CCK, carbachol, and bombesin activate distinct PLC-beta isoenzymes via G(q/11) in rat pancreatic acinar membranes

Four different isoforms of phospholipase C-beta (PLC-beta(1-4)) have been discovered, raising the important question of whether a distinct receptor activates a single PLC-beta isoenzyme or a subset of PLC-beta isoenzymes. The present study was designed to investigate activation of PLC-beta isoenzymes by three different PLC-activating agonists that bind to different receptor entities, i.e., cholecystokinin octapeptide (CCK-8), bombesin, and carbachol in rat pancreatic acinar membranes. PLC activity was measured using exogenous [H-3] phosphatidylinositol 4,5-bisphosphate as substrate. Western blot analysis of pancreatic acinar membranes revealed the presence of PLC-beta(1), -beta(3), -gamma(1), and -delta(1), but not of PLC-beta(2), -beta(4), -gamma(2), and -delta(2). Preincubation of the membranes with anti-PLC-beta(1) or -beta(3) antibody reduced agonist-induced activation of PLC. The order of sensitivity toward inhibition by anti-PLC-beta(1) antibody was CCK-8 > bombesin > carbachol. An opposite order of sensitivity was found for inhibition of PLC activity by anti-PLC-beta(3) antibody (carbachol > bombesin > CCK-8). Anti-PLC-beta(2), -beta(4), -gamma(1), -gamma(2), -delta(1), and -delta(2) antibodies had no effect. Preincubation of the membranes with an antibody raised against the COOH terminus of the alpha-subunit of G(q/ll) proteins inhibited PLC activity in response to all three different receptor agonists to a similar extent, whereas anti-G(i) alpha(1-2) and anti-G(i) alpha(3) antibodies had no effect. In conclusion, the data of the present study indicate that CCK-8 and carbachol activate PLC-beta(1) and PLC-beta(3), respectively, whereas bombesin activates both PLC-beta(1) and PLC-beta(3). Activation of PLC-beta by these receptor agonists is mediated by G(q/ll).