Altered mitochondrial sensitivity for ADP and maintenance of creatine-stimulated respiration in oxidative striated muscles from VDAC1-deficient mice

Voltage-dependent anion channels (VDACs) form the main pathway for metabolites across the mitochondrial outer membrane. The mouse vdac1 gene has been disrupted by gene targeting, and the resulting mutant mice have been examined for defects in muscle physiology. To test the hypothesis that VDAC1 constitutes a pathway for ADP translocation into mitochondria, the apparent mitochondrial sensitivity for ADP (Km(ADP)) and the calculated rate of respiration in the presence of the maximal ADP concentration (Vmax) have been assessed using skinned fibers prepared from two oxidative muscles (ventricle and soleus) and a glycolytic muscle (gastrocnemius) in control and vdac1(-/-) mice. We observed a significant increase in the apparent Km((ADP)) in heart and gastrocnemius, whereas the V(max) remained unchanged in both muscles. In contrast, a significant decrease in both the apparent Km((ADP)) and V(max) was observed in soleus. To test whether VDAC1 is required for creatine stimulation of mitochondrial respiration in oxidative muscles, the apparent Km((ADP)) and Vmax were determined in the presence of 25 mm creatine. The creatine effect on mitochondrial respiration was unchanged in both heart and soleus. These data, together with the significant increase in citrate synthase activity in heart, but not in soleus and gastrocnemius, suggest that distinct metabolic responses to altered mitochondrial outer membrane permeability occur in these different striated muscle types.     

The quantitation of ADP diffusion gradients across the outer membrane of heart mitochondria in the presence of macromolecules

We have previously provided evidence that diffusion of metabolites across the porin pores of mitochondrial outer membrane is hindered. A functional consequence of this diffusion limitation is the dynamic compartmentation of ADP in the intermembrane space. These earlier studies were done on isolated mitochondria suspended in isotonic media without macromolecules, in which intermembrane space of mitochondria is enlarged. The present study was undertaken to assess the diffusion limitation of outer membrane in the presence of 10% (w/v) dextran M20, in order to mimic the action of cytosolic macromolecules on mitochondria. Under these conditions, mitochondria have a more native, condensed configuration.Flux-dependent concentration gradients of ADP were estimated by measuring the ADP diffusion fluxes across the porin pores of isolated rat heart mitochondria incubated together with pyruvate kinase (PK), both of which compete for ADP regenerated by mitochondrial creatine kinase (mtCK) within the intermembrane space or by yeast hexokinase (HK) extramitochondrially. From diffusion fluxes and bulk phase concentrations of ADP, its concentrations in the intermembrane space were calculated using Fick's law of diffusion. Flux-dependent gradients up to 23 microM ADP (for a diffusion rate of J(Dif)=1.9 micromol ADP/min/mg mitochondrial protein) were observed. These gradients are about twice those estimated in the absence of dextran and in the same order of magnitude as the cytosolic ADP concentration (30 microM), but they are negligibly low for cytosolic ATP (5 mM). Therefore, it is concluded that the dynamic ADP compartmentation is of biological importance for intact heart cells.If mtCK generates ADP within the intermembrane space, the local ADP concentration can be clearly higher than in the cytosol resulting in higher extramitochondrial phosphorylation potentials. In this way, mtCK contributes to ensure optimal kinetic conditions for ATP-splitting reactions in the extramitochondrial compartment.     

Accurate determination of the oxidative phosphorylation affinity for ADP in isolated mitochondria

Background

Mitochondrial dysfunctions appear strongly implicated in a wide range of pathologies. Therefore, there is a growing need in the determination of the normal and pathological integrated response of oxidative phosphorylation to cellular ATP demand. The present study intends to address this issue by providing a method to investigate mitochondrial oxidative phosphorylation affinity for ADP in isolated mitochondria.

