Control of adenine nucleotide metabolism and glycolysis in vertebrate skeletal muscle during exercise

Comparison of calculated and experimental power in maximal lactate-steady state during cycling

BACKGROUND

The purpose of this study was the comparison of the calculated (MLSSC) and experimental power (MLSSE) in maximal lactate steady-state (MLSS) during cycling.

METHODS

13 male subjects (24.2 ± 4.76 years, 72.9 ± 6.9 kg, 178.5 ± 5.9 cm, V˙O2max: 60.4 ± 8.6 ml min-1 kg-1, V˙Lamax: 0.9 ± 0.19 mmol l-1 s-1) performed a ramp-test for determining the V˙O2max and a 15 s sprint-test for measuring the maximal glycolytic rate (V˙Lamax). All tests were performed on a Lode-Cycle-Ergometer. V˙O2max and V˙Lamax were used to calculate MLSSC. For the determination of MLSSE several 30 min constant load tests were performed. MLSSE was defined as the highest workload that can be maintained without an increase of blood-lactate-concentration (BLC) of more than 0.05 mmol l-1 min-1 during the last 20 min. Power in following constant-load test was set higher or lower depending on BLC.

RESULTS

MLSSE and MLSSC were measured respectively at 217 ± 51 W and 229 ± 47 W, while mean difference was -12 ± 20 W. Orthogonal regression was calculated with r = 0.92 (p < 0.001).

CONCLUSIONS

The difference of 12 W can be explained by the biological variability of V˙O2max and V˙Lamax. The knowledge of both parameters, as well as their individual influence on MLSS, could be important for establishing training recommendations, which could lead to either an improvement in V˙O2max or V˙Lamax by performing high intensity or low intensity exercise training, respectively. Furthermore the validity of V˙Lamax -test should be focused in further studies.

Regulation of post-mortem glycolysis in ruminant muscle

As a tissue, muscle has the unique ability to switch its metabolic source of ATP, the energy currency underpinning muscle function. During oxygen debt, such as that occurring immediately following the death of animals, anaerobic metabolism is initiated in an attempt to restore homeostasis within the muscle. The cascade of biochemical events that are initiated is paramount in the context of meat quality. This review revisits this reasonably well-known subject but takes a new perspective by drawing on the understanding outside the traditional discipline of meat science. Our understanding of the intrinsic regulators of glycolytic flux has improved but knowledge gaps remain. Further efforts to understand how the glycolytic enzyme kinetics are influenced by both pre- and post-slaughter factors will be beneficial in the ongoing quest to maximise fresh meat quality.

The effects of pyrene exposure on exercise performance, muscle contraction, and mitochondrial O(2) consumption in the leopard frog (Rana pipiens)

The present study exposed frogs to the PAH pyrene and measured exercise performance, muscle contractile ability, mitochondrial O(2) consumption, and membrane potential. Leopard frogs, Rana pipiens, were exposed for seven days in control or pyrene saturated water aquaria. Frogs were randomly placed into one of four groups: (i) pyrene-exposed exercised, (ii) control, exercised, (iii) pyrene-exposed, non-exercised, and (iv) control, non-exercised. Following the acute exposure, exercise duration, muscle contractile ability, blood gases and pH, glycogen levels, crossbridge formation and contractile length, and mitochondrial respiration were measured. The data revealed that pyrene exposure produced many adverse effects in Leopard frogs including significant reductions in exercise performance, muscle contractile ability, and alterations to muscle mitochondrial O(2) consumption and membrane potential. These data suggest that PAH exposure may limit survival for frogs in the wild by limiting foraging, mating, and predator avoidance behaviors.

Regulatory properties of 6-phosphofructokinase and control of glycolysis in boar spermatozoa

Glycolysis is crucial for sperm functions (motility and fertilization), but how this pathway is regulated in spermatozoa is not clear. This prompted to study the location and the regulatory properties of 6-phosphofructokinase (PFK, EC 2.7.1.11), the most important element for control of glycolytic flux. Unlike some other glycolytic enzymes, PFK showed no tight binding to sperm structures. It could readily be extracted from ejaculated boar spermatozoa by sonication and was then chromatographically purified. At physiological pH, the enzyme was allosterically inhibited by near-physiological concentrations of its co-substrate ATP, which induced co-operativity, i.e. reduced the affinity for the substrate fructose 6-phosphate. Inhibition by ATP was reinforced by citrate and H+. Above pH 8, PFK lost all its regulatory properties and showed maximum activity. However, in the physiological pH range, PFK activity was very sensitive to small changes in effectors. At near-physiological substrate concentrations, PFK activity requires activators (de-inhibitors) of which the combination of AMP and fructose 2,6-bisphosphate (F2,6P2) was most efficient as a result of synergistic effects. The kinetics of PFK suggest AMP, F2,6P2, H+, and citrate as allosteric effectors controlling PFK activity in boar spermatozoa. Using immunogold labeling, PFK was localized in the mid-piece and principal piece of the flagellum as well as in the acrosomal area at the top of the head and in the cytoplasmic droplets released from the mid-piece after ejaculation.

