Signaling through the muscarinic receptor-adenylate cyclase system of the heart is buffered against GTP over a range of concentrations.

The influence of GTP on muscarinic receptor occupancy and inhibition of adenylate cyclase activity was investigated in well washed homogenates of the rat myocardium. In these homogenates, the highly efficacious muscarinic agonist oxotremorine-M was without effect on adenylate cyclase activity in the absence of exogenous GTP but caused a maximal 38% inhibition of the enzyme in the presence of 0.1 microM GTP. Increasing the concentration of GTP to 0.1 mM caused small to moderate increases in the maximal inhibition of adenylate cyclase elicited by oxotremorine-M and in the concentration of this agonist required for half-maximal inhibition of the enzyme. In contrast, the same change in the concentration of GTP (0.1 microM to 0.1 mM) caused a relatively large increase (46-fold) in the concentration of oxotremorine-M necessary for half-maximal receptor occupancy. Similar observations were made for the highly efficacious muscarinic agonist carbachol. Our results show that GTP increases receptor coupling efficiency and decreases agonist affinity and that these two effects oppose one another, so that the level of muscarinic agonist-mediated inhibition of adenylate cyclase activity remains relatively constant over a range of concentrations of GTP. We have also used a model to predict the influence of GTP on receptor binding properties and agonist-mediated inhibition of adenylate cyclase activity and have calculated theoretical results generally consistent with the experimental observations.