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

Abstract

The mitochondrial Na⁺-Ca²⁺ exchanger (mNCE) mediates efflux of Ca²⁺ from mitochondria in exchange for influx of Na⁺. 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 IC₅₀ = 1.5 μmol/l) and increased the glucose-induced rise in mitochondrial Ca²⁺ ([Ca²⁺]_m) 2.1 times. Cellular ATP content was increased by 13% in INS-1 cells and by 49% in rat islets by CGP37157 (1 μmol/l). Krebs cycle flux was also stimulated by CGP37157 when glucose was present. Insulin secretion was increased in a glucose-dependent manner by CGP37157 in both INS-1 cells and islets. In islets, CGP37157 increased insulin secretion dose dependently (half-maximal efficacy at EC₅₀ = 0.06 μmol/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 [Ca²⁺]_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.