oscillations in a model of energy-dependent uptake by the endoplasmic reticulum

Active Ca2+ transport in living cells necessitates controlled supply of metabolic energy. Direct coupling between sarco/endoplasmic reticulum (ER) Ca2+ ATPases (SERCA) and intracellular energy-generation sites has been well established experimentally. On the basis of these experimental findings we propose a pump-driven model to investigate complex dynamic properties of a cell system. The model describes the pump process both by the Ca2+ ATPase itself and by a suitable description of the glycolysis. The associated set of differential equations shows a rich behavior, the solutions ranging from simple periodic oscillations to complex patterns such as bursting and spiking. Recent experimental results on calcium oscillations in Xenopus laevis oocytes and on dynamic patterns of intracellular Ca2+ concentrations in electrically non-excitable cells are well described by corresponding theoretical results derived within the proposed model. The simulation results are further compared to spontaneous [Ca2+] oscillations in primitive endodermal cells.

Initial aerobic power does not alter muscle metabolic adaptations to short-term training

To investigate the hypothesis that training-induced increases in muscle mitochondrial potential are not obligatory to metabolic adaptations observed during submaximal exercise, regardless of peak aerobic power (VO(2 peak)) of the subjects, a short-term training study was utilized. Two groups of untrained male subjects (n = 7/group), one with a high (HI) and the other with a low (LO) VO(2 peak) (means +/- SE; 51.4 +/- 0.90 vs. 41.0 +/- 1.3 ml. kg(-1). min(-1);P < 0.05), cycled for 2 h/day at 66-69% of VO(2 peak) for 6 days. Muscle tissue was extracted from vastus lateralis at 0, 3, and 30 min of standardized cycle exercise before training (0 days) and after 3 and 6 days of training and analyzed for metabolic and enzymatic changes. During exercise after 3 days of training in the combined HI + LO group, higher (P < 0.05) concentrations (mmol/kg dry wt) of phosphocreatine (40.5 +/- 3.4 vs. 52.2 +/- 4.2) and lower (P < 0.05) concentrations of P(i) (61.5 +/- 4.4 vs. 53.3 +/- 4.4), inosine monophosphate (0.520 +/- 0.19 vs. 0.151 +/- 0.05), and lactate (37.9 +/- 5.5 vs. 22.8 +/- 4.8) were observed. These changes were also accompanied by reduced levels of calculated free ADP, AMP, and P(i). All adaptations were fully expressed by 3 min of exercise and by 3 days of training and were independent of initial VO(2 peak) levels. Moreover, maximal activity of citrate synthase, a measure of mitochondrial capacity, was only increased with 6 days of training (5.71 +/- 0.29 vs. 7.18 +/- 0.37 mol. kg protein(-1). h(-1); P < 0. 05). These results demonstrate that metabolic adaptations to prolonged exercise occur within the first 3 days of training and during the non-steady-state period. Moreover, neither time course nor magnitude of metabolic adaptations appears to depend on increases in mitochondrial potential or on initial aerobic power.

Flying insects: model systems in exercise physiology

Insect flight is the most energy-demanding exercise known. It requires very effective coupling of adenosine triphosphate (ATP) hydrolysis and regeneration in the working flight muscles. 31P nuclear magnetic resonance (NMR) spectroscopy of locust flight muscle in vivo has shown that flight causes only a small decrease in the content of ATP, whereas the free concentrations of inorganic phosphate (Pi), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) were estimated to increase by about 3-, 5- and 27-fold, respectively. These metabolites are potent activators of glycogen phosphorylase and phosphofructokinase (PFK). Activation of glycolysis by AMP and Pi is reinforced synergistically by fructose 2,6-biphosphate (F2,6P2), a very potent activator of PFK. During prolonged flight locusts gradually change from using carbohydrate to lipids as their main fuel. This requires a decrease in glycolytic flux which is brought about, at least in part, by a marked decrease in the content of F2,6P2 in flight muscle (by 80% within 15 min of flight). The synthesis of F2,6P2 in flight muscle can be stimulated by the nervous system via the biogenic amine octopamine. Octopamine and F2,6P2 seem to be part of a mechanism to control the rate of carbohydrate oxidation in flight muscle and thus function in the metabolic integration of insect flight.