Gastric inhibitory polypeptide mechanisms of augmenting insulin secretion

The cellular mechanisms whereby gastric inhibitory polypeptide (GIP) augments glucose-dependent insulin secretion remains poorly defined. Since glucose-dependent insulin secretion is modulated by membrane associated phospholipase A2 (PLA2) and intracellular lipoxygenase (LPX) and cyclooxygenase (CO) we hypothesize that GIP's augmentation of insulin secretion involves these enzyme systems. Neonatal rat pancreatic islet cell cultures were preincubated with 5.6mM glucose for 60 minutes. The cultures were then stimulated for 60 minutes with 16mM glucose alone or with GIP with or without the addition of PLA2, LPX, and CO inhibitors. Insulin secretion significantly increased (P less than 0.05) when the glucose concentration was raised from 5.6 to 16mM glucose and this was further augmented by the addition of GIP (P less than 0.05). PLA2 inhibitors significantly (P less than 0.025) decreased 16mM glucose insulin secretion but this was restored by the simultaneous addition of GIP. LPX inhibitors significantly (P less than 0.01) decreased glucose-dependent insulin secretion and this decrease persisted despite the addition of GIP. Simultaneous treatment of islet cell cultures with GIP and CO inhibitors yielded insulin responses that were indistinguishable from CO inhibition alone. These studies suggest that GIP exerts its influence in part by modulating membrane associated PLA2 activity. Furthermore, the formation of intracellular LPX products appears to be a pivotal step in the insulinotrophic action of GIP.