Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

Abstract

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.