IMPAIRED CALCIUM-PUMP FUNCTION DOES NOT SLOW RELAXATION IN HUMAN SKELETAL-MUSCLE AFTER PROLONGED EXERCISE

This study examined the effects of prolonged exercise on human quadric eps muscle contractile function and homogenate sarcoplasmic reticulum Ca2+ uptake and Ca2+-adenosinetriphosphatase activity. Ten untrained m en cycled at 75 +/- 2% (SE) peak oxygen consumption until exhaustion. Biopsies were taken from the right vastus lateralis muscle at rest, ex haustion, and 20 and 60 min postexercise. Peak tension and half relaxa tion time of the left quadriceps muscle were measured during electrica lly evoked twitch and tetanic contractions and a maximal voluntary iso metric contraction at rest, exhaustion, and 10, 20, and 60 min postexe rcise. At exhaustion, homogenate Ca2+ uptake and Ca2+ adenosinetriphos phatase activity were reduced by 17 +/- 4 and 21 +/- 5%, respectively, and remained depressed after 60 min recovery (P less than or equal to 0.01). Muscle ATP, creatine phosphate, and glycogen were all depresse d at exhaustion (P less than or equal to 0.01). Peak tension during a maximal voluntary contraction, a twitch, and a 10-Hz stimulation were reduced after exercise by 28 +/- 3, 45 +/- 6, 65 +/- 5%, respectively (P less than or equal to 0.01), but no slowing of half relaxation time s were found. Thus fatigue induced by prolonged exercise reduced muscl e Ca2+ uptake, but this did not cause a slower relaxation of evoked co ntractions.