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 micromol/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 micromol/g of cells and 0.35 micromol 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 MgATP2- 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 Km 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.
Gaussin
V
V
Hormone and Metabolic Research Unit, International Institute of Cellular and Molecular Pathology, ICP-UCL 7529, Avenue Hippocrate, 75, B-1200 Brussels, Belgium.
Gailly
P
P
Gillis
J M
JM
Hue
L
L
D000818:Animals; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005632:Fructose / Q000494:pharmacology*; D006003:Glycogen / Q000378:metabolism*; D006006:Glycogen Synthase / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D006005:Phosphorylases / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D005632:Fructose; D006003:Glycogen; D006005:Phosphorylases; D006006:Glycogen Synthase; D008274:Magnesium