Glycogen and glycogen phosphorylase associated with sarcoplasmic reticulum: effects of fatiguing activity

The purpose of the present study was to investigate the effects of fatiguin g muscular activity on glycogen, glycogen phosphorylase (Gl?), and Ca2+ upt ake associated with the sarcoplasmic reticulum. (SR). Tetanic contractions (100 ms, 75 Hz) of the gastrocnemius and plantaris muscles, elicited once p er second for 15 min, significantly reduced force to 26.5 +/- 4.0% and whol e muscle glycogen to 23% of rested levels. SR glycogen levels were 415.4 +/ - 76.6 and 20.4 +/- 2.1 mug/mg SR protein in rested and fatigued samples, r espectively. The optical density of GP from SDS-PAGE was reduced to 21% of control, whereas pyridoxal 5'-phosphate concentration, a quantitative indic ator of GP content, was significantly reduced to 3% of control. GP activity after exercise, in the direction of glycogen breakdown, was reduced to 4% of control. Maximum SR Ca2+ uptake rate was also significantly reduced to 8 1% of control. These data demonstrate that glycogen and GP associated with skeletal muscle SR are reduced after fatiguing activity.