Evidence for the allosteric regulation of the mitochondrial K+/H+ antiporter by matrix protons

Mitochondrial Volume Regulation and Swelling Mechanisms in Cardiomyocytes

Mitochondrion, known as the "powerhouse" of the cell, regulates ion homeostasis, redox state, cell proliferation and differentiation, and lipid synthesis. The inner mitochondrial membrane (IMM) controls mitochondrial metabolism and function. It possesses high levels of proteins that account for ~70% of the membrane mass and are involved in the electron transport chain, oxidative phosphorylation, energy transfer, and ion transport, among others. The mitochondrial matrix volume plays a crucial role in IMM remodeling. Several ion transport mechanisms, particularly K+ and Ca2+, regulate matrix volume. Small increases in matrix volume through IMM alterations can activate mitochondrial respiration, whereas excessive swelling can impair the IMM topology and initiates mitochondria-mediated cell death. The opening of mitochondrial permeability transition pores, the well-characterized phenomenon with unknown molecular identity, in low- and high-conductance modes are involved in physiological and pathological increases of matrix volume. Despite extensive studies, the precise mechanisms underlying changes in matrix volume and IMM structural remodeling in response to energy and oxidative stressors remain unknown. This review summarizes and discusses previous studies on the mechanisms involved in regulating mitochondrial matrix volume, IMM remodeling, and the crosstalk between these processes.

Time and charge/pH-dependent activation of K+ channel-mediated K+ influx and K+/H+ exchange in guinea pig heart isolated mitochondria; role in bioenergetic stability

Mitochondria play an important role not only in producing energy for the cell but also for regulating mitochondrial and cell function depending on the cell's needs and environment. Uptake of cations, anions, and substrates requires a stable, polarized transmembrane charge potential (ΔΨ_m). Chemiosmosis requires ion exchangers to remove Na⁺, K⁺, Ca²⁺, PO₄^3-, and other charged species that enter mitochondria. Knowledge of the kinetics of mitochondrial (m) cation channels and exchangers is important in understanding their roles in regulating mitochondrial chemiosmosis and bioenergetics. The influx/efflux of K⁺, the most abundant mitochondrial cation, alters mitochondrial volume and shape by bringing in anions and H₂O by osmosis. The effects of K⁺ uptake through ligand-specific mK⁺ channels stimulated/inhibited by agonists/antagonists on mitochondrial volume (swelling/contraction) are well known. However, a more important role for K⁺ influx is likely its effects on H⁺ cycling and bioenergetics facilitated by mitochondrial (m) K⁺/H⁺ exchange (mKHE), though the kinetics and consequences of K⁺ efflux by KHE are not well described. We hypothesized that a major role of K⁺ influx/efflux is stimulation of respiration via the influx of H⁺ by KHE. We proposed to modulate KHE activity by energizing guinea pig heart isolated mitochondria and by altering the mK⁺ cycle to capture changes in mitochondrial volume, pH_m, ΔΨ_m, and respiration that would reflect a role for H⁺ influx via KHE to regulate bioenergetics. To test this, mitochondria were suspended in a 150 mM K⁺ buffer at pH 6.9, or in a 140 mM Cs⁺ buffer at pH 7.6 or 6.9 with added 10 mM K⁺, minimal Ca²⁺ and free of Na⁺. O₂ content was measured by a Clark electrode, and pH_m, ΔΨ_m, and volume, were measured by fluorescence spectrophotometry and light-scattering. Adding pyruvic acid (PA) alone caused increases in volume and respiration and a rapid decrease in the transmembrane pH gradient (ΔpH_m = pH_in-pH_ext) at pH_ext 6.9> > 7.6, so that ΔΨ_m was charged and maintained. BK_Ca agonist NS1619 and antagonist paxilline modified these effects, and KHE inhibitor quinine and K⁺ ionophore valinomycin depolarized ΔΨ_m. We postulate that K⁺ efflux-induced H⁺ influx via KHE causes an inward H⁺ leak that stimulates respiration, but at buffer pH 6.9 also utilizes the energy of ΔpH_m, the smaller component of the overall proton motive force, ΔμH⁺. Thus ΔpH_m establishes and maintains the ΔΨ_m required for utilization of substrates, entry of all cations, and for oxidative phosphorylation. Thus, K⁺ influx/efflux appears to play a pivotal role in regulating energetics while maintaining mitochondrial ionic balance and volume homeostasis.

