PHOSPHOCREATINE CONTENT IN SINGLE FIBERS OF HUMAN MUSCLE AFTER SUSTAINED SUBMAXIMAL EXERCISE

The effect of sustained submaximal exercise on muscle energetics has b een studied on the single-fiber level in human skeletal muscle. Seven subjects cycled to fatigue (mean 77 min) at a work rate corresponding to similar to 75% of maximal O-2 uptake. Biopsies were taken from the vastus lateralis muscle at rest, at fatigue, and after 5 mill of recov ery. Muscle glycogen decreased from 444 +/- 40 (SE) mmol glucosyl unit s/kg dry wt at rest to 94 +/- 16. Postexercise glycogen was inversely correlated (P < 0.01) to muscle content of inosine monophosphate, a ca tabolite of ATP. Phosphocreatine (PCr) in mixed-fiber muscle decreased at fatigue to 37% but was restored above the initial value (106.5%, P < 0.025) after 5 min of recovery. The overshoot was localized to type I fibers. The rapid reversal of PCr is in contrast; to the slow recov ery in contraction force. Pi increased at fatigue but less than that e xpected from the changes in PCr and other phosphate compounds. Mean PC r at rest was similar to 20% higher in type II than in type I fibers ( 86.4 +/- 3.6 and 71.6 +/- 1.8 mmol/kg dry wt, respectively, P < 0.05), but at fatigue similar PCr contents were observed in the two fiber ty pes. Reduction in PCr in all fibers at fatigue suggests that all fiber s were recruited at the end of exercise. PCr content in single fibers showed a great variability in samples at rest, exercise, and recovery. The variability was more pronounced than for ATP, and the data sugges t that it is due to interfiber physiological-biochemical differences. At fatigue ATP was maintained relatively high in all single fibers, bu t a pronounced depletion of PCr was observed in a large number of fibe rs, and this may contribute to fatigue through the associated increase s in P-i or/and free ADP. It is noteworthy that the increase in calcul ated free ADP at fatigue was similar to that after high-intensity exer cise.