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1.
Summary Several cationic dyes were found to behave as inhibitors of K + uptake in yeast. When added at high concentrations or in a K +-free medium, dyes can also produce and efflux of K +. The dyes are taken up by the cells in a process that, in different degrees, for several cations requires glucose and is inhibited to a higher degree by K + than by Na +.The inhibition of cation uptake is of the competitive type with EB and close to this type with other dyes. Ca 2+ inhibits the uptake and effects of dyes and in some cases also seems to change the inhibition kinetics on Rb + uptake closer to a pure competitive type.According to preliminary experiments, the efflux of K + seems to be of the electrogenic type, and not due to the disruption of the cells. The data indicate that, independently of the existence of other types of interaction (which do exist), dyes seem to interact with the system for monovalent cation uptake of yeast in different degrees of specificity and energy requirement. This interaction can be followed by fluorescence or metachromatic changes or reduction of the dyes as observed in the dual wavelength spectrophotometer and can be inhibited specifically by K +, but not by Na +. 相似文献
2.
The ionic requirements for K +-evoked efflux of endogenous taurine from primary cerebellar astrocyte cultures were studied. The Ca 2+ ionophore A23187 evoked taurine efflux in a dose-dependent fashion with a time-course identical to that of K +-induced efflux. The Ca 2+-channel antagonist nifedipine had no effect upon efflux induced by 10 or 50 mM K +. In addition, verapamil did not antagonize 50 mM K +-evoked efflux except at high, non-pharmacological concentrations (>100 M), and preincubation with 2 M -conotoxin had no effect on 50 mM K +-evoked efflux. Similarly, preincubation with 1 mM ouabain had no effect on the amount of taurine released by K + stimulation, but did accelerate the onset of efflux by 2–4 min. Although 2 M tetrodotoxin had no effect on K +-evoked release, replacing Na + with choline abolished the taurine efflux seen in response to K + stimulation. Together, these findings suggest that neuronal N- and L-type Ca 2+- and voltage-dependent Na +-channels are not involved in the influx of Ca 2+ which appears to be necessary for K +-evoked taurine efflux, and that in addition to Ca 2+, extracellular Na + is also required. 相似文献
3.
Summary Addition of 0.1–0.3 m A23187, a divalent cation ionophore, to human erythrocytes suspended in a 1.0 mm
45Ca 2+-containing buffer results in a small ( two fold) increase in [Ca 2+]
i
, a significant decrease in osmotic fragility, and a decrease in intracellular K + (100 mmoles/liter of cells to 70 mmoles/liter cells) without significant alteration of intracellular [Na +]. This decrease in [K +]
i
is associated with a significant decrease in packed cell volume and correlates directly with the observed alteration is osmotic fragility. Increasing extracellular K + to 125 mm prevents the A23187-induced changes in osmotic fragility, K + content and cell volume, but does not prevent the ionophore-induced uptake of 45Ca 2+. Addition of 0.1–0.3 m A23187 to toad erythrocytes leads to an increase in 45Ca 2+ uptake comparable to that observed in human erythrocytes, but does not alter osmotic fragility, cell volume or K + content. Higher concentrations of ionophore (3.0–10.0 m) cause a 30- to 50-fold increase in 45Ca 2+ uptake and concomitant change in K + content, cell volume and osmotic fragility. These changes in cell properties can be prevented by increasing extracellular [K +] to 90 mm. The difference in sensitivity of the two cell types to A23187 is attributed to the presence of additional intracellular calcium pools within toad erythrocytes that prevent an increase in cytoplasmic Ca 2+ until Ca 2+ uptake is increased substantially at the higher concentrations of A23187. 相似文献
4.