Protection against cardiac injury by small Ca(2+)-sensitive K(+) channels identified in guinea pig cardiac inner mitochondrial membrane

We tested if small conductance, Ca(2+)-sensitive K(+) channels (SK(Ca)) precondition hearts against ischemia reperfusion (IR) injury by improving mitochondrial (m) bioenergetics, if O(2)-derived free radicals are required to initiate protection via SK(Ca) channels, and, importantly, if SK(Ca) channels are present in cardiac cell inner mitochondrial membrane (IMM). NADH and FAD, superoxide (O(2)(-)), and m[Ca(2+)] were measured in guinea pig isolated hearts by fluorescence spectrophotometry. SK(Ca) and IK(Ca) channel opener DCEBIO (DCEB) was given for 10 min and ended 20 min before IR. Either TBAP, a dismutator of O(2)()(-), NS8593, an antagonist of SK(Ca) isoforms, or other K(Ca) and K(ATP) channel antagonists, were given before DCEB and before ischemia. DCEB treatment resulted in a 2-fold increase in LV pressure on reperfusion and a 2.5 fold decrease in infarct size vs. non-treated hearts associated with reduced O(2)(-) and m[Ca(2+)], and more normalized NADH and FAD during IR. Only NS8593 and TBAP antagonized protection by DCEB. Localization of SK(Ca) channels to mitochondria and IMM was evidenced by a) identification of purified mSK(Ca) protein by Western blotting, immuno-histochemical staining, confocal microscopy, and immuno-gold electron microscopy, b) 2-D gel electrophoresis and mass spectroscopy of IMM protein, c) [Ca(2+)]-dependence of mSK(Ca) channels in planar lipid bilayers, and d) matrix K(+) influx induced by DCEB and blocked by SK(Ca) antagonist UCL1684. This study shows that 1) SK(Ca) channels are located and functional in IMM, 2) mSK(Ca) channel opening by DCEB leads to protection that is O(2)(-) dependent, and 3) protection by DCEB is evident beginning during ischemia.

Regulation of the mitochondrial proton gradient by cytosolic Ca²⁺ signals

Mitochondria convert the energy stored in carbohydrate and fat into ATP molecules that power enzymatic reactions within cells, and this process influences cellular calcium signals in several ways. By providing ATP to calcium pumps at the plasma and intracellular membranes, mitochondria power the calcium gradients that drive the release of Ca²⁺ from stores and the entry of Ca²⁺ across plasma membrane channels. By taking up and subsequently releasing calcium ions, mitochondria determine the spatiotemporal profile of cellular Ca²⁺ signals and the activity of Ca²⁺-regulated proteins, including Ca²⁺ entry channels that are themselves part of the Ca²⁺ circuitry. Ca²⁺ elevations in the mitochondrial matrix, in turn, activate Ca²⁺-dependent enzymes that boost the respiratory chain, increasing the ability of mitochondria to buffer calcium ions. Mitochondria are able to encode and decode Ca²⁺ signals because the respiratory chain generates an electrochemical gradient for protons across the inner mitochondrial membrane. This proton motive force (Δp) drives the activity of the ATP synthase and has both an electrical component, the mitochondrial membrane potential (ΔΨ(m)), and a chemical component, the mitochondrial proton gradient (ΔpH(m)). ΔΨ(m) contributes about 190 mV to Δp and drives the entry of Ca²⁺ across a recently identified Ca²⁺-selective channel known as the mitochondrial Ca²⁺ uniporter. ΔpH(m) contributes ~30 mV to Δp and is usually ignored or considered a minor component of mitochondria respiratory state. However, the mitochondrial proton gradient is an essential component of the chemiosmotic theory formulated by Peter Mitchell in 1961 as ΔpH(m) sustains the entry of substrates and metabolites required for the activity of the respiratory chain and drives the activity of electroneutral ion exchangers that allow mitochondria to maintain their osmolarity and volume. In this review, we summarize the mechanisms that regulate the mitochondrial proton gradient and discuss how thermodynamic concepts derived from measurements in purified mitochondria can be reconciled with our recent findings that mitochondria have high proton permeability in situ and that ΔpH(m) decreases during mitochondrial Ca²⁺ elevations.

