Multiple Controls by Adenosine Receptors on the Adenylate Cyclase in the Rat Hepatic Membrane

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The effects of an adenosine analog, N⁶-phenyl-isopropyl-adenosine (PIA), on the glucagon-stimulated adenylate cyclase activity in rat hepatic membranes were studied. Adenosine at high concentrations (>10 μM) has been reported exclusively to inhibit the adenylate cyclase via intracellular P-sites of the hepatic membrane. The stimulation by glucagon of the enzyme was attenuated by nanomolar concentrations of PIA in the presence of low concentrations (<1.0 μM) of GTP, indicating the effect of the guanine nucleotide inhibitory system (Ni). This inhibition by PIA required the presence of sodium chloride and was antagonized with isobutyl methylxanthine, an antagonist for the extracellular R-site receptors. The inhibitory effects of PIA disappeared and reversed into a stimulatory phase with increasing concentrations of GTP, suggesting the presence of a stimulatory (Ns) and an inhibitory (Ni) guanine nucleotide system of the enzyme in the action of the adenosine. PIA concentrations over a micromolar were observed to stimulate the enzyme activity in a GTP-dependent manner, indicating the presence of the stimulatory receptor (A₂ or Ra) coupled to the Ns. These results suggest that receptors for adenosine of the inhibitory type (A₁ or Ri) and the stimulatory type (A₂ or Ra) are present on the rat hepatic membrane, showing multiple controls of the adenylate cyclase system, depending on the cellular concentrations of GTP and/or sodium chloride.