G(I)-MEDIATED STIMULATION OF TYPE-II ADENYLYL-CYCLASE IS AUGMENTED BYG(Q)-COUPLED RECEPTOR ACTIVATION AND PHORBOL ESTER TREATMENT

Synergism between G(s)- and G(i)- or G(q)-dependent signaling pathways has been demonstrated in the stimulation of type II adenylyl cyclase (AC-II). Provision of activated a, is known to allow numerous G(i)-cou pled receptors to stimulate AC-II and to potentiate the responses to G (q)-coupled receptors. To explore possible interactions between G(i)- and G(q)-coupled receptors that are independent of alpha(s), the activ ity of AC-II was determined after the activation of G(i)- and G(q)-reg ulated pathways. Human embryonic kidney 293 cells were transiently cot ransfected with cDNAs encoding AC-II and various G-protein-coupled rec eptors. Agonist-bound G(i)-coupled receptors (including the formyl pep tide, dopamine-D-2 and delta-opioid receptors) stimulated AC-II activi ty in the absence of activated alpha(s), provided that the cells were treated with 100 nM phorbol 12-myristate 13-acetate. Activation of pro tein kinase C (PKC) thus appears to relieve the requirement for the pr esence of activated alpha(s). Stimulation of PKC via G(q)-coupled rece ptors also allowed G(i)-coupled receptors to activate AC-II. Coexpress ion of the mi muscarinic receptor with the dopamine-D-2 receptor permi tted dopamine to stimulate AC-II in the presence of carbachol. The pho rbol ester-permissive and alpha(s)-independent stimulation was mediate d by G-protein beta gamma subunits because it was blocked by the beta gamma scavengers alpha(t) and beta-adrenergic receptor kinase. These r esults show that AC-II can efficiently integrate signals generated by G(q)- and G(i) coupled receptors via a mechanism that is independent o f G(s).