Change of coupling system of receptor-adenylate cyclase induced by epinephrine and GTP in plasma membranes of rat liver

Abstract

1. 1. The binding of [ 3H]epinephrine to plasma membranes was affected (temporary release of bound epinephrine and characteristic retardation of epinephrine binding) not only by GTP but also by dGTP and guanylylimidodiphosphate, whereas the binding of [ 3H]dihydroalprenolol was not affected by GTP. GTP affected the binding of [ 3H]epinephrine in the presence of α-antagonists, but not in the presence of β-antagonists, suggesting that the GTP effects are specific to β-agonists and β-receptors. 2. 2. The half-maximal release of bound [ 3H]epinephrine was found at 8.8 · 10 −6 M GTP in the absence of ATP, whereas it was found at 1.6 · 10 −6 M GTP in the presence of 0.3 mM ATP in coincidence with the half-maximal activation of adenylate cyclase by GTP in the presence of 0.3 mM ATP (as measured at 30 s of incubation). 3. 3. In the presence of 4 · 10 −5 M GTP, adenylate cyclase activity as measured at 30 s of incubation (State I) tended to increase with epinephrine concentration, showing no saturation tendency even at 1 · 10 −4 M epinephrine. The activity of State II, which is established at 4 min of incubation, was much lower than that of State I but was found to reach a plateau as the epinephrine concentration increased, showing half-maximal activation at an epinephrine concentration between 2 · 10 −6 and 2 · 10 −7 M. 4. 4. Apparent kinetic parameters ( K m and V]) for State I as assayed at 30 s of incubation suggested that GTP alone may increase V slightly, whereas epinephrine plus GTP may increase the V to a further extent and simultaneously decrease the K m. 5. 5. Adenylate cyclase of plasma membranes pretreated with epinephrine plus GTP was stimulated by GTP alone similarly to untreated membranes, but it was no longer responsive to the synergistic activation by epinephrine plus GTP. Accordingly, the binding of [ 3H]epinephrine to the pretreated plasma membranes was no longer affected by GTP. 6. 6. The results of the present study seem to support the idea that the most active and coherently coupling state (State I) of the β-receptor-adenylate cyclase system generated in the presence of epinephrine plus GTP is very labile and degenerates before reaching equilibrium. In turn, State II, in which the coherently coupling mechanism is largely impaired, seems to be established in due time. The characteristic biphasic kinetics of [ 3H]epinephrine binding in the presence of GTP seem to be related to the above change occurring in the β-receptor-adenylate cyclase system.