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

Abstract

] i on the exocytotic process (6 -8). Both pathways require glucose metabolism and appear to depend on a rise in the ATP/ADP ratio (9).Glucose metabolism in ␤ cells essentially occurs through aerobic glycolysis (10 -12). An increase in the glucose concentration is followed by an acceleration of glycolysis and an even greater stimulation of mitochondrial oxidative events, in which Ca 2ϩ may play an important role. Thus, the elevation of [Ca 2ϩ ] i is paralleled by an increase in mitochondrial Ca 2ϩ (13) that may then activate the three Ca 2ϩ -sensitive intramitochondrial dehydrogenases and promote ATP synthesis (14). Another feature of the ␤ cell metabolic organization is a low activity of lactate dehydrogenase (12,15). Cytosolic NADH formed during glycolysis is re-oxidized (and ATP synthesis concomitantly stimulated) by transfer of the reducing equivalents into mitochondria through the glycerol phosphate shuttle and the malate-aspartate shuttle (16, 17). The rate-limiting enzyme of the former shuttle, mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is located on the outer face of the mitochondrial membrane and can be activated by increases in cytosolic [Ca 2ϩ ] i (18,19). The importance of the NADH shuttles for glucose-induced insulin secretion (20) has received strong support from a recent study using islets from mice with a targeted disruption of mGPDH (21).Many ␤ cell responses to glucose are oscillatory. Oscillations of the membrane potential drive oscillations of [Ca 2ϩ ] i , leading to oscillations of insulin secretion that can be amplified by metabolic oscillations (4,(22)(23)(24)(25)(26). It is still unclear whether oscillations of glucose metabolism are intrinsic and initiate the whole chain of other pulsatile events or are entrained by the oscillations of [Ca 2ϩ ] i (27). The Ca 2ϩ sensitivity of mGPDH makes the enzyme a possible key site of the interplay between glucose metabolism and [Ca 2ϩ ] i oscillations. Thus, imposed oscillations of free Ca 2ϩ in islet mitochondrial extracts induced oscillations of mGPDH activity (28). If the hypothesis is correct, the oscillatory behavior of stimulus secretion coupling should be perturbed by mGPDH defects. The present study addressed this question with islets isolated from mGPDH knock-out (mG-PDH Ϫ/Ϫ ) mice (21). We compared the oscillations of ␤ cell

The Succinate Mechanism of Insulin Release

“…Mitochondrial glycerol-3-phosphate dehdyrogenase. Islets from mutant mice that lack mitochondrial glycerol-3-phosphate dehydrogenase activity have normal glucosestimulated insulin release (56,65). This is consistent with the succinate mechanism, which does not require an intact glycerol-3-phosphate shuttle for glucose-stimulated insulin release.…”

A high carbohydrate diet does not induce hyperglycaemia in a mitochondrial glycerol-3-phosphate dehydrogenase-deficient mouse

“…Although several previous reports have described mGPDH -/-mice as normal [14,15,16,17], our results showed that the blood concentrations of triglycerides and cholesterol in mGPDH -/-mice are increased. These results could be due to the absence of mGPDH activity in the liver, but not in the pancreas.…”

“…To determine the role of the glycerol-3-phosphate shuttle in glucose-induced insulin secretion, mice with a targeted disruption of the mGPDH gene were generated [14,15]. mGPDH -/-knockout mice are not diabetic, only when the activities of both shuttles are halted by the use of aminooxyacetate, to block the malate-aspartate shuttle, are insulin secretion, glucose metabolism, and calcium oscillations severely impaired [16,17].…”

Signals and Pools Underlying Biphasic Insulin Secretion

“…The numerous islets composing the endocrine pancreas could be functionally heterogeneous, some of them being responsible for the first phase and others for the second phase (6). This hypothesis is refuted by the observation of biphasic insulin secretion from single islets (7)(8)(9)(10). Another possibility is that, within each islet, different ␤-cells preferentially secrete during first or second phase.…”