Respiratory control and ADP:O coupling ratios of isolated chick heart mitochondria

Adenylate kinase derived ATP shapes respiration and calcium storage of isolated mitochondria

The ratio of ADP and ATP is a natural indicator of cellular bioenergetic state and thus a prominent analyte in metabolism research. Beyond adenylate interconversion via oxidative phosphorylation and ATPase activities, ADP and ATP act as steric regulators of enzymes, e.g. cytochrome C oxidase, and are major factors in mitochondrial calcium storage potential. Consideration of all routes of adenylate conversion is critical to successfully predict their abundance in an experimental system and to correctly interpret many aspects of mitochondrial function. We showcase here how adenylate kinases elicit considerable impact on the outcome of a variety of mitochondrial assays through their drastic manipulation of the adenylate profile. Parameters affected include cytochrome c oxidase activity, P/O ratio, and mitochondrial calcium dynamics. Study of the latter revealed that the presence of ATP is required for mitochondrial calcium to be shaped into a particularly dense form of mitochondrial amorphous calcium phosphate.

Acute exposure to C60 fullerene damages pulmonary mitochondrial function and mechanics

C60 fullerene (C60) nanoparticles, a nanomaterial widely used in technology, can offer risks to humans, overcome biological barriers, and deposit onto the lungs. However, data on its putative pulmonary burden are scanty. Recently, the C60 interaction with mitochondria has been described in vitro and in vivo. We hypothesized that C60 impairs lung mechanics and mitochondrial function. Thirty-five male BALB/c mice were randomly divided into two groups intratracheally instilled with vehicle (0.9% NaCl + 1% Tween 80, CTRL) or C60 (1.0 mg/kg, FUL). Twenty-four hours after exposure, 15 FUL and 8 CTRL mice were anesthetized, paralyzed, and mechanically ventilated for the determination of lung mechanics. After euthanasia, the lungs were removed en bloc at end-expiration for histological processing. Lung tissue elastance and viscance were augmented in FUL group. Increased inflammatory cell number, alveolar collapse, septal thickening, and pulmonary edema were detected. In other six FUL and six CTRL mice, mitochondria expressed reduction in state 1 respiration [FUL = 3.0 ± 1.14 vs. CTRL = 4.46 ± 0.9 (SEM) nmol O₂/min/mg protein, p = 0.0210], ATP production (FUL = 122.6 ± 18 vs. CTRL = 154.5 ± 14 μmol/100 μg protein, p = 0.0340), and higher oxygen consumption in state 4 [FUL = 12.56 ± 0.9 vs. CTRL = 8.26 ± 0.6], generation of reactive oxygen species (FUL 733.1 ± 169.32 vs. CTRL = 486.39 ± 73.1 nmol/100 μg protein, p = 0.0313) and reason ROS/ATP [FUL = 8.73 ± 2.3 vs. CTRL = 2.99 ± 0.3]. In conclusion, exposure to fullerene C60 impaired pulmonary mechanics and mitochondrial function, increased ROS concentration, and decrease ATP production.

Myoglobin-enhanced oxygen delivery to isolated cardiac mitochondria

The heart, red skeletal muscles and the nitrogen-fixing legume root nodule function in steady states of high oxygen influx, partial oxygenation of cytoplasmic myoglobin or leghemoglobin and correspondingly low oxygen partial pressure. Here, we ask: what conditions are required at the surface of actively respiring, state III, tightly coupled mitochondria to enhance oxygen flow to cytochrome oxidase? Pigeon heart mitochondria were isolated with minimal damage to the outer mitochondrial membrane and were incubated at low oxygen pressures, where respiration is oxygen limited, with solutions of each of six monomeric hemoglobins with widely divergent kinetics and equilibria in their reactions with oxygen: Busycon myoglobin, horse myoglobin, Lucina hemoglobins I and II, soybean leghemoglobin c and Gasterophilus hemoglobin. Each augments oxygen uptake. The declining fractional saturation of each hemoglobin with oxygen was monitored spectrophotometrically as mitochondrial respiration depleted the oxygen; the oxygen partial pressure at half-maximal rate of oxygen uptake was similar for each hemoglobin, supporting the conclusion that the hemoglobins did not interact with the mitochondrial surface in oxygen delivery. The oxygen pressure required to support state III mitochondrial oxygen uptake, 0.005 kPa (0.04 torr), is small compared with that obtained in the sarcoplasm and at the mitochondrial surface of the working heart, 0.32 kPa (2.4 torr). We conclude that, in normal steady states of contraction of the myoglobin-containing heart, oxygen utilization by mitochondrial cytochrome oxidase is not limited by oxygen availability.