Methodology/Principal Findings

The proposed method is based on the simultaneous monitoring of substrate oxidation (determined polarographically) and phosphorylation (determined using the glucose - hexokinase - glucose-6-phosphate dehydrogenase - NADP⁺ enzymatic system) rates, coupled to the determination of actual ADP and ATP concentrations by bioluminescent assay. This enzymatic system allows the study of oxidative phosphorylation during true steady states in a wide range of ADP concentrations. We demonstrate how the application of this method allows an accurate determination of mitochondrial affinity for ADP from both oxidation (K_mVox) and phosphorylation (K_mVp) rates. We also demonstrate that determination of K_mVox leads to an important overestimation of the mitochondrial affinity for ADP, indicating that mitochondrial affinity for ADP should be determined using phosphorylation rate. Finally, we show how this method allows the direct and precise determination of the mitochondrial coupling efficiency. Data obtained from rat skeletal muscle and liver mitochondria illustrate the discriminating capabilities of this method.

Conclusions/Significance

Because the proposed method allows the accurate determination of mitochondrial oxidative phosphorylation affinity for ADP in isolated mitochondria, it also opens the route to a better understanding of functional consequences of mitochondrial adaptations/dysfunctions arising in various physiological/pathophysiological conditions.     

Spatially distinct mitochondrial populations exhibit different mitofilin levels

Subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria exhibit unique biochemical and functional properties; however, their association with structural membrane proteins that control mitochondrial morphology and functionality in striated muscle tissue was never reported. In IMF and SS mitochondria isolated from rat heart and gastrocnemius muscle, we analysed the expression levels of mitofilin, a mitochondria-associated protein involved in organelle structure maintenance. The statistically significant higher amounts of mitofilin detected in IMF compared with SS mitochondria, 37-fold in cardiac tissue and 3.8-fold in gastrocnemius, together with the specific energetic requirements of these mitochondrial populations highlight the importance of mitofilin in oxidative phosphorylation functionality and in mitochondrial plasticity in striated muscle. The differential expression levels of mitofilin between IMF and SS also suggest that this protein can be used as a specific molecular marker to comparatively discriminate spatially distant mitochondrial populations.     

Developmental changes in regulation of mitochondrial respiration by ADP and creatine in rat heart in vivo

In saponin-skinned muscle fibers from adult rat heart and m. soleus the apparent affinity of the mitochondrial oxidative phosphorylation system for ADP (K_m = 200-400 M) is much lower than in isolated mitochondria (K_m = 10-20 M). This suggests a limited permeability of the outer mitochondrial membrane (OMM) to adenine nucleotides in slow-twitch muscle cells. We have studied the postnatal changes in the affinity of mitochondrial respiration for ADP, in relation to morphological alterations and expression of mitochondrial creatine kinase (mi-CK) in rat heart in vivo. Analysis of respiration of skinned fibers revealed a gradual decrease in the apparent affinity of mitochondria to ADP throughout 6 weeks post partum that indicates the development of mechanism which increasingly limits the access of ADP to mitochondria. The expression of mi-CK started between the 1st and 2nd weeks and reached the adult levels after 6 weeks. This process was associated with increases in creatine-activated respiration and affinity of oxidative phosphorylation to ADP thus reflecting the progressive coupling of mi-CK to adenine nucleotide translocase. Laser confocal microscopy revealed significant changes in rearrangement of mitochondria in cardiac cells: while the mitochondria of variable shape and size appeared to be random-clustered in the cardiomyocytes of 1 day old rat, they formed a fine network between the myofibrils by the age of 3 weeks. These results allow to conclude that in early period of development, i.e. within 2-3 weeks, the diffusion of ADP to mitochondria becomes progressively restricted, that appears to be related to significant structural rearrangements such as formation of the mitochondrial network. Later (after 3 weeks) the control shifts to mi-CK, which by coupling to adenine nucleotide translocase, allows to maximally activate the processes of oxidative phosphorylation despite limited access of ADP through the OMM.     

