Effects of glucagon and tolbutamide on glycogen in isolated perfused rat liver

The glycogen content of the isolated perfused liver taken from fed normal rats is remarkably stable for 3–6 hours ( Am. J. Physiol. 195: 295, 1958) and thus affords an isolated system for studies of the effect of various agents on glycogen synthesis and glycogenolysis. The effects of glucagon and tolbutamide in this system are described. Glucagon given by continuous infusion (30–225 µg/hr.) caused prompt glycogenolysis, with blood glucose concentrations rising to levels as high as 970 mg%. Hyperglycemia of 600–700 mg% did not inhibit the glycogenolytic action of glucagon; however, at glucose levels of 800– 1000 mg% glycogenolysis did not proceed to completion. High concentrations of tolbutamide, maintained before and during administration of glucagon, failed to inhibit the glycogenolytic action of the latter agent.