Inhibition of glucagon-induced glycogenolysis in isolated rat hepatocytes by the Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate.

The ability of the Rp diastereomer of adenosine cyclic 3',5'-phosphorothioate (Rp cAMPS) to inhibit glucagon-induced glycogenolysis was studied in hepatocytes isolated from fed rats. Preincubation of the cells for 20 min with progressively higher concentrations of Rp cAMPS followed by a 1 X 10(-9) M glucagon challenge resulted in a 50% inhibition of glucose production over a 30-min period at 2-3 X 10(-6) M Rp cAMPS. A maximal inhibition of 50-74% was achieved, the actual value depending upon the length of preincubation with Rp cAMPS. The inhibitory effect did not increase when the concentration of Rp cAMPS was increased from 3 X 10(-6) to 3 X 10(-4) M. Addition of 1 X 10(-5) M Rp cAMPS to the cells followed by 10(-11) to 10(-6) M glucagon shifted the glucagon concentration required for half-maximal glucose production measured at 10 min to 6-fold higher glucagon concentrations and the concentration of glucagon required for apparent maximal glucose production measured at 10 min to greater than 10-fold higher glucagon concentrations. The cAMP-dependent protein kinase activation curve was similarly shifted to higher concentrations of glucagon. These data show that Rp cAMPS acts as a cAMP antagonist capable of opposing the glucagon-induced activation of cAMP-dependent protein kinase and the concomitant activation of the glycogenolytic cascade.