Effects of Acute Variation of Fetal Glycemia on Glycogen Storage and on Glycogen Synthase and Phosphorylase Activities in the Liver of the Rat Fetus

The effects of variations of glycemia from 1.7 to 35 mM on the activity of glycogen synthase and phosphorylase, on glycogen content, and on U-¹⁴C-glucose incorporation into glycogen in the liver of the near-term rat fetus were investigated. Hypoglycemia did not affect the activities of phosphorylase and synthase; total glycogen content was not modified, but incorporation of labeled glucose was markedly decreased. This is consistent with a decreased glycogen synthesis. A slight hyperglycemia (about 5.5 mM) sharply decreased phosphorylase a (active) activity but increased slightly glycogen synthase a activity; liver glycogen content and labeled glucose incorporation were both enhanced. Higher levels of glycemia induced a decrease of phosphorylase a activity of the same order, but by contrast, glycogen synthase a activity increased progressively with increasing glycemia. Sequential study showed that hyperglycemia first induced the decrease of phosphorylase activity, then increased synthase activity. Marked hyperglycemia strongly enhanced liver glycogen content and labeled glucose incorporation. The fetal liver appears very responsive to acute variations of glycemia. The mechanisms seem to be oriented toward maximal glycogen accumulation.