The sarcoplasmic reticulum-glycogenolytic complex in mammalian fast twitch skeletal muscle. Proposed in vitro counterpart of the contraction-activated glycogenolytic pool.

Evidence is presented that the sarcoplasmic reticulum (SR)-glycogenolytic complex isolated from fast twitch skeletal muscle is a highly specific, functionally defined compartment for phosphorylase regulation. The addition of ATP alone results in prompt phosphorylase activation which demonstrates calcium dependence similar to the calcium-magnesium ATPase that catalyzes SR calcium transport suggesting that these two calcium-requiring -ystems might interact within the complex. Lowering extravesicular calcium concentration by transport of calcium into the SR lumen resulted in inactivation of phosphorylase a. This effect could be prevented by the addition of the calcium ionophore X537A which inhibits SR calcium sequestration or a calcium EGTA buffer which maintains free calcium. It was mimicked by EGTA addition. Since exogenous phosphorylase b and phosphorylase a were not activated or inactivated, respectively, by the endogenous activating enzymes or phosphatase in the SR-glycogenolytic complex, these regulatory enzymes may be compartmented. In addition, endogenous phosphorylase could be uncoupled from its activating enzymes by amylase treatment. These results suggest that the SR-glycogenolytic complex in fast twitch skeletal muscle is a compartmented system for phosphorylase activation controlled by SR calcium flux, a feature in contrast to the cardiac complex (Entman, M.L., Kaniike, K., Goldstein, M.A., Nelson, T.E., Bornet, E.P., Futch, T.W., and Schwartz, A. (1976) J. Biol. Chem. 251, 3140-3146). We suggest that the complex is the in vitro counterpart of the well documented rapid burst of glycogenolysis which ensures with the onset of contraction.