Effects of Insulin and Testosterone on Glycogen Synthesis and Glycogen Synthetase Activity in Rat Levator Ani Muscle

The temporal relationships for hormonal stimulation of glycogen synthesis are not clarified. In the present study glycogen synthesis and the activation of the rate‐limiting glycogen synthetase enzyme (EC 2.4.1.11) were investigated in levator ani muscles from prepubertal male rats during insulin stimulation in vitro for 5–240 min and testosterone stimulation in vivo for 4–36 h. For both hormones, glycogen content was gradually increased to a level of 8.5–9.0 mglg tissue. An activation of the synthetase enzyme by means of an increase of its I‐form was found after only 5 min of insulin stimulation. The increase in I‐form was continuously reduced and reached control level after 150 min of incubation. For testosterone, the I‐form was steadily increased 6–16 h after injection of the hormone. The incorporation of glucose‐ 14 C into glycogen was increased in both studies even when the I‐form was no longer increased. The level of glucose‐6‐phosphate was aiso measured and the results suggest an activation of the synthetase enzyme by changes in the concentration of this metabolite during hormonal stimulation. It is concluded that the length of stimulation is of importance for the stimulation of muscle glycogen synthesis by insulin and testosterone.