Studies on the Role of Adenosine 3',5'-Monophosphate in the Hepatic Actions of Glucagon and Catecholamines

Abstract

Abstract The effects of glucagon, epinephrine, and isoproterenol on adenosine 3',5'-monophosphate (cyclic AMP) accumulation and glucose mobilization have been studied in isolated rat livers perfused with Krebs bicarbonate buffer containing bovine erythrocytes and serum albumin. Glucagon and epinephrine produced detectable increases in liver cyclic AMP within 30 sec. With glucagon, the level of cyclic AMP rose to a maximum at 3 min and declined slowly over 1 hour. With epinephrine, the cyclic AMP concentration rapidly rose to a peak value at 1 min then declined to a lower level at 5 min and remained constant for the next 30 min. With maximally effective doses of glucagon, the cyclic AMP level was increased 11-fold at 1 min and 60-fold at 5 min. The corresponding values for epinephrine were only 4-fold and 2-fold. The lowest concentration of glucagon producing a significant elevation of cyclic AMP within 4 min was 2 x 10-10 m. A half-maximal cyclic AMP level was observed with about 1 x 10-8 m hormone. The minimally effective concentration of epinephrine for promotion of cyclic AMP accumulation was not clearly defined. A half-maximal response was observed with 1 x 10-7m hormone. By measuring changes in the glucose concentration of the effluent medium from livers from fed rats perfused without recirculation, it was possible to measure amounts of glucagon as low as 5 pmoles. The minimal concentration of glucagon producing significant glucose mobilization in the liver was 2 x 10-10 m while that of epinephrine was 1.4 x 10-8 m. These levels are similar to the minimal concentrations promoting adenosine 3',5'-monophosphate accumulation. Half-maximal glucose mobilization was observed with about 1 x 10-9 m glucagon or 1 x 10-7 m epinephrine. Norepinephrine and isoproterenol increased cyclic AMP accumulation and glucose output in livers perfused without recirculation. They were slightly less potent than epinephrine. Isoproterenol was much less potent than epinephrine or norepinephrine in stimulating glycogenolysis and gluconeogenesis in livers perfused with recirculating medium. Theophylline potentiated the effects of glucagon and epinephrine on cyclic AMP accumulation and glycogenolysis in the liver, whereas caffeine was ineffective. Serotonin, pancreozymin, and secretin did not affect glucose mobilization in livers from fed rats. Prostaglandin E1 and nicotinic acid did not alter basal or hormone-stimulated glucose production or cyclic AMP accumulation. Livers perfused with fluoride had reduced levels of cyclic AMP. Cessation of glucagon infusion resulted in a rapid fall in cyclic AMP levels, indicating rapid turnover of the nucleotide. When infusions of small amounts of glucagon, epinephrine, or cyclic AMP were stopped, glycogenolysis and gluconeogenesis declined at similar rates. The data are discussed in terms of the hypothesis that glucagon and catecholamines act in the liver by increasing the concentration of free, metabolically active cyclic AMP, and that this represents only a small fraction of the total tissue cyclic AMP.