Fructose-induced increase in intracellular free Mg2+ ion concentration in rat hepatocytes: Relation with the enzymes of glycogen metabolism

In rat hepatocytes subjected to a fructose load, ATP content decreased from 3.8 to 2.6 mu mol/g of cells. Under these conditions, the intracellular free Mg2+ ion concentration, as measured with mag-fura 2, increased from 0.25 to 0.43 mu mol/g of cells and 0.35 mu mol of Mg2+ ions were released per g of cells in the extracellular medium. Therefore the increase in the intracellular free Mg2+ ion concentration was less than expected from the decrease in ATP, indicating that approx. 80% of the Mg2+ ions released from MgATP(2-) were buffered inside the cells. When this buffer capacity was challenged with an extra Mg2+ ion load by blocking the fructose-induced Mg2+ efflux, again approx. 80% of the extra Mg2+ ion load was buffered. The remaining 20%, appearing as free Mg2+ ions in fructose-treated hepatocytes could act as second messenger for enzymes having a K-m for Mg2+ in the millimolar range. Fructose activated glycogen synthase and glycogen phosphorylase, although both the time course and the dose-dependence of activation were different. This was reflected in a stimulation of glycogen synthesis with concentrations of fructose below 5 mM. Indeed, activation of glycogen synthase reached a maximum at 30 min of incubation and was observed with small(5 mM or less) concentrations of fructose, whereas the activation of glycogen phosphorylase was almost immediate (within 5 min) and maximal with large doses of fructose. The fructose-induced activation of glycogen phosphorylase, but not that of glycogen synthase, could be related to an increase in free Mg2+ ion concentration.