The Ca 2+-dependent K + efflux from rat submandibular gland was studied using a K +-sensitive electrode. A K + efflux was induced by either adrenalin or by using the divalent cation ionophore A23187 plus added Ca 2+ to bypass the receptor mechanism. Trifluoperazine, which was used to investigate the role of calmodulin, was found to block the adrenalin-induced K + efflux but not the A23187/Ca 2+-induced K + efflux. The adrenalin-induced K + efflux was abolished by quinidine and the A23187/Ca 2+-induced K + efflux was significantly reduced by quinidine. In other experiments, the presence of indomethacin did not inhibit the adrenalin-induced K + efflux, and exogenously added arachidonic acid did not induce a K + efflux. It is concluded that neither prostaglandin synthesis, nor a cytosolic Ca 2+-calmodulin complex is involved in the agonist-induced K + efflux from rat submandibular gland. A similarity between the Ca 2+-dependent K + efflux mechanism of erythrocyte ghosts and submandibular tissue is indicated by their common response to quinidine. 相似文献
5.
- 1.
- 1. The roles of Ca2+ and Mg2+ in the transport of amino acids were examined in rat kidney cortex slices in vitro. The absence of either Ca2+ or Mg2+ from the incubation fluid was associated with increased inulin space and slightly decreased K+ content of the slices although no significant alterations of total tissue water nor Na+ content were noted. Decreased net accumulation of glycine, cycloleucine and α-aminoisobutyric acid were found upon removal of either divalent cation from the incubation fluid with no corresponding effects upon efflux from prelabeled tissues. No effects of divalent cations were noted upon lysine transport. 相似文献
6.
Summary In the presence of inhibitors for mitochondrial H +-ATPase, (Na ++K +)- and Ca 2+-ATPases, and alkaline phosphatase, sealed brush-border membrane vesicles hydrolyse externally added ATP demonstrating the existence of ATPases at the outside of the membrane (ecto-ATPases). These ATPases accept several nucleotides, are stimulated by Ca 2+ and Mg 2+, and are inhibited by N,N-dicyclohexylcarbodiimide (DCCD), but not by N-ethylmaleimide (NEM). They occur in both brushborder and basolateral membranes. Opening of brush-border membrane vesicles with Triton X-100 exposes ATPases located at the inside (cytosolic side) of the membrane. These detergent-exposed ATPases prefer ATP, are activated by Mg 2+ and Mn 2+, but not by Ca 2+, and are inhibited by DCCD as well as by NEM. They are present in brush-border, but not in basolateral membranes. As measured by an intravesicularly trapped pH indicator, ATP-loaded brush-border membrane vesicles extrude protons by a DCCD- and NEM-sensitive pump. ATP-driven H + secretion is electrogenic and requires either exit of a permeant anion (Cl –) or entry of a cation, e.g., Na + via electrogenic Na +/ d-glucose and Na +/ l-phenylalanine uptake. In the presence of Na +, ATP-driven H + efflux is stimulated by blocking the Na +/H + exchanger with amiloride. These data prove the coexistence of Na +-coupled substrate transporters, Na +/H + exchanger, and an ATP-driven H + pump in brush-border membrane vesicles. Similar location and inhibitor sensitivity reveal the identity of ATP-driven H + pumps with (a part of) the DCCD- and NEM-sensitive ATPases at the cytosolic side of the brush-border membrane. 相似文献
7.
Summary Experiments were performed to obtain information on: (i) the specific properties of Ca 2+ binding and transport in yeast (ii) the relationship between both parameters; (iii) similarities to or differences from other biological systems as measured by the effects of inhibitors; and (iv) the effects of mono and divalent cations, in order to get some insight on the specificity and some characteristics of the mechanism of the transport system for divalent cations in yeast.The results obtained gave some kinetic parameters for a high affinity system involved in the transport of Ca 2+ in yeast. These were obtained mainly by considering actual concentrations of Ca 2+ in the medium after substracting the amounts bound to the cell. A k
m
of 1.9 m and a V
max
of 1.2 nmol (100 mg·3 min) –1 were calculated.The effects of some inhibitors and other cations on Ca 2+ uptake allow one to postulate some independence between binding and transport for this divalent cation.Of the inhibitors tested, only lanthanum seems to be a potent inhibitor of Ca 2+ uptake in yeast.The effects of Mg 2+ on the uptake of Ca 2+ agree with the existence of a single transport system for both divalent cations.The actions of Na + and K + on the transport of Ca 2+ offer interesting possibilities to study further some of the mechanistic properties of this transport system for divalent cations. 相似文献
8.
