Steady state regulation of extramitochondrial Ca2+ by rat liver mitochondria: effects of Mg2+ and ATP

Lithium isotopes differentially modify mitochondrial amorphous calcium phosphate cluster size distribution and calcium capacity

Lithium is commonly prescribed as a mood stabilizer in a variety of mental health conditions, yet its molecular mode of action is incompletely understood. Many cellular events associated with lithium appear tied to mitochondrial function. Further, recent evidence suggests that lithium bioactivities are isotope specific. Here we focus on lithium effects related to mitochondrial calcium handling. Lithium protected against calcium-induced permeability transition and decreased the calcium capacity of liver mitochondria at a clinically relevant concentration. In contrast, brain mitochondrial calcium capacity was increased by lithium. Surprisingly, 7Li acted more potently than 6Li on calcium capacity, yet 6Li was more effective at delaying permeability transition. The size distribution of amorphous calcium phosphate colloids formed in vitro was differentially affected by lithium isotopes, providing a mechanistic basis for the observed isotope specific effects on mitochondrial calcium handling. This work highlights a need to better understand how mitochondrial calcium stores are structurally regulated and provides key considerations for future formulations of lithium-based therapeutics.

Alterations to the permeability of plant and animal mitochondria submitted to Ca2+ releasing agents

1. Mitochondria from different rat tissues and from plants were compared as regards their sensitivity towards Ca2+ in the presence of different Ca2+ releasing agents, and the phospholipase A2 activity was evaluated in the different mitochondrial preparations. 2. The mitochondria were exposed to Ca2+ and an oxidant such as t-butylhydroperoxide or diamide or to Ca2+ and inorganic phosphate, and plant mitochondria were seen to be much more resistant than liver, brain or kidney mitochondria of rats to the deleterious effects of these agents. 3. The phospholipase A2 activity is not directly involved in the alterations of the mitochondrial inner membrane permeability within the first 10 min of incubation under our experimental conditions. 4. The protection conferred by ATP and Mg2+ against Ca2+ efflux from mitochondria or the decrease in the mitochondrial transmembrane electrical potential was also observed under our experimental conditions, but cannot be attributed to an enhancement of the reacylation of lysophospholipids resulting from the phospholipase A2 activity.

Regulation of intracellular calcium homeostasis in Trypanosoma cruzi. Effects of calmidazolium and trifluoperazine

Trypanosoma cruzi epimastigotes maintained an intracellular free calcium concentration of about 0.15 microM, as measured with the fluorescent indicator Fura-2. The maintenance of low [Ca2+]i is energy-dependent since it is disrupted by KCN and FCCP. When the cells were permeabilized with digitonin, the steady-state free Ca2+ concentration in the absence of ATP was about 0.7 microM. The additional presence of ATP resulted in a steady-state level close to 0.1-0.2 microM which compares favorably with the concentration detected in intact cells. Intracellular Ca2+ uptake at high levels of free Ca2+ (greater than 1 microM) was due to energy-dependent mitochondrial uptake as indicated by its FCCP-sensitivity. However, as the free Ca2+ concentration was lowered from 1 microM, essentially all uptake was due to the ATP-dependent Ca2+ sequestration by the endoplasmic reticulum as indicated by its stimulation by ATP, and its inhibition by sodium vanadate. High concentrations of the calmodulin antagonist trifluoperazine, inhibited both the Ca2+ uptake by the endoplasmic reticulum and by the mitochondria, while calmidazolium released Ca2+ from both compartments. In addition, trifluoperazine and calmidazolium inhibited respiration and collapsed the mitochondrial membrane potential of T. cruzi, thus indicating non-specific effects unrelated to calmodulin.

Evidence that angiotensin II decreases mitochondrial calcium in the glomerulosa cell

