Pituitary adenylate cyclase-actviating polypeptide induces cAMP production independently from vasoactive intestinal polypeptide in osteoblast-like cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) isolated from ovine hypothalamic tissue is a novel neuropeptide which stimulates adenylate cyclase in rat anterior pituitary cell cultures. In osteoblasts, the detail in intracellular signalling systems of PACAP has not yet been clarified. In this study, we investigated the effects of PACAP on cAMP accumulation, phosphoinositide hydrolysis and Ca²⁺ influx in osteoblast-like MC3T3-E1 cells, compared with those of vasoactive intestinal polypeptide (VIP), which shows a considerable homology with PACAP in the N-terminal sequence. PACOA stimulated cAMP accumulation in a dose-dependent manner in the range between 0.1 nM and 0.1 μM in these cells. VIP also stimulated cAMP accumulation dose-dependently between 1 nM and 0.1 μM. The effect of PACAP on cAMP accumulation (ec₅₀ = 3 nM) was more potent than that of VIP (ec₅₀ = 30 nM). The cAMP accumulation stimulated by a combinatiion of PACAP (3 nM) and VIP (30 nM) was additive. [Lys¹, Pro^2,5, Arg^3,4, Tyr⁶]-VIP, an antagonist for the VIP receptor which markedly inhibited the VIP-induced cAMP accumulation, had little effect on the PACAP-induced cAMP accumulation. Either PACAP or VIP had little effect on the formation of inositol phosphates and Ca²⁺ influx in these cells. These results strongly suggest that PACAP stimulates cAMP production via an independent binding site from VIP osteoblast-like MC3AT3-E1 cells and that PACAp has no effect on the activation of protein kinase C nor the intracellular CA²⁺ mobilization in these cells.