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 tim e 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) (a verage VO2max = 66.2 and 52.0 ml . min(-1). kg(-1), respectively). To measure the metabolism of the quadriceps muscle using phosphorus nucle ar magnetic resonance spectroscopy, we developed a device which allowe d knee extension exercise inside a magnet. All the subjects performed four types of exercise: light, moderate, severe and exhausting. The en d-exercise PCr:[PCr + inorganic phosphate (P-i)] ratio decreased signi ficantly with the increase in the exercise intensity (P < 0.01). Altho ugh there was little difference in the end-exercise pH, adenosine diph osphate concentration ([ADP]) and the lowest intracellular pH during r ecovery between light and moderate exercise, significant changes were found at the two higher intensities (P < 0.01). These changes for runn ers were smaller than those for the controls (P < 0.05). The t(c) rema ined 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 b etween 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 exerci se (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 a nd general aerobic capacity. Moreover, endurance-trained runners are c haracterized by the faster PCr resynthesis at the same PCr and intrace llular pH.