Alkalinization within sarcoplasmic reticulum during the uptake of calcium ions.

The pH indicator, bromothymol blue, was incorporated into sarcoplasmic reticulum vesicles which bind more than 90% of the total added dye. The sequestered dye does not respond to changes in external pH upon addition of acid to the medium, since the decrease of absorbance at 616 nm is very slow. The absorbance of sequestered dye at 616 nm increases suddenly after triggering the transport of Ca²⁺ by ATP at a rate much higher than that of Ca²⁺ uptake, and declines when Ca²⁺ has been accumulated. When the uptake of Ca²⁺ is followed in the presence of oxalate, the absorbance of the indicator declines after the first phase of Ca²⁺ uptake. The results suggest that a transient alkalinization occurs rapidly inside the vesicles and reflects the formation of a transmembrane proton gradient responsible for sustaining the Ca²⁺ transport.     

Proton movements across the membranes of sarcoplasmic reticulum during the uptake of calcium ions

A burst of proton ejection was observed during the initial steps of Ca²⁺ uptake by sarcoplasmic reticulum vesicles. The initial rate of this proton ejection is considerably higher than the initial rate of Ca²⁺ uptake, and is independent of the amount of accumulated Ca²⁺. The ejection of protons is a transmembrane event, since it is dissipated by the ionophore X-537A, and does not occur when the ionophore is added before the initiation of the transport of Ca²⁺. The low proton permeability of the membranes is largely increased by X-537A. The studies of facilitated diffusion of protons in the presence of the ionophore permitted the estimation of the pH within the vesicles. A fast alkalinization occurs within the vesicles during the initial steps of Ca²⁺ uptake, as revealed by sequestered bromothymol blue. The change in absorbance of this dye corresponds to a change of 0.15 pH unit within the vesicles, and a maximal transmembrane ΔpH of about 0.5 may build up. Since such a gradient may not account energetically for the transmembrane gradients of Ca²⁺, I suggest that a transmembrane electrical potential may develop as a consequence of proton ejection.     

Tracking of proton flow during transition from anaerobiosis to steady state in rat liver mitochondria

(1) The hydrophobic pH indicator Bromthymol blue and the hydrophilic pH indicator Phenol red have been used to follow the redox-pump-linked proton flows during transition from anaerobiosis to static head. The domains monitored by the pH indicators, whether external or internal, and the localization of the dye, whether free or membrane bound, have been identified by recording the absorbance changes following addition of nigericin or valinomycin to anaerobic or aerobic mitochondria and the effects of permeant and impermeant buffers. (2) After addition of the H+/K+ exchanger, nigericin, to anaerobic mitochondria. Phenol red and Bromthymol blue record an alkalinization and an acidification, respectively, indicating that while the hydrophilic pH indicator faces an external domain, the hydrophobic pH indicator faces, at least partly, an internal domain. The latter effect is sensitive to phosphate and to phosphate carrier inhibitors. On the other hand, addition of nigericin to aerobic mitochondria leads to an increased Bromthymol blue absorbance, which reflects an alkalinization, indicating that the pH indicator faces an external domain. The reorientation of the dye from the internal to the external domain is a function of the uncoupler concentration and thus of the membrane potential (cf. Mitchell et al. (1968) Eur. J. Biochem. 4, 9-19). (3) The amount of oxygen required for the transition from anaerobiosis to static head has been determined by following in parallel the extent of oxidation of cytochrome aa3 and the rise of delta mu H+. With succinate as substrate, 50% levels of cytochrome oxidation are obtained at 0.125 ngatom oxygen/mg and 50% of Safranine response at about 0.2 ngatom oxygen/mg. These amounts of oxygen correspond to an H+ displacement of about 0.8-1.2 ngatom/mg on the basis of the H+/O stoichiometry. It is concluded that mitochondria are in presteady state below, and in static head above, displacement of 2-3 ngatom H+/mg. This figure is very close to the original calculation of Mitchell (Mitchell, P. (1966) Biol. Rev. 41, 445-502). (4) Transition, by oxygen pulses, of EGTA-supplemented mitochondria from anaerobiosis to either presteady state or static head state results in a response of the hydrophilic pH indicator, Phenol red, which is negligible in amount and/or kinetically unrelated to the delta mu H+ rise. The fact that H+ extrusion in the bulk aqueous phase is negligible also in presteady state excludes proton cycling as an explanation. Addition of oxygen pulses to Sr2(+)-supplemented anaerobic mitochondria results in an H+ extrusion whose amount and rate is proportional to the Sr2+ concentration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dynamic regulation of the mitochondrial proton gradient during cytosolic calcium elevations

