Phosphate transport, membrane potential, and movements of calcium in rat liver mitochondria

Cd(2+) extrusion by P-type Cd(2+)-ATPase of Staphylococcus aureus 17810R via energy-dependent Cd(2+)/H(+) exchange mechanism

Cd²⁺ is highly toxic to Staphylococcus aureus since it blocks dithiols in cytoplasmic 2-oxoglutarate dehydrogenase complex (ODHC) participating in energy conservation process. However, S. aureus 17810R is Cd²⁺-resistant due to possession of cadA-coded Cd²⁺ efflux system, recognized here as P-type Cd²⁺-ATPase. This Cd²⁺ pump utilizing cellular energy—ATP, ∆μ_H⁺ (electrochemical proton potential) and respiratory protons, extrudes Cd²⁺ from cytoplasm to protect dithiols in ODHC, but the mechanism of Cd²⁺ extrusion remains unknown. Here we propose that two Cd²⁺ taken up by strain 17810R via Mn²⁺ uniporter down membrane potential (∆ψ) generated during glutamate oxidation in 100 mM phosphate buffer (high P_iB) are trapped probably by high affinity sites in cytoplasmic domain of Cd²⁺-ATPase, forming SCdS. This stops Cd²⁺ transport towards dithiols in ODHC, allowing undisturbed NADH production, its oxidation and energy conservation, while ATP could change orientation of SCdS towards facing transmembrane channel. Now, increased number of P_i-dependent protons pumped electrogenically via respiratory chain and countertransported through the channel down ∆ψ, extrude two trapped cytoplasmic Cd²⁺, which move to low affinity sites, being then extruded into extracellular space via ∆ψ-dependent Cd²⁺/H⁺ exchange. In 1 mM phosphate buffer (low P_iB), external Cd²⁺ competing with decreased number of P_i-dependent protons, binds to ψ_s of Cd²⁺-ATPase channel, enters cytoplasm through the channel down ∆ψ via Cd²⁺/Cd²⁺ exchange and blocks dithiols in ODHC. However, Mg²⁺ pretreatment preventing external Cd²⁺ countertransport through the channel down ∆ψ, allowed undisturbed NADH production, its oxidation and extrusion of two cytoplasmic Cd²⁺ via Cd²⁺/H⁺ exchange, despite low P_iB.

Characteristics and possible functions of mitochondrial Ca(2+) transport mechanisms

Mitochondria produce around 92% of the ATP used in the typical animal cell by oxidative phosphorylation using energy from their electrochemical proton gradient. Intramitochondrial free Ca(2+) concentration ([Ca(2+)](m)) has been found to be an important component of control of the rate of this ATP production. In addition, [Ca(2+)](m) also controls the opening of a large pore in the inner mitochondrial membrane, the permeability transition pore (PTP), which plays a role in mitochondrial control of programmed cell death or apoptosis. Therefore, [Ca(2+)](m) can control whether the cell has sufficient ATP to fulfill its functions and survive or is condemned to death. Ca(2+) is also one of the most important second messengers within the cytosol, signaling changes in cellular response through Ca(2+) pulses or transients. Mitochondria can also sequester Ca(2+) from these transients so as to modify the shape of Ca(2+) signaling transients or control their location within the cell. All of this is controlled by the action of four or five mitochondrial Ca(2+) transport mechanisms and the PTP. The characteristics of these mechanisms of Ca(2+) transport and a discussion of how they might function are described in this paper.

