Effects of oxidants on membrane potential, K+ and Ca2+ currents of mouse pancreatic B-cells

Role of K(ATP) channels in β-cell resistance to oxidative stress

The importance of K(ATP) channels in stimulus-secretion coupling of β-cells is well established, although they are not indispensable for the maintenance of glycaemic control. This review article depicts a new role for K(ATP) channels by showing that genetic or pharmacological ablation of these channels protects β-cells against oxidative stress. Increased production of oxidants is a crucial factor in the pathogenesis of type 2 diabetes mellitus (T2DM). T2DM develops when β-cells can no longer compensate for the high demand of insulin resulting from excess fuel intake. Instead β-cells start to secrete less insulin and β-cell mass is diminished by apoptosis. Both, reduction of insulin secretion and β-cell mass induced by oxidative stress, are prevented by deletion or inhibition of K(ATP) channels. These findings may open up new insights into the early treatment of T2DM.

Oxidative stress and beta-cell dysfunction

Diabetes mellitus type 1 and 2 (T1DM and T2DM) are complex multifactorial diseases. Loss of beta-cell function caused by reduced secretory capacity and enhanced apoptosis is a key event in the pathogenesis of both diabetes types. Oxidative stress induced by reactive oxygen and nitrogen species is critically involved in the impairment of beta-cell function during the development of diabetes. Because of their low antioxidant capacity, beta-cells are extremely sensitive towards oxidative stress. In beta-cells, important targets for an oxidant insult are cell metabolism and K(ATP) channels. The oxidant-evoked alterations of K(ATP) channel activity seem to be critical for oxidant-induced dysfunction because genetic ablation of K(ATP) channels attenuates the effects of oxidative stress on beta-cell function. Besides the effects on metabolism, interference of oxidants with mitochondria induces key events in apoptosis. Consequently, increasing antioxidant defence is a promising strategy to delay beta cell failure in (pre)-diabetic patients or during islet transplantation. Knock-out of K(ATP) channels has beneficial effects on oxidant-induced inhibition of insulin secretion and cell death. Interestingly, these effects can be mimicked by sulfonylureas that have been used in the treatment of T2DM for many years. Loss of functional K(ATP) channels leads to up-regulation of antioxidant enzymes, a process that depends on cytosolic Ca(2+). These observations are of great importance for clinical intervention because they show a possibility to protect beta-cells at an early stage before dramatic changes of the secretory capacity and loss of cell mass become manifest and lead to glucose intolerance or even overt diabetes.

Altered excitability of intestinal neurons in primary culture caused by acute oxidative stress

Neurons were isolated from the intestine of guinea pigs and grown in primary culture for < or =15 days. Using conventional whole cell recording techniques, we demonstrated that the majority of neurons express a prolonged poststimulus afterhyperpolarization (slow AHP). These neurons also had large-amplitude (approximately 100 mV), broad-duration (approximately 2 ms) action potentials and generated a hyperpolarization activated inward current (Ih). Application of H2O2 (0.22-8.8 mM) hyperpolarized these neurons but not those lacking slow AHPs. The H2O2-induced hyperpolarization was followed by irreversible depolarization at higher concentrations (more than approximately 1 mM) of H2O2 while it was maintained after washout of submillimolar H2O2. The ionic mechanisms underlying the hyperpolarization included the suppression of Ih and the activation of an inwardly rectifying outward current, which was blocked by glybenclamide (25-50 microM) and TEA (30 mM). In addition, H2O2 suppressed the slow AHP and its underlying current. Internal perfusion of catalase and glutathione opposed the H2O2-mediated decrease in IsAHP. Our results indicate that acute oxidative stress has neuron- and conductance-specific actions in intestinal neurons that may underlie pathophysiological conditions.