Membrane potential changes accompanying Ca 2+ influx stimulated by release of Ca 2+ from intracellular stores (store-regulated Ca 2+ uptake) were monitored in BAPTA-loaded rat thymic lymphocytes using the fluorescent indicator bis(1,3-diethylthiobarbituric acid)trimethine oxonol. Depletion of [Ca 2+]
i
stores by the application of thapsigargin, ionomycin or cyclopiazonic acid induced a depolarization which was (i) dependent upon BAPTA-loading, (ii) dependent upon extracellular Ca 2+, (iii) independent of extracellular Na + and (iv) abolished by 5 m m extracellular Ni 2+. This depolarization was followed by a charybdotoxin-sensitive repolarization and subsequent hyperpolarization to values approximating the K + equilibrium potential, consistent with secondary activation of a K + conductance. These membrane potential changes temporally correlated with Ca 2+ influx from the extracellular medium as measured fluorimetrically with indo-1. The divalent cation permeability sequence was investigated by monitoring the magnitude of the depolarization observed following the addition of 4 m m Ca 2+, Mn 2+, Ba 2+ or Sr 2+ to cells pretreated with doses of thapsigargin or ionomycin known to activate the store-regulated calcium uptake pathway. On the basis of these experiments, we conclude that the store-regulated Ca 2+ uptake pathway has the following permeability sequence: Ca 2+ > Mn 2+ Ba 2+, Sr 2+ with Mn 2+ displaying significant permeability relative to Ca 2+. This pathway is distinguishable from other divalent cation uptake pathways reported in other cells types on the basis of its activation by thapsigargin and its high Mn 2+ permeability.This work is supported by grants from the American Heart Association, Louisiana Affiliate (LA-92-6-28), Louisiana Education Quality Support Fund (LEQSF(1993-96)-RD-A-31) and Tulane University Graduate Program in Molecular and Cellular Biology. 相似文献
9.
The influence of K + and Ca 2+ on Zn 2+ transport into cultured human fibroblasts was investigated. Zn 2+ uptake was markedly reduced in the presence of both valinomycin and nigericin (electrogenic and electroneutral K + ionophores, respectively), and by reduction in the transmembrane K + gradient produced by replacement of extracellular K + with Na +, suggesting that Zn 2+ may be driven by a Zn 2+/K + counter-transport system. To test the counter-transport hypothesis, we used 86Rb as an analog of K + for efflux studies. The rate of Rb + efflux was 3760 times that of Zn 2+ uptake, thus the component of K + involved in the Zn 2+ counter-transport system was only a small proportion of the total K + efflux. In investigating the effect of Ca 2+ on Zn 2+ uptake, we identified two components: (1) a basal Zn 2+ uptake pathway, independent of hormonal or growth factors which does not require extracellular Ca 2+ and (2) a Ca 2+-dependent mechanism. The absence of Ca 2+ decreased Zn 2+ uptake, while increasing extracellular C+a 2+ stimulated Zn 2+ uptake. The effect was mediated by Ca 2+ influx as the ionophores A23187 and ionomycin also stimulated Zn 2+ uptake. We could not ascribe the Ca 2+ effect to known Ca 2+ influx pathways. We conclude that Zn 2+ uptake occurs by a K +-dependent process, possibly by Zn 2+/K + counter-transport and that a component of this is also Ca 2+-dependent. 相似文献
10.
In suspension-cultured cells of Glycine max and Catharanthus roseus, marked callose synthesis can be induced by digitonin and chitosan. Leakage of a limited pool of electrolytes precedes callose formation, K + representing the major cation lost. Poly- L-ornithine, as well as the ionophores A 23187 and ionomycin, also induces some callose synthesis but to a lesser extent. Digitonin increases the net uptake of Ca 2+ from the external buffer with a time course parallel to callose synthesis but lagging behind the leakage of K +. Nifedipine partly blocks callose synthesis as well as the digitonin-induced increase in net Ca 2+ uptake. Taken together, the data support the hypothesis that addition of the various substances might indirectly lead to membrane perturbation causing the common event of an increase in net Ca 2+ uptake which results in callose deposition by a direct activition of the Ca 2+-dependent and plasma-membane-located 1,3--glucan synthase. 相似文献
11.
