Studies on Glycogen Metabolism in Normal and Diabetic Rat Heart In Vivo

Abstract

A study has been made on the key enzymes involved in synthesis and breakdown of glycogen and the major intermediates in glycogen synthesis in isolated normal and streptozotocin diabetic rat heart. A significant loss of hexokinase isoenzyme type I and type II was observed in the soluble fraction of hearts from starved and diabetic rats.No significant change was observed in total and active form of glycogen synthetase and phosphorylase in diabetic rat heart, but glycogen synthetase phosphatase activity in vitro was found to be considerably less. Treatment with insulin restored the synthetase phosphatase in vitro activity, and also significantly increased synthetase I in vivo.Tissue content of the major intermediates on the glycogen synthetic pathway, glucose 6-phosphate, glucose 1-phosphate, and UDP-glucose, was found to be significantly higher in diabetic rat heart than in normal. Insulin (4 I.U./kg for 30 min) was able to normalize the high values of tissue content of the glycogen precursors found in diabetic heart. The decreased glycogen synthesis in diabetic rat heart seems to be mainly due to the decreased activity of glycogen synthetase phosphatase.