contribute to the preservation of higher cytosolic K+ and lower Na+ under salinity stress.  相似文献   

15.
Acidification of the external medium of the yeast Saccharomyces cerevisiae, mainly caused by proton extrusion by plasma membrane H+-ATPase, was inhibited to different degrees by D2O, diethylstilbestrol, suloctidil, vanadate, erythrosin B, cupric sulfate and dicyclohexylcarbodiimide. The same pattern of inhibition was found with the uptake of amino acids, adenine, uracil, and phosphate and sulfate anions. An increase of the acidification rate by dioctanoylglycerol also increased the rates of uptake of adenine and of glutamic acid. In contrast, a decrease of the membrane potential at pH 4.5 from a mean of -40 to -20 mV caused by 20 mm KC1 had no effect on the transport rates. The ATPase-deficient mutant S. cerevisiae pmal-105 showed a markedly lower uptake of all the above solutes as compared with the wild type, while its membrane potential and pH were unchanged.Other types of acidification (spontaneous upon suspension; K+ stimulated) did not affect the secondary uptake systems.  相似文献   

16.
In various plant materials changes in turgor pressure, following hyper- or hypo-osmotic stress, were associated with the activation or inactivation of the plasma membrane H+-ATPase, respectively. To see if the turgor changes might indirectly influence H+-ATPase activity by regulating ion fluxes through plasma membrane, we investigated, in cultured cells of Arabidopsis thaliana (L.) Heynh., the early effects of hyper- and hypo-osmotic stress on Cl? fluxes in comparison, in the case of hyper-osmotic treatment, with its effect on net H+ extrusion. The results obtained showed that hyper-osmotic stress (200 mM mannitol) quickly reduced Cl? efflux (?70%) from cells preloaded with 36C1? for 18 h. This inhibiting effect was independent of the simultaneous mannitol-induced stimulation of Cl? influx and rapidly reversible after removal of the hyper-osmotic treatment. The inhibition of Cl? efflux was associated with a stimulation of net H+ extrusion, and these two effects showed the same dependence on the external mannitol concentration. Fusicoccin (FC, 20 µM), which stimulated H+ extrusion to about the same extent as 200 mM mannitol, did not affect Cl? efflux. When cells preloaded with 36C1? for 18 h in the presence of mannitol (from 25 up to 200 mM) were eluted in a mannitol-free medium an early and strong increase in Cl? efflux was found. The increase of Cl- efflux was already detectable for a small hypo-osmotic jump (25 mM), and was reduced (?50%) by the anion channel inhibitor A9C (300 µM). These results lead to exclude a direct causal relationship mediated by Em changes between the effects of osmoticum on Cl? efflux and net H+ extrusion, and favour the view that the changes in turgor pressure induced by hyper/hypo-osmotic stress may respectively induce an early inactivation/activation of stretch-sensitive anion channels.  相似文献   

17.
The present work was aimed at studying the kinetics and nature of the l-DOPA transporter in opossum kidney (OK) cells. Saturation experiments were performed in OK cells incubated for 6 min with increasing concentrations of l-DOPA (10 to 2500 μm); non-linear analysis of the saturation curve revealed for l-DOPA aKmof 129 μm (114, 145) and aVmaxof 30.0±0.4 nmol mg protein?16 min?1The uptake of l-DOPA (250 μm) was inhibited in a concentration-dependent manner by cyanine 863, an organic cation inhibitor, with aKivalue of 638 (430, 947) μmthe organic anion inhibitor 4,4′-diisothiocyanostilbene-2,2′-disulphonic acid (DIDS), was devoid of effect upon the uptake of l-DOPA. The uptake of l-DOPA (250 μm) was significantly (P<0.02) decreased (25% reduction) when cells were incubated in the presence of 137 mm K+plus 5 mm Na+when compared with the control condition (137 mm Na+plus 5 mm K+); substitution of NaCl by choline chloride (137 mm) did not affect l-DOPA uptake. Similarly, inwardly or outwardly directed proton gradients of 0.5 pH units (7.9, 7.4, 6.9, 6.4 and 5.9) were found not to change l-DOPA uptake. In conclusion, the l-DOPA uptake system in OK cells has the characteristics of an organic cation potential-dependent and proton-independent transporter.  相似文献   

