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 μmol/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 μmol/g of cells and 0.35 μmol 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.