RAPID AND DELAYED-EFFECTS OF EPIDERMAL GROWTH-FACTOR ON GLUCONEOGENESIS

Most reports on the effects of epidermal growth factor (EGF) on glucon eogenesis have indicated that such effects depend on the substrate use d and are only observable after a lag time of 30-40 min. Recently, an immediate and transient effect of EGF on glucose synthesis was describ ed in a perfused liver system. Here we extend the study of the effect of EGF on gluconeogenesis to isolated hepatocytes from fasted rats. Th e delayed effect of EGF on gluconeogenesis was studied by adding the s ubstrate 40 min after the peptide. Under these conditions EGF increase d glucose synthesis from pyruvate, decreased it when the substrate was lactate or glycerol and did not modify gluconeogensis from fructose o r dihydroxyacetone. EGF did not affect the metabolic flux through glyc olysis, determined as the production of lactate + pyruvate from 30 mM glucose. Furthermore, EGF did not modify the metabolic flux through py ruvate kinase, determined as the production of lactate + pyruvate from 1 mM dihydroxyacetone. The differing effects of EGF on gluconeogenesi s depending on the substrate used can be explained by the effects of E GF on the cytosolic redox state (measured as the lactate/pyruvate rati o). About 20 min after the addition of EGF, the mitochondrial redox st ate (measured as the 3-hydroxybutyrate/acetoacetate ratio) decreased. This effect of EGF was blocked by ammonium, which also abolished the e ffect of the peptide on gluconeogenesis. Thus the effect of EGF at the mitochondrial level appears to be necessary for its effects on glucon eogenesis. Taken together, our results indicate that the delayed effec ts of EGF on gluconeogenesis are secondary to the effects of the pepti de at both the mitochondrial and cytosolic levels. In addition to thes e delayed effects, we observed that EGF rapidly and transiently stimul ated glucose synthesis from lactate, decreased the cytosolic redox sta te and increased oxygen consumption. All of these rapid effects requir ed the presence of extracellular calcium and disappeared in the presen ce of rotenone, suggesting that this rapid effect of EGF on gluconeoge nesis is secondary to the stimulation of mitochondrial respiration.