Lithium's Effects on Rat Liver Glucose Metabolism in Vivo

Abstract

Oral administration of lithium carbonate to fed-healthy rats strongly decreased liver glycogen content, despite the simultaneous activation of glycogen synthase and the inactivation of glycogen phosphorylase. The effect seemed to be related to a decrease in glucose 6-phosphate concentration and to a decrease in glucokinase activity. Moreover, in these animals lithium markedly decreased liver fructose 2,6-bisphosphate, which could be a consequence of the fall in glucose 6-phosphate and of the inactivation of 6-phosphofructo-2-kinase. Liver pyruvate kinase activity and blood insulin also decreased after lithium administration. Lower doses of lithium carbonate had less intense effects. Lithium administration to starved-healthy and fed-streptozotocin-diabetic rats caused a slight increase in blood insulin, which was simultaneous with increases in liver glycogen, glucose 6-phosphate, and fructose 2,6-phosphate. Glucokinase, 6-phosphofructo-2-kinase, and pyruvate kinase activities also increased after lithium administration in starved-healthy and fed-diabetic rats. Lithium treatment activated glycogen synthase and inactivated glycogen phosphorylase in a manner similar to that observed in fed-healthy rats. Glycemia was not modified in any group of animals. These results indicate that lithium acts on liver glycogen metabolism in vivo in at least two different ways: one related to changes in insulinemia, and the other related to the direct action of lithium on the activity of some key enzymes of liver glucose metabolism.