Mitochondrial and myoplasmic [Ca2+] in single fibres from mouse limb muscles during repeated tetanic contractions

Previous studies on single fast-twitch fibres from mouse toe muscles have shown marked fatigue-induced changes in the free myoplasmic [Ca2+] ([Ca2+]i), while mitochondrial [Ca2+] remained unchanged. We have now investigated whether muscle fibres from the legs of mice respond in a similar way. Intact, single fibres were dissected from the soleus and extensor digitorum longus (EDL) muscles of adult mice. To measure [Ca2+]i, indo-1 was injected into the isolated fibres. Mitochondrial [Ca2+] was measured using Rhod-2 and confocal laser microscopy. Fatigue was induced by up to 1000 tetanic contractions (70 Hz) given at 2 s intervals. In soleus fibres, there was no significant decrease in tetanic [Ca2+]i at the end of the fatiguing stimulation, whereas tetanic force was significantly reduced by about 30 %. In 10 out of 14 soleus fibres loaded with Rhod-2 and subjected to fatigue, mitochondrial [Ca2+] increased to a maximum after about 50 tetani; this increase was fully reversed within 20 min after the end of stimulation. The force-frequency curve of the non-responding soleus fibres was shifted to higher frequencies compared to that of the responding fibres. In addition, eight out of nine Rhod-2-loaded EDL fibres showed similar changes in mitochondrial [Ca2+] during and after a period of fatiguing stimulation. The stimulation-induced increase in mitochondrial [Ca2+] was reduced when mitochondria were depolarised by application of carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone, whereas it was increased by application of an inhibitor of the mitochondrial Na+/Ca2+ exchange (CGP-37157). In conclusion, isolated slow-twitch muscle fibres show only modest changes in tetanic force and [Ca2+]i during repeated contractions. The increase in mitochondrial Ca2+ does not appear to be essential for activation of mitochondrial ATP production, nor does it cause muscle damage.

Changes in mitochondrial Ca2+ detected with Rhod-2 in single frog and mouse skeletal muscle fibres during and after repeated tetanic contractions

The present study investigated mitochondrial Ca2+ uptake and release in intact living skeletal muscle fibres subjected to bouts of repetitive activity. Confocal microscopy was used in conjunction with the Ca2+-sensitive dye Rhod-2 to monitor changes in mitochondrial Ca2+ in single Xenopus or mouse muscle fibres. A marked increase in the mitochondrial Ca2+ occurred in Xenopus fibres after 10 tetani applied at 4 s intervals. The mitochondrial Ca2+ continued to increase with increasing number of tetani. After the end of tetanic stimulation, mitochondrial Ca2+ declined to 50% of the maximal increase within 10 min and thereafter took up to 60 min to return to its original value. Depolarization of the mitochondria with FCCP greatly attenuated the rise in the mitochondrial Ca2+ evoked by repetitive tetanic stimulation. In addition, FCCP slowed the rate of decay of the tetanic Ca2+ transient which in turn led to an elevation of resting cytosolic Ca2+. Accumulation of Ca2+ in the mitochondria was accompanied by a modest mitochondrial depolarization. In contrast to the situation in Xenopus fibres, mitochondria in mouse toe muscle fibres did not show any change in the mitochondrial Ca2+ during repetitive stimulation and FCCP had no effect on the rate of decay of the tetanic Ca2+ transient. It is concluded that in Xenopus fibres, mitochondria play a role in the regulation of cytosolic Ca2+ and contribute to the relaxation of tetanic Ca2+ transients. In contrast to their important role in Xenopus fibres, mitochondria in mouse fast-twitch skeletal fibres play little role in Ca2+ homeostasis.

Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)]

This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.

On the mechanism of rebounding of calcium in liver mitochondria

Rat liver mitochondria are able to temporarily lower the steady state concentration of external Ca2+ after having accumulated a pulse of added Ca2+. This could be due to inhibition of efflux or/and stimulation of influx of Ca2+. This question has been addressed in mitochondria respiring on succinate +/- malonate. In the presence of malonate the depression of the membrane potential during Ca2+ uptake is more extensive and the rate of Ca2+ uptake slower. There were no discernible differences in the rates of efflux either after inhibition of the calcium uniporter by Ruthenium Red or by studying efflux of preloaded 45Ca-labeled Ca2+. The efflux was not changed by diltiazem or cyclosporin A to inhibit Ca2+ exchange on the Ca2+/nNA+ antiporter or efflux through the permeability transition pore. It is concluded that the rebounding is due mainly to stimulation of the calcium uniporter.

