Skeletal muscle energy metabolism during prolonged, fatiguing exercise

Abstract

Febbraio, Mark A., and Jane Dancey. Skeletal muscle energy metabolism during prolonged, fatiguing exercise. J. Appl. Physiol. 87(6): 2341-2347.-A depletion of phosphocreatine (PCr), fall in the total adenine nucleotide pool (TAN ϭ ATP ϩ ADP ϩ AMP), and increase in TAN degradation products inosine 5Ј-monophosphate (IMP) and hypoxanthine are observed at fatigue during prolonged exercise at 70% maximal O 2 uptake in untrained subjects [J. Baldwin, R. J. Snow, M. F. Carey, and M. A. Febbraio. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R295-R300, 1999]. The present study aimed to examine whether these metabolic changes are also prevalent when exercise is performed below the blood lactate threshold (LT). Six healthy, untrained humans exercised on a cycle ergometer to voluntary exhaustion at an intensity equivalent to 93 Ϯ 3% of LT (ϳ65% peak O 2 uptake). Muscle biopsy samples were obtained at rest, at 10 min of exercise, ϳ40 min before fatigue (FϪ40 ϭ143 Ϯ 13 min), and at fatigue (F ϭ 186 Ϯ 31 min). Glycogen concentration progressively declined (P Ͻ 0.01) to very low levels at fatigue (28 Ϯ 6 mmol glucosyl U/kg dry wt). Despite this, PCr content was not different when FϪ40 was compared with F and was only reduced by 40% when F was compared with rest (52.8 Ϯ 3.7 vs. 87.8 Ϯ 2.0 mmol/kg dry wt; P Ͻ 0.01). In addition, TAN concentration was not reduced, IMP did not increase significantly throughout exercise, and hypoxanthine was not detected in any muscle samples. A significant correlation (r ϭ 0.95; P Ͻ 0.05) was observed between exercise time and glycogen use, indicating that glycogen availability is a limiting factor during prolonged exercise below LT. However, because TAN was not reduced, PCr was not depleted, and no correlation was observed between glycogen content and IMP when glycogen stores were compromised, fatigue may be related to processes other than those involved in muscle high-energy phosphagen metabolism.total adenine nucleotides; phosphocreatine; lactate threshold; glycogen IT IS WELL ESTABLISHED that fatigue during prolonged exercise coincides with low intramuscular glycogen stores (2,3,9,12,13,25,32,34,38). Although there are several possible reasons as to the requirement for carbohydrate in the maintenance of contractile force (for review, see Ref. 18), it is widely accepted that metabolic processes are limited by carbohydrate availability. It has been suggested that as muscle glycogen stores are progressively compromised during exercise, flux through glycolysis is reduced, leading to a fall in pyruvate formation and a reduction in tricarboxylic acid cycle intermediates, in turn resulting in an impairment in ATP provision via oxidative phosphorylation (32,34). Because ATP demand during prolonged exercise is maintained, such a decrease in ATP provision leads to transient ADP formation and ATP generation from alternative pathways, including creatine phosphokinase (CPK) and adenylate kinase (AK) (32). Because CPK has a much higher activity in skeletal muscle compared with AK (7), ph...

Effect of high wavelengths low intensity light during dark period on physical exercise performance, biochemical and haematological parameters of swimming rats

“…We interpreted these results considering a multi-factorial view, once the glycogen stores and haematological variables measured in our experiment could directly influence this result. It is well known that the glycogen is the major energy substrate for our proposed exercise and some authors have associated the inability to continue exercise at the required intensity to a lack of energetic substrates, mainly glycogen depletion (15). In our study, the skeletal muscle glycogen content was decreased by the experimental luminosity (Table I).…”

Skeletal muscle energetics assessed by 31P-NMR in prepubertal girls with a familial predisposition to obesity

“…In one study with the exercise set at a moderate intensity (65% VO 2 peak), PCr decreased by 24% at the submaximal exercise intensity level, and by 39% at fatigue. 35 Hargreaves et al 36 used intermittent exercise for 30 s and decreased PCr by 39%. Studies using maximal exercise depleted PCr by 58%, 37 63% 38 and 76%.…”

Metabolismo do glicogênio muscular durante o exercício físico: mecanismos de regulação

“…Esses estudos constituíram a base atual do conhecimento sobre o metabolismo do glicogênio muscular durante o exercício 9 , sendo utilizados na sustentação da maior parte das publicações subseqüentes [10][11][12] . Entre os principais achados deste grupo estão: a correlação linear entre o tempo de fadiga em uma determinada intensidade (%VO 2max ) e as concentrações iniciais de glicogênio no músculo (Figura 1), bem como a redução dos estoques de glicogênio (g/100g músculo seco) de forma semi-logarítmica em função do tempo, tendendo a se aproximar de zero no mesmo instante em que passa a ser difícil a manutenção da intensidade do exercício.…”

“…Uma importante e significativa parcela destina-se a manter o funcionamento da bomba de cálcio e interfere, apenas indiretamente, no processo de contração -relaxamento 41,42 . Alguns autores sugerem que, mesmo com glicogênio total intracelular suficiente para manter a atividade muscular, a depleção dos depósitos próxi-mos à bomba de cálcio pode ocorrer precocemente, impossibilitando a continuidade do exercício 12 . Apesar de evidências indiretas sugerirem a existência desse mecanismo em humanos 43 , infelizmente, dentro do nosso conhecimento, não existem estudos que possam comprovar essa hipótese.…”