Evidence for a role of superoxide generation in glucose-induced beta-cell dysfunction in vivo

Prolonged elevation of glucose can adversely affect beta-cell function. In vitro studies have linked glucose-induced beta-cell dysfunction to oxidative stress; however, whether oxidative stress plays a role in vivo is unclear. Therefore, our objective was to investigate the role of oxidative stress in an in vivo model of glucose-induced beta-cell dysfunction.

Wistar rats were infused intravenously with glucose for 48 h to achieve 20 mmol/l hyperglycemia with/without co-infusion of one of the following antioxidants: taurine (2-amino ethanesulfonic acid) (TAU), an aldehyde scavenger; N-acetylcysteine (NAC), a precursor of glutathione; or tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) (TPO), a superoxide dismutase mimetic. This was followed by islet isolation or hyperglycemic clamp.

A 48-h glucose infusion decreased glucose-stimulated insulin secretion (GSIS) and elevated reactive oxygen species (ROS), total superoxide, and mitochondrial superoxide in freshly isolated islets. TPO prevented the increase in total and mitochondrial superoxide and the beta-cell dysfunction induced by high glucose. However, TAU and NAC, despite completely normalizing H(2)DCF-DA (dihydro-dichlorofluorescein diacetate)-measured ROS, did not prevent the increase in superoxide and the decrease in beta-cell function induced by high glucose. TPO but not TAU also prevented beta-cell dysfunction induced by less extreme hyperglycemia (15 mmol/l) for a longer period of time (96 h). To further investigate whether TPO is effective in vivo, a hyperglycemic clamp was performed. Similar to the findings in isolated islets, prolonged glucose elevation (20 mmol/l for 48 h) decreased beta-cell function as assessed by the disposition index (insulin secretion adjusted for insulin sensitivity), and co-infusion of TPO with glucose completely restored beta-cell function.

These findings implicate superoxide generation in beta-cell dysfunction induced by prolonged hyperglycemia.