Joint participation of mitochondria and sarcoplasmic reticulum in the formation of tubular aggregates in gastrocnemius muscle of CK-/- mice

Tubular aggregates are specific subcellular structures that appear in skeletal muscle fibres under different pathological conditions. The origin of the tubular aggregates is generally ascribed to proliferating membranes of sarcoplasmic reticulum. There are, however, histochemical indications for the presence of mitochondrial enzymes in tubular aggregates suggesting contribution of mitochondria to the genesis of tubular aggregates. In this study we used an immunocytochemical detection technique to assess participation of mitochondria and of sarcoplasmic reticulum in derivation of tubular aggregates. The fast skeletal muscle fibres (m. gastrocnemius) of mice bearing the double invalidation for both the mitochondrial and the cytosolic isoforms of creatine kinase (CK), an enzyme involved in energetics of muscle cells, were employed as a model muscle with tubular aggregates (Steeghs et al., Cell 89, 93-103, 1997). Immunogold labelling of the bc1 complex, a specific integral protein of the inner mitochondrial membrane, provided strong signals in both the mitochondria and tubular aggregates but not in other ultrastructural components of muscle fibres. A similar strong immunogold signal was obtained when labelling for SERCA1, a specific enzyme of the sarcoplasmic reticulum membrane, in regions of typical occurrence of the sarcoplasmic reticulum and in tubular aggregates. In double labelling experiments, we found simultaneous labelling of tubular aggregates with both the bc1 and SERCA1 antibodies. It is concluded, that in CK-/- mouse both the inner mitochondrial membrane and the membrane of the sarcoplasmic reticulum participate in the formation of tubular aggregates.     

Lack of myostatin alters intermyofibrillar mitochondria activity, unbalances redox status, and impairs tolerance to chronic repetitive contractions in muscle

Loss of myostatin (mstn) function leads to a decrease in mitochondrial content, a reduced expression of cytochrome c oxidase, and a lower citrate synthase activity in skeletal muscle. These data suggest functional or ultrastructural mitochondrial abnormalities that can impact on muscle endurance characteristics in such phenotype. To address this issue, we investigated subsarcolemmal and intermyofibrillar (IMF) mitochondrial activities, skeletal muscle redox homeostasis, and muscle fiber endurance quality in mstn-deficient mice [mstn knockout (KO)]. We report that lack of mstn induced a decrease in the coupling of IMF mitochondria respiration, with significantly higher basal oxygen consumption. No lysis of mitochondrial cristae or excessive swelling were observed in mstn KO mice compared with wild-type (WT) mice. Concerning redox status, mstn KO gastrocnemius exhibited a significant decrease in lipid peroxidation levels (-56%; P < 0.01 vs. WT) together with a significant upregulation of the antioxidant glutathione system. In contrast, superoxide dismutase and catalase activities were altered in mstn KO, gastrocnemius and soleus with a reduction of up to 80% compared with WT animals. The force production observed after contractile endurance test was significantly lower in extensor digitorum longus and soleus muscles of mstn KO mice compared with the controls (17 ± 3 and 36 ± 5% vs. 28 ± 4 and 56 ± 5%, respectively, P < 0.05). Together, these findings indicate that, besides an increased skeletal muscle mass, genetic mstn inhibition has differential effects on redox homeostasis and mitochondrial function that would have functional consequences on muscle response to endurance exercise.     

