The effect of high-intensity exhaustive exercise studied in isolated mitochondria from human skeletal muscle

Six young men performed five 1-min bicycle exercise bouts to exhaustion. Muscle lactate increased to congruent with 114 mmol x kg(-1) dwt and pH decreased to congruent with 6.6. Mitochondria were prepared from a needle biopsy sample taken from m. vastus lateralis immediately after the last exercise bout. No significant effect of exhaustion on the proton permeability and amount of cytochromes c and aa3 in isolated mitochondria was detected. The activities of the following enzymes and systems were not altered either: citrate synthase, succinate dehydrogenase, cytochrome oxidase, succinate + glutamate respiration, malate + glutamate respiration, the respiratory chain, and the reactions involved in ATP synthesis. Thus, the mitochondria did not appear globally altered upon exhaustion. However, the following NAD-linked activities were significantly lowered: pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, glutamate dehydrogenase and fatty acid beta-oxidation. The activities of alpha-glycerophosphate dehydrogenase and exo-NADH oxidase, enzymes that might catalyze the oxidation of sarcoplasmic NADH, were increased. These changes may be due to the action of reactive oxygen species, protons and Ca2+. Transient opening of the permeability transition pore may also be involved. Some effects may have been reversed during isolation of the mitochondria and the changes in mitochondrial function in situ upon exhaustion may have been more extensive than observed.

Aerobic metabolism of human quadriceps muscle: in vivo data parallel measurements on isolated mitochondria

The aim of the present study was to examine whether parameters of isolated mitochondria could account for the in vivo maximum oxygen uptake (VO2max) of human skeletal muscle. VO2max and work performance of the quadriceps muscle of six volunteers were measured in the knee extensor model (range 10-18 mmol O2 x min(-1) x kg(-1) at work rates of 22-32 W/kg). Mitochondria were isolated from the same muscle at rest. Strong correlations were obtained between VO2max and a number of mitochondrial parameters (mitochondrial protein, cytochrome aa3, citrate synthase, and respiratory activities). The activities of citrate synthase, succinate dehydrogenase, and pyruvate dehydrogenase, measured in isolated mitochondria, corresponded to, respectively, 15, 3, and 1.1 times the rates calculated from VO2max. The respiratory chain activity also appeared sufficient. Fully coupled in vitro respiration, which is limited by the rate of ATP synthesis, could account for, at most, 60% of the VO2max. This might be due to systematic errors or to loose coupling of the mitochondrial respiration under intense exercise.

Human quadriceps muscle mitochondria: a functional characterization

Human quadriceps mitochondria were isolated from ca. 80 mg tissue in ca. 45% yield. The preparation is described with respect to content of mitochondrial markers and nine different respiratory activities. The specific state 3 activities were high in comparison with literature data, indicating high integrity and purity of the preparation. Examples of state 3 rates, in micromol O min(-1) g protein(-1) (25 degrees C): pyruvate + malate, 400; succinate, 514; malate + glutamate, 444. The notion of high integrity was also supported by the reproducibility of the preparation and the magnitude of the respiratory control ratios and the P/O ratios. The mitochondria most likely had lost ca. 30% of their cytochrome c upon isolation, but it was substantiated that this loss had not influenced the state 3 rates. Functional assays of single reactions or groups of reactions could be based on respiration experiments. The respiratory chain activity, for instance, was measured as respiration of NADH in freeze-permeabilized mitochondria (1263 micromol O min(-1) g protein(-1)). Comparison of uncoupled rates of respiration and state 3 rates indicated that the ATP synthesis exerted major flux control over respiration of succinate + glutamate, malate + glutamate and pyruvate + malate. These reactions, showing very similar rates of ATP synthesis, could be used as a functional assay of ATP synthesis (1200 micromol ATP min(-1) g protein(-1)). Respiration of succinate, palmitoyl-carnitine + malate, or glutamate could not support the maximal rate of ATP synthesis and the upstream reactions probably exerted major flux control in these cases. The specific activities appeared very constant in this group of young men, only the respiratory activity with glutamate might show biological variation.

