Glycogen content of mouse hemidiaphragm as a biologic indicator of insulin action

Effects of insulin on glycogen metabolism of serum-deprived BHK-21 fibroblasts

We have studied the effects of insulin on glycogen metabolism of cultured BHK-21 fibroblasts. Addition of insulin to cells cultured in 0.5% serum stimulated incorporation of 5mM 14C-glucose into glycogen and increased cellular glycogen content, without inducing proliferation. With serum-deprived cells incubated for 2 hr, maximal stimulation of incorporation of glucose into glycogen was 3.5-fold and the half-maximal dose of insulin was 5nM. After culture for 24 hr with 100nM insulin, incorporation was increased 13-fold and glycogen content was increased by 44%. Incorporation of glucose into glycogen was reduced by agents which elevate cellular adenosine 3',5-cyclic monophosphate (cyclic AMP): 10 microM prostaglandin E1 (PGE1), 0.25 mM 1-methyl-3-isobutylxanthine (MIX), 10 microM L-epinephrine (L-EPI), and 10 microM L-isoproterenol (L-ISO). Culture with 100nM insulin for 2 hr stimulated incorporation three-fold in the presence of any of these compounds, but insulin did not affect cellular cyclic AMP. At all times from 1 hr to 24 hr after addition of insulin to serum-deprived cultures, the 1-min uptake of 10 microM 3H-2-deoxyglucose (3H-2DG) was increased. There were small (30 to 40%) increases in glycogen synthase I activity at 15 and 30 min but not 4 hr after addition of insulin. After 24 hr with insulin, total glycogen synthase and phosphorylase activities were increased approximately two-fold, without changes in their activation states. We conclude that insulin promotes glycogenesis in serum-deprived BHK-21 cells. This response is mediated principally by increased entry of glucose into cells, and is not mediated by a change in cellular cyclic AMP concentration.