Glycogen Synthetase and Phosphorylase Activity in Slow and Fast Twitch Skeletal Muscle Fibres in Man

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Abstract
Glycogen synthetase (I + D) and phosphorylase (a + b) activity was determined in human skeletal muscle biopsies with different muscle fiber composition and in dissected and pooled batches of the 2 main muscle fiber types: slow twitch (ST) and fast twitch (FT), respectively. Glycogen synthetase (I + D) activity remained unchanged as the percent of FT fibers increased, but phosphorylase (a + b) activity was increased. A similar activity pattern was found in dissected and pooled FT fibers as compared to ST fibers, i.e., the same glycogen synthetase activity but heightened phosphorylase activity.