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

The purpose of the present study was to investigate the effects of fatiguing muscular activity on glycogen, glycogen phosphorylase (GP), and Ca(2+) uptake associated with the sarcoplasmic reticulum (SR). Tetanic contractions (100 ms, 75 Hz) of the gastrocnemius and plantaris muscles, elicited once per second for 15 min, significantly reduced force to 26.5 +/- 4.0% and whole muscle glycogen to 23% of rested levels. SR glycogen levels were 415.4 +/- 76.6 and 20.4 +/- 2.1 microg/mg SR protein in rested and fatigued samples, respectively. The optical density of GP from SDS-PAGE was reduced to 21% of control, whereas pyridoxal 5'-phosphate concentration, a quantitative indicator 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 Ca(2+) uptake rate was also significantly reduced to 81% of control. These data demonstrate that glycogen and GP associated with skeletal muscle SR are reduced after fatiguing activity.