Cardioprotective signaling to mitochondria

Mitochondria are central players in the pathophysiology of ischemia-reperfusion. Activation of plasma membrane G-coupled receptors or the Na,K-ATPase triggers cytosolic signaling pathways that result in cardioprotection. Our working hypothesis is that the occupied receptors migrate to caveolae, where signaling enzymes are scaffolded into signalosomes that bud off the plasma membrane and migrate to mitochondria. The signalosome-mitochondria interaction then initiates intramitochondrial signaling by opening the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). MitoK(ATP) opening causes an increase in ROS production, which activates mitochondrial protein kinase C epsilon (PKCvarepsilon), which inhibits the mitochondrial permeability transition (MPT), thus decreasing cell death. We review the experimental findings that bear on these hypotheses and other modes of protection involving mitochondria.

The direct physiological effects of mitoK(ATP) opening on heart mitochondria

The mitochondrial ATP-sensitive K+ channel (mitoK(ATP)) has been assigned multiple roles in cell physiology and in cardioprotection. Each of these roles must arise from basic consequences of mitoK(ATP) opening that should be observable at the level of the mitochondrion. MitoK(ATP) opening has been proposed to have three direct effects on mitochondrial physiology: an increase in steady-state matrix volume, respiratory stimulation (uncoupling), and matrix alkalinization. Here, we examine the evidence for these hypotheses through experiments on isolated rat heart mitochondria. Using perturbation techniques, we show that matrix volume is the consequence of a steady-state balance between K+ influx, caused either by mitoK(ATP) opening or valinomycin, and K+ efflux caused by the mitochondrial K+/H+ antiporter. We show that increasing K+ influx with valinomycin uncouples respiration like a classical uncoupler with the important difference that uncoupling via K+ cycling soon causes rupture of the outer mitochondrial membrane and release of cytochrome c. By loading the potassium binding fluorescent indicator into the matrix, we show directly that K+ influx is increased by diazoxide and inhibited by ATP and 5-HD. By loading the fluorescent probe BCECF into the matrix, we show directly that increasing K+ influx with either valinomycin or diazoxide causes matrix alkalinization. Finally, by comparing the effects of mitoK(ATP) openers and blockers with those of valinomycin, we show that four independent assays of mitoK(ATP) activity yield quantitatively identical results for mitoK(ATP)-mediated K+ transport. These results provide decisive support for the hypothesis that mitochondria contain an ATP-sensitive K+ channel and establish the physiological consequences of mitoK(ATP) opening for mitochondria.

Electroneutral K+/H+ exchange in mitochondrial membrane vesicles involves Yol027/Letm1 proteins

YOL027c in yeast and LETM1 in humans encode integral proteins of the inner mitochondrial membrane. They have been implicated in mitochondrial K+ homeostasis and volume control. To further characterize their role, we made use of submitochondrial particles (SMPs) with entrapped K+- and H+-sensitive fluorescent dyes PBFI and BCECF, respectively, to study the kinetics of K+ and H+ transport across the yeast inner mitochondrial membrane. Wild-type SMPs exhibited rapid, reciprocal translocations of K+ and H+ driven by concentration gradients of either of them. K+ and H+ translocations have stoichiometries similar to those mediated by the exogenous K+/H+ exchanger nigericin, and they are shown to be essentially electroneutral and obligatorily coupled. Moreover, [K+] gradients move H+ against its concentration gradient, and vice-versa. These features, as well as the sensitivity of K+ and H+ fluxes to quinine and Mg2+, qualify these activities as K+/H+ exchange reactions. Both activities are abolished when the yeast Yol027p protein is absent (yol027Delta mutant SMPs), indicating that it has an essential role in this reaction. The replacement of the yeast Yol027p by the human Letm1 protein restores K+/H+ exchange activity confirming functional homology of the yeast and human proteins. Considering their newly identified function, we propose to refer to the yeast YOL027c gene and the human LETM1 gene as yMKH1 and hMKH1, respectively.