Human skeletal muscle mitochondrial capacity

Under aerobic work, the oxygen consumption and major ATP production occur in the mitochondria and it is therefore a relevant question whether the in vivo rates can be accounted for by mitochondrial capacities measured in vitro. Mitochondria were isolated from human quadriceps muscle biopsies in yields of approximately 45%. The tissue content of total creatine, mitochondrial protein and different cytochromes was estimated. A number of activities were measured in functional assays of the mitochondria: pyruvate, ketoglutarate, glutamate and succinate dehydrogenases, palmitoyl-carnitine respiration, cytochrome oxidase, the respiratory chain and the ATP synthesis. The activities involved in carbohydrate oxidation could account for in vivo oxygen uptakes of 15-16 mmol O2 min-1 kg-1 or slightly above the value measured at maximal work rates in the knee-extensor model of Saltin and co-workers, i.e. without limitation from the cardiac output. This probably indicates that the maximal oxygen consumption of the muscle is limited by the mitochondrial capacities. The in vitro activities of fatty acid oxidation corresponded to only 39% of those of carbohydrate oxidation. The maximal rate of free energy production from aerobic metabolism of glycogen was calculated from the mitochondrial activities and estimates of the DeltaG or ATP hydrolysis and the efficiency of the actin-myosin reaction. The resultant value was 20 W kg-1 or approximately 70% of the maximal in vivo work rates of which 10-20% probably are sustained by the anaerobic ATP production. The lack of aerobic in vitro ATP synthesis might reflect termination of some critical interplay between cytoplasm and mitochondria.

Optimization of preparation of mitochondria from 25-100 mg skeletal muscle

A method for isolation of mitochondria from 25-100 mg skeletal muscle is described. The instrumental developments include a refined homogenization setup, special handling techniques, and equipment for biopsy storage. The preparation medium was a standard ionic medium. All fractions were assayed for marker enzymes and the data used in optimization of the yield. It was observed that the homogenization procedure exerts strong control on the integrity of the isolated mitochondria. The method was developed with pigeon breast muscle as the model tissue and used virtually unaltered for preparation from muscles of pigs, rats, and humans. The relative yield was 40-50% and the mitochondria were well coupled and showed high rates of phosphorylating respiration.

Small scale preparation of skeletal muscle mitochondria, criteria of integrity, and assays with reference to tissue function

Mitochondria prepared in small scale from skeletal muscle were studied with respiration measurements and low temperature spectroscopy. The method of preparation was developed for 25-100 mg tissue with pigeon breast muscle as model organ. The yield was 40%. Data collected during the developmental work were used to evaluate criteria of mitochondrial quality. The cytochrome c conservation, i.e. cytochrome c per mitochondrial quantity in the preparation relative to that in the tissue, is a most useful test parameter. It is bounded between 0-100%. Proportionality between the state 3 rate and the cytochrome c conservation was not rejected by statistical tests. The respiratory control ratio (RCR) was also highly correlated to the cytochrome c conservation. These correlations might be extrapolated to 100% conservation to give hypothetical tissue values. The cause for the correlations is discussed. The P/O ratio showed only weak dependence on the cytochrome c conservation and the state 4 rate showed no dependence. Other, rather insensitive test parameters are also discussed. The pigeon breast muscle mitochondria isolated by the final method showed cytochrome c conservation of 73 +/- 9% (n = 16). They are compared with pig biceps femoris mitochondria prepared by the same method. The two types of mitochondria show many similarities. Some differences may be explained by a different amount of inner mitochondrial membrane relative to mitochondrial protein. The pig tissue contains ten times less mitochondrial protein than the pigeon tissue does.

