Cytochemical localization of glycogen phosphorylase activity in rat liver

Abstract

Glycogen phosphorylase (GP) is a key enzyme in liver glycogen breakdown. GP activity is altered with its state of phosphorylation. In the current study, the intralobular distribution of GP activity was observed histochemically in frozen sections of rat liver during fasting and after stimulation of glycogen synthesis.Normal and adrenalectomized (ADX) rats were fasted overnight to reduce liver glycogen to minimal levels. Fasted ADX rats received 2 mg dexamethasone (DEX) 0-8 h prior to sacrifice to stimulate glycogen synthesis. Liver was removed, rapidly frozen in isopentane cooled in liquid nitrogen, then cryostat sectioned. In order to determine sites of GP activity, sections were incubated in medium that contained glucose 1-phosphate as substrate. Under the incubation conditions used, GP synthesized glycogen as the reaction product. Glycogen was identified by two staining methods: 1) iodine staining has been shown to be rather specific for newly synthesized glycogen produced during the histochemical procedure (Figs.1-6), whereas 2) periodic acid-Schiff (PAS) stained both native and nascent glycogen.