Anoxia induces matrix shrinkage accompanied by an increase in light scattering in isolated brain mitochondria

It is important to monitor mitochondrial conditions, and light scattering (LS) measurements have been applied to the detection of morphological changes in mitochondria in vivo. Little is known about the morphological and LS responses of brain mitochondria to oxygen withdrawal, a critical factor in cell death. We have therefore investigated the morphological and LS responses of isolated brain mitochondria to anoxia. Anoxia induced an increase in LS, reflecting mitochondrial matrix shrinkage. This response was reversible, but was reduced by adding digitonin, which disrupted the outer membrane selectively. This suggested that integrity of the outer membrane was necessary for the matrix response. We further examined the effects of Mg2+ and ATP on the responses because both exist in cells and modulate the changes in matrix volume. Although Mg2+ and ATP reduced the rates of increase and decrease in LS, respectively, the magnitudes of the increases in LS caused by anoxia stayed at over 80% of the control level (no Mg2+) in the presence of Mg2+ and ATP. This suggested that the increase in LS occurred in cells containing Mg2+ and ATP during anoxia. In contrast, that caused by inhibitors of the electron transport chain was reduced to below 30% of the control level in the presence of Mg2+. The present in vitro study provides a basis for interpretation of LS signals from mitochondria in brain research during oxygen withdrawal.

Temperature Dependence of the Mitochondrial Inner Membrane Anion Channel

The mitochondrial inner membrane anion channel (IMAC) carries a wide variety of anions and is postulated to be involved in mitochondrial volume homeostasis in conjunction with the K+/H+ antiporter, thus allowing the respiratory chain proton pumps to drive salt efflux. How it is regulated is uncertain; however, it is inhibited by matrix Mg2+ and matrix protons. Previously determined values for the IC50 suggested that the channel would be closed under physiological conditions. In a previous study (Liu, G., Hinch, B., Davatol-Hag, H., Lu, Y., Powers, M., and Beavis, A. D. (1996) J. Biol. Chem. 271, 19717-19723), it was demonstrated that the channel is highly temperature-dependent, and that a large component of this sensitivity resulted from an effect on the pIC50 for protons. We have now investigated the effect of temperature on the inhibition by Mg2+ and have found that it too is temperature-dependent. When the temperature is raised from 20 degrees C to 45 degrees C, the IC50 increases from 22 to 350 microm at pH 7.4 and from 80 to 1.5 mm at pH 8.4, respectively. The Arrhenius plot for the IC50 is linear with a slope = -80 kJ/mol. The IC50 is also strongly pH-dependent, and at 37 degrees C increases from 90 microm at pH 7.4 to 1230 microm at pH 8.4. In view of the extremely rapid fluxes that IMAC is capable of conducting at 37 degrees C, we conclude that inhibition by matrix Mg2+ and protons is necessary to limit its activity under physiological conditions. We conclude that the primary role of Mg2+ is to ensure IMAC is poised to allow regulation by small changes in pH in the physiological range. This control is mediated by a direct effect of H+ on the activity, in addition to an indirect effect mediated by a change in the Mg2+ IC50. The question that remains is not whether IMAC can be active at physiological concentrations of Mg2+ and H+, but what other factors might increase its sensitivity to changes in mitochondrial volume.

Mitochondrial potassium transport: the K(+) cycle

Potassium transport plays three distinct roles in mitochondria. Volume homeostasis to prevent excess matrix swelling is a housekeeping function that is essential for maintaining the structural integrity of the organelle. This function is mediated by the K(+)/H(+) antiporter and was first proposed by Peter Mitchell. Volume homeostasis to prevent excess matrix contraction is a recently discovered function that maintains a fully expanded matrix when diffusive K(+) influx declines due to membrane depolarization caused by high rates of electron transport. Maintaining matrix volume under these conditions is important because matrix contraction inhibits electron transport and also perturbs the structure-function of the intermembrane space (IMS). This volume regulation is mediated by the mitochondrial ATP-sensitive K(+) channel (mitoK(ATP)). Cell signaling functions to protect the cell from ischemia-reperfusion injury and also to trigger transcription of genes required for cell growth. This function depends on the ability of mitoK(ATP) opening to trigger increased mitochondrial production of reactive oxygen species (ROS). This review discusses the properties of the mitochondrial K(+) cycle that help to understand the basis of these diverse effects.