The present studies were performed using primary monolayer cultures of bovine glomerulosa cells to determine whether the elevation in cytosolic calcium concentration produced by angiotensin II was accompanied by an elevation in mitochondrial calcium. Exchangeable mitochondria calcium content was assessed indirectly by measuring the changes in cytosolic calcium concentration and calcium efflux produced by the mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Total mitochondrial calcium content was also assessed directly by atomic absorption spectroscopy. CCCP had a direct effect to promote calcium release from an oligomycin/antimycin-sensitive (mitochondrial) calcium pool in permeabilized cells. In intact cells, CCCP caused rapid reductions in cellular ATP content and the ratio of ATP to ADP. Still, its effects on calcium dynamics were exerted primarily at the mitochondrial level as evidenced by inhibition with ruthenium red, but not dantrolene. As expected, angiotensin II produced a rapid increase in calcium efflux and an equally rapid and sustained increase in cytosolic calcium concentration. Nonetheless, CCCP-stimulated elevations in cytosolic calcium concentration and calcium efflux were reduced by angiotensin II in a concentration-dependent manner. Total mitochondrial calcium content was also lower in angiotensin-treated than in control cells. These results indicate that angiotensin II causes a net decrease in mitochondrial calcium stores. On the basis of these data, it is proposed that alterations in calcium metabolism initiated by angiotensin II are exerted not only at the membrane and cytosolic levels but also at the level of the mitochondria. Changes in mitochondrial calcium dynamics may directly contribute to the regulation of mitochondrial steroidogenic enzymes by angiotensin II.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

Manganese and calcium efflux kinetics in brain mitochondria. Relevance to manganese toxicity

Manganese shares the uniport mechanism of mitochondrial calcium influx, accumulates in mitochondria and is cleared only very slowly from brain. Using dual-label isotope techniques, we have investigated both Mn2+ and Ca2+ mitochondrial efflux kinetics. We report that (1) there is no significant Na(+)-dependent Mn2+ efflux from brain mitochondria; (2) Mn2+ inhibits both Na(+)-dependent and Na(+)-independent Ca2+ efflux in brain, in a mode that appears to be primarily competitive and with apparent Ki values of 5.1 and 7.9 nmol/mg respectively; and (3) Ca2+ does not appear to inhibit Mn2+ efflux from brain mitochondria. Findings (1) and (2) suggest the possibility of mitochondrial accumulation of both Mn2+ and Ca2+ in Mn2(+)-intoxicated brain.

Characteristics of Ca2+ transport by Trypanosoma cruzi mitochondria in situ

The use of digitonin to permeabilize Trypanosoma cruzi plasma membrane has allowed the study of Ca2+ transport and oxidative phosphorylation in mitochondria in situ (R. Docampo and A. E. Vercesi (1989) J. Biol. Chem. 264, 108-111). The present results show that these mitochondria are able to build up and retain a membrane potential as indicated by a tetraphenylphosphonium-sensitive electrode. Ca2+ uptake caused membrane depolarization compatible with the existence of an electrogenically mediated Ca2+ transport mechanism in these mitochondria. Addition of Ca2+ or ethylene glycol bis (beta-aminoethyl ether) N-N'-tetraacetic acid to these preparations under steady-state conditions was followed by Ca2+ uptake or release, respectively, tending to restore the original Ca2+ "set point" at about 0.9 microM. In addition, large amounts of Ca2+ were retained by T. cruzi mitochondria even after addition of thiols and NAD(P)H oxidants such as t-butyl hydroperoxide, diamide, and the 1,2-naphthoquinone beta-lapachone. However, when ascorbate plus N,N,N',N'-tetramethyl-p-phenylenediamine in the presence of antimycin A was used as subtrate, beta-lapachone caused pyridine nucleotide oxidation, and Ca2+ accumulation by these mitochondria was considerably lower than in control preparations, this effect being dose-dependent.

Mechanisms of mitochondrial calcium transport

Mitochondria are known to possess a rapid calcium uptake mechanism or uniport and both sodium-dependent and sodium-independent efflux mechanisms. Whether sodium-independent calcium efflux is mediated and whether sodium-dependent calcium efflux can be found in liver mitochondria have been questioned. Kinetics results relevant to the answers of these questions are discussed below. A slow, mediated, sodium-independent calcium efflux mechanism is identified which shows second order kinetics. This mechanism, which shows "nonessential activation" kinetics, has a Vmax around 1.2 nmol calcium per mg protein per min and a half maximal velocity around 8.4 nmol calcium per mg protein. A slow, sodium-dependent calcium efflux mechanism is identified, which is first order in calcium and second order in sodium. This mechanism has a Vmax around 2.6 nmol of calcium per mg protein per min. The sodium dependence is half saturated at an external sodium concentration of 9.4 mM, and the calcium dependence is half saturated at an internal calcium concentration of 8.1 nmol calcium per mg protein. The cooperativity of the sodium dependence effectively permits a terreactant system to be fit by a bireactant model in which [Na] only appears as the square of [Na]. This liver system shows simultaneous, as opposed to ping-pong, kinetics. It is also found to be sensitive to inhibition by tetraphenyl phosphonium, magnesium, and ruthenium red. A model is proposed in which mitochondrial calcium transport could function to "shape the pulses" of cytosolic calcium. Simultaneously, mitochondria may mediate a "calcium memory" coupled perhaps to activation of cytosolic events through calmodulin or perhaps to activation of electron transport through the activation of specific dehydrogenases by intramitochondrial calcium.