Inhibitory effect of dipyridamole and its derivatives on lipid peroxidation in mitochondria

Dipyridamole (DIP), 2,6-bis(diethanolamino)-4,8-dipiperidino-[5,4-d] pyrimidine, is a coronary vasodilator widely used in clinics. It has also been reported to have coactivator activity for a number of antitumour drugs and antioxidant activity in membrane systems. In recent years we have been studying the spectroscopic properties of this drug and several of its derivatives as well as their interaction with charged micelles and phospholipid monolayers. A strong interaction of DIP and DIP derivatives with these model membrane systems and a dependence of the strength of the interaction upon the chemical structure of the DIP derivative was observed. Here, the antioxidant effect of DIP and the derivatives, RA14, RA47, and RA25, was compared. We observed that although it strongly inhibits the iron-induced lipoperoxidation on mitochondria (IC50 = 1 microM), it shows no protection against an organic oxidant, cumene hydroperoxide. The order of hydrophobicity of the DIP derivatives, DIP > RA14 > RA47 > RA25, correlates very well with both the values of the association constants of these derivatives to micelles, their localization in the micelles, and phospholipid films and their antioxidant effect on mitochondria. So, a very good correlation of the structure of the drug in regarded to the nature of its substituents with the biological activity is observed. Essentially the same result was observed either measuring the lipid peroxidation or the membrane fluidity by ESR, suggesting that the effect of DIP and DIP derivatives is probably associated to their binding to the lipid bilayer and not to interaction with membrane proteins.

Transport of calcium by mitochondria

The identification of intramitochondrial free calcium ([Ca2+]m) as a primary metabolic mediator [see Hansford (this volume) and Gunter, T. E., Gunter, K. K., Sheu, S.-S., and Gavin, C. E. (1994) Am. J. Physiol. 267, C313-C339, for reviews] has emphasized the importance of understanding the characteristics of those mechanisms that control [Ca2+]m. In this review, we attempt to update the descriptions of the mechanisms that mediate the transport of Ca2+ across the mitochondrial inner membrane, emphasizing the energetics of each mechanism. New concepts within this field are reviewed and some older concepts are discussed more completely than in earlier reviews. The mathematical forms of the membrane potential dependence and concentration dependence of the uniporter are interpolated in such a way as to display the convenience of considering Vmax to be an explicit function of the membrane potential. Recent evidence for a transient rapid conductance state of the uniporter is discussed. New evidence concerning the energetics and stoichiometries of both Na(+)-dependent and Na(+)-independent efflux mechanisms is reviewed. Explicit mathematical expressions are used to describe the energetics of the system and the kinetics of transport via each Ca2+ transport mechanism.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

Calcium release associated with discharge of specific granule contents from human neutrophils

Neutrophil discharge of calcium into the ambient medium was measured, using an ion-sensitive electrode, after cells were stimulated with phorbol myristate acetate. Dose-dependent calcium efflux was observed over the same range of stimulus concentrations associated with specific granule marker release. Calcium efflux was preserved when cells were treated with vanadate to inhibit the plasma membrane calcium ATPase. However, less calcium was released than in previous studies employing a complete secretagogue to discharge both specific and azurophil granule contents. These studies suggest that calcium is stored in both the azurophil and the specific granules of the neutrophil, and can be mobilized from both sites during degranulation.

Measurement of the matrix free Ca2+ concentration in heart mitochondria by entrapped fura-2 and quin2

A method was developed to monitor continuously the matrix free Ca2+ concentration ([Ca2+]m) of heart mitochondria by use of the fluorescent Ca2+ indicators, fura-2 and quin2. The acetoxymethyl esters of fura-2 and quin2 were accumulated in and hydrolysed by isolated mitochondria. An increase of the mitochondrial Ca content from 0.3 nmol/mg of protein to 6 nmol/mg corresponded to a rise of [Ca2+]m from 30 to 1000 nM. The results indicate that physiological fluctuations of the mitochondrial Ca content elicit changes of [Ca2+]m in that range which regulates the matrix dehydrogenases.

