Mechanisms of phospholipase C activation by the vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide type 2 receptor

The vasoactive intestinal polypeptide/pituitary adenylate cyclase-activatin g polypeptide type 2 (VPAC(2)) receptor was shown to induce both [H-3]inosi tol phosphate ([H-3]InsP) and cAMP production in transfected COS7 cells and in GH(3) cells where it is natively expressed. Neither cholera toxin nor f orskolin could elicit an equivalent [H-3]InsP response, suggesting independ ent coupling of the two pathways. The VPAC(2) receptor-mediated [H-3]InsP r esponse was partially inhibited by pertussis toxin (Ptx) and by the G beta gamma -sequestering C-terminal fragment of GRK2 (GRK2-ct) in COS7 and GH, c ells, whereas responses of control receptors were unaffected. Blockers of r eceptor-activated Ca2+ influx pathways (Co2+ and SKF 96365) also partially inhibited VPAC(2) receptor-mediated [H-3]InsP responses. This inhibition wa s not present in the component of the response remaining after Ptx treatmen t. A range of blockers of voltage-sensitive Ca2+ channels were ineffective, consistent with the reported lack of these channels in COS7 cells. The dat a suggest that the VPAC(2) receptor may couple to phospholipase C through b oth Ptx-insensitive and Ptx-sensitive G proteins (G(q/11) and C-i/o, respec tively) to generate [H-3]InsP. In addition to G beta gamma, G(i/o) activati on appears to require receptor-activated Ca2+ entry. This is consistent wit h the possibility that not only G alpha (q/11)- responsive and G beta gamma -responsive isoforms of phospholipase C but Ca2+-responsive forms may cont ribute to the overall [H-3]InsP response.