Decrease in total and free magnesium concentration following traumatic brain injury in rats

31P magnetic resonance spectroscopy was used to determine the intracellular free Mg2+ concentration prior to and following fluid percussion induced traumatic brain injury in rats. Prior to injury, cerebral intracellular free Mg2+ concentration in the rat was 0.93 +/- 0.19 mM (mean +/- SE; n = 5). Following injury, free Mg2+ in the injured cortex declined by 70% within the first hour, and did not recover over the next 3 hours. Total Mg2+ also declined by 10% over this time period; however, there were no changes in brain Na+ or tissue water content. Because of its primary role in cellular metabolism, the early decline in tissue Mg2+ following brain trauma may be a critical factor in the development of irreversible tissue injury.

Regulation of free Ca2+ by subcellular fractions of rat incisor odontoblasts

A miniaturized Ca2+ electrode system was developed to monitor small and fast fluctuations of Ca2+ activity in the micromolar range in 100 microliters volumes. This was used to study Ca2+ influx/efflux cycling in suspensions of rat-odontoblast and liver-cell mitochondria and microsomes, as well as in whole odontoblasts with plasma membranes made permeable by digitonin. The steady-state free-Ca2+ activity maintained by mitochondria was pCa 6.2-6.4, and that of microsomes pCa 6.4-6.6. These levels were held upon repeated additions of Ca2+ and EGTA. The odontoblast mitochondria and microsomes had an intracellular Ca2+ buffering capacity similar to that of liver cells. The steady-state pCa level maintained in suspensions of digitonin-permeabilized whole odontoblasts was 6.4-6.6. Thus, this study gave no evidence for any specialized intracellular handling of Ca2+ in cells involved in mineralization.

t-Butylhydroperoxide-induced Ca2+ efflux from liver mitochondria in the presence of physiological concentrations of Mg2+ and ATP

Isolated rat liver mitochondria, energized either by succinate oxidation or by ATP hydrolysis, present a transient increase in the rate of Ca2+ efflux concomitant to NAD(P)H oxidation by hydroperoxides when suspended in a medium containing 3 mM ATP, 4 mM Mg2+ and acetate as permeant anion. This is paralleled by an increase in the steady-state concentration of extramitochondrial Ca2+, a small decrease in delta psi and an increase in the rate of respiration and mitochondrial swelling. With the exception of mitochondrial swelling all other events were found to be reversible. If Ca2+ cycling was prevented by ruthenium red, the changes in delta psi, the rate of respiration and the extent of mitochondrial swelling were significantly diminished. In addition, there was no significant decrease in the content of mitochondrial pyridine nucleotides. Mitochondrial coupling was preserved after a cycle of Ca2+ release and re-uptake under these experimental conditions. It is concluded that hydroperoxide-induced Ca2+ efflux from intact mitochondria is related to the redox state of pyridine nucleotides.

Citrate effects on the Ca2+-loading capacity of isolated rat liver mitochondria: interaction of citrate and ATP

The maximal amounts of Ca2+ being accumulated (delta Ca2+max) and H+ emitted (delta H+max) by Ca2+-loading mitochondria, with succinate (+rotenone) as respiratory substrate, were evaluated. delta Ca2+max was increased by providing either citrate or ATP to a Pi- and Mg2+-free medium. With citrate, delta H+max was only scarcely increased, so that the effect of the proton-carrying anion resulted essentially from an increase in the Ca2+/H+ ratio, i.e., from preservation of membrane potential. With ATP (+/- oligomycin), the Ca2+/H+ ratio was unaltered; i.e., the increase of delta Ca2+max was paralleled by a related increase in delta H+max. Mitochondria appeared to retain Ca at higher delta pH, i.e., at lower membrane potential, in the presence of ATP. With citrate and ATP together, both the Ca2+/H+ ratio and delta H+max were largely increased, and the product of these two terms, delta Ca2+max, was considerably enlarged. The effect of either citrate or ATP was markedly reinforced in the presence of the other anion. In addition to increasing the Ca2+/H+ ratio, citrate contributed to increasing delta H+max in the presence of ATP, i.e., apparently sensitized mitochondria to the action of ATP. A citrate-induced depression of Ca2+ cycling across the inner membrane, even though pronounced, did not account for the sensitization. Supraadditive effects of citrate and ATP persisted in the presence of MgCl2 and Pi, under conditions of massive Ca2+ loading, and may contribute to the high capacity of mitochondria, in situ, to retain calcium.