Bcl-2 and tBid proteins counter-regulate mitochondrial potassium transport

The mechanism of cytochrome c release from mitochondria in apoptosis remains obscure, although it is known to be regulated by bcl-2 family proteins. Here we describe a set of novel apoptotic phenomena--stimulation of the mitochondrial potassium uptake preceding cytochrome c release and regulation of such potassium uptake by bcl-2 family proteins. As a result of increased potassium uptake, mitochondria undergo moderate swelling sufficient to release cytochrome c. Overexpression of bcl-2 protein prevented the mitochondrial potassium uptake as well as cytochrome c release in apoptosis. Bcl-2 was found to upregulate the mitochondrial potassium efflux mechanism--the K/H exchanger. Specific activation of the mitochondrial K-uniporter led to cytochrome c release, which was inhibited by bcl-2. tBid had an opposite effect-it stimulated mitochondrial potassium uptake resulting in cytochrome c release. The described counter-regulation of mitochondrial potassium transport by bcl-2 and Bid suggests a novel view of a mechanism of cytochrome c release from mitochondria in apoptosis.

Matrix free Mg(2+) and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis in order to carry out oxidative phosphorylation. It has been postulated that the concentration of free Mg(2+) ([Mg(2+)]) serves as the sensor of matrix volume and regulates a K(+)-extruding K(+)/H(+) antiport (K. D. Garlid. J. Biol. Chem. 255: 11273-11279, 1980). To test this hypothesis, the fluorescent probe furaptra was used to monitor [Mg(2+)] and free Ca(2+) concentration ([Ca(2+)]) in the matrix of isolated beef heart mitochondria, and K(+)/H(+) antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 microM matrix [Mg(2+)] and 2.2 microM [Ca(2+)] were determined for the K(+)/H(+) antiport. Untreated mitochondria average 670 microM matrix [Mg(2+)], a value that would permit <1% of maximum K(+)/H(+) antiport activity. Hypotonic swelling results in large decreases in matrix [Mg(2+)], but swelling due to accumulation of acetate salts does not alter [Mg(2+)]. Swelling in phosphate salts decreases matrix [Mg(2+)], but not to levels that permit appreciable antiport activity. We conclude that 1) it is unlikely that matrix [Mg(2+)] serves as the mitochondrial volume sensor, 2) if K(+)/H(+) antiport functions as a volume control transporter, it is probably regulated by factors other than [Mg(2+)], and 3) alternative mechanisms for mitochondrial volume control should be considered.

Mitochondrial transport of cations: channels, exchangers, and permeability transition

This review provides a selective history of how studies of mitochondrial cation transport (K+, Na+, Ca2+) developed in relation to the major themes of research in bioenergetics. It then covers in some detail specific transport pathways for these cations, and it introduces and discusses open problems about their nature and physiological function, particularly in relation to volume regulation and Ca2+ homeostasis. The review should provide the basic elements needed to understand both earlier mitochondrial literature and current problems associated with mitochondrial transport of cations and hopefully will foster new interest in the molecular definition of mitochondrial cation channels and exchangers as well as their roles in cell physiology.

