Regulation of Liver Glycogen Synthetase Activity

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Glycogen synthetase activity of the rat liver tissue has been studied particularly on its alterations in activity when some kinds of hormones, i. e. insulin, glucagon, prednisolone, or glucose were administered to the rat, and also some in vitro experiments have been performed by using liver slices of the rat. The enzyme activity was assayed by measuring the incorporation rate of UDPG-14C into glycogen as described by Villar-Palasi et al. 1) The enzyme activity was markedly increased by oral administration of glucose to rats fasted for 24 hours, showing the peak of the activity 60 min later and a gradual return to the fasting level 4 hours later. Besides, when the activity was assayed with removal of G6P in the incubation medium, such an increase in the activity was much more pronounced than that of the total activity assayed by adding G6P. It is therefore suggested that the glucose administration accelerates conversion of the dependent type (D type) of the enzyme to the independent type (I type). However, any effect of the glucose administration was not observed in the alloxan diabetic rat. This suggests that secretion of insulin stimulated by the glucose administration may cause an increase of the enzyme activity. 2) Insulin (0.4u) actually induced the activity of the I type in the normal liver tissue of the rat. Nevertheless, glycogen l6rel and glucose-14C incorporation into glycogen were decreased in the liver tissue. This paradoxical finding is likely to be due to the lowered levels of glucose and G6P in the liver tissues caused by insulin. 3) Glucagon (100μg) showed an effect to lower glycogen level and reduced glucose-14C incorporation into glycogen, and also markedly decreased activity of the enzyme. And the I type was much more markedly affected. Such a response to glucose was blocked by glucagon simultaneously administered, indicating a counteraction between insulin and glucagon. 4) An activity of the I type was increased by prednisolone, though the total activity was less affected. 5) To observe the effects of these hormones on glycogen synthetase activity in vitro, liver slices were incubated with 5 ml of Krebs-Ringer bicarbonate buffer containing 10 mM of glucose at 37°C for 30 min. Levels of the enzyme activity in the liver slices which were incubated 30 min with 2mU/ml of insulin after preincubation for 30 min, was slightly but significantly higher than that in control slices. Without preincubation, the effect of insulin was not consistent. Glucagon 2μg/ml had an effect to decrease the enzyme activity in the slices which were obtained from the rats sacrificed one hour after glucose administration. In other conditions, glucagon had no consistent effect on the enzyme activity. 7) The addition of 2×10-4M or 2×10-3M or 3'-5' cyclic AMP in 30 min after preincubation showed an decrease in the enzyme activity in the slices.
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Regulation of Liver Glycogen Synthetase Activity

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