Treatment of Swiss 3T3 cells with a subsaturating concentration of recombinant Pasteurella multocida toxin (rPMT) markedly potentiated the production of inositol phosphates induced by bombesin, vasopressin, and endothelin but not by platelet-derived growth factor (PDGF) (AA and BB homodimers). Similarly, the neuropeptides but not PDGF caused a shift in the dose-dependent increase in inositol phosphates induced by rPMT. The rate of accumulation of inositol phosphates induced by bombesin was increased 2-fold by rPMT treatment while that of PDGF was unaffected. rPMT treatment also enhanced bombesin-induced inositol(1,4,5)trisphosphate, the direct product of phosphatidylinositol 4,5-bisphosphate hydrolysis. In contrast, treatment of cells with rPMT had no effect on the tyrosine phosphorylation of phospholipase C gamma. Depletion of protein kinase C increased rPMT-induced inositol phosphates in a manner similar to that observed for bombesin but not PDGF. Thus, rPMT selectively potentiates neuropeptide-mediated inositol phosphate production. The action of rPMT on phosphatidylinositol 4,5-bisphosphate hydrolysis persisted in streptolysin O-permeabilized cells. Addition of guanosine 5'-O-(beta-thiodiphosphate) to permeabilized cells markedly reduced rPMT-induced inositol phosphates in a time- and dose-dependent manner. rPMT also increased the sensitivity of phospholipase C for free calcium. Our results strongly suggest that the action of rPMT facilitates the coupling of G protein to phospholipase C.