The Oscillatory Behavior of Pancreatic Islets from Mice with Mitochondrial Glycerol-3-phosphate Dehydrogenase Knockout

Glucose stimulation of pancreatic β cells induces oscillations of the membrane potential, cytosolic Ca2+ ([Ca2+]i), and insulin secretion. Each of these events depends on glucose metabolism. Both intrinsic oscillations of metabolism and repetitive activation of mitochondrial dehydrogenases by Ca2+ have been suggested to be decisive for this oscillatory behavior. Among these dehydrogenases, mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH), the key enzyme of the glycerol phosphate NADH shuttle, is activated by cytosolic [Ca2+]i. In the present study, we compared different types of oscillations in β cells from wild-type and mGPDH−/− mice. In clusters of 5–30 islet cells and in intact islets, 15 mm glucose induced an initial drop of [Ca2+]i, followed by an increase in three phases: a marked initial rise, a partial decrease with rapid oscillations and eventually large and slow oscillations. These changes, in particular the frequency of the oscillations and the magnitude of the [Ca2+] rise, were similar in wild-type and mGPDH−/− mice. Glucose-induced electrical activity (oscillations of the membrane potential with bursts of action potentials) was not altered in mGPDH−/− β cells. In single islets from either type of mouse, insulin secretion strictly followed the changes in [Ca2+]i during imposed oscillations induced by pulses of high K+ or glucose and during the biphasic elevation induced by sustained stimulation with glucose. An imposed and controlled rise of [Ca2+]i in β cells similarly increased NAD(P)H fluorescence in control and mGDPH−/− islets. Inhibition of the malate-aspartate NADH shuttle with aminooxyacetate only had minor effects in control islets but abolished the electrical, [Ca2+]i and secretory responses in mGPDH−/− islets. The results show that the two distinct NADH shuttles play an important but at least partially redundant role in glucose-induced insulin secretion. The oscillatory behavior of β cells does not depend on the functioning of mGPDH and on metabolic oscillations that would be generated by cyclic activation of this enzyme by Ca2+.