Oxygen-bridged dinuclear ruthenium amine complex specifically inhibits Ca2+ uptake into mitochondria in vitro and in situ in single cardiac myocytes

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Inhibitory properties of ruthenium amine complexes on mitochondrial calcium uptake

The recent finding that the inhibition of Ca2+-stimulated respiration by ruthenium red is mainly due to a binuclear ruthenium complex (Ru360) present in the commercial samples of the classical inhibitor ruthenium red (Ying et. al., 1991), showed that this complex is the more potent and specific inhibitor of the mitochondrial calcium uniporter. This work was aimed to provide insights into the mechanism by which Ru360 and other ruthenium-related compounds inhibits calcium uptake. Ruthenium red and a synthesized analog (Rrphen) were compared with Ru360. The inhibition by this binuclear complex was noncompetitive, with a Ki of 9.89 nM. The number of specific binding sites for Ru360 was 6.2 pmol/mg protein. Ruthenium red and Ru360 were mutually exclusive inhibitors. Bound La3+ was not displaced by Ru360. Rrphen was the least effective for inhibiting calcium uptake. The results support the notion of a specific binding site in the uniporter for the polycationic complexes and a negative charged region from the phospholipids in the membrane, closely associated with the uniporter inhibitor-binding site.

Ruthenium red modifies the cardiac and skeletal muscle Ca(2+) release channels (ryanodine receptors) by multiple mechanisms

The effects of ruthenium red (RR) on the skeletal and cardiac muscle ryanodine receptors (RyRs) were studied in vesicle-Ca(2+) flux, [(3)H]ryanodine binding, and single channel measurements. In vesicle-Ca(2+) flux measurements, RR was more effective in inhibiting RyRs at 0.2 microM than 20 microM free Ca(2+). [(3)H]Ryanodine binding measurements suggested noncompetitive interactions between RR inhibition and Ca(2+) regulatory sites of RyRs. In symmetric 0.25 M KCl with 10-20 microM cytosolic Ca(2+), cytosolic RR decreased single channel activities at positive and negative holding potentials. In close to fully activated skeletal (20 microM Ca(2+) + 2 mM ATP) and cardiac (200 microM Ca(2+)) RyRs, cytosolic RR induced a predominant subconductance at a positive but not negative holding potential. Lumenal RR induced a major subconductance in cardiac RyR at negative but not positive holding potentials and several subconductances in skeletal RyR. The RR-related subconductances of cardiac RyR showed a nonlinear voltage dependence, and more than one RR molecule appeared to be involved in their formation. Cytosolic and lumenal RR also induced subconductances in Ca(2+)-conducting skeletal and cardiac RyRs recorded at 0 mV holding potential. These results suggest that RR inhibits RyRs and induces subconductances by binding to cytosolic and lumenal sites of skeletal and cardiac RyRs.

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.

Altered mitochondrial oxidative phosphorylation in hippocampal slices of kainate-treated rats

Mitochondria provide the main neuronal energy supply and are important organelles for the sequestration of intracellular Ca2+. This indicates a possible important role for mitochondria in modulating neuronal excitability in normal function as well as in disease. Therefore, we have investigated mitochondrial oxidative phosphorylation in the kainate model of epilepsy. We measured the oxygen consumption of single 400-micron rat hippocampal slices applying high resolution respirometry and determined mitochondrial NAD(P)H autofluorescence signal changes in single slices by laser-excited fluorescence spectroscopy. We observed an about 2-fold higher (p<0.001) basal glucose oxidation rate in slices from kainate-treated animals. This increased endogenous energy consumption was found to be unrelated to spontaneous activity since it was not sensitive to the inhibitors of the sodium-potassium ATPase ouabain and of the mitochondrial adenine nucleotide translocator atractyloside. This finding suggested an increased mitochondrial energy turnover in kainate-induced epilepsy. Furthermore, the uncoupler-stimulated oxygen consumption of the slices was approximately 1.3-fold higher (p<0.01) in the kainate model. In accordance with the respirometric data, fluorescence spectroscopy showed decreased reduction levels of the mitochondrial NAD-system in glucose oxidizing slices from kainate-treated rats. The preincubation of epileptic hippocampal slices with either BAPTA AM, ruthenium red or TPP+ increased the atractyloside sensitivity of glucose oxidation to about 1.4-fold (p<0.01). These observations indicate that the increased mitochondrial energy turnover in hippocampal slices from kainate-treated rats is most possibly caused by futile Ca2+-cycling.