Age-dependent regulation of skeletal muscle mitochondria by the thrombospondin-1 receptor CD47

CD47, a receptor for thrombospondin-1, limits two important regulatory axes: nitric oxide-cGMP signaling and cAMP signaling, both of which can promote mitochondrial biogenesis. Electron microscopy revealed increased mitochondrial densities in skeletal muscle from both CD47 null and thrombospondin-1 null mice. We further assessed the mitochondria status of CD47-null vs WT mice. Quantitative RT-PCR of RNA extracted from tissues of 3 month old mice revealed dramatically elevated expression of mRNAs encoding mitochondrial proteins and PGC-1α in both fast and slow-twitch skeletal muscle from CD47-null mice, but modest to no elevation in other tissues. These observations were confirmed by Western blotting of mitochondrial proteins. Relative amounts of electron transport enzymes and ATP/O₂ ratios of isolated mitochondria were not different between mitochondria from CD47-null and WT cells. Young CD47-null mice displayed enhanced treadmill endurance relative to WTs and CD47-null gastrocnemius had undergone fiber type switching to a slow-twitch pattern of myoglobin and myosin heavy chain expression. In 12 month old mice, both skeletal muscle mitochondrial volume density and endurance had decreased to wild type levels. Expression of myosin heavy chain isoforms and myoglobin also reverted to a fast twitch pattern in gastrocnemius. Both CD47 and TSP1 null mice are leaner than WTs, use less oxygen and produce less heat than WT mice. CD47-null cells produce substantially less reactive oxygen species than WT cells. These data indicate that loss of signaling from the TSP1-CD47 system promotes accumulation of normally functioning mitochondria in a tissue-specific and age-dependent fashion leading to enhanced physical performance, lower reactive oxygen species production and more efficient metabolism.     

Computational model of excitable cell indicates ATP free energy dynamics in response to calcium oscillations are undampened by cytosolic ATP buffers

Mitochondria in excitable cells are recurrently exposed to pulsatile calcium gradients that activate cell function. Rapid calcium uptake by the mitochondria has previously been shown to cause uncoupling of oxidative phosphorylation. To test (i) if periodic nerve firing may cause oscillation of the cytosolic thermodynamic potential of ATP hydrolysis and (ii) if cytosolic adenylate (AK) and creatine kinase (CK) ATP buffering reactions dampen such oscillations, a lumped kinetic model of an excitable cell capturing major aspects of the physiology has been developed. Activation of ATP metabolism by low-frequency calcium pulses caused large oscillation of the cytosolic, but not mitochondrial ATP/ADP, ratio. This outcome was independent of net ATP synthesis or hydrolysis during mitochondrial calcium uptake. The AK/CK ATP buffering reactions dampened the amplitude and rate of cytosolic ATP/ADP changes on a timescale of seconds, but not milliseconds. These model predictions suggest that alternative sources of capacitance in neurons and striated muscles should be considered to protect ATP-free energy-driven cell functions.     

Adenine nucleotides in foetal rat liver cells. Compartmentation and variation with age.

The digitonin method for the separation of cytosolic and mitochondrial fractions was applied to liver cells isolated from foetal rats. The cytosolic [ATP]/[ADP] ratio approximately doubles during the last 4 days of gestation, whereas the mitochondrial ratio remains constant. In the presence of oligomycin and added glucose, the cytosolic [ATP]/[ADP] ratio does not increase with age, but is still considerably higher than the mitochondrial ratio. Without added glucose, and when the glycogen content of foetal liver is still very low (more than 3 days before birth), the cytosolic [ATP]/[ADP] ratio in the presence of oligomycin becomes very low and equal to the mitochondrial ratio. It is concluded that the increasein the cytosolic [ATP]/[ADP] ratio during the last 4 days of gestation is solely due to enhanced mitochondrial activity in this period. Atractyloside and bongkrekic acid do not influence the O2 consumption, nor the [ATP]/[ADP] ratios in either compartment of foetal liver cells. Respiration of isolated foetal mitochondria, however, is strongly inhibited by both compounds. The implications of these findings are discussed.     

Subcellular distribution and kinetic properties of cytosolic and non-cytosolic hexokinases in maize seedling roots: implications for hexose phosphorylation.