Summary The purpose of this study was to examine the effect of three classes of Ca 2+ antagonists, diltiazem, verapamil and nifedipine on Na +-Ca 2+ exchange mechanism in the sarcolemmal vesicles isolated from canine heart. Na +-Ca 2+ exchange and Ca 2+ pump (ATP-dependent Ca 2+ uptake) activities were assessed using the Millipore filtration technique. sarcolemmal vesicles used in this study are estimated to consist of several subpopulations wherein 23% are inside-out and 55% are right side-out sealed vesicles in orientation. The affect of each Ca 2+ antagonist on the Na +-dependent Ca 2+ uptake was studied in the total population of sarcolemmal vesicles, in which none of the agents depressed the initial rate of Ca 2+ uptake until concentrations of 10 M were incubated in the incubation medium. However, when sarcolemmal vesicles were preloaded with Ca 2+ via ATP-dependent Ca 2+ uptake, cellular Ca 2+ influx was depressed only by verapamil (28%) at 1 M in the efflux medium with 8 mM Na +. Furthermore, inhibition of Ca 2+ efflux by verapamil was more pronounced in the presence of 16 mM Na + in the efflux medium. The order of inhibition was; verapamil > diltiazem > nifedipine. These results indicate that same forms of Ca 2+-antagonist drugs may affect the Na +-Ca 2+ exchange mechanism in the cardiac sarcolemmal vesicles and therefore we suggest this site of action may contribute to their effects on the myocardium. 相似文献
13.
Summary This study concerns the properties of rapid K + and Cl transport pathways that are present in the (H ++K +)-ATPase membrane from stimulated, and secreting, gastric oxyntic cells. Ion permeabilities in the isolated stimulation-associated vesicles were monitored via the rates of H + efflux under conditions of exclusive H +/K + counterflux or H +–Cl co-efflux, as well as by comparison of equilibration rates for 86Rb and 36Cl under conditions of equilibrium exchange and unidirectional salt flux. These latter studies suggest that Rb + and Cl pathways are conductive and independent. In spite of the functional independence of the ion pathways, several divalent cations inhibit Rb + and Cl isotopic exchange as well as the H + efflux that is dependent on either K + or anion (Cl, SCN, NO 2) fluxes. Zn 2+ is the more potent inhibitor, reducing by 50% the sensitive component of K +, Cl, and NO 2 fluxes at about 20 m; Mn 2+ has a similar effect at 200 m. Ni 2+ and Co 2+ were roughly equipotent to Mn 2+ while Mg 2+ and Ca 2+ had not inhibitory effect. These results suggest that the stimulation-induced permeabilities, while functioning independently, may be physically linked, i.e., residing within a single entity. In similar studies carried out in (H ++K +)-ATPase vesicles obtained from nonstimulated cells, no vestiges of sensitivity to the inhibitory divalent cations could be detected. The implications of these findings for the physiology of the oxyntic cell in the context of a model for membrane fusion are discussed. 相似文献
14.
