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 a-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 a-amylase and a-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 (CC14) 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 a-amylolysis is an indication of an increased fragility of gsd hepatocytes, which becomes prominent when these cells are incubated in vitro.Kinetic data on the two forms of liver phosphorylase indicate that the rate of glycogenolysis in the liver is determined by the concentration of phosphorylase a [l]. However, this relationship has only been directly demonstrated with isolated hepatocytes incubated with or without glucagon [2].

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.

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