Permeability transition pore regulates both mitochondrial membrane potential and agonist-evoked Ca2+ signals in oligodendrocyte progenitors

Abstract

In this study, we investigated the importance of mitochondrial permeability transition pore (PTP) in agonist-evoked cytosolic Ca2+ ([Ca2+](c)) signals in oligodendrocyte progenitor cells (OP cells). We measured transmembrane potential across the mitochondrial inner membrane (Delta Psi(m)) and [Ca2+](c) in the immediate vicinity simultaneously using tetramethylrhodamine ethyl ester (TMRE) and calcium green respectively. Stimulation of OP cells with methacholine evoked robust [Ca2+](c) signals in approximately 80% of cells which were either oscillatory or showed a peak followed by a plateau. Elevations in [Ca2+](c) induced by supramaximal concentrations of the agonist (> 200 mu M) were accompanied by changes in Delta Psi(m) in 33-42% of the total mitochondria investigated. the mitochondria that responded either depolarized (26-29%), hyperpolarized (7-13%) or showed no change (58-67%). Thus, of the responsive mitochondria, most (70%) depolarized during agonist-evoked [Ca2+](c) signals. Blockade of PTP with cyclosporin A (CSA) reduced the number of mitochondria that depolarized with a corresponding increase in the number that hyperpolarized. in addition, CSA or its analogue methyl valine-4- CSA (MeVal-CSA), reduced the frequency of agonist-evoked global [Ca2+](c) oscillations. in resting cells, CSA (63%) and MeVal-CSA (77%) hyperpolarized a majority of the mitochondria suggesting that PTP is constitutively active and may show flickering openings. Such hyperpolarizations were not mimicked by either cyclosporine H or verapamil and were inhibited by Ru360, which blocks the mitochondrial uniporter. This observation suggested that in resting cells, Ca2+ ions might redistribute between cytosol and mitochondrial matrix through the uniporter and the PTP. Taken together, these data suggest that PTP may play an important role in regulating Delta Psi(m) and local [Ca2+](c) signals during agonist stimulation in OP cells. (C) Harcourt Publishers Ltd 1999.