The control of glycolysis in contracting muscle is not fully understood. The aim of the present study was to examine whether activation of glycolysis is mediated by factors related to the energy state or by a direct effect of Ca 2+ on the regulating enzymes. Extensor digitorum longus muscles from rat were isolated, treated with cyanide to inhibit aerobic ATP production and stimulated (0.2 s trains every 4 s) until force was reduced to 70% of initial force (control muscle, referred to as Con). Muscles treated with BTS ( N -benzyl- p -toluene sulfonamide), an inhibitor of cross-bridge cycling without affecting Ca 2+ transients, were stimulated for an equal time period as Con. Energy utilization by the contractile apparatus (estimated from the observed relation between ATP utilization and force–time integral) was 60% of total. In BTS, the force–time integral and ATP utilization were only 38 and 58% of those in Con respectively. Glycolytic rate in BTS was only 51% of that in Con but the relative contribution of ATP derived from PCr (phosphocreatine) and glycolysis and the relation between muscle contents of PCr and Lac (lactate) were not different. Prolonged cyanide incubation of quiescent muscle (low Ca 2+ ) did not change the relation between PCr and Lac. The reduced glycolytic rate in BTS despite maintained Ca 2+ transients, and the unchanged PCr/Lac relation in the absence of Ca 2+ transients, demonstrates that Ca 2+ is not the main trigger of glycogenolysis. Instead the preserved relative contribution of energy delivered from PCr and glycolysis during both conditions suggests that the glycolytic rate is controlled by factors related to energy state.
Uncoupled respiration (UCR) is an essential property of muscle mitochondria and has several functions in the cell. We hypothesized that endurance training may alter the magnitude and properties of UCR in human muscle. Isolated mitochondria from muscle biopsies taken before and after 6 weeks of endurance exercise training ( n = 8) were analysed for UCR. To investigate the role of uncoupling protein 2 (UCP2) and UCP3 in UCR, the sensitivity of UCR to UCP-regulating ligands (non-esterified fatty acids and purine nucleotides) and UCP2 and UCP3 mRNA expression in muscle were examined. Oleate increased the mitochondrial oxygen consumption rate, an effect that was not attenuated by GDP and/or cyclosporin A. The effect of oleate was significantly greater after compared with before training. Training had no effect on UCP2 or UCP3 mRNA levels, but after training the relative increase in respiration rate induced by oleate was positively correlated with the UCP2 mRNA level. In conclusion, we show that the sensitivity of UCR to non-esterified fatty acids is up-regulated by endurance training. This suggests that endurance training causes intrinsic changes in mitochondrial function, which may enhance the potential for regulation of aerobic energy production, prevent excess free radical generation and contribute to a higher basal metabolic rate.
Muscle biopsies taken from the musculus quadriceps femoris of man were analysed for pH, ATP, ADP, AMP, creatine phosphate, creatine, lactate and pyruvate. Biopsies were taken at rest, after circulatory occlusion and after isometric contraction. Muscle pH decreased from 7.09 at rest to 6.56 after isometric exercise to fatigue. Decrease in muscle pH was linearly related to accumulation of lactate plus pyruvate. An increase of 22μmol of lactate plus pyruvate per g of muscle resulted in a fall of 0.5pH unit. The apparent equilibrium constant of the creatine kinase reaction (apparent K CK ) increased after isometric contraction and a linear relationship between log(apparent K CK ) and muscle pH was obtained. The low content of creatine phosphate in muscle after contraction as analysed from needle-biopsy samples is believed to be a consequence of an altered equilibrium state of the creatine kinase reaction. This in turn is attributed mainly to a change in intracellular pH.