The capacity of anaerobically grown bacteria to exchange the 2H+ of the cell for the K+ of the medium and to maintain a high K+ distribution between cell and medium

The N,N'-dicyclohexylcarbodiimide sensitive exchange of 2H+ of a cell for K+ of medium stable to pH, K+ activity and temperature changes has been discovered in anaerobically grown gram-negative Escherichia coli, Salmonella typhimurium. S. enteritidis, Proteus mirabilis, P. vulgaris, anaerobic gram-positive bacteria Streptococcus faecalis, Lactobacillus salivarius, L. lactis in the presence of exogenic energy source. This exchange in gram-negative bacteria is operating only at increase of medium osmolarity. The high K+ distribution between cell and medium has been reached during the exchange of 2H+ for one K+ and the corresponding potassium equilibrium potential is much more than the measured delta psi. In aerobically grown E. coli, S. typhimurium, Brevibacterium flavum and aerobic Micrococcus luteus exchange of 2H+ for K+ does not take place, the K+ distribution is lower and in good conformity with the measured delta psi. It is assumed that exchange of 2H+ for K+ in anaerobic bacteria is carried out by the H+-ATPase complex and the Trk (or Trk-like) system of K+ absorption united into the same membrane supercomplex which functions as the H+-K+-pump and supports the high K+ distribution between cell and medium.     

The character of K+ uptake in anaerobically grown S. typhimurium

Tre character of K+ uptake in anaerobically grown S. typhimurium LT-2 is studied. In the alkaline media with glucose and moderate K+ activity these bacteria uptake K+ in two steps, the first of which has a high rate of K+ uptake, Km 2.1 mM and Vmax 0.44 mM/g. min and is sensitive to the medium osmolarity. Bacteria transfer from the media with high osmolarity to that with low one leads to a decrease of K+ uptake at the first step. The second increase of the medium osmolarity turns on the rapid K+ uptake only at alkaline pH. K+ uptake at the first step is inhibited by DCC and protonophores. In the absence of phosphate in the medium arsenate blocks K+ uptake at the first step, and when phosphate is available arsenate decreases K+ uptake. Valinomycin decreases the rate of K+ uptake. K+ uptake at the first step in S. typhimurium proceeds via Trk-like system which requires for K+ uptake both ATP and delta mu H+.     

The constitutive K+ pump in Serratia marcescens

Transport of K+ and H+ in the anaeronically and aerobically grown bacterium Serratia marcescens has been studied. The volumes of one cell of the anaerobically and aerobically grown bacterium were 3.7 X 10(-13) cm3 and 2.4 X 10(-13) cm3, respectively. Irrespective of the growth conditions the bacteria manifested the same respiration rate. However, the values of membrane potential for the anaerobically and aerobically grown bacterium were different and equal to -130 mV and -175 mV (interior negative), respectively, in the absence of an exogenic energy source. KCN + DCCD decreases delta psi down to almost zero in both species. DCCD alone decreases delta psi partially in anaerobes and increases delta psi in aerobes, whereas KCN alone reduces delta psi partially in both species. The introduction of glucose into the medium containing K+ reduces the absolute value of delta psi to [-160] mV in aerobes and to [-20] mV in anaerobes. The effect is not observed without external K+. In the presence of arsenate a delta psi is not reduced after the addition of glucose. At pH 7.5-7.8 the ATP level in aerobes grows notably faster than in anaerobes. The H+ extrusion becomes intensified when K+ uptake is activated by the increase in external osmotic pressure. Apparent Km and Vmax for K+ accumulation are 1.2 mM and 0.4 mM.min-1.g-1. The decrease of delta psi by glucose or KCN + DCCD have no effect on the K+ uptake whereas CCCP inhibits potassium accumulation. At the same time, arsenate stabilizes the delta psi value, but blocks K+ uptake. The accumulation of K+ correlates with the potassium equilibrium potential of -200 mV calculated according to the Nernst equation, whereas the delta psi measured was not more than [-25] mV. The calculated H+/ATP stoichiometry was 3.3 for aerobes. It was assumed that a constitutive K+ pump having a K+/ATP ratio equal to 2 or 3 operates in S. marcescens membranes.     

