Induction of electroneutral exchanges of H+ with Na+, divalent and organic cations in rat liver mitochondria

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.

Effect of butylhydroxytoluene and related compounds on permeability of the inner mitochondrial membrane

Mitochondrial inner membrane contains a latent pore (PTP) that when opened uncouples mitochondrial energy transduction and allows rapid equilibration of low-molecular-weight solutes between the matrix and exterior. Based on sensitivity of the PTP to well-known free radical scavenger butylhydroxytoluene (BHT), it has been proposed that increased steady-state level of oxygen radicals, and subsequent radical attack of proteins and lipids, is a central event in activation of this pore (Novgorodov et al., J. Bioenerg. Biomembr. 19, 191-202, 1987; Carbonera and Azzone, Biochim. Biophys. Acta 943, 245-255, 1988). Present studies revealed that DBT, a derivative of BHT devoid of radical scavenging activity, exerts an analogous effect on the permeability of the inner membrane. Inhibition of the Ca2+-induced PTP opening is essentially complete at dose range of 50-60 nmol/mg protein with IC50 values of about 32 and 23 nmol/mg protein for DBT and BHT, respectively. Electron microscopy and osmotic experiments utilizing polyethylene glycols with different Stokes radii showed that the apparent lack of inhibition seen at high concentrations of these compounds results from cyclosporin A- and Ca2+-insensitive pore formation in the inner membrane. Experiments employing antioxidants with similar structure but dissimilar hydrophobicity provided evidence for localization of the antioxidant binding sites within the hydrophobic zone of the inner membrane or in the matrix space. The data obtained do not refute the notion that oxygen radicals modulate the PTP, but rather indicate that BHT operates independently of its free radical scavenging activity. Overall, the sensitivity to BHT and other antioxidants is not always a reliable criterion for the involvement of free radical reactions in the processes under study.

The effect of respiration on the permeability of the mitochondrial membrane to ions

1. The rates of cation uptake, for either organic cations such as tetrapropylammonium, TPA+, at variable tetraphenylboron concentrations, TPB-, or inorganic cations such as Mn2+, or K+ plus valinomycin, have been measured in mitochondria either respiring, under uncoupler titrations, or non-respiring, under variable K+ diffusion potentials. 2. The flow-force relationship for the respiration-coupled ion fluxes during titrations with uncouplers is almost identical to that obtained for the K(+)-diffusion driven fluxes. Similar results are obtained when TPA+ is replaced with inorganic cations, either monovalent such as K+ (+valinomycin), or divalent such as Mn2+. 3. By applying the Eyring analysis, as developed by Garlid et al. (Garlid, K.D., Beavis, A.D. and Ratkje, S.K. (1989) Biochim. Biophys. Acta 976, 109-121), from the flux-voltage relationships the values for the permeability coefficients and for the energy barriers have been obtained for the transport of the ion pair TPA(+)-TPB-, of Mn2+ and of K+ plus valinomycin, in non-respiring and in respiring, coupled and uncoupled, mitochondria. 4. The findings that the rates of respiration-coupled ion fluxes, at all values of membrane potential, are similar to the rates of the K+ diffusion potential-coupled ion fluxes and the similar pattern of the flux-voltage relationships during the titrations with uncouplers and artificial gradients indicate that the membrane permeability for ions is not modified by respiration.

Evidence for the existence of an inner membrane anion channel in mitochondria

Mitochondria normally exhibit very low electrophoretic permeabilities to physiologically important anions such as chloride, bicarbonate, phosphate, succinate, citrate, etc. Nevertheless, considerable evidence has accumulated which suggests that heart and liver mitochondria contain a specific anion-conducting channel. In this review, a postulated inner membrane anion channel is discussed in the context of other known pathways for anion transport in mitochondria. This anion channel exhibits the following properties. It is anion-selective and inhibited physiologically by protons and magnesium ions. It is inhibited reversibly by quinine and irreversibly by dicyclohexylcarbodiimide. We propose that the inner membrane anion channel is formed by inner membrane proteins and that this pathway is normally latent due to regulation by matrix Mg2+. The physiological role of the anion channel is unknown; however, this pathway is well designed to enable mitochondria to restore their normal volume following pathological swelling. In addition, the inner membrane anion channel provides a potential futile cycle for regulated non-shivering thermogenesis and may be important in controlled energy dissipation.

