Intracellular mediators of bombesin action on rat pancreatic acinar cells.

The effects of bombesin on physiological responses (amylase secretion, protein synthesis) and intracellular mediators [inositol 1,4,5-trisphosphate (1,4,5-IP3), [Ca2+]i, and diacylglycerol] were studied in isolated rat pancreatic acini and compared with the actions of cholecystokinin (CCK). Bombesin stimulated amylase secretion to the same extent as CCK. However, it failed to reproduce the inhibition of amylase secretion by high concentrations of CCK and likewise did not inhibit incorporation of [3H]leucine into protein in contrast to high concentrations of CCK. Low concentrations of bombesin (1-100 pM) induced repetitive oscillations in [Ca2+]i, whereas higher concentrations of bombesin (1-10 nM) induced a large transient increase in [Ca2+]i followed by a small sustained plateau. Bombesin (1-100 nM) induced an early peak of 1,4,5-IP3 at 5-15 s but was without measurable effect at lower concentrations. These effects on [Ca2+]i and 1,4,5-IP3 were similar to those seen with CCK except that bombesin was approximately 10-fold less potent than CCK. Bombesin induced an increase in acinar 1,2-diacylglycerol with a biphasic time course similar to CCK. However, the magnitude of the response to bombesin was much smaller than the response to CCK. The results suggest that bombesin receptors initiate similar intracellular messengers as does CCK. However, CCK induces a larger increase of diacylglycerol and probably an as yet unidentified messenger responsible for its inhibitory effects.