Summary ATP-dependent 45Ca 2+ uptake was investigated in purified plasma membranes from rat pancreatic acinar cells. Plasma membranes were purified by four subsequent precipitations with MgCl 2 and characterized by marker enzyme distribution. When compared to the total homogenate, typical marker enzymes for the plasma membrane, (Na +,K +)-ATPase, basal adenylate cyclase and CCK-OP-stimulated adenylate cyclase were enriched by 43-fold, 44-fold, and 45-fold, respectively. The marker for the rough endoplasmic reticulum was decreased by fourfold compared to the total homogenate. Comparing plasma membranes with rough endoplasmic reticulum, Ca 2+ uptake was maximal with 10 and 2 mol/liter free Ca 2+, and half-maximal with 0.9 and 0.5 mol/liter free Ca 2+. It was maximal at 3 and 0.2 mmol/liter free Mg 2+ concentration, at an ATP concentration of 5 and 1 mmol/liter, respectively, and at pH 7 for both preparations. When Mg 2+ was replaced by Mn 2+ or Zn 2+ ATP-dependent Ca 2+ uptake was 63 and 11%, respectively, in plasma membranes; in rough endoplasmic reticulum only Mn 2+ could replace Mg 2+ for Ca 2+ uptake by 20%. Other divalent cations such as Ba 2+ and Sr 2+ could not replace Mg 2+ in Ca 2+ uptake. Ca 2+ uptake into plasma membranes was not enhanced by oxalate in contrast to Ca 2+ uptake in rough endoplasmic reticulum which was stimulated by 7.3-fold. Both plasma membranes and rough endoplasmic reticulum showed cation and anion dependencies of Ca 2+ uptake. The sequence was K +>Rb +>Na +>Li +>choline + in plasma membranes and Rb +K +Na +>Li +>choline + for rough endoplasmic reticulum. The anion sequence was Cl –Br –I –>SCN –>NO
3
–
>isethionate – >cyclamate –>gluconate –>SO
4
2–
glutarate – and Cl –>Br>gluconate>SO
4
2–
>NO
3
–
>I –>cyclamate –SCN –, respectively. Ca 2+ uptake into plasma membranes appeared to be electrogenic since it was stimulated by an inside-negative K + and SCN diffusion potential and inhibited by an inside-positive diffusion potential. Ca 2+ uptake into rough endoplasmic reticulum was not affected by diffusion potentials. We assume that the Ca 2+ transport mechanism in plasma membranes as characterized in this study represents the extrusion system for Ca 2+ from the cell that might be involved in the regulation of the cytosolic Ca 2+ level. 相似文献
15.
Summary The effects of cAMP, ATP and GTP on the Ca 2+-dependent K + channel of fresh (1–2 days) or cold-stored (28–36 days) human red cells were studied using atomic absorption flame photometry of Ca 2+-EGTA loaded ghosts which had been resealed to monovalent cations in dextran solutions. When high-K + ghosts were incubated in an isotonic Na + medium, the rate constant of Ca 2+-dependent K + efflux was reduced by a half on increasing the theophylline concentration to 40 mm. This effect was observed in ghosts from both fresh and stored cells, but only if they were previously loaded with ATP. The inhibition was more marked when Mg 2+ was added together with ATP, and it was abolished by raising free Ca 2+ to the micromolar level. Like theophylline, isobutyl methylxanthine (10 mm) also affected K + efflux. cAMP (0.2–0.5 mm), added both internally and externally (as free salt, dibutyryl or bromide derivatives), had no significant effect on K + loss when the ghost free-Ca 2+ level was below 1 m, but it was slightly inhibitory at higher concentrations. The combined presence of cAMP (0.2 mm) plus either theophylline (10 mm), or isobutyl methylxanthine (0.5 mm), was more effective than cAMP alone. This inhibition showed a strict requirement for ATP plus Mg 2+ and it, was not overcome by raising internal Ca 2+. Ghosts from stored cells seemed more sensitive than those from fresh cells, to the combined action of cAMP and methylxanthines. Loading ATP into ghosts from fresh or stored cells markedly decreased K + loss. Although this effect was observed in the absence of added Mg 2+ (0.5 mm EDTA present), it was potentiated upon adding 2 mm Mg 2+. The K + efflux from ATP-loaded ghosts was not altered by dithio- bis-nitrobenzoic acid (10 mm) or acridine orange (100 m), while it was increased two-to fourfold by incubating with MgF 2 (10 mm), or MgF 2 (10 mm)+theophylline (40 mm), respectively. By contrast, a marked efflux reduction was obtained by incorporating 0.5 mm GTP into ATP-containing ghosts. The degree of phosphorylation obtained by incubating membranes with (- 32P)ATP under various conditions affecting K + channel activity, was in direct correspondence to their effect on K + efflux. The results suggest that the K + channel of red cells is under complex metabolic control, via cAMP-mediated and nonmediated mechanisms, some which require ATP and presumably, involve phosphorylation of the channel proteins. 相似文献
16.
