31P nuclear magnetic resonance study on changes in phosphocreatine and the intracellular pH in rat skeletal muscle during exercise at various inspired oxygen contents.

We measured ATP, phosphocreatine (PCr), inorganic phosphate (Pi), and the intracellular pH in rat hindlimb muscles during submaximal isometric exercise with various O2 deliveries using 31P nuclear magnetic resonance spectroscopy (31P NMR) to evaluate changes in energy metabolism in relation to O2 availability. Delivery of O2 to muscles was altered by controlling the fractional concentration of inspired oxygen (F1O2) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial pressure of oxygen and carbon dioxide, and bicarbonate at the femoral artery. The steady-state ratio of PCr: (PCr + Pi) during exercise decreased as a function of F1O2 even at 0.21. Significant acidification of the intracellular pH during exercise occurred at 0.08 F1O2. Change in the PCr: (PCr + Pi) ratio demonstrated that the oxidative capacity, i.e. the maximal rate of the oxidative phosphorylation reaction, in muscle was not limited by O2 delivery at 0.50 F1O2, but was significantly limited at 0.21 F1O2 or below. Change in the intracellular pH at 0.08 F1O2 could be interpreted as an increase in lactate, suggesting activation of glycolysis. Correlation between the PCr: (PCr + Pi) ratio and the intracellular pH revealed the existence of a critical PCr: (PCr + Pi) ratio and pH for glycolysis activation at around 0.4 and 6.7, respectively.