Differential activation of phospholipase D by VPAC and PAC1 receptors.

To investigate the phospholipase D (PLD) responses of the VIP/PACAP receptors, VPAC1 and VPAC2, and the PACAP-specific PAC1 receptors (short and "hop" intracellular loop 3 (i3) splice variants), stable CHO cell lines expressing similar levels of each wildtype receptor were generated (except for the VPAC1 receptor clone which showed considerably lower expression and lesser responses in signalling assays). All clones caused activation of PLD in response to agonists, as monitored by [3H]phosphatidylbutanol production. The PLD responses of the PAC1 "hop", but not the "null" receptor, were sensitive to the ARF inhibitor, brefeldin A (BFA) (as were VPAC1 and VPAC2 responses). Chimeric constructs of VPAC2 receptors containing i3 of either PAC1 hop or PAC1 null receptors were transiently expressed in COS 7 cells and PLD responses were measured. Only the PLD response of the hop construct was sensitive to BFA. This suggests that i3 motifs in certain Group II GPCRs may play a key role in determining their linkage to ARF-dependent PLD activation.