Using fluorescently tagged markers for organelles in conjunction with confocal microscopy, we have studied the effects of agonist-induced Ca2+ signals on the motility of mitochondria and the ER (endoplasmic reticulum). We observed that the muscarinic agonist carbachol produced a rapid, simultaneous and reversible cessation of the movements of both organelles, which was dependent on a rise in cytosolic Ca2+. This rise in Ca2+ was shown to cause a fall in cellular ATP levels, and the effect of carbachol on organelle movement could be mimicked by depleting ATP with metabolic inhibitors in the absence of any such rise in Ca2+. However, a Ca2+-sensing process independent of ATP appears also to be involved, because we identified conditions where the ATP depletion was blocked (by inhibitors of the Ca2+ pumps), but the organelle movements still ceased following a rise in cytosolic Ca2+. We conclude that the co-ordinated cessation of mitochondria and ER motility is a process regulated by the cytosolic concentration of both Ca2+ and ATP, and that these two parameters are likely to synergize to regulate the localization of the two organelles, and to facilitate the transfer of Ca2+ between them.

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