β-Cell toxicity of ATP-sensitive K + channel-blocking insulin secretagogues

Abstract

A prolonged exposure of isolated pancreatic islets to insulin secretagogues, the imidazolines phentolamine, alinidine and idazoxan (100 μM each), the sulfonylurea tolbutamide (500 μM), or the alkaloid quinine (100 μM) resulted in morphological damage of 4–18% of β-cells compared to less than 2% in controls. Thus, the question arose whether K ATP channel-blocking insulin secretagogues are β-cell toxic as has already been suggested for sulfonylureas. The concentration- and time-dependency of the secretagogue-associated toxicity was documented by viability assays in insulin-secreting HIT T15 cells. Treatment for 24 h with idazoxan reduced MTT conversion by 50% at 100 μM and by 98% at 1000 μM. Phentolamine and quinine reduced viability comparably at 1000 μM, but were less toxic at 100 μM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000 μM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100 μM of idazoxan and phentolamine, whereas at 1000 μM signs of necrosis predominated. Alinidine, tolbutamide and quinine treatment did not increase markers of apoptotic cell death. Blocking Ca 2+ influx by D600 did not diminish secretagogue-associated toxicity. Electron microscopy confirmed the validity of these observations for β-cells in intact mouse islets. In summary, β-cell toxicity of the tested insulin secretagogues varied widely and did not depend on a prolonged Ca 2+ influx via L-type Ca 2+ channels. Thus, secretagogue-mediated closure of K ATP channels is apparently not per se β-cell toxic.