Summary Exposure of porcine renal brush-border membrane vesicles to 1.2% cholate and subsequent detergent removal by dialysis reorients almost all N-ethylmaleimide (NEM)-sensitive ATPases from the vesicle inside to the outside. ATP addition to cholate-pretreated, but not to intact, vesicles causes H + uptake as visualized by the pH indicator, acridine organge. The reoriented H +-pump is electrogenic because permeant extravesicular anions or intravesicular K + plus valinomycin enhance H + transport. ATP stimulates H + uptake with an apparent K
m
of 93 m. Support of H + uptake and P
i
liberation by ATP>GTPITP> UTP indicates a preference for ATP and utilization of other nucleotides at lower efficiency. ADP is a potent, competitive inhibitor of ATP-driven H + uptake,( K
i
, 24 m). Mg 2+ and Mn 2– support ATP-driven H + uptake, but Ca 2+, Ba 2+ and Zn 2+ do not. I mm Zn 2+ inhibits MgATP-driven H + transport completely. NEM-sensitive P
i
liberation is stimulated by Mg 2+ and Mg 2– and, unlike H + uptake, also by Ca 2+ suggesting Ca 2+-dependent ATP hydrolysis unrelated to H + transport. The inside-out oriented H +-pump is relatively insensitive toward oligomycin, azide, N,N-dicyclohexylcarbodiimide (DCCD) and vanadate, but efficiently inhibited by NEM (apparent K
i
, 0.77 m), and 4-chloro-7-nitro-benzoxa-1,3-diazole (NBD-Cl; apparent K
i
, 0.39 m). Taken together, the H +-ATPase of proximal tubular brush-border membranes exhibits characteristics very similar to those of vacuolar type ( V-type) H +-ATPases. Hence, V-type H +-ATPases occur not only in intracellular organelles but also in specialized plasma membrane areas. 相似文献
17.
Summary The presence of a coupled Na +/Ca 2+ exchange system has been demonstrated in plasma membrane vesicles from rat pancreatic acinar cells. Na +/Ca 2+ exchange was investigated by measuring 45Ca 2+ uptake and 45Ca 2+ efflux in the presence of sodium gradients and at different electrical potential differences across the membrane (=) in the presence of sodium. Plasma membranes were prepared by a MgCl 2 precipitation method and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the plasma membrane, (Na ++K +)-ATPase was enriched by 23-fold. Markers for the endoplasmic reticulum, such as RNA and NADPH cytochrome c reductase, as well as for mitochondria, the cytochrome c oxidase, were reduced by twofold, threefold and 10-fold, respectively. For the Na +/Ca 2+ countertransport system, the Ca 2+ uptake after 1 min of incubation was half-maximal at 0.62 mol/liter Ca 2+ and at 20 mmol/liter Na + concentration and maximal at 10 mol/liter Ca 2+ and 150 mmol/liter Na + concentration, respecitively. When Na + was replaced by Li +, maximal Ca 2+ uptake was 75% as compared to that in the presence of Na +. Amiloride (10 –3 mol/liter) at 200 mmol/liter Na + did not inhibit Na +/Ca 2+ countertransport, whereas at low Na + concentration (25 mmol/liter) amiloride exhibited dose-dependent inhibition to be 62% at 10 –2 mol/liter. CFCCP (10 –5 mol/liter) did not influence Na +/Ca 2+ countertransport. Monensin inhibited dose dependently; at a concentration of 5×10 –6 mol/liter inhibition was 80%. A SCN – or K + diffusion potential (=), being positive at the vesicle inside, stimulated calcium uptake in the presence of sodium suggesting that Na +/Ca 2+ countertransport operates electrogenically, i.e. with a stoichiometry higher than 2 Na + for 1 Ca 2+. In the absence of Na +, did not promote Ca 2+ uptake. We conclude that in addition to ATP-dependent Ca 2+ outward transport as characterized previously (E. Bayerdörffer, L. Eckhardt, W. Haase & 1. Schulz, 1985, J. Membrane Biol.
84:45–60) the Na +/Ca 2+ countertransport system, as characterized in this study, represents a second transport system for the extrusion of calcium from the cell. Furthermore, the high affinity for calcium suggests that this system might participate in the regulation of the cytosolic free Ca 2+ level. 相似文献
18.
