Glycogen depletion in rat skeletal muscle fibers at different intensities and durations of exercise.

Abstract

Total muscle glycogen depletion, the glycogen depletion pattern (PAS staining) in the different fiber types of skeletal muscle, and several other measures of carbohydrate metabolism were studied in rats that ran 162.2 m at varying speeds (22.5–80.5 m/min) or swam from 0.5–4 hrs. Muscle glycogen declined as an increasing function of exercise intensity during running whereas during swimming there was a near linear decline in muscle glycogen throughout the 4 hrs of exercise. Blood lactate did not increase until running speed exceeded a load that would required a VO2 of about 60% of aerobic capacity. a peak lactate of 21.15 mM occurred after the rats ran at 67m/min. Liver glycogen declined steadily at a rate of about 0.6 mg×g−1×min−1 during the first 2 hrs of swimming. During this time blood glucose was maintained at or above resting levels. During the final 2 hrs of swimming glycogenolysis in the liver declined to about 0.09 mg×g−1×min−1 and there was then a sharp decrease in blood glucose to a final value of 68.7 mg/100 ml. At low running speeds and during the first hour of swimming the greatest loss in PAS staining occured in fast-twitch-oxidative-glycolytic (FOG) and slow-twitch-oxidative (SO) fibers. At running speeds above about 50 m/min a rapid loss of PAS staining was observed in the fast-twitch-glycolytic (FG) fibers. These higher speeds are above those that should elicit VO2 max for the rat. In the late stages of swimming FG fibers also demonstrated a loss of glycogen. These data suggest that at low work intensities there is a primary reliance upon oxidative fibers for contractile activity and that a major use of anaerobic fibers only occurs at high work levels or when the aerobic fibers are depleted of glycogen during prolonged-low intensity work.