Bioavailability of recombinant human growth hormone in different concentrations and formulations

Whether the bioavailability of growth hormone depends on the concentration or formulation of the preparation was evaluated in 18 growth hormone-deficient patients. The design was a single-blinded, randomized cross-over study, where the patients were given a single, fixed dose subcutaneous injection of growth homrone (3 IU/m2) of 3 different preparations: (1) 4 IU/ml in a bicarbonate buffer dissolved in 0.9% benzyl alcohol (approximately 1.37 mg/ml), (2) 5.9 IU/ml in a phosphate buffer dissolved in 1.5% benzyl alcohol (approximately 2 mg/ml) and (3) 11 7 IU/ml in a phosphate buffer dissolved in 1.5% benzyl alcohol (approximately 4 mg/ml). Conventional growth hormone-therapy was withdrawn 2 days before each study period. Blood samples were drawn over a 24-hr period and assessed for growth hormone, serum insulin-like growth factor I (IGF-I), insulin and glucose. The geometric mean values (+/- geometric S.D) of the relative absorption fractions were F5.9 IU/4 IU = AUC5.9 IU/AUC4 IU = (+/- 1.139) (P = 0.66), F11.7 IU/AUC4 IU = AUC11.7 IU/AUC4 IU (1.14 +/- 1.21) (P = 0.009) AND F11.7 IU/5.9 IU = AUC11.7 IU/AUC5.9 IU = 1.12 (+/- 1.17) (P = 0.005), respectively. The 90% confidence intervals were contained within the limits of 0.80-1.25 accepted for bioequivalence. Geometric mean values (+/- geometric S.D.) of the relative observed maximum concentration, Cmax was for Cmax 5.9 IU/Cmax 4 IU = 1.04 (+/- 1.19) (P = 0.32), Cmax 11.7 IU/Cmax 4 IU = 1.24 (+/- 1.21) (P = 0.0002) and Cmax 11.7 IU/Cmax 5.9 IU = 1.19 (+/- 1.29) (P = 0.012). The median and the range values for the observed time to reach Cmax was tmax 5.9 IU/tmax 4 IU = 0.63 (0.04-1.00), tmax 11.7 IU/tmax 4 IU = 0.59 (0.06-1.0) and tmax 11.7 IU/tmax 5.9 IU = 0.90 (0.51-18.00). There were no significant differences in IGF-I, glucose and insulin profiles. Based on the upper limits of the 90% confidence intervals for relative AUC's the conclusion is that the three different preparations were bioequivalent.

Characterization of mitochondria from pig muscle: higher activity of exo-NADH oxidase in animals suffering from malignant hyperthermia

Mitochondria were isolated from biopsies of the biceps femoris muscle of Danish landrace pigs. Three groups of animals were compared: (1) normal pigs; (2) pigs that were homozygous with respect to the gene Hal(n)/Hal(n) coding for the porcine malignant hyperthermia syndrome; and (3) heterozygote animals. A newly developed micro-method for preparation and assaying of small quantities of intact mitochondria was employed. With this technique mitochondria from biopsies weighing less than 100 mg were examined with respect to cytochrome content as well as phosphorylating and respiratory activities, including the nonphosphorylating exo-NADH oxidase activity. The mitochondria, prepared in a yield of 48%, showed high respiratory activities with tricarboxylic acid-cycle intermediates and pyruvate, and somewhat lower activity with palmitoyl-carnitine as substrate. The ATP synthase activity was about 1000 micromol ATP/min per g of protein and the maximal respiratory activity approx. 700 micromol of O2/min per g of protein. No differences among the three groups of animals were detected, except for the exo-NADH oxidase activities, which were 43, 78 and 107 micromol of O2/min per g of protein in the groups of normal, heterozygous and homozygous animals respectively. It is concluded that the exo-NADH oxidase activity may be a genetic manifestation of malignant hyperthermia and may play a significant role in the heat production characteristic of the syndrome.

Respiration measurements in small scale

The design and operation of a small (< 40 microliters) reaction vessel with membrane-covered oxygen electrode is described. The equipment's productivity and experimental versatility is similar to that of equipment using 100 times more biological material. Control experiments showed that respiration experiments are made with virtually no systematic errors and random errors of less than 2%.

