Active mitochondria surrounding the pancreatic acinar granule region prevent spreading of inositol trisphosphate-evoked local cytosolic Ca2+ signals

Abstract

Agonist-evoked cytosolic Ca2+ spikes in mouse pancreatic acinar cells are specifically initiated in the apical secretory pole and are mostly confined to this region. The role played by mitochondria in this process has been investigated. Using the mitochondria-specific fluorescent dyes MitoTracker Green and Rhodamine 123, these organelles appeared as a bright belt concentrated mainly around the secretory granule area. We tested the effects of two different types of mitochondrial inhibitor on the cytosolic Ca2+ concentration using simultaneous imaging of Ca2+-sensitive fluorescence (Fura 2) and electrophysiology. When carbonyl cyanide m-chrorophenylhydrazone (CCCP) was applied in the presence of the Ca2+-releasing messenger inositol 1,4,5-trisphosphate (IP3), the local repetitive Ca2+ responses in the granule area were transformed into a global rise in the cellular Ca2+ concentration. In the absence of IP3, CCCP had no effect on the cytosolic Ca2+ levels. Antimycin and antimycin + oligomycin had the same effect as CCCP. Active mitochondria, strategically placed around the secretory pole, block Ca2+ diffusion from the primary Ca2+ release sites in the granule-rich area in the apical pole to the basal part of the cell containing the nucleus. When mitochondrial function is inhibited, this barrier disappears and the Ca2+ signals spread all over the cytosol.