The hepatic glycogenolysis induced by reversible ischaemia or KCN is exclusively catalysed by phosphorylase a.

1. Ischaemia was applied for 30 min to the liver of Wistar rats and of gsd/gsd rats, which have a genetic deficiency of phosphorylase kinase. The rate of glycogenolysis corresponded closely to the concentration of phosphorylase a. The loss of glycogen from Wistar livers was accounted for by the intrahepatic increase in glucose plus lactate. Further, the accumulation of oligosaccharides was negligible in the gsd/gsd liver. 2. Isolated hepatocytes from Wistar and gsd/gsd rats were incubated for 40 min in the presence of either KCN or glucagon. Again, the production of glucose plus lactate was strictly dependent on the presence of phosphorylase a. However, the catalytic efficiency of phosphorylase a was about 2-fold higher in the presence of KCN. 3. We conclude that the hepatic glycogenolysis induced by anoxia and by KCN is solely mediated by phosphorylase a. The higher catalytic activity of phosphorylase a under these circumstances could be due to an increased concentration of the substrate Pi.

1-Deoxynojirimycin and related compounds inhibit glycogenolysis in the liver without affecting the concentration of phosphorylase a.

Administration in vivo of the alpha-glucosidase inhibitors 1-deoxynojirimycin and its derivatives BAY m 1099 (miglitol) and BAY o 1248 resulted in a dose- and time-dependent decrease in the rate of hepatic glycogenolysis induced by glucagon. This represents a direct effect on the liver, since it could be reproduced on isolated hepatocytes. The amount of glucose produced by hepatocytes over a period of 10-20 min after addition of glucagon was decreased by about 70, 60 and 45% in the presence of maximally effective concentrations of BAY o 1248, deoxynojirimycin, and BAY m 1099, respectively. Half-maximal effects were observed at inhibitor concentrations between 20 and 100 microM. The concentrations of phosphorylase a and glycogen synthase a were not affected by inclusion of the alpha-glucosidase inhibitors in the hepatocyte suspensions. Thus, the antiglycogenolytic action of these compounds is not mediated by an altered activation state of the rate-limiting enzymes of glycogenolysis and of glycogen synthesis.

An assessment of the importance of intralysosomal and of alpha-amylolytic glycogenolysis in the liver of normal rats and of rats with a glycogen-storage disease.

Mechanisms of glycogenolysis have been investigated in a comparative study with Wistar rats and gsd rats, which maintain a high glycogen concentration in the liver as a result of a genetic deficiency of phosphorylase kinase. In Wistar hepatocytes the rate of glycogenolysis, as modulated by glucagon and by glucose, was proportional to the concentration of phosphorylase a. In suspensions of gsd hepatocytes the rate of glycogenolysis was far too high as compared with the low level of phosphorylase a; in addition, only a minor fraction of the glycogen lost was recovered as glucose and lactate, owing to the accumulation of oligosaccharides. When the gsd hepatocytes were incubated in the presence of an inhibitor of alpha-amylase (BAY e 4609) glycogenolysis and the formation of oligosaccharides virtually ceased; the production of glucose plus lactate, already modest in the absence of BAY e 4609, was further decreased by 40%, owing to the suppression of a pathway for glucose production by the successive actions of alpha-amylase and alpha-glucosidase. Evidence was obtained that gsd hepatocytes are more fragile, and that amylolysis of glycogen occurred in damaged cells and/or in the extracellular medium. This may even occur in vivo, since quick-frozen liver samples from anesthetized gsd rats contained severalfold higher concentrations of oligosaccharides than did similar samples from Wistar rats. However, administration of a hepatotoxic agent (CCl4) caused hepatic glycogen depletion in Wistar rats, but not in gsd rats. The administration of phloridzin and of vinblastine, which have been proposed to induce glycogenolysis in the lysosomal system, did not decrease the hepatic glycogen level in gsd rats. Taken together, the data indicate that only the phosphorolytic degradation of glycogen is metabolically important, and that alpha-amylolysis is an indication of an increased fragility of gsd hepatocytes, which becomes prominent when these cells are incubated in vitro.