P/O ratios of mitochondrial oxidative phosphorylation

Mitochondrial mechanistic P/O ratios are still in question. The major studies since 1937 are summarized and various systematic errors are discussed. Values of about 2.5 with NADH-linked substrates and 1.5 with succinate are consistent with most reports after apparent contradictions are explained. Variability of coupling may occur under some conditions but is generally not significant. The fractional values result from the coupling ratios of proton transport. An additional revision of P/O ratios may be required because of a report of the structure of ATP synthase (D. Stock, A.G.W. Leslie, J.E. Walker, Science 286 (1999) 1700-1705) which suggests that the H+/ATP ratio is 10/3, rather than 3, consistent with P/O ratios of 2.3 with NADH and 1.4 with succinate, values that are also possible.

Increased state 4 mitochondrial respiration and swelling in early post-ischemic reperfusion of rat heart

Isolated rat hearts were exposed to 30 min ischemia or to 30 min ischemia followed by 2, 5 or 40 min reperfusion and mitochondria were isolated at these different time points. ADP-stimulated, succinate-dependent respiration rate (state 3) was not significantly changed at the different time points examined. In contrast, state 4 (non-ADP-stimulated) respiration rate was significantly increased after 30 min ischemia, and it increased further during the first post-ischemic reperfusion period. Mitochondrial swelling, as evaluated under conditions of the major controlled ion channels (i.e. permeability transition pore and ATP-dependent mitochondrial K(+) channel) closed, significantly increased in parallel. It is suggested that the inner mitochondrial membrane permeability is increased under exposure of the heart to ischemia and early reperfusion, and that the phenomenon is reversible upon subsequent long periods of reperfusion.

Endotoxin-induced mitochondrial damage correlates with impaired respiratory activity

OBJECTIVE

This study was designed to determine whether mitochondrial function in a systemic organ is acutely impaired in a resuscitated model of sepsis (endotoxemia, lipopolysaccharide) and the relationship, if any, between this impairment and the extent of mitochondrial ultrastructural damage that occurs.

DESIGN

Perspective, controlled laboratory investigation.

SETTING

Animal laboratory in a university research institute.

SUBJECTS

Adult male cats.

INTERVENTIONS

A well-established feline model of acute endotoxemia was used wherein measures were taken to minimize tissue hypoxia. After lipopolysaccharide (3 mg/kg intravenously, n = 9) or isotonic saline vehicle (control, n = 5) administration, liver samples were obtained at 4 hrs posttreatment, and mitochondrial ultrastructure and respiratory function were assessed. Mitochondrial ultrastructural injury was graded on a scale of 0 (no injury) to 5 (severe injury), and mitochondrial respiration was evaluated by using standard techniques.

MEASUREMENTS AND MAIN RESULTS

Significant mitochondrial injury was apparent by 4 hrs, but only in the lipopolysaccharide-treated group (2.5 +/- 0.2 vs. 1.3 +/- 0.2, p <.001) and despite maintenance of tissue oxygen availability. In addition, lipopolysaccharide treatment reduced the rate of state 3 (adenosine 5'-diphosphate-dependent) respiration, especially at complex IV (40% inhibition), and increased the rate of state 4 (adenosine 5'-diphosphate-independent) respiration, reflecting partial uncoupling of mitochondrial oxidative phosphorylation. Finally, a significant correlation was demonstrated between the severity of ultrastructural injury and the magnitude of mitochondrial respiratory dysfunction after lipopolysaccharide treatment and despite resuscitation efforts.

CONCLUSION

Mitochondrial function is significantly impaired during acute sepsis, and this impairment is strongly associated with the extent of mitochondrial ultrastructural abnormalities present in the tissues. These findings in conjunction with those previously shown suggest that mitochondrial functional impairment may contribute to the pathogenesis of altered oxygen metabolism in systemic organs during sepsis.