Mitochondria extrude protons across their inner membrane to generate the mitochondrial membrane potential (ΔΨ(m)) and pH gradient (ΔpH(m)) that both power ATP synthesis. Mitochondrial uptake and efflux of many ions and metabolites are driven exclusively by ΔpH(m), whose in situ regulation is poorly characterized. Here, we report the first dynamic measurements of ΔpH(m) in living cells, using a mitochondrially targeted, pH-sensitive YFP (SypHer) combined with a cytosolic pH indicator (5-(and 6)-carboxy-SNARF-1). The resting matrix pH (～7.6) and ΔpH(m) (～0.45) of HeLa cells at 37 °C were lower than previously reported. Unexpectedly, mitochondrial pH and ΔpH(m) decreased during cytosolic Ca(2+) elevations. The drop in matrix pH was due to cytosolic acid generated by plasma membrane Ca(2+)-ATPases and transmitted to mitochondria by P(i)/H(+) symport and K(+)/H(+) exchange, whereas the decrease in ΔpH(m) reflected the low H(+)-buffering power of mitochondria (～5 mm, pH 7.8) compared with the cytosol (～20 mm, pH 7.4). Upon agonist washout and restoration of cytosolic Ca(2+) and pH, mitochondria alkalinized and ΔpH(m) increased. In permeabilized cells, a decrease in bath pH from 7.4 to 7.2 rapidly decreased mitochondrial pH, whereas the addition of 10 μm Ca(2+) caused a delayed and smaller alkalinization. These findings indicate that the mitochondrial matrix pH and ΔpH(m) are regulated by opposing Ca(2+)-dependent processes of stimulated mitochondrial respiration and cytosolic acidification.     

Acidification and Recovery Results in Nuclear Accumulation of Supravital Dyes during Interphase

Recent studies using real time imaging demonstrated relative nuclear insulation for ion-size particles. We show here that acidification and recovery converted the insulated interphase nuclei of KB carcinoma and nontumorigenic Chang cells into intense nuclear accumulating states marked by sequestration of the exogenous supravital dyes neutral red, methylene blue, and brilliant cresyl blue. The phenomenon was not affected by Na⁺-free and HCO₃-free conditions nor by the presence of cationic and anionic antiport regulators of cytosolic pH. Cytological, microspectrophotometric, and flow cytometric evaluation of whole cell populations showed that the nuclear influx was abolished by omitting the pH recovery response, and by modulating the recovery response. The abolition of nuclear influx in the presence of the P-ATPase and F_o-ATPase inhibitors, vanadyl(IV) ions and oligomycin, respectively, suggest that H⁺-translocating ATPase pumps are involved in regulating cytosolic acidification in Na⁺-free and HCO₃-conditions vanadyl(IV) inhibited nuclear uptake of supravital dyes in a dose dependent manner. Nuclear uptake of dyes, however, was not affected by up to 1 mM of genistein even though tyrosine-specific phosphorylation and DNA synthesis were abolished. Upgradient nuclear influx involving proton pump is novel. KB cancer cells and nontumorigenic Chang cells had differential dye accumulations induced by acidification and recovery.     

Kinetics and stoichiometry of light-induced proton release and uptake from purple membrane fragments, Halobacterium halobium cell envelopes, and phospholipid vesicles containing oriented purple membrane

We have used flash spectroscopy and pH indicator dyes to measure the kinetics and stoichiometry of light-induced proton release and uptake by purple membrane in aqueous suspension, in cell envelope vesicles and in lipid vesicles. The preferential orientation of bacteriorhodopsin in opposite directions in the envelope and lipid vesicles allows us to show that uptake of protons occurs on the cytoplasmic side of the purple membrane and release on the exterior side.

