Distribution of glycogen in the rat heart

Myocardial glycophagy flux dysregulation and glycogen accumulation characterize diabetic cardiomyopathy

Diabetic heart disease morbidity and mortality is escalating. No specific therapeutics exist and mechanistic understanding of diabetic cardiomyopathy etiology is lacking. While lipid accumulation is a recognized cardiomyocyte phenotype of diabetes, less is known about glycolytic fuel handling and storage. Based on in vitro studies, we postulated the operation of an autophagy pathway in the myocardium specific for glycogen homeostasis - glycophagy. Here we visualize occurrence of cardiac glycophagy and show that the diabetic myocardium is characterized by marked glycogen elevation and altered cardiomyocyte glycogen localization. We establish that cardiac glycophagy flux is disturbed in diabetes. Glycophagy may represent a potential therapeutic target for alleviating the myocardial impacts of metabolic disruption in diabetic heart disease.

Spatial Structure of Glycogen Molecules in Cells

Glycogen is a strongly branched polymer of α-D-glucose, with glucose residues in the linear chains linked by 1→4-bonds (~93% of the total number of bonds) and with branching after every 4-8 residues formed by 1→6-glycosidic bonds (~7% of the total number of bonds). It is thought currently that a fully formed glycogen molecule (β-particle) with the self-glycosylating protein glycogenin in the center has a spherical shape with diameter of ~42 nm and contains ~ 55,000 glucose residues. The glycogen molecule also includes numerous proteins involved in its synthesis and degradation, as well as proteins performing a carcass function. However, the type and force of bonds connecting these proteins to the polysaccharide moiety of glycogen are significantly different. This review presents the available data on the spatial structure of the glycogen molecule and its changes under various physiological and pathological conditions.

Some aspects of cardiac heterogeneity

It is remarkable that the heart, which obviously functions as a homogenous pump, shows such a high degree of heterogeneity. There are often contradictory or controversial results as far as transmural gradients are concerned (78). These discrepancies or inconsistencies, however, can at least in part be explained by species differences. Of particular interest is the heterogeneity that is due to the separation into two hearts. In view of the differences in the morphology and function of the left and right heart, it is surprising that the whole organ works in a rather homogenous fashion. In the past, the right heart has received not as much attention as the left heart. This is in particular true for small laboratory animals. It becomes more and more evident that compared to the left ventricle, the right ventricle reacts to various stimuli in a quantitatively different manner. More research on the right heart is therefore needed. Collectively, the data summarized in this article may be interpreted in that the heart not only works as a homogeneous unit, but may even benefit from the heterogeneity of its different parts or subunits under normal as well as under various pathophysiological conditions. Analysis of the processes involved in creating cardiac heterogeneity can provide new insights into the mechanism of adaptation of the heart.

The conduction system in Pompe's disease

We report our findings in the microscopic examination of the conduction system in four infants with glycogen storage disease, one of whom had adequate electrophysiologic studies. The electrophysiologic studies in the latter case showed P-A and A-H intervals at the lower limits of normal, but the H-V interval was just above the normal mean. This suggests that the rapid conduction was not localized in the anatomic counterpart of the H-V interval. The short P-R interval in the ECG may be related to the enlargement of cells, which may in turn be related to increased glycogen content. The relationship of glycogen per se to the speed of conduction is unknown. We found that the summit of the ventricular septum bulged, probably because of the generally increased cell size, and that the topography of the atrioventricular conducting system was different from normal. This is possibly related both to an increase in the cell sizes of the specialized conducting tissue itself and to deforming effects of this bulging summit of the ventricular septum. New microscopic details of the components of the conducting system are described in these cases.

Glycogen concentration in working and nonworking ventricles of isolated dog hearts

Glycogen utilization in working and nonworking ventricles was studied at high (over 70 mg/100 ml) and low (27 to 61 mg/100 ml) arterial glucose concentrations and after insulin or epinephrine addition in 16 isolated ventricle preparations of dog hearts. Coronary perfusion and hemodynamic determinants of right ventricular work were controlled, and the left ventricle was kept unloaded. Time courses of change in ventricular glycogen concentration were determined during monitoring of heart rate, workload, arterial oxygen saturation, and coronary perfusion pressure. Epicardial samples for glycogen analysis were taken from each ventricle, and glucose uptake from circulating blood was determined. Glycogen loss was greater in working right than in nonworking left ventricles. In spontaneously fibrillating hearts, this difference was not observed, and there was greater glycogenolysis than during coordinated contraction. Insulin administration early in experiments led to equivalent glycogen loss in working right and nonworking left ventricles. There was glycogen preservation in both ventricles of fibrillating hearts. Epinephrine augmented glycogen loss in fibrillating hearts; depletion was never complete. Myocardial glucose uptake, corrected for red cell glycolysis, was proportional to initial arterial glucose concentration.

Regional Distribution of Glycogen and Phosphorylase in the Ventricles of the Heart

In rat, rabbit, dog, and ox hearts, the glycogen concentration varied throughout the walls. The concentration gradient decreased from endocardium to epicardium in the right and left ventricles. In the septum, the interior part of the wall had a lower concentration of glycogen than tissue sections closer to the endocardial surfaces. The maximum values for glycogen concentration in endocardial sections were from two to six times higher than the minimum values. This gradient was much greater than the 30% differences observed between apex and base of the ventricles. Since our studies have demonstrated a very large variation for glycogen content in layers of myocardium parallel to sides of the ventricular wall, there is an obvious need for caution in sampling hearts for glycogen analysis.

In the ox heart, the right bundle branch and A-V node were dissected out, and the glycogen values were compared with those for the rest of the myocardium. Glycogen values of the conduction system, especially of the bundle of His, were considerably higher than for any part of the myocardium studied, indicating that the myocardial gradients observed may be related to distribution of the Purkinje fibers.

Phosphorylase values decreased from endocardium to epicardium in a pattern similar to that observed for glycogen, indicating a possible function in association with glycogen concentration.

In the hearts perfused with a glucose-free medium by the Langendorff technique, glycogen diminished throughout the heart, but the general pattern of gradients was preserved. This suggested that glycogen is utilized as an energy source for contraction.