Contributions of mitochondria to animal physiology: from homeostatic sensor to calcium signalling and cell death

Over recent years, it has become clear that mitochondria play a central role in many key aspects of animal physiology and pathophysiology. Their central and ubiquitous task is clearly the production of ATP. Nevertheless, they also play subtle roles in glucose homeostasis, acting as the sensor for substrate supply in the transduction pathway that promotes insulin secretion by the pancreatic -cell and that modulates the excitability of the hypothalamic glucose-sensitive neurons involved in appetite control. Mitochondria may also act as sensors of availability of oxygen, the other major mitochondrial substrate, in the regulation of respiration. Mitochondria take up calcium, and the high opacity mitochondrial calcium uptake pathway provides a mechanism that couples energy demand to increased ATP production through the calcium-dependent upregulation of mitochondrial enzyme activity. Mitochondrial calcium accumulation may also have a substantial impact on the spatiotemporal dynamics of cellular calcium signals, with subtle differences of detail in different cell types. Recent work has also revealed the centrality of mitochondrial dysfunction as an irreversible step in the pathway to both necrotic and apoptotic cell death. This review looks at recent developments in these rapidly evolving areas of cell physiology in an attempt to draw together disparate areas of research into a common theme.

Cytosolic and mitochondrial Ca2+ signals in patch clamped mammalian ventricular myocytes

1. Ventricular myocytes isolated from ferret or cat were loaded with the acetoxymethyl ester form of indo-1 (indo-1 AM) such that approximately 75% of cellular indo-1 was mitochondrial. The intramitochondrial indo-1 concentration was 0.5-2 mM. 2. Myocytes were also voltage clamped (membrane capacitance, Cm = 100 pF) and a typical wash-out time constant of cytosolic indo-1 by a patch pipette was found to be approximately 300 s. Depolarizations to +110 mV produced graded and progressive cellular Ca2+ load via Na(+)-Ca2+ exchange. 3. During these relatively slow Ca2+ transients, cell contraction (delta L) paralleled fluorescence ratio signals (R) such that delta L could be used as a bioassay of cytosolic [Ca2+] ([Ca2+]c), where [Ca2+]CL is the inferred signal which is delayed by approximately 200 ms from true [Ca2+]c. 4. In myocytes without Mn2+ quench, the kinetics of the total cellular indo-1 signal, delta R (including cytosolic and mitochondrial components), match delta L during stimulations at low basal [Ca2+]i. However, after progressive Ca2+ loading, delta R kinetics deviate from delta L dramatically. The deviation can be completely blocked by a potent mitochondrial Ca2+ uniport blocker, Ru360. 5. When cytosolic indo-1 is quenched by Mn2+, initial moderate stimulation triggers contractions (delta L), but no change in indo-1 signal, indicating both the absence of cytosolic Ca(2+)-sensitive indo-1 and unchanged mitochondrial [Ca2+] (delta [Ca2+]m). Subsequent stronger stimulation evoked larger delta L and also delta R. The threshold [Ca2+]c for mitochondrial Ca2+ uptake was 300-500 nM, similar to that without Mn2+ quench. 6. At high Ca2+ loads where delta [Ca2+]m is detected, the time course of [Ca2+]m was different from that of [Ca2+]c. Peak [Ca2+]m after stimulation has an approximately 1 s latency with respect to [Ca2+]c, and [Ca2+]m decline is extremely slow. 7. Upon a Ca2+ influx which increased [Ca2+]c by 0.4 microM and [Ca2+]m by 0.2 microM, total mitochondrial Ca2+ uptake was approximately 13 mumol (1 mitochondria)-1. 8. With Mn2+ quench of cytosolic indo-1, there was no mitochondrial uptake of Mn2+ until the point at which mitochondrial Ca2+ uptake became apparent. However, after mitochondrial Ca2+ uptake starts, mitochondria continually take up Mn2+ even during relaxation, when [Ca2+]c is low. 9. It is concluded that mitochondria in intact myocytes do not take up detectable amounts of Ca2+ during individual contractions, unless resting [Ca2+]c exceeds 300-500 nM. At high cell Ca2+ loads and [Ca2+]c, mitochondrial Ca2+ transients occur during the twitch, but with much slower kinetics than those of [Ca2+]c.