Hexose phosphorylation by hexokinases plays an important role in glycolysis, biosynthesis and control of sugar-modulated genes. Several cytosolic hexokinase and fructokinase isoforms have been characterized and organelle-bound hexokinases have also been detected in higher plants. In this study a hexokinase activity is described that is inhibited by ADP (K(i)=30 microM) and mannoheptulose (K(i) congruent with 300 microM) in non-cytosolic fractions (mitochondria, Golgi apparatus and microsomes) obtained from preparations of seedling roots of maize (Zea mays L.). The catalytic efficiency (Vmax/Km) for both ATP and glucose in all non-cytosolic hexokinase fractions is more than one order of magnitude higher than that of cytosolic hexokinase and fructokinases. Low (30%) or no ADP and mannoheptulose inhibition is observed with hexokinase and fructokinase activities derived from the cytosolic compartment obtained after ion exchange and affinity chromatography. The soluble fructokinase (FK) shows fructose cooperativity (Hill n>2). The Vmax/Km ratio is about 3-fold higher for ATP than for other NTPs and no difference for hexose phosphorylation efficiencies is found between cytosolic hexokinase and fructokinase isoforms (FK1, FK2) with ATP as substrate. The K(i) for fructose inhibition is 2 mM for FK1 and 25 mM for FK2. The data indicate that low energy-charge and glucose analogues preferentially inhibit the membrane-bound hexokinases possibly involved in sugar-sensing, but not the cytosolic hexokinases and fructokinases.     

Lack of dystrophin is associated with altered integration of the mitochondria and ATPases in slow-twitch muscle cells of MDX mice

The potential role of dystrophin-mediated control of systems integrating mitochondria with ATPases was assessed in muscle cells. Mitochondrial distribution and function in skinned cardiac and skeletal muscle fibers from dystrophin-deficient (MDX) and wild-type mice were compared. Laser confocal microscopy revealed disorganized mitochondrial arrays in m. gastrocnemius in MDX mice, whereas the other muscles appeared normal in this group. Irrespective of muscle type, the absence of dystrophin had no effect on the maximal capacity of oxidative phosphorylation, nor on coupling between oxidation and phosphorylation. However, in the myocardium and m. soleus, the coupling of mitochondrial creatine kinase to adenine nucleotide translocase was attenuated as evidenced by the decreased effect of creatine on the Km for ADP in the reactions of oxidative phosphorylation. In m. soleus, a low Km for ADP compared to the wild-type counterpart was found, which implies increased permeability for that nucleotide across the mitochondrial outer membrane. In normal cardiac fibers 35% of the ADP flux generated by ATPases was not accessible to the external pyruvate kinase-phosphoenolpyruvate system, which suggests the compartmentalized (direct) channeling of that fraction of ADP to mitochondria. Compared to control, the direct ADP transfer was increased in MDX ventricles. In conclusion, our data indicate that in slow-twitch muscle cells, the absence of dystrophin is associated with the rearrangement of the intracellular energy and feedback signal transfer systems between mitochondria and ATPases. As the mechanisms mediated by creatine kinases become ineffective, the role of diffusion of adenine nucleotides increases due to the higher permeability of the mitochondrial outer membrane for ADP and enhanced compartmentalization of ADP flux.     

Desmin cytoskeleton linked to muscle mitochondrial distribution and respiratory function

