LOCALIZATION OF GLYCOGEN-PHOSPHORYLASE ACTIVITY IN LIVER OF FASTED NORMAL AND ADRENALECTOMIZED RATS AND IN FASTED ADRENALECTOMIZED RATS AFTER INJECTION OF DEXAMETHASONE

Background: The intralobular distribution of activity of glycogen phos phorylase (GP), a key enzyme in the breakdown of glycogen, was evaluat ed to determine changes during early glycogen synthesis, Hepatic GP ac tivity was localized in normal and adrenalectomized (ADX) rats after f asting overnight and in fasted ADX rats stimulated to synthesize glyco gen by administration of dexamethasone (DEX) 2-8 h prior to sacrifice, Methods: Cryostat sections were incubated in medium containing approp riate substrate for demonstration of GP activity as indicated by glyco gen synthesized by the enzyme during incubation. Results: In sections from fasted normal rats, GP activity in hepatocytes varied from undete ctable to substantial amounts with no notable periportal to pericentra l gradient evident, In contrast, GP activity in sections from adrenale ctomized fasted rats was concentrated in discrete aggregates in random hepatocytes throughout lobules, Two hours after DEX injection, GP enz yme activity occurred as single aggregates or in a dispersed pattern i n many hepatocytes, By 4 h after DEX administration, most cells displa yed GP enzyme activity, the concentration of which appeared to be grea ter in pericentral cells than in periportal cells, Eight hours after i njection of DEX, GP enzyme activity had increased and appeared more ev enly distributed throughout the lobules, Conclusions: These results su ggest that GP activity became concentrated in limited regions of selec ted hepatocytes in fasted ADX rats, DEX stimulation of glycogen synthe sis in these rats resulted in increased GP activity that was concentra ted in pericentral cells after 4 h, After 8 h, activity increased and was more evenly distributed throughout the lobules, The increase in GP enzyme activity concurrent with overall glycogen synthesis suggests t hat the enzyme may participate in glycogen turnover. (C) 1997 Wiley-Li ss, Inc.