Aims/hypothesis  

In skeletal muscle, the storage of glycogen by insulin is regulated by glycogen synthase, which is regulated by glycogen synthase kinase 3 (GSK3). Here we examined whether adrenergic receptor activation, which can increase glucose uptake, regulates glycogen synthesis in L6 skeletal muscle cells.

Methods  

We used L6 cells and measured glycogen synthesis (as incorporation of d-[U-¹⁴C]glucose into glycogen) and GSK3 phosphorylation following adrenergic activation.

Results  

Insulin (negative logarithm of median effective concentration [pEC₅₀] 8.2 ± 0.3) and the β-adrenergic agonist isoprenaline (pEC₅₀ 7.5 ± 0.3) induced a twofold increase in glycogen synthesis in a concentration-dependent manner. The α₁-adrenergic agonist cirazoline and α₂-adrenergic agonist clonidine had no effect. Both insulin and isoprenaline phosphorylated GSK3. The β-adrenergic effect on glycogen synthesis is mediated by β₂-adrenoceptors and not β₁-/β₃-adrenoceptors, and was not mimicked by 8-bromo-cyclic AMP or cholera toxin, and also was insensitive to pertussis toxin, indicating no involvement of cyclic AMP or inhibitory G-protein (G_i) signalling in the β₂-adrenergic effect on glycogen synthesis. 12-O-tetra-decanoylphorbol-13-acetate (TPA) increased glycogen synthesis 2.5-fold and phosphorylated GSK3 fourfold. Inhibition of protein kinase C (PKC) isoforms with 12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrollo(3,4-c)-carbazole (Gö6976; inhibits conventional and novel PKCs) or 2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide (Gö6983; inhibits conventional, novel and atypical PKCs) inhibited the stimulatory TPA effect, but did not significantly inhibit glycogen synthesis mediated by insulin or isoprenaline. Inhibition of phosphatidylinositol 3-kinase (PI3K) with wortmannin inhibited the effects of insulin and isoprenaline on glycogen synthesis.

Conclusions/interpretation  

These results demonstrate that in L6 skeletal muscle cells adrenergic stimulation through β₂-adrenoceptors, but not involving cyclic AMP or G_i, activates a PI3K pathway that stimulates glycogen synthesis through GSK3.