Vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activatingpeptide receptor subtypes in mouse calvarial osteoblasts: Presence of VIP-2 receptors and differentiation-induced expression of VIP-1 receptors

Three distinct complementary DNAs for vasoactive intestinal peptide (VIP) a nd pituitary adenylate cyclase-activating peptide (PACAP) receptors have be en cloned and designated VIP-1 receptor (VIP-1R), VIP-2 receptor (VIP-SR), and PACAP receptor (PACAP-R). In the present study, we have characterized t he binding sites on primary mouse calvarial osteoblasts for VIP and related peptides. By analyzing the cAMP response, the rank order of response obser ved was PACAP 38 > PACAP 27 > helodermin > VIP > helospectin > glucagon > P HI >>> secretin. The VIP-2R/PACAP-R antagonist, PACAP 6-38, inhibited both VIP and PACAP-stimulated cAMP formation. Binding studies using an atomic fo rce microscopy (AFM) technique showed high affinity binding for VIP and PAC AP 38, but not for secretin. Radioligand binding studies using I-125-VIP an d I-125-PACAP 38 demonstrated a more specific and higher affinity binding f or PACAP 38 than for VIP. Secretin failed to inhibit both I-125-VIP and I-1 25-PACAP 38 binding. RT-PCR demonstrated that undifferentiated mouse calvar ial osteoblasts express messenger RNA for VIP-SR, but not for VIP-1R or PAC AP-R, When the osteoblasts were cultured for 20 days to induce bone noduli formation, VIP-1R, in addition to VIP-2R, were expressed when the nodules s tarted to mineralize at 12 days. Taken together, these data demonstrate tha t mouse calvarial osteoblasts express functional VIP-SR with higher affinit y binding for PACAP than for VIP and that the VIP-1R expression is induced during osteoblastic differentiation.