In suspensions of isolated purple membrane, approximately one proton per cycling bacteriorhodopsin molecule appears transiently in the aqueous phase with a half-rise time of 0.8 ms and a half-decay time of 5.4 ms at 21 °C.

In cell envelope preparations which consist of vesicles with a preferential orientation of purple membrane, as in whole cells, and which pump protons out, the acidification of the medium has a half-rise time of less than 1.0 ms, which partially relaxes in approx. 10 ms and fully relaxes after many seconds.

Phospholipid vesicles, which contain bacteriorhodopsin preferentially oriented in the opposite direction and pump protons in, show an alkalinization of the medium with a time constant of approximately 10 ms, preceded by a much smaller and faster acidification. The alkalinization relaxes over many seconds.

The initial fast acidification in the lipid vesicles and the fast relaxation in the envelope vesicles are accounted for by the misoriented fractions of bacteriorhodopsin. The time constants of the main effects, acidification in the envelopes and alkalinization in the lipid vesicles correlate with the time constants for the release and uptake of protons in the isolated purple membrane, and therefore show that these must occur on the outer and inner surface respectively. The slow relaxation processes in the time range of several seconds must be attributed to the passive back diffusion of protons through the vesicle membrane.     

Amine Accumulation in Acidic Vacuoles Protects the Halotolerant Alga Dunaliella salina Against Alkaline Stress

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 NH₄⁺ 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.     

Vesicular ATPase inserted into the plasma membrane of motor terminals by exocytosis alkalinizes cytosolic pH and facilitates endocytosis

Key components of vesicular neurotransmitter release, such as Ca(2+) influx and membrane recycling, are affected by cytosolic pH. We measured the pH-sensitive fluorescence of Yellow Fluorescent Protein transgenically expressed in mouse motor nerve terminals, and report that Ca(2+) influx elicited by action potential trains (12.5-100 Hz) evokes a biphasic pH change: a brief acidification (～ 13 nM average peak increase in [H(+)]), followed by a prolonged alkalinization (～ 30 nM peak decrease in [H(+)]) that outlasts the stimulation train. The alkalinization is selectively eliminated by blocking vesicular exocytosis with botulinum neurotoxins, and is prolonged by the endocytosis-inhibitor dynasore. Blocking H(+) pumping by vesicular H(+)-ATPase (with folimycin or bafilomycin) suppresses stimulation-induced alkalinization and reduces endocytotic uptake of FM1-43. These results suggest that H(+)-ATPase, known to transfer cytosolic H(+) into prefused vesicles, continues to extrude cytosolic H(+) after being exocytotically incorporated into the plasma membrane. The resulting cytosolic alkalinization may facilitate vesicular endocytosis.     

Consequences of loss of Vph1 protein-containing vacuolar ATPases (V-ATPases) for overall cellular pH homeostasis

In yeast cells, subunit a of the vacuolar proton pump (V-ATPase) is encoded by two organelle-specific isoforms, VPH1 and STV1. V-ATPases containing Vph1 and Stv1 localize predominantly to the vacuole and the Golgi apparatus/endosomes, respectively. Ratiometric measurements of vacuolar pH confirm that loss of STV1 has little effect on vacuolar pH. Loss of VPH1 results in vacuolar alkalinization that is even more rapid and pronounced than in vma mutants, which lack all V-ATPase activity. Cytosolic pH responses to glucose addition in the vph1Δ mutant are similar to those in vma mutants. The extended cytosolic acidification in these mutants arises from reduced activity of the plasma membrane proton pump, Pma1p. Pma1p is mislocalized in vma mutants but remains at the plasma membrane in both vph1Δ and stv1Δ mutants, suggesting multiple mechanisms for limiting Pma1 activity when organelle acidification is compromised. pH measurements in early prevacuolar compartments via a pHluorin fusion to the Golgi protein Gef1 demonstrate that pH responses of these compartments parallel cytosolic pH changes. Surprisingly, these compartments remain acidic even in the absence of V-ATPase function, possibly as a result of cytosolic acidification. These results emphasize that loss of a single subunit isoform may have effects far beyond the organelle where it resides.     

