Dual pathways of receptor-mediated cyclic GMP generation in NG108-15 cells as differentiated by susceptibility to islet-activating protein, pertussis toxin.

The cellular cGMP content increased in response to a variety of receptor agonists, which activate [e.g., prostaglandin (PG) E1, E2, and F2 alpha] or inhibit (e.g., alpha-adrenergic, muscarinic, and opiate agonists) adenylate cyclase in neuroblastoma X glioma hybrid NG108-15 cells. The responses were additive when PGF2 alpha and enkephalin were mixed. The inhibitory guanine nucleotide regulatory protein (Ni) is involved in adenylate cyclase inhibition; this function of Ni is lost when it is ADP-ribosylated by islet-activating protein (IAP), pertussis toxin [H. Kurose, T. Katada, T. Amano, and M. Ui (1983) J. Biol. Chem. 258, 4870-4875]. The cGMP rise induced by stimulation of the receptors linked to adenylate cyclase inhibition was also diminished by IAP; the time course and dose response for the IAP-induced diminution were the same between adenylate cyclase inhibition and cGMP generation. Ni thus appears to mediate guanylate cyclase activation as well as adenylate cyclase inhibition initiated via the same receptors. Melittin also increased cGMP. No additivity was shown when enkephalin and melittin were combined, suggesting that phospholipase A2 might play a role in Ni-mediated guanylate cyclase activation. On the other hand, the PGF2 alpha-induced cGMP rise was associated with increased incorporation of 32Pi into phosphatidylinositol; was not affected by cholera toxin, IAP or forskolin; and showed no additivity when combined with A23187, which increased cGMP by itself. PGs would occupy receptors linked to phosphatidylinositol breakdown, thereby increasing the availability of intracellular Ca2+, which is responsible for guanylate cyclase activation. Thus, dual pathways are proposed for a receptor-mediated cGMP rise in NG108-15 cells.