Beta-cell toxicity of ATP-sensitive K+ channel-blocking insulin secretagogues

A prolonged exposure of isolated pancreatic islets to insulin secretagogues, the imidazolines phentolamine, alinidine and idazoxan (100microM each), the sulfonylurea tolbutamide (500microM), or the alkaloid quinine (100microM) 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 24h with idazoxan reduced MTT conversion by 50% at 100microM and by 98% at 1000microM. Phentolamine and quinine reduced viability comparably at 1000microM, but were less toxic at 100microM. On the other hand, the imidazoline alinidine and the sulfonylurea tolbutamide were only moderately toxic (less than 40% viability loss at 1000microM). The imidazoline efaroxan appeared even to be non-toxic. Apoptotic DNA fragmentation and DEVD-caspase activation was observed at 100microM of idazoxan and phentolamine, whereas at 1000microM 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 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 Ca(2+) influx via L-type Ca(2+) channels. Thus, secretagogue-mediated closure of K(ATP) channels is apparently not per se beta-cell toxic.