Proton movements and electric potential generation in reconstituted ATPase proteoliposomes from the thermophilic cyanobacterium Synechococcus 6716

ATP hydrolysis-induced proton translocation and electric potential generation have been studied in ATPase proteoliposomes by means of various optical probes. The proteoliposomes consisted of reconstituted ATPase complex and native lipid mixture isolated from the thermophilic cyanobacterium Synechococcus 6716 [Van Walraven et al. (1983) Eur. J. Biochem. 137, 101-106]. The native cartenoids and added oxonol VI served as probes for the electric membrane potential generated by the net charge separation (negative outside, positive inside). Their responses, with similar half-times as 9-tetradecylamino-6-chloro-2-methoxyacridine, are sensitive to valinomycin and stimulated by nigericin, as expected. The proton concentrations of extraliposomal and intraliposomal aqueous spaces were monitored by neutral red and cresol red; for internal measurements these pH indicators were trapped inside the vesicles during detergent dialysis. Internal acidification and external alkalinization induced by ATP hydrolysis are inhibited by nigericin and enhanced by valinomycin; at the commonly used higher valinomycin concentrations the neutral red response becomes transient, while the much slower cresol red response is diminished right from its onset. At smaller preset pH gradients both ATP hydrolysis activity and neutral red response are diminished. At increasing MgCl2 concentrations the neutral red responses are slowed down and the cresol red responses are slightly enhanced; this is observed for both internal and external dye responses. Neutral red permeation through the membrane is insignificant under our experimental conditions but is enhanced at temperatures below the lipid-phase transition. In the case of externally added neutral red the non-permeant buffer Hepes is only effective at high MgCl2 concentration, whereas some external cresol red response is visible only at high MgCl2 concentration in the presence of Hepes. The kinetics of the pH indicator and electric potential probe responses clearly distinguish fast interfacial and intra-membrane proton displacements from slow bulk proton equilibration. The data are summarized in a model that supports the importance of localized proton displacements for the primary energy-transducing events.     

Uptake of the fluorescent indicator atebrin into acidic vacuoles in the halotolerant alga Dunaliella satina

The fluorescent indicator atebrin (3-chloro-9-(4-diethylamino-1-methylbutyl)-7-methyoxy-acridine) is taken up by Dunaliella salina cells at alkaline external pH and accumulates in acidic vacuoles. The uptake is unaffected by light, by photosynthetic inhibitors, by protonophores or by ionophores; however, the dye can be released by amines, indicating that it is specifically accumulating in acidic vacuoles. Amines induce a biphasic enhancement of atebrin fluorescence — a fast phase, accompanied by redistribution within the cell, consistent with release of the dye from the vacuoles to the cytoplasm, and a slow phase, correlated with release of atebrin from the cells. These results are interpreted to indicate a slow equilibration of atebrin across the plasma membrane and a fast equilibration across the vacuolar membrane. Part of the dye cannot be released by the amines, and appears to be internally bound. Atebrin uptake is inhibited by cholesteryl hemisuccinate and is stimulated by lysophosphatidylcholine, indicating that modification of the lipid composition of the plasma membrane affects the permeability to atebrin. Analysis of the pH dependence of atebrin uptake indicates that the dye enters the cells by fluid-phase permeation. Different stresses enhance the rate of atebrin uptake and release, indicating that they modify plasma-membrane structure or composition. Atebrin may serve as a specific marker for acidic vacuoles, as an indicator for amine uptake, and as a probe for subtle changes in the permeability of the plasma membrane.Abbreviations Atebrin 3-chloro-9-(4-diethylamino-1-methylbutyl)-7-methoxy-acridine - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea - SF-6847 3,5-ditertbutyl-4-hydroxybenzylidenemalonitrile     

Effect of phase transition on the kinetics of dye transport in phospholipid bilater structures.

