Inhibition of Mitochondrial Na+-Ca2+ Exchanger Increases Mitochondrial Metabolism and Potentiates Glucose-Stimulated Insulin Secretion in Rat Pancreatic Islets

Abstract

؉ . We show that inhibition of the mNCE enhances mitochondrial oxidative metabolism and increases glucose-stimulated insulin secretion in rat islets and INS-1 cells. The benzothiazepine CGP37157 inhibited mNCE activity in INS-1 cells (50% inhibition at IC 50 ‫؍‬ 1.5 mol/l) and increased the glucose-induced rise in mitochondrial Ca 2؉ ([Ca 2؉ ] m ) 2.1 times. Cellular ATP content was increased by 13% in INS-1 cells and by 49% in rat islets by CGP37157 (1 mol/l). Krebs cycle flux was also stimulated by CGP37157 when glucose was present. Insulin secretion was increased in a glucosedependent manner by CGP37157 in both INS-1 cells and islets. In islets, CGP37157 increased insulin secretion dose dependently (half-maximal efficacy at EC 50 ‫؍‬ 0.06 mol/l) at 8 mmol/l glucose and shifted the glucose dose response curve to the left. In perifused islets, mNCE inhibition had no effect on insulin secretion at 2.8 mmol/l glucose but increased insulin secretion by 46% at 11 mmol/l glucose. The effects of CGP37157 could not be attributed to interactions with the plasma membrane sodium calcium exchanger, L-type calcium channels, ATP-sensitive K ؉ channels, or [Ca 2؉ ] m uniporter. In hyperglycemic clamp studies of Wistar rats, CGP37157 increased plasma insulin and C-peptide levels only during the hyperglycemic phase of the study. These results illustrate the potential utility of agents that affect mitochondrial metabolism as novel insulin secretagogues. Diabetes 52:965-973, 2003 M itochondrial oxidative metabolism plays an important role in the insulin secretory process in pancreatic ␤-cells. Stimulus-secretion coupling in the ␤-cell depends on the metabolism of glucose and the subsequent mitochondrial oxidative phosphorylation that generates ATP. ATP closes ATPsensitive K ϩ (K ATP ) channels, causing depolarization of the ␤-cell membrane, opening of voltage-dependent calcium channels, and Ca 2ϩ influx (1,2). Although two main processes, glycolysis and oxidative phosphorylation, are responsible for ATP synthesis during glucose metabolism, oxidative phosphorylation is the predominant pathway in the pancreatic ␤-cell (3,4). Insulin secretion induced by other secretagogues, such as leucine and glyceraldehydes, is also mediated by the production of ATP (5,6). The critical regulatory role of oxidative ATP production in glucose-stimulated insulin secretion (GSIS) is underscored by the observation that disrupting mitochondrial oxidative metabolism blocks nutrient-mediated insulin secretion. For example, inhibition of oxidative phosphorylation (7,8), blockade of NADH transport into mitochondria (9,10), or elimination of mitochondrial DNA from ␤-cells in vitro (11-15) or in vivo (16) all prevent GSIS. We tested the hypothesis that enhancing oxidative ATP production in response to glucose can increase insulin secretion. The normal feed-forward regulatory role of mitochondrial Ca 2ϩ ([Ca 2ϩ ] m ) in the ␤-cell was exploited to enhance oxidative metabolism.The influx of calcium into the cytoplasm of the ␤-cell in response to a nutrient l...

The mitochondrial Na+/Ca2+ exchanger plays a key role in the control of cytosolic Ca2+ oscillations

“…In addition, this new function of the mitochondrial Na + /Ca 2+ exchanger allows it to perform a fine tuning of the cytosolic Ca 2+ oscillatory pattern. This effect adds to the previously shown modulation of Ca 2+ entry [36] and K Ca current [37] by this exchange system. It is interesting to note here that a functional cross-talk between the plasma membrane Na + /Ca 2+ exchanger and InsP 3 R has also been recently described [38].…”

Pathways in Beta-Cell Stimulus-Secretion Coupling as Targets for Therapeutic Insulin Secretagogues

“…The rise in ␤-cell [Ca 2ϩ ] i produced by glucose and other secretagogues is followed by an influx of Ca 2ϩ into mitochondria. It has been proposed that, by activating matrix dehydrogenases, Ca 2ϩ augments the production of ATP and other putative messenger molecules (34,35 (36). It also increased glucose-induced insulin secretion by rat islets and augmented plasma insulin levels at certain time points of a hyperglycemic clamp.…”

Taurine increases glucose sensitivity of UCP2-overexpressing β-cells by ameliorating mitochondrial metabolism

“…In contrast, the Ca 2ϩ /H ϩ exchanger mechanism is dominant in liver and kidney, which are major tissues involved in clearance of metal ions from the body (37). Blocking ␤-cell Na ϩ /Ca 2ϩ exchangers has been reported to increase GSIS (12), whereas the reverse is true in the case of PTP blockage by the immunosuppressive drug CysA (23). This difference in regulation of insulin secretion might be due to the distinct physiological roles of these compounds, with PTP being intimately involved in other important functions such as apoptosis (37).…”

“…Because intramitochondrial Ca 2ϩ concentration ([Ca 2ϩ ] m ) has been observed to activate several dehydrogenases coupled to the Krebs or tricarboxylic acid cycle (30,39), determinants of [Ca 2ϩ ] m may be critical for controlling ␤-cell metabolic rate. Blocking of [Ca 2ϩ ] m efflux in pancreatic ␤-cells with CGP-37157, a mitochondria-specific Na ϩ /Ca 2ϩ exchanger antagonist, has been recently shown to increase GSIS (23). Despite the important role of taurine in [Ca 2ϩ ] m , its influence has not been emphasized in the [Ca 2ϩ ] m dynamics in ␤-cells.…”