Some characteristics of Ca2+ transport in plant mitochondria

Coupled mitochondria isolated from the white leaves of cabbage (Brassica Oleracea, var. capitata) were inactive in respiration-coupled Ca2+ accumulation, in contrast to mitochondria isolated from etiolated corn (Zea mays) which showed the ability to take up Ca2+ from the medium, although with a much lower activity than liver mitochondria. The addition of corn mitochondria to aerobic medium containing succinate as respiratory substrate and a free Ca2+ concentration of 40 microM resulted in Ca2+ uptake with a decrease in free Ca2+ concentration until a steady state of about 2.0 microM was reached and maintained constant for several minutes. Perturbation of this steady state by the addition of Ca2+ or EGTA was followed by Ca2+ uptake or release, respectively, until the steady state was attained at the original extramitochondrial free Ca2+ concentration. These results indicate that corn but not cabbage mitochondria, as with some animal mitochondria, have the ability to buffer external Ca2+ and may be involved in the maintenance of Ca2+ homeostasis in the cell.

Tissue-specific modulation of the mitochondrial calcium uniporter by magnesium ions

This paper analyzes the kinetics of the Ca2+ uniporter of mitochondria from rat heart, kidney and liver operating in a range of Ca2+ concentrations near the steady-state value (1-4 microM). Heart mitochondria exhibit the lowest activity, and physiological Mg2+ concentrations inhibit the mitochondrial Ca2+ uniporter by approx. 50% in heart and kidney, and by 20% in liver. At physiological Ca2+ and Mg2+ concentrations the external free Ca2+ maintained by respiring mitochondria in vitro is higher in heart and kidney with respect to liver mitochondria. This behaviour could represent an adaptation of different mitochondria to their specific intracellular environment.

Inhibition of ruthenium red-induced Ca2+ efflux from liver mitochondria by the antibiotic X-537A

It has been reported (Becker, G.L., Fiskum, G. and Lehninger, A.L. (1980) J. Biol. Chem. 255, 9009-9012) that respiring rat liver mitochondria suspended in KC1 medium containing ATP, Mg2+ and phosphate, maintain a steady state extramitochondrial free Ca2+ concentration of about 0.5 microM. The results reported here show that the addition of the antibiotic X-537A, at concentrations far below those required for ionophorous activity, caused a perturbation in this steady state, lowering the extramitochondrial free Ca2+ concentration by about 0.20 microM. This shift in steady state was clarified by a study of X-537A inhibition of the Ca2+ efflux induced by ruthenium red; a half-maximum effect was observed at approximately 25 nM X-537A. No effect on Ca2+ transport through the influx uniporter was observed. The possibility of a generalized stabilizing action of the antibiotic on the mitochondrial membrane seems to be ruled out by its effectiveness at very low concentrations.

Intramitochondrial and extramitochondrial free calcium ion concentrations of suspensions of heart mitochondria with very low, plausibly physiological, contents of total calcium

The 2-oxoglutarate dehydrogenase of intact rat heart mitochondria is activated by Ca2+, with 50% activation at approximately 0.5 nmol of total Ca/mg of mitochondrial protein, in the presence of Pi and Mg2+. Mitochondrial Ca contents in excess of 2 nmol/mg of protein result in 100% activation of the enzyme. Investigation of Ca2+ release from the mitochondria using the metallochromic indicator Arsenazo III defines a S0.5 of 5.4 +/- 0.4 nmol of Ca/mg of protein, when the endogenous Ca content of the mitochondria is progressively depleted with EGTA, prior to the initiation of the release process being studied. The subsequent determination of matrix free Ca2+ concentration by the "null-point" technique has allowed expression of these results in terms of free concentration rather than Ca content, with an activity coefficient of approximately 0.001 for matrix Ca2+. From the above, Ca2+ efflux from heart mitochondria is not saturated at the mitochondrial Ca contents or Ca2+ concentrations which give effective regulation of dehydrogenase activity. A consequence is that heart mitochondria do not buffer the pCa of the extramitochondrial medium at these Ca contents (less than 2 nmol/mg of protein), and this is shown in direct measurements of extramitochondrial pCa. This is taken to question the physiological significance of mitochondrial buffering of cytosolic free Ca2+ in normal heart.

