Effects of GTP on cyclic AMP concentrations in intact Ehrlich ascites tumor cells

Experiments were conducted to test the hypothesis that intracellular concentrations of guanosine triphosphate (GTP) regulate the activity and hormonal sensitivity of adenylate cyclase in intact cells. By appropriate treatments, the GTP concentrations of Ehrlich ascites tumor cells could be varied between 28% and 680% of control values. Cyclic AMP concentrations were measured before and after addition of epinephrine in cells containing this range of GTP concentratins. Basal cyclic AMP concentrations were unaffected by changes in GTP concentrations. In cells containing lowered concentrations of GTP, the cyclic AMP concentration following addition of epinephrine was half that in control cells. Elevation of GTP concentrations above normal and had no effect on cyclic AMP concentrations following epinephrine treatment.