In vivo glucose-, glucagon-, and cAMP-induced changes in liver glycogen synthase phosphatase activity.

In normal fed rats, glycogen synthase D phosphatase activity in a glycogen pellet preparation was only partially inhibited (approximately 50%) by high concentrations of EDTA. However, the proportion of phosphatase activity inhibited by EDTA was markedly and rapidly (15 s) increased following glucagon or cAMP administration. Epinephrine administration did not alter the proportion of activity inhibited by EDTA. Glucose administration rapidly (2 min) reduced the proportion of synthase phosphatase activity inhibitable by EDTA. That is, the effect of glucose was just the opposite of that produced by glucagon or cAMP. Insulin administration had no effect on phosphatase activity. Synthase phosphatase activity assayed in the absence of EDTA was similar in all groups except for a moderate increase after glucose administration. Addition of Mg2+ completely reversed EDTA inhibition. Phosphorylase phosphatase activity in each group was not modified by addition of EDTA, although the percentage of phosphorylase in the alpha form was higher in glucagon-treated and lower in the glucose-treated animals as expected. These data suggest the presence of rapidly interconvertible forms of either synthase phosphatase or its substrate synthase D, detectable as a change in EDTA inhibitability and subject to glucose and glucagon control.