Ultrastructural studies have previously suggested potential association of intermediate filaments (IFs) with mitochondria. Thus, we have investigated mitochondrial distribution and function in muscle lacking the IF protein desmin. Immunostaining of skeletal muscle tissue sections, as well as histochemical staining for the mitochondrial marker enzymes cytochrome C oxidase and succinate dehydrogenase, demonstrate abnormal accumulation of subsarcolemmal clumps of mitochondria in predominantly slow twitch skeletal muscle of desmin-null mice. Ultrastructural observation of desmin-null cardiac muscle demonstrates in addition to clumping, extensive mitochondrial proliferation in a significant fraction of the myocytes, particularly after work overload. These alterations are frequently associated with swelling and degeneration of the mitochondrial matrix. Mitochondrial abnormalities can be detected very early, before other structural defects become obvious. To investigate related changes in mitochondrial function, we have analyzed ADP-stimulated respiration of isolated muscle mitochondria, and ADP-stimulated mitochondrial respiration in situ using saponin skinned muscle fibers. The in vitro maximal rates of respiration in isolated cardiac mitochondria from desmin-null and wild-type mice were similar. However, mitochondrial respiration in situ is significantly altered in desmin-null muscle. Both the maximal rate of ADP-stimulated oxygen consumption and the dissociation constant (K(m)) for ADP are significantly reduced in desmin-null cardiac and soleus muscle compared with controls. Respiratory parameters for desmin-null fast twitch gastrocnemius muscle were unaffected. Additionally, respiratory measurements in the presence of creatine indicate that coupling of creatine kinase and the adenine translocator is lost in desmin-null soleus muscle. This coupling is unaffected in cardiac muscle from desmin-null animals. All of these studies indicate that desmin IFs play a significant role in mitochondrial positioning and respiratory function in cardiac and skeletal muscle.     

Adenine nucleotide compartmentation in foetal rat hepatocytes. Effects of atractyloside, oligomycin, calcium ionophore and adrenergic agonists

In the presence of lactate plus pyruvate, or glucose or alanine as substrates, ATP/ADP ratios in the cytosol were higher than in mitochondria in isolated rat foetal hepatocytes. The cytosolic ATP/ADP ratios were dependent on substrate (lactate + pyruvate greater than glucose greater than alanine). Oleate increased the cytosolic ATP/ADP ratios in the presence of the other substrates studied. Atractyloside decreased the cytosolic ATP/ADP ratios, oligomycin decreasing these values in both compartments. Isoproterenol, phenylephrine and Ca2+ ionophore decreased the cytosolic ATP/ADP ratio, without altering this value in mitochondria.     

What controls the outer mitochondrial membrane permeability for ADP: facts for and against the role of oncotic pressure

In our study 10% of bovine serum albumin was added to the physiological incubation medium to mimic the oncotic pressure of the cellular cytoplasm and to test for its effect on the respiration of isolated rat heart mitochondria, saponin- or saponin plus crude collagenase (type IV)-treated heart muscle fibers and saponin-treated rat quadriceps muscle fibers. Pyruvate and malate were used as substrates. We found that albumin slightly decreased the maximal ADP-stimulated respiration rate only for saponin-treated heart muscle fibers. The apparent Km ADP of oxidative phosphorylation increased significantly, by 70-100%, for isolated heart mitochondria, saponin plus collagenase-treated heart muscle fibers and for saponin-treated quadriceps muscle fibers but remained unchanged for saponin-treated heart muscle fibers. The saponin-treated heart muscle fibers were characterized by a very high control apparent Km ADP value (234+/-24 microM ADP) compared with other preparations (14-28 microM ADP). The results suggest that in vivo the oncotic pressure is not the relevant factor causing the low outer mitochondrial membrane permeability for ADP in cardiomyocytes, in contrast to quadriceps muscle cells. It is likely that the outer mitochondrial membrane-bound protein(s) which is supposed to remain in saponin-treated heart muscle fibers is responsible for this property of the membrane.     

Rapid thyroid-hormone effect on mitochondrial and cytosolic ATP/ADP ratios in the intact liver cell.