Identification of a mitochondrial Na+/H+ exchanger

The electroneutral exchange of protons for Na+ and K+ across the mitochondrial inner membrane contributes to organellar volume and Ca2+ homeostasis. The molecular nature of these transporters remains unknown. In this report, we characterize a novel gene (YDR456w; renamed NHA2) in Saccharomyces cerevisiae whose deduced protein sequence is homologous to members of the mammalian Na+/H+ exchanger gene family. Fluorescence microscopy showed that a Nha2-green fluorescent protein chimera colocalizes with 4',6-diamidino-2-phenylindole staining of mitochondrial DNA. To assess the function of Nha2, we deleted the NHA2 gene by homologous disruption and found that benzamil-inhibitable, acid-activated 22Na+ uptake into mitochondria was abolished in the mutant strain. It also showed retarded growth on nonfermentable carbon sources and severely reduced survival during the stationary phase of the cell cycle compared with the parental strain, consistent with a defect in aerobic metabolism. Sequence comparisons revealed that Nha2 has highest identity to a putative Na+/H+ exchanger homologue (KIAA0267; renamed NHE6) in humans. Northern blot analysis demonstrated that NHE6 is ubiquitously expressed but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle, and heart. Fluorescence microscopy showed that a NHE6-green fluorescent protein chimera also accumulates in mitochondria of transfected HeLa cells. These data indicate that NHA2 and NHE6 encode homologous Na+/H+ exchangers and suggest they may be important for mitochondrial function in lower and higher eukaryotes, respectively.

Effect of leader peptides on the permeability of mitochondria

Peptides with sequences based on the leader sequence of yeast cytochrome c oxidase subunit IV (pCOX IV-(1-25)) activate the electrophoretic uptake of K+ and other cations such as tetraethylammonium and lysine by rat liver mitochondria with EC50 = 11-15 microM. Uptake of these cations is dependent on respiration and is prevented by uncoupling agents, and the Vmax for K+ is 1.2-1.5 micromol/min/mg. Albeit more slowly, the non-electrolytes mannitol and sucrose are also transported by this pathway. Treatment of the peptides with proteinase K eliminates the stimulatory effect. Since the stimulated rate is not inhibited by ATP or by cyclosporin, we conclude that this pathway is not related to the mitochondrial KATP channel or the Ca2+-dependent permeability transition pore. Transport is stimulated by pCOX IV-(1-23), pCOX IV-(1-22), and pCOX IV-(1-12)Y, but not by a 13-amino acid peptide representing the nuclear location sequence of the SV40 large T antigen, which is responsible for directing that protein to the nucleus. Spermine, which has four positive charges, also has no stimulatory effect, and an amphiphilic 22-residue peptide derived from antithrombin III with seven net charges is only one-twentieth as effective as pCOX IV-(1-22). Thus, these data indicate that the sequence/structure is important for activation of transport. We also demonstrate that mitochondrial uncoupling, previously reported to be induced by these peptides, actually reflects coupled accumulation of salt. In view of our findings, it is also likely that the lytic effects attributed to these peptides are secondary to swelling and are not due to membrane damage per se. Finally, we show that, in non-ionic media, the peptide is an inhibitor of cytochrome c oxidase.

Investigations of the inhibitory effect of propranolol, chlorpromazine, quinine, and dicyclohexylcarbodiimide on the swelling of yeast mitochondria in potassium acetate. Evidences for indirect effects mediated by the lipid phase

The mode of action of propranolol, chlorpromazine, and quinine, three cationic drugs inhibiting swelling of yeast mitochondria in potassium acetate, was investigated by looking at their effect on fluorescent probes of the polar heads and of the nonpolar moiety of the membranes, under inhibitory conditions of swelling. As expected, propranolol and chlorpromazine exhibited specificity for anionic phospholipids since they increased the binding of the anionic probe 1-anilino 8-naphthalenesulfonate (ANS). Although propranolol did not release 1,6-diphenyl-1,3,5-hexatriene (DPH) from the hydrophobic moiety of the membrane, it increased the excimer/monomer fluorescence ratio of 10-(1-pyrene)decanoate, suggesting that it induced a limitation in the movements of the aliphatic chains of phospholipids. Opposite to propranolol, chlorpromazine removed DPH from the membrane, suggesting that it bound essentially to the hydrophobic moiety. However, chloramphenicol, which was also able to remove DPH but did not increase the binding of ANS, did not inhibit swelling. Inhibition by chlorpromazine therefore appeared to be related to its binding to the hydrophobic moiety of anionic phospholipids. Quinine had no effect on membrane properties: at inhibitory concentrations of swelling in potassium acetate, it did not inhibit swelling in ammonium phosphate (mediated by the phosphate/H+ cotransporter), whereas propranolol and chlorpromazine did, suggesting a more specific effect of quinine on (a) protein(s) involved in the K+/H+ exchange. Dicyclohexylcarbodiimide (DCCD), which irreversibly inhibits the swelling in potassium acetate, bound to ethanolamine heads; despite this effect, DCCD had no major consequences on the binding of the probes. Consequently, propranolol and chlorpromazine are of no help for characterizing protein(s) catalyzing the K+/H+ exchange, although their effect on lipids seems to involve limited zones of the inner mitochondrial membrane. Quinine and DCCD, although they also bind to lipids, may inhibit the activity by acting on a limited number of proteins.