Changes in membrane potential and transport of ions through the S. typhimurium LT2 membrane induced by bacteriophages

Bacteriophages P22 and dp8 cause the membrane potential depolarization for 10-30 mV, reversal rapid H+ influx into bacteria and K+ exit from S. typhimurium LT2, these effects depend on infection plural and are observed only in the presence of Ca+2 in the medium. delta psi depolarization and K+ efflux induced by phage dp8 are increased with the growth of Mg+2 concentration from 0 to 2 mM. Changes of delta pH and also Na+,Ca+2 concentrations are not observed. In the presence of glucose phage infection leads to changes in H(+)-K(+)-exchange. The phages P22 and dp8 adsorption on bacteria causes changes in the form or turn of the channels in S. typhimurium membrane.     

Electrochemical potential difference for H+-ions as a regulator of redox profile of membrane during ATP-dependent ion transport in E. coli

The importance of delta mu H+ for transport of K+ via K(+)-ionophore and H(+)-K(+)-pump was studied. It was shown that the operation of the pump was decelerated by oxidant ferrycyanide, whereas sulfhydryl reagent dithiothreitol (DTT) drastically accelerated ATP driven ion exchange. Introduction of protonophore CCCP into the medium completely blocked the pump operation. However, the addition of DTT after CCCP restored the high level activity of the pump. At the same time DTT was unable to restore K+ accumulation after CCCP in aerobically grown bacteria for which the K+ uptake was performed across the electrical field gradient. Thus it was established that delta mu H+ was necessary for ATP driven ionic systems as a regulator of the membrane redox state.     

N,N'-dicyclohexylcarbodiimide-sensitive K+-dependent ATPase activity in Escherichia coli

ATPase activity sensitive to N,N'-dicyclohexylcarbodiimide and dependent on K+ content in medium is observed only in anaerobically grown Escherichia coli and as the analysis of mutants with defects in different subunits of (F0F1) H+-ATPase and in potassium transport shows only under the structural integrity of both F0F1 and K+-ionophore (the Trk system). The obtained results confirm the data on the H+/K+-exchange and indicate that the F0F1 and Trk systems in anaerobically grown bacteria unite into the same membrane supercomplex inside which the direct energy transfer occurs without a mediation of delta-mu H+.     

Effect of temperature on H+-K+ exchange in Escherichia coli bacteria during their anaerobic growth

The H(+)-K(+)-exchange in E.coli grown under anaerobic conditions at temperatures from 17 to 37 degrees C was studied. The Arrhenius plots for both the N,N'-dicyclohexylcarbodiimide-sensitive release of H+ and K+ uptake by cells transferred into a fresh medium containing a carbon source (glucose) are nonlinear. The activation energy values for the transport of these cations at different temperatures significantly differ. It is shown that as the temperature decreases, the accumulation of K+ by cells is reduced. In this process, the initial rate of K+ absorption through the TrkA system, the time of accumulation of these cations by cells and the osmosensitivity of K+ uptake substantially decrease. At temperatures below 20 degrees C, the absorption becomes insensitive to the secondary osmoshock. However, the stoichiometry of N,N'-dicyclohexylcar-bodiimide-sensitive cation fluxes remains unchanged and is equal to 2H+:K+. It is assumed that the H(+)-K(+)-exchange proceeds by the operation of an ensemble of oligomers, formed from the protomers of F0F1 and TrkA, which rearrange by the action of temperature, whereas F0F1 and TrkA in each protomer do not change.     

Characteristics of K+ uptake in S. enteritidis bacteria

Bacterial Salmonella enteritidis var. Issatchenko in media without exogenic energy source uptakes K+ in one step with Km 2.1 mM and Vmax 0.08 mM min-1/10(12) cells. This K+ uptake does not depend on pH and osmotic shock and is not inhibited by DCC. Endogenic energy source (glucose) leads to K+ uptake with Km 2.8 mM and Vmax 0.10 mM min-1/10(12) cells, and secretion of H+. The ratio of the DCC-sensitive fluxes of H+ to K+ equals 2. Arsenate and protonophores depress the K+ uptake. Valinomycin decreases the rate of K+ uptake. It is assumed that K+ uptake takes place via the Trk-like system, which works as a separate system as supercomplex with the H+-ATPase complex.     