Ruthenium red-insensitive Ca2+ uptake and release by mitochondria

Calcium uptake by rat liver mitochondria driven by an artificial pH gradient is ruthenium red insensitive, electrically neutral, and inhibited by the local anesthetic, nupercaine. This pH-driven Ca2+ transport is also inhibited by NH3, Pi, and acetate. Direct measurements of Pi indicate it is not translocated with Ca2+ during pH-driven Ca2+ uptake. Calcium is therefore not transported by a Ca2+-Pi symport mechanism. Ruthenium red-insensitive Ca2+ efflux is similar in its inhibition by nupercaine and its kinetics, and is also electroneutral. This suggests that the Ca2+ uptake described here occurs via reversal of the principal pathway of mitochondrial Ca2+ release. From the available data, pH-driven Ca2+ uptake (and presumably Ca2+ efflux) is hypothesized to occur by Ca2+ symport with unidentified anions. Protons may move counter to Ca2+ or reversibly dissociate from cotransported anions, which therefore couples Ca2+ transport to the pH gradient.

Na+/H+ antiporters

Na+/H+ antiports or exchange reactions have been found widely, if not ubiquitously, in prokaryotic and eukaryotic membranes. In any given experimental system, the multiplicity of ion conductance pathways and the absence of specific inhibitors complicate efforts to establish that the antiport observed actually results from the activity of a specific secondary porter which catalyzes coupled exchanged of the two ions. Nevertheless, a large body of evidence suggests that at least some prokaryotes possess a delta psi-dependent, mutable Na+/H+ antiporter which catalyzes Na+ extrusion in exchange for H+; in other bacterial species, the antiporter my function electroneutrally, at least at some external pH values. The bacterial Na+/H+ antiporter constitutes a critical limb of Na+ circulation, functioning to maintain a delta mu Na+ for use by Na+-coupled bioenergetic processes. The prokaryotic antiporter is also involved in pH homeostasis in the alkaline pH range. Studies of mutant strains that are deficient in Na+/H+ antiporter activity also indicate the existence of a relationship, e.g., a common subunit or regulatory factor, between the Na+/H+ antiporter and Na+/solute symporters in several bacterial species. In eukaryotes, an electroneutral, amiloride-sensitive Na+/H+ antiport has been found in a wide variety of cell and tissue types. Generally, the normal direction of the antiport appears to be that of Na+ uptake and H+ extrusion. The activity is thus implicated as part of a complex system for Na+ circulation, e.g., in transepithelial transport, and might have some role in acidification in the renal proximal tubule. In many experimental systems, the Na+/H+ antiport appears to influence intracellular pH. In addition to a role in general pH homeostasis, such Na+-dependent changes in intracellular pH could be part of the early events in a variety of differentiating and proliferative systems. Reconstitution and structural studies, as well as detailed analysis of gene loci and products which affect the antiport activity, are in their very early stages. These studies will be important in further clarification of the precise structural nature and role(s) of the Na+/H+ antiporters. In neither prokaryotes nor eukaryotes systems is there yet incontrovertible evidence that a specific protein carrier, that catalyzes Na+/H+ antiport, is actually responsible for any of the multitude of effects attributed to such antiporters. The Na+-H+ exchange might turn out to be side reactions of other porters or the additive effects of several conductance pathways; or, as appears most likely in at least some bacteria and in renal tissue, the antiporter may be a discrete, complex carr

The effect of the energy state of mitochondria on the kinetics of unidirectional cation fluxes

Unidirectional fluxes of triphenylmethylphosphonium and of Cs+ as its valinomycin complex were studied using trace concentrations of the cations. The rate constants of influx and efflux were estimated mainly at 0 degrees C from the uptake kinetics in respiring mitochondria and the in/out ratios in the steady state. The efflux rate constants in the energized state were also measured after dilution of the mitochondrial suspension in the steady state, and in deenergized mitochondria from the efflux rates of cations after inhibition of respiration. It was found that the energy state of mitochondria had little effect on the rate constants of efflux, while the rate of influx was strongly stimulated by respiration. The former finding is not readily explained by the classical chemiosmotic theory, since a transmembrane potential, negative on the inside, formed on energization would be expected to strongly inhibit the efflux of cations. The data may be explained by a pump-and-leak model in which localized electrical fields in hydrophobic domains of the membrane are coupled to the pumping of hydrophobic cations against an electrochemical gradient, while leaks would effect efflux.

