Abstract: The alterations in muscle metabolism were investigated in response to repeated sessions of heavy intermittent exercise performed over 16 h. Tissue samples were extracted from the vastus lateralis muscle before (B) and after (A) 6 min of cycling at approximately 91% peak aerobic power at repetitions one (R1), two (R2), nine (R9), and sixteen (R16) in 13 untrained volunteers (peak aerobic power = 44.3 [+ or -] 0.66 mL x [kg.sup.-1] x [min.sup.-1], mean [+ or -] SE). Metabolite content (mmol x [(kg dry mass).sup.-1]) in homogenates at R1 indicated decreases (p < 0.05) in ATP (21.9 [+ or -] 0.62 vs. 17.7 [+ or -] 0.68) and phosphocreatine (80.3 [+ or -] 2.0 vs. 8.56 [+ or -] 1.5) and increases (p < 0.05) in inosine monophosphate (IMP, 0.077 [+ or -] 0.12 vs. 3.63 [+ or -] 0.85) and lactate (3.80 [+ or -] 0.57 vs. 84.6 [+ or -] 10.3). The content ([micro]mol x [(kg dry mass).sup.-1]) of calculated free ADP ([[ADP.sub.f]], 86.4 [+ or -] 5.5 vs. 1014 [+ or -] 237) and free AMP ([[AMP.sub.f]], 0.32 [+ or -] 0.03 vs. 78.4 [+ or -] 31) also increased (p < 0.05). No differences were observed between R1 and R2. By R9 and continuing to R16, pronounced reductions (p < 0.05) at A were observed in IMP (72.2%), [[ADP.sub.f]] (58.7%), [[AMP.sub.f]] (85.5%), and lactate (41.3%). The 16-hour protocol resulted in an 89.7% depletion (p < 0.05) of muscle glycogen. Repetition-dependent increases were also observed in oxygen consumption during exercise. It is concluded that repetitive heavy exercise results in less of a disturbance in phosphorylation potential, possibly as a result of increased mitochondrial respiration during the rest-to-work non-steady-state transition.
Key words: metabolism, adaptation, repetitive activity, oxidative phosphorylation, mitochondrial sensitivity.
Resume: On a examine les modifications du metabolisme musculaire en reponse a des seances repetees d'un exercice intermittent intense effectue sur une periode de 16 h. On a preleve des echantillons de tissus dans le muscle vaste externe du membre inferieur avant (B) et apres (A) un effort sur bicyclette de 6 min, a approximativement 91 % de la capacite aerobique maximale aux repetitions 1 (R1), 2 (R2), 9 (R9) et 16 (R16) chez 13 volontaires non entraines (la capacite aerobique maximale = 44,3 [+ or -] 0,66 mL x [kg.sup.-1] x [min.sup.-1], (moyenne [+ or -] e.t). La concentration de metabolites (mmol x [kg.sup.-1] p.s.) dans les homogenats a R1 a indique des diminutions (p < 0,05) d'ATP (21,9 [+ or -] 0,62 vs. 17,7 [+ or -] 0,68) et de phosphocreatine (80,3 [+ or -] 2,0 vs. 8,56 [+ or -] 1,5), et des augmentations (p < 0,05) d'inosine monophosphate (IMP, 0,077 [+ or -] 0,12 vs. 3,63 [+ or -] 0,85) et de lactate (3,80 [+ or -] 0,57 vs. 84,6 [+ or -] 10,3). Les concentrations (cmol x [kg.sup.-1] p.s.) d'ADP libre ([[ADP.sub.f]], 86,4 [+ or -] 5,5 vs. 1014 [+ or -] 237) et d'AMP libre ([[AMP.sub.f]], 0,32 [+ or -] 0,03 vs. 78,4 [+ or -] 31) calculees ont aussi augmente (p < 0,05). Il n'y a eu aucune difference entre R1 et R2. De R9 a R16, des diminutions marquees (p < 0,05) d'IMP (72,2 %), d'[[ADP.sub.f]] (58,7 %), d'[[AMP.sub.f]] (85,5 %) et de lactate (41,3 %) ont ete observees a (A). Le protocole de 16 h a entraine une depletion de 89,7 % (p < 0,05) du glycogene musculaire. Des augmentations dependantes des repetitions ont aussi ete notees durant l'exercice la capacite aerobique. On conclut qu'un exercice intense repete perturbe moins le potentiel de phosphorylation, probablement en raison d'une augmentation de la respiration mitochondriale durant la transition repos-effort a l'etat non stationnaire.
Mots-cles: metabolisme, adaptation, activite repetitive, phosphorylation oxydative, sensibilite mitochondriale.