Relation between H2 production and energy-dependent metabolism of H+ and K+ ions in E.coli

Anaerobically grown E. coli escape H2 into the medium during the operation of H(+)-K(+)-pump exchanging 2H+ from a cell for one K+ of the medium. Anaerobic cells grown in the nitrate medium as well as the aerobically grown bacteria possessed neither 2H+/K+ exchange system, nor the ability for H2 production. Introduction of N,N'-dicyclohexylcarbodiimide into the medium, the removal of external K+ or the decrease of external osmotic pressure blocked both the functioning of H(+)-K(+)-pump and H2 production. The substitution of glucose by lactate reduced the activity of bacteria without change in pump operation and H2 production. It is assumed that formate-hydrogen lease and H(+)-K(+)-pump are working in collaboration.     

Electroneutral H+-K+ exchange in liver mitochondria. Regulation by membrane potential

The paper analyzes the factors affecting the H+-K+ exchange catalyzed by rat liver mitochondria depleted of endogenous Mg2+ by treatment with the ionophore A23187. The exchange has been monitored as the rate of K+ efflux following addition of A23187 in low-K+ media. (1) The H+-K+ exchange is abolished by uncouplers and respiratory inhibitors. The inhibition is not related to the depression of delta pH, whereas a dependence is found on the magnitude of the transmembrane electrical potential, delta psi. Maximal rate of K+ efflux is observed at 180-190 mV, whereas K+ efflux is inhibited below 140-150 mV. (2) Activation of H+-K+ exchange leads to depression of delta pH but not of delta psi. Respiration is only slightly stimulated by the onset of H+-K+ exchange in the absence of valinomycin. These findings indicate that the exchange is electroneutral, and that the delta psi control presumably involves conformational changes of the carrier. (3) Incubation in hypotonic media at pH 7.4 or in isotonic media at alkaline pH results in a marked activation of the rate of H+-K+ exchange, while leaving unaffected the level of Mg2+ depletion. This type of activation results in partial 'uncoupling' from the delta psi control, suggesting that membrane stretching and alkaline pH induce conformational changes on the exchange carrier equivalent to those induced by high delta psi. (4) The available evidence suggests that the activity of the H+-K+ exchanger is modulated by the electrical field across the inner mitochondrial membrane.     

Characterization of a H+-ATPase in rat brain synaptic vesicles. Coupling to L-glutamate transport

Synaptic vesicles contain a H+-ATPase that generates a proton electrochemical gradient (delta mu H+) required for the uptake of neurotransmitters into the organelles. In this study, the synaptic vesicle H+-ATPase was examined for structural and functional similarities with other identified ATPases that generate a delta mu H+ across membranes. The synaptic vesicle H+-ATPase displayed immunological similarity with the 115-, 72-, and 39-kDa subunits of a vacuolar-type H+-ATPase purified from chromaffin granules. Functionally, the ATP-dependent H+ pumping across synaptic vesicles and ATP hydrolysis were sensitive to the sulfhydryl-modifying reagents, N-ethylmaleimide and 4-chloro-7-nitrobenz-2-oxa-1,3-diazole, at concentrations known to affect vacuolar-type H+-ATPases. In addition, as with vacuolar-type H+-ATPases, the presence of NO3-, SO4(2-), or F- inhibited the generation of a delta mu H+, but addition of vanadate or oligomycin had no effect. The delta mu H+ is a function of the pH gradient (delta pH) and membrane potential (delta psi sv) across the synaptic vesicle. Acidification (delta pH) of the synaptic vesicle interior was enhanced in the presence of permeant anions, such as Cl-, or the K+ ionophore, valinomycin. In the absence of permeant anions, the H+-ATPase generated a delta psi sv that effected the transport of L-glutamate into the synaptic vesicles. Dissipation of delta psi sv by incubation with increased external Cl- or nigericin resulted in the abolition of glutamate uptake, despite the continued maintenance of a delta mu H+ across the synaptic vesicle as a substantial delta pH. The results suggest that the synaptic vesicle H+-ATPase is of a vacuolar type and energizes the uptake of anionic glutamate by virtue of the delta psi sv component of the delta mu H+ it generates.     

N,N'-dicyclohexylcarbodiimide-sensitive K+-dependent H+ secretion from anaerobically grown E. coli cells

Dependence of N,N'-dicyclohexylcarbodiimide (DCC)-sensitive H+ secretion of K+ activity was discovered. This dependence took place only in anaerobically grown bacteria, and only at the structural intact DCC-sensitive H+-ATPase complex and K+-ionophore Trk.     

