Guanine‐nucleotide‐dependent inhibition of adenylate cyclase of rabbit heart by glucagon

The present study demonstrates an inhibitory effect of glucagon on the adenylate cyclase system of rabbit heart. Inhibition was maximal (22–40%) at 0.1 –0.01 μM glucagon and required the presence of 0.01 –0.1 mM GTP or guanosine 5′‐[β,γ‐imido]triphosphate (Guo PP [NH] P ). Reduced or no inhibitor effect of glucagon was observed: (a) after limited proteolysis of plasma membrane proteins by trypsin, (b) in the presence of 1 mM Mn 2+ , (c) in the absence of Na + , and (d) during the first 10 min of incubation if Guo PP [NH] P was the activating ligand. With GTP as the activating ligand, inhibition of cyclase by glucagon occurred without delay. These data are consistent with a mediation of glucagon inhibition by a guanine‐nucleotide‐binding protein. In the presence of ethanol (0.2 M) or benzyl alcohol (0.05 M), agents which are known to increase the fluidity of biological membranes, glucagon increased the enzyme activity in a guanine‐nucleotide‐dependent manner. Activation of cyclase in the presence of alcohols was maximal (30–60%) at 0.1–1.0 μM glucagon and 0.01 mM guanine nucleotides. Data suggest that glucagon receptors can interact with both the activatory and inhibitory guanine‐nucleotide‐binding proteins and the physical state of membranes may play a role in determining which interaction will be preferential.