Mechanisms of phospholipase C activation by the vasoactive intestinal polypeptide/pituitary adenylate...

The vasoactive intestinal polypeptide/pituitary adenylate cyclase-activating polypeptide type 2 (VPAC(2)) receptor was shown to induce both [(3)H]inositol phosphate ([(3)H]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 [(3)H]InsP response, suggesting independent coupling of the two pathways. The VPAC(2) receptor-mediated [(3)H]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(3) cells, whereas responses of control receptors were unaffected. Blockers of receptor-activated Ca(2+) influx pathways (Co(2+) and SKF 96365) also partially inhibited VPAC(2) receptor-mediated [(3)H]InsP responses. This inhibition was not present in the component of the response remaining after Ptx treatment. A range of blockers of voltage-sensitive Ca(2+) 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 G(i/o), respectively) to generate [(3)H]InsP. In addition to G beta gamma, G(i/o) activation appears to require receptor-activated Ca(2+) 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 also Ca(2+)-responsive forms may contribute to the overall [(3)H]InsP response.