Effects of over-stimulation by glucose on pancreatic beta-cell functioning : studies in vitro with diazoxide

Abstract: Long term hyperglycemia impairs pancreatic ß-cell function. In vivo studies in rats have shown a decreased glucose-induced insulin response in the perfused pancreas after high glucose infusions. If insulin secretion was blocked by diazoxide in the presence of high glucose, the subsequent insulin response to glucose was restituted, indicating that overstimulation was an important factor behind the impairment. Long term exposure of rat pancreatic islets to high glucose in vitro also resulted in a decreased glucose-induced insulin response, which could be prevented by previous exposure to diazoxide. The after-effect of diazoxide was not present after low glucose culture and not tightly coupled to its interaction with K+-ATP channels. Omitting Ca2+ during cooling completely mimicked the effect of diazoxide indicating that diazoxide acts on both the level of Ca2+ inflow and exocytosis. Also in human islets cultured at 27 mM glucose for 48 h previous diazoxide restituted a glucose-induced insulin response in final incubations. Diazoxide also protected against specific [Ca2+]i abnormalities seen after high glucose culture such as a rise in basal [Ca2+]i and a loss of slow oscillations. On the other hand, diazoxide did not restore a glucose-induced rise in [Ca2+ ]i. Diazoxide protected against over-stimulation also with regard to the adenylate cyclase-cAMP system. Exposing pancreatic islets from GK rats and GK hybrids to diazoxide increased intracellular insulin content. In contrast previous exposure to diazoxide failed to improve an impaired insulin response to glucose Culture of human pancreatic islets at high glucose increased intraislet PI/I (proinsulin/insulin) ratios and PI/I ratios of secretion. Co-culture with diazoxide normalized these parameters. The inclusion of palmitate enhanced PI/I ratios of secretion. This effect was additive to that of glucose. In conclusion, over-stimulation by long term elevated glucose induces several effects that may be harmful to the ß-cell, indicating that inducing "ß-cell rest" may offer a new approach in the treatment of type 2 diabetes.