Cation transport systems in mitochondria: Na+ and K+ uniports and exchangers

It is now well established that mitochondria contain three antiporters that transport monovalent cations. A latent, allosterically regulated K+/H+ antiport appears to serve as a cation-extruding device that helps maintain mitochondrial volume homeostasis. An apparently unregulated Na+/H+ antiport keeps matrix [Na+] low and the Na(+)-gradient equal to the H(+)-gradient. A Na+/Ca2+ antiport provides a Ca(2+)-extruding mechanism that permits the mitochondrion to regulate matrix [Ca2+] by balancing Ca2+ efflux against influx on the Ca(2+)-uniport. All three antiports have well-defined physiological roles and their molecular properties and regulatory features are now being determined. Mitochondria also contain monovalent cation uniports, such as the recently described ATP- and glibenclamide-sensitive K+ channel and ruthenium red-sensitive uniports for Na+ and K+. A physiological role of such uniports has not been established and their properties are just beginning to be defined.

On the regulation of Na+/H+ and K+/H+ antiport in yeast mitochondria: evidence for the absence of an Na(+)-selective Na+/H+ antiporter

Unlike mammalian mitochondria, yeast mitochondria swell spontaneously in both NaOAc and KOAc. This swelling reflects the activity of an electroneutral cation/H+ antiport pathway. Transport of neither salt is stimulated by depletion of endogenous divalent cations; however, it can be inhibited by addition of exogenous divalent cations (Mg2+ IC50 = 2.08 mM, Ca2+ IC50 = 0.82 mM). Transport of both Na+ and K+ can be completely inhibited by the amphiphilic amines propranolol (IC50 = 71 microM) and quinine (IC50 = 199 microM) with indistinguishable IC50 values. Dicyclohexylcarbodiimide inhibits with a second-order rate constant of 1.6 x 10(-4) (nmol DCCD/mg)-1 min-1 at 0 degrees C; however, with both Na+ and K+ inhibition reaches a maximum of about 46%. The remaining transport can still be inhibited by propranolol. Transport of both cations is sensitive to pH; yielding linear Hill plots and Dixon plots with a pIC50 value of 7.7 for both Na+ and K+. These properties are qualitatively the same as those of the non-selective K+/H+ antiporter of mammalian mitochondria. However, the remarkable similarity between the data obtained in Na+ and K+ media suggests that an antiporter akin to the Na(+)-selective Na+/H+ antiporter of mammalian mitochondria, which is inhibited by none of these agents, is absent in yeast. In an attempt to reveal the activity of a propranolol-insensitive Na(+)-selective antiporter, we compared the rates of Na+/H+ and K+/H+ antiport in the presence of sufficient propranolol to block the K+/H+ antiporter. Between pH 4.6 and 8.8 no difference could be detected. Consequently, we conclude that yeast mitochondria lack the typical Na(+)-selective Na+/H+ antiporter of mammalian mitochondria.