(+)-Cyanidanol-3 prevents the functional deterioration of rat liver mitochondria induced by Fe2+ ions

The clinically effective hepatoprotective flavonoid, (+)-Cyanidanol-3, prevented the Fe2+-induced functional deterioration of rat liver mitochondria. Fe2+ treatment of mitochondria resulted in increased lipid peroxidation (MDA-formation), decreased mitochondrial membrane potential, impaired Ca2+ uptake capacity and caused large amplitude swelling of mitochondria. All of the consequences of Fe2+ treatment were inhibited by (+)-Cyanidanol-3 in a concentration dependent manner. The mitochondrial protective action of the drug is comparable with its free radical scavenging property.

The Ba2+ sensitivity of the Na+-induced Ca2+ efflux in heart mitochondria: the site of inhibitory action

The Na+-induced Ca2+ release from rat heart mitochondria was measured in the presence of Ruthenium red. Ba2+ effectively inhibited the Na+-induced Ca2+ release. At 10 mM Na+ 50% inhibition was reached by 1.51 +/- 0.48 (S.D., n = 8) microM Ba2+ in the presence of 0.1 mg/ml albumin and by 0.87 +/- 0.25 (S.D., n = 3) microM Ba2+ without albumin. In order to inhibit, it was not required that Ba2+ ions enter the matrix. 140Ba2+ was not accumulated in the mitochondrial matrix space; further, in contrast to liver mitochondria, Ba2+ inhibition was immediate. The Na+-induced Ca2+ release was inhibited by Ba2+ non-competitively, with respect of the extramitochondrial Na+. The double inhibitor titration of the Na+-Ca2+ exchanger with Ba2+ in the presence and absence of extramitochondrial Ca2+ revealed that the exchanger possesses a common binding site for extramitochondrial Ca2+ and Ba2+, presumably the regulatory binding site of the Na+-Ca2+ exchanger, which was described by Hayat and Crompton (Biochem. J. 202 (1982) 509-518). All these observations indicate that Ba2+ acts at the cytoplasmic surface of the inner mitochondrial membrane. The inhibitory properties of Ba2+ on the Na+-dependent Ca2+ release in heart mitochondria are basically different from those found on Na+-independent Ca2+ release in liver mitochondria (Lukács, G.L. and Fonyó, A. (1985) Biochim. Biophys. Acta 809, 160-166).

Ba2+ ions inhibit the release of Ca2+ ions from rat liver mitochondria

The release of Ca2+ from respiring rat liver mitochondria following the addition of either ruthenium red or an uncoupler was measured by a Ca2+-selective electrode or by 45Ca2+ technique. Ba2+ ions are asymmetric inhibitors of both Ca2+ release processes. Ba2+ ions in a concentration of 75 microM inhibited the ruthenium red and the uncoupler induced Ca2+ release by 80% and 50%, respectively. For the inhibition, it was necessary that Ba2+ ions entered the matrix space: Ba2+ ions did not cause any inhibition of Ca2+ release if addition of either ruthenium red or the uncoupler preceded that of Ba2+. The time required for the development of the inhibition of the Ca2+ release and the time course of 140Ba2+ uptake ran in parallel. Ba2+ accumulation is mediated through the Ca2+ uniporter as 140Ba2+ uptake was competitively inhibited by extramitochondrial Ca2+ and prevented by ruthenium red. Due to the inhibition of the ruthenium red insensitive Ca2+ release, Ba2+ shifted the steady-state extramitochondrial Ca2+ concentration to a lower value. Ba2+ is potentially a useful tool to study mitochondrial Ca2+ transport.