Protection of rat pancreatic islets by potassium channel openers against alloxan, sodium nitroprusside and interleukin-1β mediated suppression—possible involvement of the mitochondrial membrane potential

Abstract

Aims/hypothesis. We aimed to study the effects of two K ATP channel openers (KCO), diazoxide and the more potent compound NNC 55-0118, on beta-cell suppression and/or toxicity induced by alloxan, sodium nitroprusside and IL-1β. Methods. Islets from rats were exposed to 0.3 mmol/l diazoxide or NNC 55-0118 for 30 min and either alloxan (0.5 mmol/l), sodium nitroprusside (0.5 mmol/l) or IL-1β (12.5 or 25 U/ml) were added and the incubation continued for 30 min. Islets were then washed and incubated for 24 h before examination. Results. After exposure to alloxan, islets showed reduced glucose oxidation rate and impaired glucosestimulated insulin release. NNC 55-0118 counteracted the effects of alloxan, while diazoxide was less effective. After treatment with sodium nitroprusside, islet glucose oxidation rates were reduced and this was prevented by pretreatment with NNC 55-0118. In shortterm experiments the potassium channel openers (KCOs) did not influence the IL-1β effect on insulin secretion. However, long-term addition (24 h) of NNC 55-0118 counteracted IL-1β induced inhibition of the glucose oxidation rate. It was shown, using the fluorescent probe JC-1, that the mitochondrial membrane potential was reduced by the potassium channel openers (KCOs), most strongly by NNC 55-0118. Nevertheless culture with KCOs for 72 h did not cause irreversible damage to the islets. Conclusion/interpretation. Potassium channel openers (KCOs), in particular NNC 55-0118, prevented the toxic effects of alloxan and sodium nitroprusside. IL-1β mediated suppression was reduced by NNC 55-0118 provided the long-term addition of the potassium channel opener (KCO). The protective mechanism of potassium channel openers (KCOs) might involve a decrease of the mitochondrial membrane potential. [Diabetologia (2003) 46:80-88] Keywords Alloxan, interleukin-1β, mitochondria, nitric oxide, pancreatic islets, potassium channel opener, sodium nitroprusside. Environmental as well as immunological events are thought to be of importance for the destruction of the insulin-producing beta-cells in the islets of Langerhans in Type 1 diabetes [1]. The mechanisms and mediators of this process are not fully understood. There is evidence suggesting that a period of reduced activity following the start of insulin treatment is beneficial in new onset diabetes [2,3] and that the cellular activity affects the susceptibility to damage in vitro [4,5,6]. Inhibition of insulin secretion by the use of potassium channel openers (KCOs) provides a means of inducing 'beta-cell rest'. The drugs open the ATP-sensitive potassium channel (K ATP channel) and hyperpol-

Effects of Diazoxide on Gene Expression in Rat Pancreatic Islets Are Largely Linked to Elevated Glucose and Potentially Serve to Enhance β-Cell Sensitivity

“…Diazoxide enhances ischemic preconditioning in the heart (12) and neurons (13,14) through a mitochondrial interaction that may be coupled to reduction of mitochondrial reactive oxygen species production (14). In ␤-cells diazoxide decreases mitochondrial membrane potential in acute experiments (15,16) and may inhibit succinate oxidation (17). We did not observe any effect on gene expression for succinate dehydrogenase, represented in the array by four genes encoding for separate subunits.…”

Glucose-stimulated DNA Synthesis through Mammalian Target of Rapamycin (mTOR) Is Regulated by KATP Channels

“…Overall, these results suggested that diazoxide might be exerting effects independent of the activation of plasma membrane K ATP channels that are responsible for the potentiation of glucose-stimulated DNA synthesis. Findings by Kullin et al (31) also reported that K ATP channel openers including diazoxide decreased the vulnerability of rat islets to free radicals induced by alloxan, nitroprusside, or IL-1␤. It was proposed that K ATP channel openers by their ability to decrease Ca 2ϩ influx due to ␤-cell membrane hyperpolarization in combination with their direct effect to decrease the mitochondrial membrane potential reduced ATP production necessary for the activation of apoptotic pathways.…”

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels in Pancreatic β-Cells

“…Recent reports have shown that K ATP channel activation using diazoxide or other selective openers induces ␤-cell rest and protection against the effects of high-fat diet (40)(41)(42). Chronic studies where HCN channel function or expression is manipulated are required to determine the potential roles of these channels on ␤-cell function.…”