Inverse Effects of d-Galactosamine and Inorganic Phosphate on Glycogenolysis in Isolated Rat Hepatocytes

d-Galactosamine (2 mM) lowered the intracellular Pi level of isolated rat hepatocytes within 20 min from 3.6 to 2.3 and the ATP content from 2.34 to 1.86 mmol per kg wet cells. At the same time, the rate of glycogenolysis decreased to 55% and of glucose output to 45%. Inorganic phosphate added to the incubation medium raised the intracellular Pi level in a dose-dependent manner. At an extracellular Pi concentration of 20 mM, the intracellular Pi content rose within 20 min to 6.4 and the ATP content to 2.96 mmol per kg wet cells. This increase was accompanied by a two-fold stimulation of glycogenolysis and release of glucose. d-Galactosamine (2 mM) and potassium phosphate (20 mM) supplied simultaneously counteracted their individual effects. Since the activities of the enzymes involved in glucose formation from glycogen including the glucagon-stimulated adenyl cyclase activity were unaltered by d-galactosamine treatment, it is concluded that in isolated hepatocytes phosphorolysis of glycogen is the limiting step in its conversion to glucose, that the intracellular Pi level is a limiting factor of phosphorylase a activity, that the intracellular Pi content and the rate of glycogenolysis can be increased by raising the extracellular Pi concentration, and that the net influx from elevated medium Pi levels can effectively counteract the phosphate trapping capacity of galactosamine, but not that of 20 mM fructose.