Citrate transport in Klebsiella pneumoniae

Sodium ions were specifically required for citrate degradation by suspensions of K. pneumoniae cells which had been grown anaerobically on citrate. The rate of citrate degradation was considerably lower than the activities of the citrate fermentation enzymes citrate lyase and oxaloacetate decarboxylase, indicating that citrate transport is rate limiting. Uptake of citrate into cells was also Na+ -dependent and was accompanied by its rapid metabolism so that the tricarboxylic acid was not accumulated in the cells to significant levels. The transport could be stimulated less efficiently by LiCl. Li+ ions were cotransported with citrate into the cells. Transport and degradation of citrate were abolished with the uncoupler [4-(trifluoromethoxy)phenylhydrazono]propanedinitrile (CCFP). After releasing outer membrane components and periplasmic binding proteins by cold osmotic shock treatment, citrate degradation became also sensitive towards monensin and valinomycin. The shock procedure had no effect on the rate of citrate degradation indicating that the transport is not dependent on a binding protein. Citrate degradation and transport were independent of Na+ ions in K. pneumoniae grown aerobically on citrate and in E. coli grown anaerobically on citrate plus glucose. An E. coli cit+ clone obtained by transformation of K. pneumoniae genes coding for citrate transport required Na specifically for aerobic growth on citrate indicating that the Na-dependent citrate transport system is operating. Na+ and Li+ were equally effective in stimulating citrate degradation by cell suspensions of E. coli cit+. Citrate transport in membrane vesicles of E. coli cit+ was also Na+ dependent and was energized by the proton motive force (delta micro H+). Dissipation of delta micro H+ or its components delta pH or delta psi by ionophores either totally abolished or greatly inhibited citrate uptake. It is suggested that the systems energizing citrate transport under anaerobic conditions are provided by the outwardly directed cotransport of metabolic endproducts with protons yielding delta pH and by the decarboxylation of oxaloacetate yielding delta pNa+ and delta psi. In citrate-fermenting K. pneumoniae an ATPase which is activated by Na+ was not found. The cells contain however a proton translocating ATPase and a Na+/H+ antiporter in their membrane.     

Effects of K+ on the proton motive force of Bradyrhizobium sp. strain 32H1.

In previous studies, respiring Bradyrhizobium sp. strain 32H1 cells grown under 0.2% O2, conditions that derepress N2 fixation, were found to have a low proton motive force of less than -121 mV, because of a low membrane potential (delta psi). In contrast, cells grown under 21% O2, which do not fix N2, had high proton motive force values of -175 mV or more, which are typical of respiring bacteria, because of high delta psi values. In the present study, we found that a delta psi of 0 mV in respiring cells requires growth in relatively high-[K+] media (8 mM), low O2 tension, and high internal [K+]. When low-[O2], high-[K+]-grown cells were partially depleted of K+, the delta psi was high. When cells were grown under 21% O2 or in media low in K+ (50 microM K+), the delta psi was again high. The transmembrane pH gradient was affected only slightly by varying the growth or assay conditions. In addition, low-[O2], high-[K+]-grown cells had a greater proton permeability than did high-[O2]-grown cells. To explain these findings, we postulate that cells grown under conditions that derepress N2 fixation contain an electrogenic K+/H+ antiporter that is responsible for the dissipation of the delta psi. The consequence of this alteration in K+ cycling is rerouting of proton circuits so that the putative antiporter becomes the major pathway for H+ influx, rather than the H+-ATP synthase.     

Factors and processes involved in membrane potential build-up in yeast: diS-C3(3) assay.

