Molecular Pharmacology, Vol 16, 941-949, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Regulation of Glycogenolysis in Isolated Rat Hepatocytes by the Specific Activation of Type I Cyclic AMP-Dependent Protein Kinase

¹ Department of Pharmacology, University of Arizona Health Sciences Center, Tucson, Arizona 85724

Incubation of isolated hepatocytes with concentrations of dibutyryl cyclic AMP from 0.1 µM to 10 µM led to a dose-dependent increase in the cyclic AMP-dependent protein kinase activity ratio. A concentration of 10 µM dibutyryl cyclic AMP increased the activity ratio from 0.24 to 0.85. Each concentration led to an optimal protein kinase activation within 5 min of incubation. Incubation of hepatocytes with 5 µM dibutyryl cyclic AMP resulted in a maximal production of glucose compared to control cultures. A concentration of 1 µM dibutyryl cyclic AMP resulted in a near maximal stimulation of glycogenolysis for the initial 60-min incubation period. Concentrations of dibutyryl cyclic AMP from 0.1 µM to 5 µM increased the protein kinase activity ratio from 0.24 to 0.59 and stimulated glycogenolysis in a dose-dependent manner as determined 20 min after the addition of dibutyryl cyclic AMP. Increasing the dibutyryl cyclic AMP to 10 µM increased the activity ratio from 0.59 to 0.85 but did not result in any further increase in glucose production. Concentrations of dibutyryl cyclic AMP from 0.1 µM to 5 µM resulted in the specific activation of type I protein kinase. As the concentration of dibutyryl cyclic AMP in the hepatocytes incubation was further increased to 10 µM, the type II holoenzyme was activated. A similar sequential activation of type I and type II protein kinase occurred with liver cells incubated with increasing concentrations of glucagon. Therefore, maximal stimulation of glycogenolysis was coupled with the selective activation of type I cyclic AMP-dependent protein kinase.