Carbamylcholine inhibits β-adrenergic receptor-coupled G_s protein function proximal to adenylate cyclase

The specific mechanism by which the inhibitory guanine nucleotide binding protein (G_i) mediates the inhibition of adenylate cyclase activity is still unclear. The subunit dissociation model, based on studies in purified or reconstituted systems, suggests that the βγ subunit, which is dissociated with activation of G_i, inhibits the function of the stimulatory guanine nucleotide binding protein (G_s) by reducing the concentration of the free α_s subunit. In the present study, G_s, protein function is determined by measuring cholera toxin-blockable, isoproterenol-induced increases in guanosine triphosphate (GTP) binding capacity to rat cardiac ventricle membrane preparations. Carbamylcholine totally inhibited this β-adrenergic receptor-coupled G_s protein function. Pretreatment of the cardiac ventricle membrane preparation with pertussis toxin prevented this muscarinic agonist effect. These results confirm the possibility of an inhibitory agonist-receptor coupled effect through G_i on G_s protein function proximal to the catalytic unit of adenylate cyclase in an intact membrane preparation.