18.
Synechococcus R-2 is a unicellular blue-green alga. The cells will grow on Rb+ as a substitute for K+ but at a slower rate (t2~ 15 h versus 12 h). Potassium is not, strictly speaking, an essential element for Synechococcus. Rubidium duxes (using 86Rb+) are much slower than those of potassium, about 1 nmol m?2 s?1 in the light (0.35 mol m?3 Rb+). 86Rb+ fluxes in the dark are about 0.1 nmol m?2 s?1. These fluxes are very slow compared to those of Na+ and other ions. Isotopic influx of Rb+ can supply sufficient Rb+ to keep up with the demands for growth, but the net dux needed to keep up with growth in the light is a large proportion of the total observed dux. Kinetic studies of Rb+ uptake versus [Rb+] show two uptake phases consistent with a high-affinity and a low-affinity system. Both systems appear to be light-activated. Transport of Rb+ appears to be passive at pHo 10 in the light and dark. There is no case for active transport of Rb+ at pHo 7.5 in the light, but a marginal case for active uptake in the dark (about 3 kJ mol?1). There is only a small effect of Na+ upon Rb+ transport. 86Rb+ should not be used in place of 42K+ in K+ nutrition studies as the details of Rb+ transport are different to those of K+ transport.  相似文献   

19.
Abstract A method is described for perfusing xylem vessels in tap root segments of the halophyte P. maritima. Use of excised segments allowed recording of the trans-root potential (TRP) at both ends of a segment. It was shown that there can be a spatial variation of electrogenic ion pump activity along the xylem in one root segment. The pH of perfusion solutions, differing in buffering capacity, was adjusted by the root segment to pH 5.1–5.6 during How through the xylem. This pH range was similar to that of sap produced by root pressure. The K+ activity in the outflow solution (K+out) was rather constant at 12–13 mol m?l3 despite input K+ activities ranging from 8 to 20 mol m?l3. Addition of fusicoccin (10?l2 mol m?l3) to the perfusion solution induced a strong acidification of the xylem sap, a decrease in K+out and an increase in Na+out. Inhibition of aerobic respiration through anoxia inhibited electrogenic proton pumping into the xylem and led to an increase in K+out and a decrease in Na+out. It is suggested that transport of K+ and Na+ to the shoot of the halophyte P. maritima is regulated in the tap root by means of ion exchange between xylem vessels and xylem parenchyma and that this exchange is energized by proton translocating ATPases.  相似文献   

20.
Synechococcus R-2 (PCC 1942) actively accumulates sulphate in the light and dark. Intracellular sulphate was 1.35 ± 0.23 mol m?3 (light) and 0.894 ± 0.152 mol m?3 (dark) under control conditions (BG-11 media: pHo, 7.5; [SO42?]o, 0.304 mol m?3). The sulphate transporter is different from that found in higher plants: it appears to be an ATP-driven pump transporting one SO42?/ATP [ΔμSO42?i,o=+ 27.7 ± 0.24 kJ mol?1 (light) and + 24 ± 0.34 kj mol?1 (dark)]. The rate of metabolism of SO42?at pHo, 7.5 was 150 ± 28 pmol m?2 s?1 (n = 185) in the light but only 12.8 ± 3.6 pmol m?2 s?1 (n = 61) in the dark. Light-driven sulphate uptake is partially inhibited by DCMU and chloramphenicol. Sulphate uptake is not linked to potassium, proton, sodium or chloride transport. The alga has a constitutive over-capacity for sulphate uptake [light (n= 105): Km= 0.3 ± 0.1 mmol m?3, Vmax, = 1.8 ± 0.6 nmol m?2 s?1; dark (n= 56): Km= 1.4 ± 0.4 mmol m?3, Vmax= 41 ± 22 pmol m?2 s?1]. Sulphite (SO32?) was a competitive inhibitor of sulphate uptake. Selenate (SeO42?) was an uncompetitive inhibitor.  相似文献   

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