CONTROL OF THE RATE OF PHOSPHOCREATINE RESYNTHESIS AFTER EXERCISE IN TRAINED AND UNTRAINED HUMAN QUADRICEPS MUSCLES

We examined the effect of differences in exercise intensity on the time constant (t(c)) of phosphocreatine (PCr) resynthesis after exercise and the relationships between t(c) and maximal oxygen uptake (VO2max) in endurance-trained runners (n = 5) and untrained controls (n = 7) (average VO2max = 66.2 and 52.0 ml . min(-1). kg(-1), respectively). To measure the metabolism of the quadriceps muscle using phosphorus nuclear magnetic resonance spectroscopy, we developed a device which allowed knee extension exercise inside a magnet. All the subjects performed four types of exercise: light, moderate, severe and exhausting. The end-exercise PCr:[PCr + inorganic phosphate (P-i)] ratio decreased significantly with the increase in the exercise intensity (P < 0.01). Although there was little difference in the end-exercise pH, adenosine diphosphate concentration ([ADP]) and the lowest intracellular pH during recovery between light and moderate exercise, significant changes were found at the two higher intensities (P < 0.01). These changes for runners were smaller than those for the controls (P < 0.05). The t(c) remained constant after light and moderate exercise and then lengthened in proportion to the increase in intensity (P < 0.05). The runners had a lower t(c) at the same PCr and pH than the controls, particularly at the higher intensity (P < 0.05). There was a significant correlation between t(c) and [ADP] in light exercise and between t(c) and both end-exercise PCr and pH in severe and exhausting exercise (P < 0.05). The threshold of changes in pH and t(c) was a PCr:(PCr + P-i) ratio of 0.5. There was a significant negative correlation between the VO2max and t(c) after all levels of exercise (P < 0.05). However, in the controls a significant correlation was found in only light and moderate exercise (P < 0.05), These findings suggest the validity of the use of t(c) at an end-exercise PCr:(PCr + P-i) ratio of more than 0.5 as a stable index of muscle oxidative capacity and the correlation between local and general aerobic capacity. Moreover, endurance-trained runners are characterized by the faster PCr resynthesis at the same PCr and intracellular pH.