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.

Regulation of the monovalent cation permeability of brain mitochondria

The Na+ and K+ conductances of rat brain mitochondria were estimated from rates of metabolically dependent swelling and uncoupling of respiration. These were maximal in the presence of EDTA plus Pi. Pi could not be replaced with acetate. Na+ conductance was greater than that of K+ and was therefore examined in greater detail. According to the influences of N-ethylmaleimide, internal Pi (exogenous and perhaps endogenous) promoted Na+ permeability. Treatment with the ionophore A23187 obviated the Pi requirement although EDTA was still necessary. The stimulation by EDTA with Pi or A23187 and inhibition by exogenous Mg2+ suggested endogenous polyvalent cations could also regulate Na+ conductance. The influence of these substances upon endogenous Mg2+ (and Ca2+) levels is consistent with such a role of membrane-bound Mg2+. Low levels of ruthenium red (150 pmol/mg) inhibit Na+ permeation, indicating that the number of 'sites' or 'channels' involved may be small. The Ca2+ uniport is not directly involved in Na+ flow according to its greater sensitivity to inhibition by ruthenium red.

Mechanism of mitochondrial transport of thallous ions

Rat liver mitochondria were found to swell under nonenergized conditions when suspended in media containing 30-40 mM TINO3. Respiration on succinate caused a rapid contraction of mitochondria swollen under nonenergized conditions. In the presence of thallous acetate, there was a rapid initial swelling under nonenergized conditions until a plateau was reached; respiration on succinate then caused a further swelling. Trace amounts of 204Tl (less than 100 microM) equilibrated fairly rapidly across the mitochondrial membrane. The influx of Tl+ was able to promote the decay not only of a valinomycin-induced K+-diffusion potential but also of respiration-generated fields in the inner membrane in accordance with the electrophoretic nature of Tl+ movement. Efflux of Tl+ showed a half-time of about 10 sec at 20 degrees C and was not affected appreciably by the energy state. Efflux was retarded by Mg2+ and by lowering the temperature. The data indicate that Tl+ when present at high concentrations, 30mM or more, distributes across the mitochondrial inner membrane both in response to electrical fields and to delta pH. In energized mitochondria the uptake of Tl+ would occur electrophoretically, while Tl+/H+ exchange would constitute a leak. In the presence of NO3-, the movements of Tl+ are determined by that of NO3-, indicating short-range coupling of electrical forces. At low concentrations of Tl+, 5 mM or less, there was no indication of a Tl+/H+ exchange, which appears to be induced by high concentrations of Tl+.

Induction of Na+/K+ exchange in swollen heart mitochondria

Heart mitochondria swollen passively in nitrate salts contract in a respiration-dependent reaction which can be attributed to an endogenous cation/H+ exchange component (or components). The rate of contraction increases with increased extent of passive swelling in both Na+ and K+ salts. Since nearly constant internal cation concentrations are maintained during osmotic swelling, this result suggests that both Na+/H+ and K+/H+ exchange is enhanced by increased matrix volume. Endogenous Mg2+ is also lost with increased matrix volume, and this observation, in conjunction with other evidence available in the literature, suggests that monovalent cation/H+ exchanges may be regulated by divalent cations. Passive exchange of Na+/K+, 42K+/K+, and 24Na+/Na+ can be readily demonstrated in mitochondria swollen in nitrate. All these exchanges are low or not detectable in unswollen control mitochondria, and it appears that they are manifestations of the activated cation/H+ component (or components) functioning in the absence of delta pH.