Energetics of heart mitochondria during acute phase of Trypanosoma cruzi infection in rats

The energetics of heart mitochondria was studied in the acute phase of Trypanosoma cruzi infection in rats. Wistar rats were infected with 2 x 10(5) trypomastigote forms of the Y strain of T. cruzi, and heart mitochondria and submitochondrial particles isolated after 7 and 25 days of infection. Ultrastructure of mitochondria seemed to be preserved, but cytochrome c levels were significantly depressed. Respiratory control ratios (RCR) were decreased for glutamate and succinate oxidations, as a consequence of inhibition of respiration in state 3 and/or of stimulation of respiration in state 4. Stimulation of hydrolytic activity of FoF1-ATPase by energization of mitochondria was approx. 2-fold higher in relation to controls. Mitochondrial ATP concentration remained constant. In conclusion, during the acute phase of T. cruzi infection in rats there is an energy impairment at the level of heart mitochondria, but their ultrastructure and ATP concentration seem to be preserved; the maintenance of ATP may be due to an adaptative mechanism of the cell which includes inhibition of the hydrolytic activity of FoF1-ATPase.

Mechanistic stoichiometry of mitochondrial oxidative phosphorylation

P/O ratios of rat liver mitochondria were measured with particular attention to systematic errors. Corrections for energy loss during oxidative phosphorylation were made by measurement of respiration as a function of mitochondrial membrane potential. The corrected values were close to 1, 0.5, and 1 at the three coupling sites, respectively. These values are consistent with recent measurements of mitochondrial proton transport.

The effect of inorganic phosphate on chick (Gallus domesticus) heart mitochondrial volume and cation content

1. In the absence of exogenous Ca(II), Pi induces a swelling change that is kinetically first order with k = 1.08 +/- 0.1 min-1. The first-order rate constant is independent of [Pi] over the range of 0.5-45 mM. 2. In the presence of exogenous substrate, the volume change induced by Pi is monophasic and can be reversed by ADP. 3. The swelling process and the approach to steady state is accompanied by controlled losses of both K+ and Mg(II) from within the mitochondria. 4. The loss of K+ is biphasic as a function of time with ki = 14.1 +/- 1.6 and k2 = 4.4 +/- 0.34 nmol min-1 mg mitochondria-1. 5. The loss of Mg(II) is monophasic and the rate at which this cation is released decreases as a function of time. Ca(II) fluxes are not involved in the volume occurring secondary to Pi uptake. 6. In the absence of exogenous substrate, Pi induces a triphasic change in mitochondrial volume. 7. The sequence of volume changes corresponds to an initial first-order swelling secondary to the addition of Pi, a contraction apparently triggered by the loss of approximately 85% of total intra-mitochondrial Mg(II), and a second larger swelling phase that cannot be reversed with ADP. 8. The Pi-induced swelling of chick heart mitochondria is not inhibited by EGTA and does not depend on the provision of exogenous Ca(II). 9. The Ca(II) and Mg(II) ions released from within the mitochondria are responsible for activating divalent cation-dependent ATPases which cosediment with isolated chick heart mitochondria.

Respiration and mitochondrial ATPase in energized mitochondria during isoproterenol-induced cell injury of myocardium

1. Respiration of mitochondria, membrane potential and mitochondrial ATPase under energized conditions were studied in rat myocardium during cell injury induced by treatment with isoproterenol. 2. Increase in the state 4 rate of respiration and ADP:O ratio, as well as decrease in the state 3 rate and Respiratory Control Ratio (RCR) were found. 3. The optimum pH for RCR and for maximum ATPase activity was shifted to lower values. 4. The state 3 respiration was more sensitive to oligomycin inhibition. 5. The mitochondria showed lower ability to generate membrane potential. 6. An increase in the K0.5 values for catalytic sites II and III of mitochondrial ATPase at pH 7.4 and 5.5 was found. 7. These results are consistent with alterations on the integrity of mitochondrial membrane, and corroborate with the hypothesis of changes on the mitochondrial ATPase during isoproterenol-induced cell injury of myocardium.