Effects of diabetes on cardiac glycogen metabolism in rats

The effects of diabetes on myocardial glycogen metabolism in rats were examined and compared with those of fasting. Male Wistar rats were divided into three groups: controls, streptozotocin-induced diabetics, and one-week fasted. Isolated rat hearts were subjected to substrate-free 30-min Langendorff perfusion followed by 60-min working heart perfusion with glucose alone or in combination with insulin or insulin plus beta-hydroxybutyrate (BHB). Myocardial glycogen contents were determined before or 30 min after Langendorff perfusion, or 60 min after working heart perfusion. Before Langendorff perfusion, tissue glycogen concentrations in control, diabetic, and fasted hearts were 3.3 +/- 0.2, 10.0 +/- 0.9, and 5.7 +/- 0.5 (mg/g wet weight), respectively. In diabetic rats, the myocardial glycogen concentration was markedly decreased after working heart perfusion of any of the substrate combinations, even those with insulin and BHB. In contrast, myocardial glycogen in control or fasted rats was not reduced after the addition of glucose with insulin, and/or glucose with insulin and BHB. These results suggest that degradation of tissue glycogen occurs in isolated perfused hearts from diabetic rats, while a clearly different response is shown by fasted hearts.