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-activating polypeptide type 2 (VPAC(2)) receptor was shown to induce both [H-3]inositol 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 forskolin could elicit an equivalent [H-3]InsP response, suggesting independent coupling of the two pathways. The VPAC(2) receptor-mediated [H-3]InsP response 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, cells, whereas responses of control receptors were unaffected. Blockers of receptor-activated Ca2+ influx pathways (Co2+ and SKF 96365) also partially inhibited VPAC(2) receptor-mediated [H-3]InsP responses. This inhibition was not present in the component of the response remaining after Ptx treatment. A range of blockers of voltage-sensitive Ca2+ channels were ineffective, consistent with the reported lack of these channels in COS7 cells. The data suggest that the VPAC(2) receptor may couple to phospholipase C through both Ptx-insensitive and Ptx-sensitive G proteins (G(q/11) and C-i/o, respectively) to generate [H-3]InsP. In addition to G beta gamma, G(i/o) activation appears to require receptor-activated Ca2+ entry. This is consistent with the possibility that not only G alpha (q/11)- responsive and G beta gamma -responsive isoforms of phospholipase C but Ca2+-responsive forms may contribute to the overall [H-3]InsP response.