No methods are currently available for fully reliable monitoring of membrane potential changes in suspensions of walled cells such as yeast. Our method using the Nernstian cyanine probe diS-C3(3) monitors even relatively fast changes in membrane potential delta psi by recording the shifts of probe fluorescence maximum lambda max consequent on delta psi-dependent probe uptake into, or exit from, the cells. Both increased [K+]out and decreased pHout, but not external NaCl or choline chloride depolarise the membrane. The major ion species contributing to the diS-C3(3)-reported membrane potential in S. cerevisiae are thus K+ and H+, whereas Na+ and Cl- do not perceptibly contribute to measured delta psi. The strongly pHout-dependent depolarisation caused by the protonophores CCCP and FCCP, lack of effect of the respiratory chain inhibitors rotenone and HQNO on the delta psi, as well as results obtained with a respiration-deficient rho- mutant show that the major component of the diS-C3(3)-reported membrane potential is the delta psi formed on the plasma membrane while mitochondrial potential forms a minor part of the delta psi. Its role may be reflected in the slight depolarisation caused by the F1F0-ATPase inhibitor azide in both rho- mutant and wildtype cells. Blocking the plasma membrane H(+)-ATPase with the DMM-11 inhibitor showed that the enzyme participates in delta psi build-up both in the absence and in the presence of added glucose. Pore-forming agents such as nystatin cause a fast probe entry into the cells signifying membrane damage and extensive binding of the probe to cell constituents reflecting obviously disruption of ionic balance in permeabilised cells. In damaged cells the probe therefore no longer reports on membrane potential but on loss of membrane integrity. The delta psi-independent probe entry signalling membrane damage can be distinguished from the potential-dependent diS-C3(3) uptake into intact cells by being insensitive to the depolarising action of CCCP.     

Proton motive force in growing Streptococcus lactis and Staphylococcus aureus cells under aerobic and anaerobic conditions.

Measurements of the electrochemical gradient of hydrogen ions, which gives rise to the proton motive force (PMF), were carried out with growing Streptococcus lactis and Staphylococcus aureus cells. The facultative anaerobe was chosen in order to compare the PMF of cells growing aerobically and anaerobically. It was expected that during aerobic growth the cells would have a higher PMF than during anaerobic growth, because the H+-translocating ATPase (BF0F1) operates in the direction of H+ influx and ATP synthesis during respiration, whereas under anaerobic conditions the BF0F1 hydrolyzes glycolytically generated ATP and establishes the proton gradient by extruding H+. The electrical component of the PMF, delta psi, and the chemical gradient of H+, delta pH, were measured with radiolabeled tetraphenylphosphonium and benzoate ions. In both S. lactis and S. aureus cells, the PMF was constant during the exponential phase of batch growth and decreased in the stationary phase. In both species of bacteria, the exponential-phase PMF was not affected by varying the growth rate by adding different sugars to the medium. The relative contributions of delta psi and delta pH to the PMF, however, depended on the pH of the medium. The internal pH of S. aureus was constant at pH 7.4 to 7.6 under all conditions of growth tested. Under aerobic conditions, the delta psi of exponential phase S. aureus remained fairly constant at 160 to 170 mV. Thus, the PMF was 250 to 270 mV in cells growing aerobically in media at pH 6 and progressively lower in media of higher pH, reaching 195 to 205 mV at pH 7. Under anaerobic conditions, the delta psi ranged from 100 to 120 mV in cells at pH 6.3 to 7, resulting in a PMF of 150 to 140 mV. Thus, the mode of energy metabolism (i.e., respiration versus fermentation) and the pH of the medium are the two important factors influencing the PMF of these gram-positive cells during growth.     

Active electrogenic transport H+ in plasma membrane vesicles of cow parsnip phloem cells

Generation of electric (delta psi) and chemical (delta pH) components of electrochemical proton gradient delta muH+, in plasma membrane vesicles of Heracleum sosnovskyi phloem cells was investigated. ATP-dependent generation of delta psi at pH 6.0 in the presence of Mg2+ and K+ was established with the help of fluorescent probes AU+ and ANS-. Protonophore CCCP and proton ATPase inhibitor DCCD suppressed generation, whereas oligomycin, the inhibitor of mitochondrial ATPases did not affect it. Measurings of delta psi value indicated its oscillations within the limits from 10 to 60 mV. ATP-dependent generation of delta pH was established by means of fluorescent probe 9-AA. The effect was eliminated by CCCP and stimulated by K+, that may testify to the transformation of a part of delta psi into delta pH at antiport H+/K+. Existence of H+-ATPase in the plasma membranes of higher plant cells insuring generation of delta muH+ is supposed.     

