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J. Cell Biol.,
Volume 142, Number 4, August 24, 1998 975-988

* Department of Physiology, Digital imaging of mitochondrial potential in
single rat cardiomyocytes revealed transient depolarizations of mitochondria discretely localized within the
cell, a phenomenon that we shall call "flicker." These
events were usually highly localized and could be restricted to single mitochondria, but they could also be
more widely distributed within the cell. Contractile
waves, either spontaneous or in response to depolarization with 50 mM K+, were associated with propagating
waves of mitochondrial depolarization, suggesting that
propagating calcium waves are associated with mitochondrial calcium uptake and consequent depolarization. Here we demonstrate that the mitochondrial
flicker was directly related to the focal release of calcium from sarcoplasmic reticular (SR) calcium stores
and consequent uptake of calcium by local mitochondria. Thus, the events were dramatically reduced by (a)
depletion of SR calcium stores after long-term incubation in EGTA or thapsigargin (500 nM); (b) buffering
intracellular calcium using BAPTA-AM loading; (c)
blockade of SR calcium release with ryanodine (30 µM); and (d) blockade of mitochondrial calcium uptake by microinjection of diaminopentane pentammine
cobalt (DAPPAC), a novel inhibitor of the mitochondrial calcium uniporter. These observations demonstrate that focal SR calcium release results in calcium
microdomains sufficient to promote local mitochondrial calcium uptake, suggesting a tight coupling of calcium signaling between SR release sites and nearby mitochondria.
Department of Biochemistry, University College London, London WC1E 6BT, United Kingdom
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