Insulin-sparing effects of troglitazone in rat pancreatic islets

Thiazolidinediones (TZDs) have been suggested to act beneficially on pancreatic islet function and on beta-cell viability but data concerning direct effects on isolated islets are controversial. Therefore, we have examined parameters of pancreatic insulin and glucagon secretion and biosynthesis in TZD-exposed rat pancreatic islets under physiological glucose level conditions and under conditions of glucolipotoxicity. Primary rat islets were incubated for 2.5 h with or without troglitazone (10 muM) in 5.6 mM glucose (standard glucose levels) and 16.7 mM glucose (high glucose levels); a subgroup was additionally treated with oleate (200 muM) to simulate acute glucolipotoxicity. Insulin and glucagon secretion, intracellular content and their respective mRNAs were quantified. Newly synthesized insulin was determined by pulse-labeling experiments. Troglitazone reduced insulin secretion at standard and high glucose levels by about one-third (Pless than or equal to0.05). Insulin content was decreased at 5.6 mM glucose but increased at 16.7 mM glucose by the presence of troglitazone (Pless than or equal to0.05). Newly synthesized insulin mRNA and preproinsulin mRNA decreased by about 20% at standard glucose levels (Pless than or equal to0.05). Glucagon secretion was augmented by troglitazone in islets under high glucose conditions by an additional 50% (Pless than or equal to0.05). No clear beneficial troglitazone effects were observed under glucolipotoxic conditions. The reduced insulin secretion and biosynthesis at standard glucose levels can be interpreted as an insulin-sparing effect. Troglitazone effects were less pronounced at high glucose alone or in combination with oleate. From a clinical point of view, these results indicate a greater benefit of troglitazone for beta-cell function in hyperinsulinemic, but normoglycemic patients with insulin resistance or early type 2 diabetes without major insulin secretion deficits and/or pronounced hyperglycemia.