Binding of 8-anilino-1-naphthalenesulfonate to dimyristoyl-L-alpha-lecithin bilayers enhances the fluorescence quantum yield of the dye molecule by 100-fold. By following the generation of fluorescence after a rapid mixing in a stopped-flow apparatus (mixing time 2 msec), kinetics of the binding of the fluorescence probe to the phospholipid vesicles has been investigated in the temperature range where the crystal-liquid crystal phase transition of the bilayer structures occurs. No reactions depending on the dye or the vesicle concentrations were detected. This suggests that the initial adsorption of the dye was very rapid. Two kinetic phases which appear in the 50 msec and the second time ranges are unimolecular. The faster one has a small amplitude and is observable in the entire temperature range studied. In the phase transition region the slower reaction becomes the major kinetic phase. It also increases the apparent concentration of bound dye by a factor of 2. These observations suggest that the 50-msec reaction has detected a reorientation of the probe molecule after the initial binding, and that the slow reaction represents a transport of the dye molecule into the inner layer of the lipid vesicle. The transport reaction is extremely temperature sensitive and exhibits a maximum rate at the midpoint of the bilayer phase transition (Tm = 24.1 degrees). the Arrhenius plot of the transport reaction shows a maximum at the Tm. the same temperature dependence was also observed for the bromothymol blue transport reaction. However, no such effects were detected for less amphiphilic molecules such as tetracycline, chlortetracycline, and pyrene. In the latter systems only a slight bending of the Arrhenius plots were seen at the phase transition temperature. Since the kinetics of the transport of 8-anilino-1-naphthalenesulfonate is sensitive to the physical state of the phospholipid bilayers this reaction may be used for probing membrane structures.     

Intracellular pH modulates inner segment calcium homeostasis in vertebrate photoreceptors

Neuronal metabolic and electrical activity is associated with shifts in intracellular pH (pH(i)) proton activity and state-dependent changes in activation of signaling pathways in the plasma membrane, cytosol, and intracellular compartments. We investigated interactions between two intracellular messenger ions, protons and calcium (Ca2(+)), in salamander photoreceptor inner segments loaded with Ca2(+) and pH indicator dyes. Resting cytosolic pH in rods and cones in HEPES-based saline was acidified by ～0.4 pH units with respect to pH of the superfusing saline (pH = 7.6), indicating that dissociated inner segments experience continuous acid loading. Cytosolic alkalinization with ammonium chloride (NH?Cl) depolarized photoreceptors and stimulated Ca2(+) release from internal stores, yet paradoxically also evoked dose-dependent, reversible decreases in [Ca2(+)](i). Alkalinization-evoked [Ca2(+)](i) decreases were independent of voltage-operated and store-operated Ca2(+) entry, plasma membrane Ca2(+) extrusion, and Ca2(+) sequestration into internal stores. The [Ca2(+)](i)-suppressive effects of alkalinization were antagonized by the fast Ca2(+) buffer BAPTA, suggesting that pH(i) directly regulates Ca2(+) binding to internal anionic sites. In summary, this data suggest that endogenously produced protons continually modulate the membrane potential, release from Ca2(+) stores, and intracellular Ca2(+) buffering in rod and cone inner segments.     

Na+/H+ exchange in Ehrlich ascites tumor cells. Regulation by extracellular ATP and 12-O-tetradecanoylphorbol 13-acetate

The effects of extracellular ATP and/or the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on the intracellular pH of Ehrlich ascites tumor cells were measured using both distribution of [14C]5,5-dimethyloxazolidine-2,4-dione, and the fluorescent indicator 5(6)-carboxyfluorescein. Micromolar concentrations of extracellular ATP induce a biphasic change in the intracellular pH characterized by a rapid acidification of 0.04 pH units followed by an alkalinization of 0.11 pH units. Concurrently with the alkalinization, an increase in the total cellular [Na+] from 37.5 to 45.0 mM is observed. The pH change is half-maximally activated by 0.5-2.5 microM extracellular ATP. The intracellular alkalinization, but not the initial acidification, phase requires extracellular Na+, with half-maximal alkalinization in the presence of 24-32 mM Na+, and is inhibited by amiloride. Exposure of Ehrlich ascites tumor cells to TPA alone produces a slight alkalinization of approximately 0.04 pH units. Conversely, preincubation of the cells with TPA partially inhibits the ATP-induced changes in intracellular pH. Under identical conditions TPA also inhibits the ATP-induced increase in the cytosolic [Ca2+]. The half-maximal dose for both effects is produced by 3-10 nM TPA. These data indicate that extracellular ATP triggers the activation of Na+/H+ exchange. Furthermore, activation of protein kinase C mediates at least part of the Na+/H+ exchange, although a second mechanism may also exist.     

