The Glucagon-sensitive Adenyl Cyclase System in Plasma Membranes of Rat Liver

Abstract

Abstract Studies have been made of the effects of nucleotides on binding of 125I-glucagon at its specific binding sites in plasma membranes of rat liver. GTP and GDP, equally and at a minimal concentration of 0.05 µm, stimulate the rate and degree of dissociation of bound labeled hormone, decrease uptake of glucagon by the membranes, and decrease the affinity of the binding sites for glucagon. These effects of the nucleotides are concentration-dependent, reversible, and rapid in onset. Divalent metal ions are not required for the actions of the nucleotides which act equally at 0° or 30°. 5'-Methylene guanylyl-diphosphonate, a nonphosphorylating analogue of GTP, mimicked the effects of GTP or GDP on glucagon binding although at 100 times the concentration of the natural nucleotides. Based on these observations and the finding that the nucleotides do not act competitively with glucagon, it is suggested that GTP or GDP regulate glucagon binding by an allosteric type of action. This action of the guanyl nucleotides is inhibited by a sulfhydryl reagent (p-chloromercuri-benzoate), which also inhibits binding of glucagon. Sodium fluoride, which stimulates adenyl cyclase activity in liver membranes, has no effect on either the binding of glucagon or on the actions of guanyl nucleotides on this process. ATP, ADP, UTP, and CTP act similarly to the guanyl nucleotides on the glucagon binding process but only at concentrations greater than 0.1 mm. Cyclic 3', 5'-GMP, 5'-GMP, and the corresponding adenine nucleotides are inactive on the glucagon-binding process.