P-31 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 (P-i), and the intracellular pH in rat hindlimb muscles during submaximal isomet ric exercise with various O-2 deliveries using P-31 nuclear magnetic r esonance spectroscopy (P-31 NMR) to evaluate changes in energy metabol ism in relation to O-2 availability. Delivery of O-2 to muscles was al tered by controlling the fractional concentration of inspired oxygen ( FIO2) at 0.50, 0.28, 0.21, 0.11 and 0.08 with monitoring partial press ure of oxygen and carbon dioxide, and bicarbonate at the femoral arter y. The steady-state ratio of PCr : (PCr + P-i) during exercise decreas ed as a function of FIO2 even at 0.21. Significant acidification of th e intracellular pH during exercise occurred at 0.08 FIO2. Change in th e PCr: (PCr + P-i) ratio demonstrated that the oxidative capacity, i.e . the maximal rate of the oxidative phosphorylation reaction, in muscl e was not limited by O-2 delivery at 0.50 FIO2, but was significantly limited at 0.21 FIO2 or below. Change in the intracellular pH at 0.08 FIO2 could be interpreted as an increase in lactate, suggesting activa tion of glycolysis. Correlation between the PCr:(PCr + P-i) ratio and the intracellular pH revealed the existence of a critical PCr : (PCr P-i) ratio and pH for glycolysis activation at around 0.4 and 6.7, re spectively.