Energized Uptake of Sugars from the Apoplast of Leaves: A Study of Some Plants Possessing Different Minor Vein Anatomy

Solutions of sucrose, glucose, raffinose, and stachyose were fed via the petiole to detached leaves of plant species known to transfer sugars during photosynthesis into the phloem using either the apoplastic or the symplastic pathway of phloem loading. Symplastic phloem loaders, which translocate raffinose-type oligosaccharides and sucrose in the phloem, and apoplastic plants, translocating exclusively sucrose, were selected for this study. As the sugars arrived with the transpiration stream in the leaf blade within little more than a minute, dark respiration increased. Almost simultaneously, fluorescence of a potential-indicating dye, which had been infiltrated into the leaves, indicated membrane depolarization. Another fluorescent dye used to record the apoplastic pH revealed apoplastic alkalinization that occurred with a slight lag phase after respiration and membrane depolarization responses. Occasionally, alkalinization was preceded by transient apoplastic acidification. Whereas membrane depolarization and apoplastic acidification are interpreted as initial responses of the proton motive force across the plasma membrane to the advent of sugars in the leaf apoplast, the following apoplastic alkalinization showed that sugars were taken up from the apoplast into the symplast in cotransport with protons. This was true not only for glucose and sucrose, but also for raffinose and stachyose. Similar observations were made for sugar uptake not only in leaves of plants known to export sugars by symplastic phloem loading but also of plants using the apoplastic pathway. Increased respiration during sugar uptake revealed tight coupling between respiratory ATP production and ATP consumption by proton-translocating ATPase of the plasma membrane, which exports protons into the apoplast, thereby compensating for the proton loss in the apoplast when protons are transported together with sugars into the symplast. The extent of stimulation of respiration by sugars indicated that sugar uptake was not limited to phloem tissue. Ratios of the extra CO₂ released during sugar uptake to the amounts of sugars taken up were variable, but lowest values were lower than 0.2. When a ratio of 0.2 is taken as a basis to calculate rates of sugar uptake from observed maxima of sugar-dependent increases in respiration, rates of sugar uptake approached 350 nmol/(m² leaf surface s). Sugar uptake rates were half-saturated at sugar concentrations in the feeding solutions of about 10–25 mM indicating a low in vivo affinity of sugar uptake systems for sugars.     

Oxygen-pulse curves in rat liver mitochondrial suspensions. Some observations and deductions.

1. The inference, implicit in the chemiosmotic hypothesis, that protons move into the bulk phase during ATP synthesis was investigated. 2. Incubation of rat liver mitochondria in the presence of the cation exchanger CM-Sephadex C-50 caused alkalinization in the medium, though total ATP synthesis remained unchanged. The addition of N-ethylmaleimide prevented the alkalinization, but there was still no indication of protons passing into the medium. The expected proton movement [Mitchell & Moyle (1967) Biochem. J. 105, 1147--1162] was readily detected when as an equivalent acid pulse. 3. Analysis of delta H+ decay curves after O2 pulses (3 micrograms-atoms of O/g of protein) indicated the presence of fast and slow components of decay, with first-order rate constants (k) of 0.24s-1 and 0.032s-1. The fast decay was finite and was eliminated in the presence of N-ethylmaleimide. 4. These observations are interpreted as evidence for the development of unmasking of fixed charges on the outer surface of the mitochondrial inner membrane during energization and for the existence of proton-retentive electrical fields (rho-zones) on this surface. The charge concentration is calculated as about 1 charge/10nm2. 5. A cycle of changes in a single fixed-charge molecule is proposed which mediates both Ca2+ uptake and the first step in the utilization of the rho-zone protonmotive force, delta p rho.     

Active transport of alanine by thermostable membrane vesicles isolated from a thermophilic bacterium.