Oxygen taxis and proton motive force in Salmonella typhimurium

The aerotactic response of Salmonella typhimurium SL3730 has been quantitatively correlated with a change in the proton motive force (delta p) as measured by a flow-dialysis technique. At pH 7.5, the membrane potential (delta psi) in S. typhimurium changed from -162 +/- 13 to -111 +/- 15 mV when cells grown aerobically were made anaerobic, and it returned to the original value when the cells were returned to aerobiosis. The delta pH across the membrane was zero. At pH 5.5, delta psi was -70 mV in aerobiosis and -20 mV in anaerobiosis, and delta pH was -118 and -56 mV for aerobic and anaerobic cells, respectively. A decrease in delta p resulted in increased tumbling, and an increase in delta p resulted in a smooth swimming response at either pH. Inhibition of aerotaxis at pH 7.5 by various concentrations of KCN correlated with a decreased delta p, due to a decreased delta psi in aerobiosis and little change in delta psi in anaerobiosis. At concentrations up to 100 mM, 2,4-dinitrophenol decreased delta psi, but did not inhibit aerotaxis because the difference between delta psi in aerobic and anaerobic cells remained constant. Considered as a whole, the results indicate that aerotaxis in S. typhimurium is mediated by delta p.     

Finding of a KCl-independent, electrogenic, and ATP-driven H+-pumping activity in rat light gastric membranes and its effect on the membrane K+ transport activity

Resting rat light gastric membranes prepared through 2H2O and Percoll gradient centrifugations were enriched not only with (H+-K+)-ATPase and K+ transport activity (Im, W. B., Blakeman, D. P., and Davis, J. P. (1985) J. Biol. Chem. 260, 9452-9460), but also with a K+-independent, ATP-dependent H+-pumping activity. This intravesicular acidification has been ascribed to an oligomycin-insensitive H+-ATPase which differed from (H+-K+)-ATPase in several respects. The H+-ATPase is electrogenic, apparently of lower capacity, required a lower optimal ATP concentration (4 microM for the H+-ATPase and 500 microM for (H+-K+)-ATPase), of lower sensitivity to vanadate and sulfhydryl agents such as p-chloromercuribenzoate and N-ethylmaleimide, and insensitive to SCH 28,080, a known competitive inhibitor of (H+-K+)-ATPase with respect to K+. Operation of the H+-ATPase, however, appeared to interfere with the K+ transport activity in the light gastric membranes, probably through development of intravesicular positive membrane potential; for example, micromolar levels of Mg2+-ATP fully inhibited K+ uptake and stimulated K+ efflux as measured with 86Rb+. Involvement of (H+-K+)-ATPase in the K+ transport is not likely, since the inhibitory effect of Mg2+-ATP continued even after removal of the nucleotide with an ATP-scavenging system. Moreover, nigericin, an electroneutral H+/K+ exchanger, could bypass the inhibitory effect of Mg2+-ATP and equilibrate the membrane vesicles with 86Rb+ while valinomycin, an electrogenic K+ ionophore, could not. Finally, the H+-ATPase could possibly be involved in the acid secretory process, since its H+-pumping activity was removed from the light gastric membrane fraction upon carbachol treatment, along with the K+ transport and (H+-K+)-ATPase activities. We have speculated that the H+-ATPase is responsible for maintaining the K+-permeable intracellular membrane vesicles acidic and K+ free during the resting state of acid secretion and may contribute to basal acid secretion.     

Effect of lysophosphatidylcholine on K+ transport in rat heavy gastric membranes enriched with (H+-K+)-ATPase

K+- and ATP-dependent H+-accumulation in rat heavy gastric membrane vesicles enriched with (H+-K+)-ATPase was markedly stimulated by amphiphiles like lysophosphatidylcholine and Zwittergent 3-14 at concentrations of 10(-5) M. Their stimulatory effect was dependent on K+-concentration in the medium and was abolished by SCH 28,080, a specific inhibitor of (H+-K+)-ATPase. Lysophosphatidylcholine at the optimal dose (3 X 10(-5) M) showed dual effects on K+-dependent membrane functions; it stimulated the rate of K+-uptake by nearly 60%, but partially inhibited SCH 28,080-sensitive and K+-dependent ATP-hydrolysis (about 20% reduction). These data indicate that H+-pumping through (H+-K+)-ATPase in the inside-out gastric membrane vesicles was facilitated by the stimulatory effect of lysophosphatidylcholine on membrane K+-transport in spite of its partial inhibition of ATP-hydrolysis. It appears that the rate limiting step for operation of the ATPase is the availability of K+ ions in the luminal side of the pump. We propose that ionic amphiphiles may modulate K+-transport in rat heavy gastric membranes through specific interactions with the putative K+-transporter.