beta-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 muM each), the sulfonylurea tolbutamide (500 muM), or the alkaloid quinine (100 muM) resulted in morphological damage of 4-18% of beta-cells compared to less than 2% in controls. Thus, the question arose whether K-ATP channel-blocking insulin secretagogues are beta-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 muM and by 98% at 1000 muM. Phentolamine and quinine reduced viability comparably at 1000 muM, but were less toxic at 100 muM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000 muM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100 muM of idazoxan and phentolamine, whereas at 1000 muM signs of necrosis predominated. Alinidine, tolbutamide and quinine treatment did not increase markers of apoptotic cell death. Blocking Ca2+ influx by D600 did not diminish secretagogue-associated toxicity. Electron microscopy confirmed the validity of these observations for beta-cells in intact mouse islets. In summary, beta-cell toxicity of the tested insulin secretagogues varied widely and did not depend on a prolonged Ca2+ influx via L-type Ca2+ channels. Thus, secretagogue-mediated closure of K-ATP channels is apparently not per se beta-cell toxic. (C) 2004 Elsevier Inc. All rights reserved.