Ubiquinone reduction pattern in pigeon heart mitochondria. Identification of three distinct ubiquinone pools

Intact pigeon heart mitochondria showed 10-30% ubiquinone reduction in the absence of substrates. This reduction could not be ascribed to endogenous substrates, as judged by lack of effect of inhibitors and uncouplers and by the very low endogenous respiratory rate. Addition of NADH in the presence of antimycin caused further reduction of about 10% ubiquinone, apparently coupled to the rotenone- and antimycin-sensitive exo-NADH oxidase system [Rasmussen (1969) FEBS Lett. 2, 157-162]. Citric acid cycle substrates reduced most of the remaining ubiquinone in the presence of antimycin; 15-20% of the total ubiquinone content was still in the oxidized form under the most reducing conditions. Three pools of ubiquinone therefore appeared to be present in heart mitochondria: a metabolically inactive pool consisting of reduced as well as oxidized ubiquinone, a pool coupled to oxidation of added (cytoplasmic) NADH, and the well-known pool coupled to citric acid cycle oxidations. Ferricyanide selectively oxidized the ubiquinol reduced by added NADH, indicating that this pool is situated on the outer surface of the mitochondrial inner membrane. Ubiquinone reduction levels were determined with a new method, which is described in detail.

The NADH oxidase system (external) of muscle mitochondria and its role in the oxidation of cytoplasmic NADH

An exo-NADH oxidase system [NADH oxidase system (external)], effecting intact-mitochondrial oxidation of added NADH, was studied in pigeon heart mitochondria. Breast muscle mitochondria showed an equal specific activity of the system. The exo-NADH oxidase activity (200 micron mol of NADH/min per g of protein) equalled two-thirds of the State-3 respiratory activity with malate + pyruvate or one-seventh of the total NADH oxidase activity of heart mitochondria. The activity was not caused by use of proteinase in the preparation procedure and all measured parameters were very reproducible from preparation to preparation. The activity is therefore most likely not due to preparation artefacts. The exo-NADH oxidase system is present in all mitochondria in the preparation and is not confined to a subpopulation. The system reduced all cytochrome anaerobically and direct interaction with all cytochrome oxidase was demonstrated by interdependent cyanide inhibition. The exo-NADH oxidase system seems to be located at the outer surface of the mitochondrial inner membrane because, for instance, only this system was rapidly inhibited by rotenone, and ferricyanide could act as acceptor in the rotenone-inhibited system (reductase activity = 20 times oxidase activity). In the presence of antimycin, added NADH reduced only a part of the b-cytochromes. Freezing and thawing the mitochondria, one of the methods used for making them permeable to NADH, destroyed this functional compartmentation. The characteristics of the exo-NADH oxidase system and the malate-aspartate shuttle are compared and the evidence for the shuttle's function in heart in vivo is re-evaluated. It is proposed that oxidation of cytoplasmic NADH in red muscles primarily is effected by the exo-NADH oxidase system.

Three functionally different cytochrome b redox centres in pigeon heart mitochondria

Three functionally different cytochrome b redox centres, apparently of high metabolic activity, were detected in intact pigeon heart mitochondria; cytochrome b(1), b(m) and b(h), with maxima of absorption at 556.6 (State 5), 560.6, and 564.5 nm, respectively (alpha-bands, 77K). 2. Cytochrome (b(l) was reduced in the presence of either antimycin or HQNO (2-heptyl-4-hydroxyquinoline N-oxide). The absorption maximum was shifted by dithionite, cyanide, NNN'N'-tetramethyl-p-phenylenediamine + ascorbate, HQNO and antimycin. The spectra obtained on simultaneous or successive addition of HQNO and antimycin favoured the assumption of a common binding site for the two inhibitors. 3. Cytochrome b(m) was reduced in the presence of HQNO, but not in the presence of antimycin. No shifts of absorption maximum was observed. 4. Cytochrome b(h) was reduced in the presence of antimycin. HQNO was unable to cause reduction of this cytochrome by endogenous substrates. The absorption maximum was shifted to lower wavelength by organic solvents. It was inseparable from that of cytochrome b(m) in the presence of 0.4% ethanol. 5. The pattern of reduction in the presence of HQNO or antimycin demonstrates the functional difference of the three redox centres and appears incompatible wih a linear respiratory chain.