1. Thermostable membrane vesicles which were capable of active transport of alanine dependent on either respiration or an artificial membrane potential were isolated from the thermophilic aerobic bacterium PS3. 2. Uptake of alanine was dependent on the oxidation of ascorbate-phenazine methosulfate or on generated or exogenous NADH, but succinate and malate failed to drive the uptake. The optimum temperature for respiration-driven uptake of alanine was 45 to 60 degrees. 3. Potassium ion-loaded vesicles were prepared by incubating vesicles at 55 degrees in 0.5 M potassium phosphate. The addition of valinomycin elicited rapid and transient uptake of alanine under the test conditions. Uptake of alanine in response to valinomycin was progressively enhanced by the addition of dicylohexylcarbodiimide, but was completely abolished in the presence of a proton conductor or synthetic permeable cation. The effect of dicyclohexylcarbodiimide was dependent on its concentration and was maximal at a concentration of 0.4 mM. 4. The proton permeability of membrane vesicles was reduced by the addition of dicyclohexylcarbodiimide. A small but significant difference was found in the initial rates of proton uptake in the presence of dicyclohexylcarbodiimide with and without alanine. The results suggest that protons alanine are transported simultaneously in a stoichiometric ratio of 1 : 1. 5. The uptake of alanine was also driven by a pH gradient induced by an instantaneous pH drop in a suspension of alkali-loaded vesicles. Thus, alanine accumulation was driven not only by an electrical potential but also by a pH gradient. 6. Addition of ATP resulted in the inhibition of alanine uptake dependent on artificial membrane potential. ATP hydrolysis by membrane ATPase created a membrane potential which was inside-positive, and this might decrease the effective membrane potential (generated by K+ efflux mediated by valinomycin) available to drive alanine uptake.     

Transmembrane distribution of lipophilic cations in response to an electrochemical potential in reconstituted cytochromec oxidase vesicles and in vesicles exhibiting a potassium ion diffusion potential

It has been shown previously that biogenic amines and a number of pharmaceutical agents can redistribute across vesicle membranes in response to imposed potassium ion or proton gradients. Surprisingly, drug accumulation is observed for vesicles exhibiting either a pH gradient (interior acidic) or a membrane potential (interior negative), implying that these compounds can traverse the lipid bilayer as either the neutral or charged species. This interpretation, however, is complicated by the fact that vesicles exhibiting a membrane potential (interior negative) accumulate protons in response to this potential, thereby creating a pH gradient (interior acidic). This raises the possibility that in both vesicle systems drug redistribution occurs in response to the proton gradient present. We have therefore compared the uptake of several lipophilic cations by reconstituted cytochromec oxidase vesicles and by similar vesicles exhibiting a potassium ion diffusion potential. While turnover of the oxidase generates a membrane potential of comparable magnitude to the potassium ion diffusion system, it is associated with a proton gradient of opposite polarity (interior basic). Both systems show rapid uptake of the permanently charged lipophilic cation, tetraphenylphosphonium, but only the potassium ion diffusion system accumulates the lipophilic amines doxorubicin and propranolol. This provides compelling evidence that such weak bases redistribute only in response to pH gradients and not membrane potential.     

ATP4- permeabilizes the plasma membrane of mouse macrophages to fluorescent dyes

Extracellular ATP induces cation fluxes in thioglycolate-elicited mouse peritoneal macrophages and the J774 macrophage cell line apparently due to ligation of a plasma membrane receptor for ATP4-. We report that ATP permeabilizes the plasma membrane of J774 cells to 6-carboxyfluorescein (376 Da), lucifer yellow (457 Da), and fura-2 (831 Da) but not to trypan blue (961 Da), Evans blue (961 Da), or larger dye conjugates. We employed fluorescence microscopy and quantitative fluorimetry to study entry of lucifer yellow into the cytoplasm of J774 cells. Permeabilization to lucifer yellow appears to be mediated by the same ATP4- receptor that induces cation fluxes because it was inhibited by divalent cations and low pH, was mediated by the nonhydrolyzable analog adenosine 5'-(beta, gamma-imido)triphosphate, and because a variant J774 cell line resistant to ATP-induced Rb+ efflux did not take up lucifer yellow when exposed to ATP. ATP permeabilization was reversed within 5 min by removal of ATP or by addition of divalent cations. ATP also caused a transient increase in lucifer yellow uptake by pinocytosis. These data suggest that ATP4- ligates a receptor on macrophages which induces the formation of a channel admitting molecules less than or equal to 831 daltons into the cytoplasmic matrix and that removal of ATP4- from the medium causes rapid channel closure.