Endocrine Abstracts

Aim: To identify the role of unmetabolised arachidonic acid (AA) produced by phospholipase A₂ (PLA₂) enzymes, and its 12-lipoxygenase (12-LOX) and cyclooxygenase (COX) products in the insulin secretory response of human islets of Langerhans. Material and Methods: Expression of COX-1 and COX-2 mRNAs was determined by RT-PCR. Insulin secretion from perifused human islets was measured by radioimmunoassay, in the presence of inhibitors of PLA₂ (AACOCF₃, PACOCF₃), COX and LOX (phenidone), 12-LOX (baicalein) and COX-2 (NS-398). Results: Both COX-1 and COX-2 mRNAs were amplified by RT-PCR from unstimulated human islets. Insulin release from human islets increased significantly (P<0.01) in response to 20mM glucose, with a maximal response of 825±91% basal (2mM glucose) secretion, followed by a sustained plateau of 361±38% basal (n=10). Neither the profile nor the magnitude of the glucose-induced insulin secretory response was significantly affected by 100µM of AACOCF₃ or PACOCF₃. Exposure of human islets to AA (50µM) transiently stimulated basal insulin secretion (peak 298±63% basal, P<0.05, n=6) and returned to basal levels within 20 minutes, but subsequent exposure to AA in the presence of phenidone led to a further, significant increase in insulin secretion. A similar profile was obtained when human islets were exposed to NS-398. Exposure of human islets to NS-398 or baicalein in the absence of AA transiently increased basal insulin secretion (246±26% basal; 247±18% basal respectively, n=3). Conclusion: COX-2 is constitutively expressed in human islets and we have shown, for the first time, that human islets also express COX-1 mRNA. Secretion data provide compelling evidence for a stimulatory role for unmetabolised AA in the stimulation of insulin secretion from human islets of Langerhans, but they also indicate that AA generation is not obligatory for nutrient-induced insulin secretion.

Work was funded by Eli Lilly International Foundation.