Aims/hypothesis Generation of reduction equivalents is a prerequisite for nutrient-stimulated insulin secretion. Mitochondrial succinate dehydrogenase (SDH) fulfils a dual function with respect to mitochondrial energy supply: (1) the enzyme is part of mitochondrial respiratory chains; and (2) it catalyses oxidation of succinate to fumarate in the Krebs cycle. The aim of our study was to elucidate the significance of SDH for beta cell stimulus-secretion coupling (SSC). Methods Mitochondrial variables, reactive oxygen species (ROS) and cytosolic Ca 2+ concentration ([Ca 2+ ] c ) were measured by fluorescence techniques and insulin release by radioimmunoassay in islets or islet cells of C57Bl/6N mice. Results Inhibition of SDH with 3-nitropropionic acid (3-NPA) or monoethyl fumarate (MEF) reduced glucosestimulated insulin secretion. Inhibition of the ATP-sensitive K + channel (K ATP channel) partly prevented this effect, whereas potentiation of antioxidant defence by superoxide dismutase mimetics (TEMPOL and mito-TEMPO) or by nuclear factor erythroid 2-related factor 2 (Nrf-2)-mediated upregulat i o n o f a n t i o x i d a n t e n z y m e s ( o l t i p r a z , t e r tbutylhydroxyquinone) did not diminish the inhibitory influence of 3-NPA. Blocking SDH decreased glucose-stimulated increase in intracellular FADH 2 concentration without alterations in NAD(P)H. In addition, 3-NPA and MEF drastically reduced glucose-induced hyperpolarisation of mitochondrial membrane potential, indicative of decreased ATP production. As a consequence, the glucose-stimulated rise in [Ca 2+ ] c was significantly delayed and reduced. Acute application of 3-NPA interrupted glucose-driven oscillations of [Ca 2+ ] c . 3-NPA per se did not elevate intracellular ROS, but instead prevented glucose-induced ROS accumulation. Conclusions/interpretation SDH is an important regulator of insulin secretion and ROS production. Inhibition of SDH interrupts membrane-potential-dependent SSC, pointing to a pivotal role of mitochondrial FAD/FADH 2 homeostasis for the maintenance of glycaemic control.

Atrial natriuretic peptide (ANP) influences glucose homeostasis and possibly acts as a link between the cardiovascular system and metabolism, especially in metabolic disorders like diabetes. The current study evaluated effects of ANP on β-cell function by the use of a β-cell-specific knockout of the ANP receptor with guanylate cyclase activity (βGC-A-KO). ANP augmented insulin secretion at the threshold glucose concentration of 6 mmol/L and decreased K single-channel activity in β-cells of control mice but not of βGC-A-KO mice. In wild-type β-cells but not β-cells lacking functional K channels (SUR1-KO), ANP increased electrical activity, suggesting no involvement of other ion channels. At 6 mmol/L glucose, ANP readily elicited Ca influx in control β-cells. This effect was blunted in β-cells of βGC-A-KO mice, and the maximal cytosolic Ca concentration was lower. Experiments with inhibitors of protein kinase G (PKG), protein kinase A (PKA), phosphodiesterase 3B (PDE3B), and a membrane-permeable cyclic guanosine monophosphate (cGMP) analog on K channel activity and insulin secretion point to participation of the cGMP/PKG and cAMP/PKA/Epac (exchange protein directly activated by cAMP) directly activated by cAMP Epac pathways in the effects of ANP on β-cell function; the latter seems to prevail. Moreover, ANP potentiated the effect of glucagon-like peptide 1 (GLP-1) on glucose-induced insulin secretion, which could be caused by a cGMP-mediated inhibition of PDE3B, which in turn reduces cAMP degradation.