The isolation of lymphocyte mitochondria and their regulation of extramitochondrial free Ca2+ concentration

1. A method for the isolation of functionally intact mitochondria from lymphocytes is described. It involves digitonin breakage of the plasma membrane, followed by differential centrifugation. The yield was 36 mg of mitochondrial protein/200 g of pig mesenteric lymph node (6 mg of mitochondrial protein/10(9) lymphocytes). The mitochondrial had a respiratory-control ratio of 2--3.5 with succinate as substrate. 2. Ca2+ transport by these mitochondria was investigated. They were able to regulate the extramitochondrial free [Ca2+] very precisely, by buffering any displacements from the steady-state. The exact extramitochondrial free [Ca2+] of this steady-state depended on the conditions of incubation. In a medium designed to resemble the cytoplasmic environment, with added Ca2+, lymphocyte mitochondria maintained a steady-state free [Ca2+] of 0.63 microM (pCa of 6.2). The rates of Ca2+ uptake and efflux under these conditions, with both lymphocyte and liver mitochondria, were very much lower than those in a less complex medium. 3. Lymphocyte mitochondria were shown to possess an Na+-independent Ruthenium Red-insensitive efflux pathway similar to that of liver mitochondria. Ruthenium Red totally inhibited the electrophoretic uniporter. Although Na+ had no effect on the steady-state maintained by lymphocyte mitochondria, they were shown to possess an Na+/H+ antiporter.

ATP prevents both hydroperoxide-induced hydrolysis of pyridine nucleotides and release of calcium in rat liver mitochondria

The mechanism of the hydroperoxide-induced release of calcium from rat liver mitochondria was investigated. In calcium-loaded mitochondria both the hydroperoxide-induced hydrolysis of pyridine nucleotides and the release of calcium is inhibited by intramitochondrial ATP. In submitochondrial particles, in the absence of ATP a hydrolytic product of NAD+, probably ADP-ribose, binds covalently to a protein of the inner mitochondrial membrane. It is proposed that a compound deriving from NAD(P)+ is an important factor in the calcium release mechanism in rat liver mitochondria.

Evidence for two compartments of exchangeable calcium in isolated rat liver mitochondria obtained using a 45Ca exchange technique in the presence of magnesium, phosphate, and ATPase at 37 degrees C

The distribution of calcium between isolated rat liver mitochondria and the extramitochondrial medium at 37 degrees C and in the presence of 2 mM inorganic phosphate, 3 mM ATP, 0.05 or 1.1 mM free magnesium and a calcium buffer, nitrilotriacetic acid, was investigated using a 45Ca exchange technique. The amounts of 40Ca in the mitochondria and medium were allowed to reach equilibrium before initiation of the measurement of 45Ca exchange. At 0.05 mM free magnesium and initial extramitochondrial free calcium concentrations of between 0.15 and 0.5 microM, the mitochondria accumulated calcium until the extramitochondrial free calcium concentration was reduced to 0.15 microM. Control experiments showed that the mitochondria were stable under the incubation conditions employed. The 45Ca exchange data were found to be consistent with a system in which two compartments of exchangeable calcium are associated with the mitochondria. Changes in the concentration of inorganic phosphate did not significantly affect the 45Ca exchange curves, whereas an increase in the concentration of free magnesium inhibited exchange. The maximum rate of calcium outflow from the mitochondria was estimated to be 1.7 nmol/min per mg of protein, and the value of K0.5 for intramitochondrial exchangeable calcium to be about 1.6 nmol per mg of protein. Ruthenium Red decreased the fractional transfer rate for calcium inflow to the mitochondria while nupercaine affected principally the fractional transfer rates for the transfer of calcium between the two mitochondrial compartments. The use of the incubation conditions and 45Ca exchange technique described in this report for studies of the effects of agents which may alter mitochondrial calcium uptake or release (e.g., the pre-treatment of cells with hormones) is briefly discussed.