The effect of thyroid-hormone application on cytosolic and mitochondrial ATP/ADP ratio was investigated in rat liver in vivo and in the isolated perfused organ. In vivo the ATP/ADP ratio in livers from hypothyroid rats was 0.84 +/- 0.08 in the mitochondrial matrix and 5.6 +/- 0.9 in the cytosol, as was observed in euthyroid controls. In contrast, hyperthyroidism was followed by a significant decrease in the mitochondrial and by an increase in the cytosolic ATP/ADP ratio (to 0.34 +/- 0.06 and 11.3 +/- 2.8 respectively). In the perfused liver from hypothyroid animals, addition of L-3,3',5-tri-iodothyronine in the perfusate also provoked, within 2 h, a significant decrease in the mitochondrial ATP/ADP ratio, whereas the cytosolic ratio was unaffected. From these and previous data in the isolated perfused liver and in isolated mitochondria from hypothyroid and tri-iodothyronine-treated rats it is concluded that thyroid hormones increase mitochondrial respiration and ATP regeneration, which is associated with an acceleration of mitochondrial adenine nucleotide transport and significant alterations in the mitochondrial and cytosolic ATP/ADP ratios.     

Increase in the adenine nucleotide translocase content of duckling subsarcolemmal mitochondria during cold acclimation

Intermyofibrillar and subsarcolemmal mitochondria were isolated from duckling gastrocnemius muscle. The adenine nucleotide translocase (ANT) content of subsarcolemmal mitochondria was found to be half of that present in intermyofibrillar mitochondria. In addition, cold acclimation resulted in a 1.7-fold increase in subsarcolemmal mitochondrial ANT content, with intermyofibrillar mitochondrial ANT remaining constant. This change in mitochondrial ANT content correlates with the previously reported cold-induced change in the sensitivity of mitochondria to palmitate-inhibited ATP synthesis [Roussel et al. (1998) FEBS Lett. 439, 258-262]. It is suggested that the mitochondrial ANT content enhances or reduces the fatty acid uncoupling activity in tissue, depending on the energetic state of mitochondria.     

Seasonal cycles of mitochondrial ADP sensitivity and oxidative capacities in trout oxidative muscle

Mitochondria from red myotomal muscle of rainbow trout, Oncorhynchus mykiss, showed seasonal cycles of their maximal rates of substrate oxidation (nmol · min⁻¹ mg⁻¹ mitochondrial protein) and their apparent ADP affinity (Km_app), as well as in the thermal sensitivity of these properties. Increases in the maximal capacity of pyruvate oxidation were sufficient to compensate for seasonal changes in temperature, except during the winter months when rates at habitat temperature were depressed relative to other periods. The ADP affinity of isolated mitochondria was highest during cold months. Thus, the Km_app for ADP at habitat temperature showed less seasonal variation than the ADP Km_app at a given temperature. A loss in ADP affinity with decreasing temperature occurred through much of the year, and only was definitively suppressed in December and July. Both the ADP affinity and the maximal oxidative capacities of muscle mitochondria seem to be regulated parameters. Accepted: 10 June 1999     

Influence of mitochondrial content on the sensitivity of respiratory control

This study evaluated the sensitivity of mitochondrial respiratory control as a function of tissue oxidative capacity. The mitochondrial content of rat skeletal muscle was increased by exercise training or decreased by hypothyroidism. Muscles of the lower hindlimb were stimulated to tetanically contract in situ for 3 min at one of four frequencies to elicit a 30-fold range of oxygen consumption rates. Freeze-clamped sections of fast-twitch red gastrocnemius muscle were extracted and analyzed for metabolite levels. The sensitivity of respiratory control was examined for three models of cytosolic respiratory control (ADPf, ATP/ADPf, and ATP/(ADPf X Pi]; for each proposed model, sensitivity went up as mitochondrial content increased. Thus, a smaller change in cytosolic modulator (e.g., ADPf) is required as oxidative capacity increases. Increases in the sensitivity of cytosolic respiratory control resulted in lower flux through the near-equilibrium energy exchange reactions of creatine kinase and myokinase such that calculated free concentrations of ADP and AMP were less. Other energetically important reactions/pathways were also affected. Accumulation of lactate and the deamination of AMP to IMP were lower in tissues with higher mitochondrial content. In summary, changes in oxidative capacity directly influence the sensitivity of cytosolic respiratory control and this, in turn, has important consequences for maintenance of cellular energy balance.