[Traduit par la Redaction]
Introduction
In previous work from our laboratory, we have examined the metabolic and substrate adaptations that occur in working muscle in response to a variety of short-term training models (Green et al. 1992, 1999, 2000; Cadefau et al. 1994). We have been able to demonstrate that extensive changes occur, all typical of the trained state, at least qualitatively, soon after the onset of regular exercise. The most dramatic illustration of the early onset of the modifications that reduce the metabolic strain (i.e., metabolic adaptations) that occur was in response to 16 bouts of heavy exercise involving 6 min of cycling performed once per hour (Green et al. 2000). We have found that in response to this stimulus, there was less of a reduction in the content of the high energy phosphate compounds (phosphorylation potential), as indicated by the reduced depletion of ATP and phosphocreatine (PCr) and the reduced accumulation of free adenosine diphosphate ([[ADP.sub.f]]), free adenosine monophosphate ([[AMP.sub.f]]), and inosine monophosphate (IMP) in response to a standardized cycling task. In addition, we have found that another landmark property associated with training, namely, muscle lactate content, was reduced (Green et al. 2000). All these changes occurred without alteration in steady-state oxygen uptake ([??][O.sub.2]).
Particularly intriguing was the fact that the adaptations in metabolic behaviour occurred in the absence of change in mitochondrial potential, as assessed by the failure in the maximal activity of a number of representative enzymes of the citric acid cycle and [beta]-oxidation to be up-regulated (Green et al. 2000). Increases in mitochondrial potential have long been promoted as fundamental to the metabolic and substrate adaptations that occur in muscle in response to training (Holloszy and Coyle 1984; Gollnick 1986). According to this theory, the increase in mitochondrial potential that occurs with training allows a given level of mitochondrial oxidative phosphorylation (OXPHOS) to occur during exercise with less of an increase in the principal stimuli, such as [[ADP.sub.f]], [[AMP.sub.f]], inorganic phosphate ([P.sub.i]), or some combination thereof (Holloszy and Coyle 1984). The increased mitochondrial sensitivity, defined as the ability to maintain OXPHOS at a higher phosphorylation potential, observed following training has been postulated to occur as a result of the increase in the enzyme protein levels allowing for a decreased flux per respiratory chain at a given level of OXPHOS. The improved phosphorylation potential observed during exercise following training has also been hypothesized to decrease the recruitment of glycogenolysis and glycolysis resulting in a reduction in glycogen depletion and in lactate accumulation (Holloszy and Coyle 1984).
The fact that we have been able to demonstrate that all of these adaptations are not necessarily coupled and that some may occur early in training in the absence of increases in mitochondrial potential suggests that the regulatory mechanisms governing metabolic behaviour during exercise may be more complex than previously envisioned. It has been suggested that the regulation of metabolism must be viewed as a system involving a set of interactive components with properties that depend both on the interactions within each metabolic pathway and segment and on the interactions that occur with other systems (Connett et al. 1990).
It is possible that the metabolic adaptations that we have observed in response to a brief period of training do not reflect adaptations to the mitochondria per se, but to the supply of substrates and cofactors, such as oxygen, reducing equivalents, and oxidizable substrates (Wilson 1995). Accordingly, adaptations in one or more of these factors could allow a given level of mechanical work to be performed with less of a disturbance in phosphorylation potential. We have identified one such possibility from our short-term training model. We have observed that in the transition from rest to steady-state exercise, pulmonary [??][O.sub.2] kinetics were increased (Phillips et al. 1995a) in association with an increase in blood flow kinetics (Shoemaker et al. 1996). Moreover, the increase during exercise was related to a more protected phosphorylation potential during the non-steady state (Green et al. 1995). These findings suggest that an increased supply of [O.sub.2] occurs during the rest-to-work transition, resulting in an increased OXPHOS with a decreased dependency in high-energy phosphate transfer reactions and glycolysis.
Typically, studies involving training-induced adaptations are designed to examine the responses 24-48 h after the last training session. The approach that we have used in our short-term training experiments was similar in design. For the intermittent exercise study, in which we have shown many of the adaptations typically observed with extended periods of training, the standardized exercise test and tissue sampling was performed 36-48 h after the 16-hour intermittent work session (Green et al. 2000). It is possible that similar metabolic adaptations may occur during the extended sessions of heavy intermittent exercise. There is evidence that prior heavy exercise can speed pulmonary [??][O.sub.2] kinetics during subsequent exercise (Gurd et al. 2005). If such is the case, increases in [??][O.sub.2] should occur in the non-steady-state period with increases in the number of repetitions of heavy exercise. The increase in pulmonary [??][O.sub.2] kinetics should …
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