Properties of the inner membrane anion channel in intact mitochondria

The mitochondrial inner membrane possesses an anion channel (IMAC) which mediates the electrophoretic transport of a wide variety of anions and is believed to be an important component of the volume homeostatic mechanism. IMAC is regulated by matrix Mg2+ (IC50 = 38 microM at pH 7.4) and by matrix H+ (pIC50 = 7.7). Moreover, inhibition by Mg2+ is pH-dependent. IMAC is also reversibly inhibited by many cationic amphiphilic drugs, including propranolol, and irreversibly inhibited by N,N'-dicyclohexylcarbodiimide. Mercurials have two effects on its activity: (1) they increase the IC50 values for Mg2+, H+, and propranolol, and (2) they inhibit transport. The most potent inhibitor of IMAC is tributyltin, which blocks anion uniport in liver mitochondria at about 1 nmol/mg. The inhibitory dose is increased by mercurials; however, this effect appears to be unrelated to the other mercurial effects. IMAC also appears to be present in plant mitochondria; however, it is insensitive to inhibition by Mg2+, mercurials, and N,N'-dicyclohexylcarbodiimide. Some inhibitors of the adenine nucleotide translocase also inhibit IMAC, including Cibacron Blue, agaric acid, and palmitoyl CoA; however, atractyloside has no effect.

Regulation of the mitochondrial Na+/Ca2+ antiport by matrix pH

The effect of matrix pH (pHi) on the activity of the mitochondrial Na+/Ca2+ antiport has been studied using the fluorescence of SNARF-1 to monitor pHi and Na(+)-dependent efflux of accumulated Ca2+ to follow antiport activity. Heart mitochondria respiring in a KCl medium maintain a large delta pH (interior alkaline) and show optimal Na+/Ca2+ antiport only when the pH of the medium (pH0) is acid. Addition of nigericin to these mitochondria decreases delta pH and increases the membrane potential (delta psi). Nigericin strongly activates Na+/Ca2+ antiport at values of pH0 near 7.4 but inhibits antiport activity at acid pH0. When pHi is evaluated in these protocols, a sharp optimum in Na+/Ca2+ antiport activity is seen near pHi 7.6 in the presence or absence of nigericin. Activity falls off rapidly at more alkaline values of pHi. The effects of nigericin on Na+/Ca2+ antiport are duplicated by 20 mM acetate and by 3 mM phosphate. In each case the optimum rate of Na+/Ca2+ antiport is obtained at pHi 7.5 to 7.6 and changes in antiport activity do not correlate with changes in components of the driving force of the reaction (i.e., delta psi, delta pH, or the steady-state Na+ gradient). It is concluded that the Na+/Ca2+ antiport of heart mitochondria is very sensitive to matrix [H+] and that changes in pHi may contribute to the regulation of matrix Ca2+ levels.

Regulation of mitochondrial K+/H+ antiport activity by hydrogen ions

The effect of matrix pH (pHi) on the activity of the mitochondrial K+/H+ antiport has been studied using the fluorescence of 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF) to monitor pHi and passive swelling in K+ acetate to follow antiport activity. Heart mitochondria suspended in hypotonic K+ acetate in the absence of respiration show an initial delta pH of -0.4 (interior acid) that decays slowly. Addition of A23187 to deplete matrix Mg2+ results in a further acid shift in pHi followed by equilibration of delta pH. This equilibration appears to depend on K+/H+ antiport and is slow at acid pHi but very rapid when the matrix is alkaline. Swelling of Mg(2+)-depleted mitochondria in K+ acetate is multiphasic with a slow initial rate, a period of maximum swelling, and a final period in which the rate declines. At constant external pH (pH0), the initial rate of swelling is faster with increasing pHi and the time to the onset of the maximum swelling rate decreases. The maximum swelling rate is initiated at pHi 7.4 when pH0 is 7.8 and at pHi 7.1 when pH0 is 7.4. The maximum rate of swelling increases linearly with increasing pH0 in the range from 7.0 to 8.2. This rate also shows a linear relationship to the value of pHi at the time the maximum rate is attained. Dixon plots of the reciprocal of the maximum swelling rate vs [H+]0 suggest that external [H+] is a noncompetitive inhibitor of K+ entry on the antiport. It is concluded that K+/H+ antiport in Mg(2+)-depleted heart mitochondria can be regulated by matrix [H+] (see Beavis, A. D., and Garlid, K. D. (1990) J. Biol. Chem. 265, 2538-2545), but that this antiport is also sensitive to external [H+] or to delta pH when it acts in the direction of K+ uptake.