Washing corn ( Zea mays L.) root tissue in water causes loss of about one-third of the exchangeable Ca 2+ over the first 10 to 15 minutes. Upon transfer to K +-containing solutions, the tissue shows a short period of rapid K + influx which subsequently declines. Addition of 0.1 millimolar Ca 2+ decreases the initial rapid K + influx, but increases the sustained rate of K + and Cl − uptake. It was confirmed (Elzam and Hodges 1967 Plant Physiol 42: 1483-1488) that 0.1 millimolar Ca 2+ is more effective than higher concentrations for the initial inhibition, and that Mg 2+ will substitute. The inhibition arises from a mild shock affect of restoring Ca2+. With 0.1 millimolar Ca2+ net H+ efflux is blocked for 10 to 15 minutes and the cells are depolarized by about 30 millivolts. However, 1 millimolar Ca2+ rapidly produces increased K+ influx and blocks net H+ efflux for only a few minutes; blockage is preceded by a brief net H+ influx which may restore and increase ion transport by reactivating the plasmalemma H+-ATPase. Stimulation of electrogenic H+-pumping with fusicoccin eliminates the shock responses and minimizes Ca2+ effects on K+ influx. Fusicoccin also strongly decreases Ca2+ influx, but has no effect on Ca2+ efflux. Ice temperatures and high pH decreased Ca2+ efflux, but uncoupler and chlorpromazine did not. It is suggested that the inhibitory and promotive actions of Ca2+ are manifested through decreases or increases in the protonmotive force. 相似文献
19.
Summary Micromolar concentrations of silver ion activate large Ca 2+ fluxes across the plasma membrane of intact rod outer segments isolated from bovine retinas (intact ROS). The rate of Ag +-induced Ca 2+ efflux from intact ROS depended on the Ag + concentration in a sigmoidal manner suggesting a cooperative mechanism with a Hill coefficient between 2 and 3. At a concentration of 50 m Ag + the rate of Ca 2+ efflux was 7×10 6 Ca 2+/outer segment/sec; this represents a change in total intracellular Ca 2+ by 0.7 mm/outer segment/sec. Addition of the nonselective ionophore gramicidin in the absence of external alkali cations greatly reduced the Ag +-induced Ca 2+ efflux from intact ROS, apparently by enabling internal alkali cations to leak out. Adding back alkali cations to the external medium restored Ag +-induced Ca 2+ efflux when gramicidin was present. In the presence of gramicidin, Ag +-induced Ca 2+ efflux from intact ROS was blocked by 50 m tetracaine or l- cis diltiazem, whereas without gramicidin both blockers were ineffective. Both l- cis diltiazem and tetracaine are blockers of one kinetic component of cGMP-induced Ca 2+ flux across ROS disk membranes. The ion selectivity of the Ag +-induced pathway proved to be broad with little discrimination between the alkali cations Li +, Na +, K +, and Cs + or between Ca 2+ and Mg 2+. The properties of the Ag +-induced pathway(s) suggest that it may reflect the cGMP-dependent conductance opened in the absence of cGMP by silver ions. 相似文献
20.
Summary In storage tissue of Beta vulgaris L., carbonyl cyanide m-chlorophenylhydrazone or cyanide+salicylhydroxamic acid reduce cell electropotentials from about –200 to below –100 mV. The relationship between potential and cellular ATP level is examined during treatment with different concentrations of inhibitiors. At low ATP levels the potential rises sharply with increases in ATP, but above an ATP level of approximately 50% of the uninhibited level the potential changes very little with ATP concentration. A plot of membrane potential vs. 86Pb + influx or of potential vs. net K + uptake indicates that as the level of inhibition is decreased, the potential tends to reach a limit while cation influx and net uptake continue to increase. Resistance measurements, although subject to difficulties of interpretation, indicate no change in conductance with potential, ion flux, or ATP level. Thus the membrane potential should directly reflect electrogenic pump activity, attributed to active uncoupled H + efflux. K + uptake can occur against its electrochemical gradient and is attributed to a coupled K + influx/H + efflux pump. The results show that the electrogenic pump activity is independent of the K +/H + exchange rate. Thus electrogenic H + efflux and K +/H + exchange may represent different transport systems, or different modes of operation of a single pump with variable stoichiometry. 相似文献
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