J. Biol. Chem., Vol. 261, Issue 36, 16927-16931, Dec, 1986
Effects of lithium ions on glycogen synthase and phosphorylase in rat hepatocytes
F Bosch, AM Gomez-Foix, J Arino and JJ Guinovart
Incubation of hepatocytes from fasted rats with LiCl provoked a
concentration- and time-dependent activation of glycogen synthase. This
effect was observed in the absence of glucose in the incubation medium. No
changes in the intracellular concentrations of ATP or glucose-6- phosphate
were detected. Lithium was also able to activate glycogen synthase in the
absence of extracellular calcium. If hepatocytes were incubated with
lithium and insulin, an additive effect of both agents on glycogen synthase
activity was observed. LiCl was also effective in activating the enzyme in
hepatocytes obtained from fed rats. When hepatocytes were incubated with
[33P]phosphate and then treated with LiCl, a decrease in the amount of
[32P]phosphate incorporated in the enzyme was observed. This
dephosphorylation affected two CNBr fragments of the enzyme (CB-2 and
CB-1), suggesting that several phosphorylation sites were involved. Lithium
was also able to activate glycogen phosphorylase from both fasted and fed
rats. Phosphorylase activation was concentration- and time-dependent,
either in the presence or absence of calcium in the incubation medium.
These findings demonstrate that although lithium appears to mimic the
effects of insulin on glycogen synthase activity, its mechanism of action
must be different from that of the hormone.