Prostacyclin receptor-independent inhibition of phospholipase C activity by non-prostanoid prostacyclin mimetics

1 Chinese hamster ovary (CHO) cells were transiently transfected with the m ouse prostacyclin (mIP) receptor to examine IP agonist-mediated stimulation of [H-3]-cyclic AMP and [H-3]-inositol phosphate production. 2 The prostacyclin analogues, cicaprost, iloprost, carbacyclin and prostagl andin E-1, stimulated adenylyl cyclase activity with EC50 values of 5, 6, 2 5 and 95 nm, respectively. These IP agonists also stimulated the phospholip ase C pathway with 10-40 fold lower potency than stimulation of adenylyl cy clase. 3 The non-prostanoid prostacyclin mimetics, octimibate, BMY 42393 and BMY 4 5778, also stimulated adenylyl cyclase activity, with EC50 values of 219, 1 66 and 398 nm, respectively, but failed to stimulate [H-3]-inositol phospha te production. 4 Octimibate, BMY 42393 and BMY 45778 inhibited iloprost-stimulated [H-3]-i nositol phosphate production in a non-competitive manner. 5 Activation of the endogenously-expressed P-2 purinergic receptor by ATP l ed to an increase in [H-3]-inositol phosphate production which was inhibite d by the non-prostanoid prostacyclin mimetics in non-transfected CHO cells. Prostacyclin analogues and other prostanoid receptor ligands failed to inh ibit ATP-stimulated [H-3]-inositol phosphate production. 6 A comparison between the IP receptor-specific non-prostanoid ONO-1310 and the structurally-related EP3 receptor-specific agonist ONO-AP-324, indicat ed that the inhibitory effect of non-prostanoids was specific for those com pounds known to activate IP receptors. 7 The non-prostanoid prostacyclin mimetics also inhibited phospholipase C a ctivity when stimulated by constitutively-active mutant G alpha qRC, G alph a 14RC and G alpha (16)QL transiently expressed in CHO cells. These drugs d id not inhibit adenylyl cyclase activity when stimulated by the constitutiv ely-active mutant G alpha (s)QL. 8 These results suggest that non-prostanoid prostacyclin mimetics can speci fically inhibit [H-3]inositol phosphate production by targeting G(q/11) and /or phospholipase C in CHO cells, and that this effect is independent of IP receptors.