Ca uptake in mitochondria occurs via the reverse action of the Na /Ca exchanger in metabolically inhibited MDCK cells

Smets, Ilse, Adrian Caplanusi, Sanda Despa, Zsolt Molnar, Mihai Radu, Martin vandeVen, Marcel Ameloot, and Paul Steels. Ca uptake in mitochondria occurs via the reverse action of the Na /Ca exchanger in metabolically inhibited MDCK cells. Am J Physiol Renal Physiol 286: F784–F794, 2004. First published December 9, 2003; 10.1152/ajprenal.00284.2003.—In ischemic or hypoxic tissues, elevated Ca levels have emerged as one of the main damaging agents among other Ca -independent mechanisms of cellular injury. Because mitochondria, besides the endoplasmic reticulum, play a key role in the maintainance of cellular Ca homeostasis, alterations in the mitochondrial Ca content ([Ca ]m) were monitored in addition to changes in cytosolic Ca concentration ([Ca ]i) during metabolic inhibition (MI) in renal epithelial MadinDarby canine kidney (MDCK) cells. [Ca ]i and [Ca ]m were monitored via, respectively, fura 2 and rhod 2 measurements. MI induced an increase in [Ca ]i reaching 631 78 nM in 20 min, followed by a decrease to 118 9 nM in the next 25 min. A pronounced drop in cellular ATP levels and a rapid increase in intracellular Na concentrations in the first 20 min of MI excluded Ca efflux in the second phase via plasma membrane ATPases or Na /Ca exchangers (NCE). Mitochondrial rhod 2 intensities increased to 434 46% of the control value during MI, indicating that mitochondria sequester Ca during MI. The mitochondrial potential ( m) was lost in 20 min of MI, excluding mitochondrial Ca uptake via the m-dependent mitochondrial Ca uniporter after 20 min of MI. Under Na -free conditions, or when CGP-37157, a specific inhibitor of the mitochondrial NCE, was used, no drop in [Ca ]i was seen during MI, whereas the MI-induced increase in mitochondrial rhod 2 fluorescence was strongly reduced. To our knowledge, this study is the first to report that in metabolically inhibited renal epithelial cells mitochondria take up Ca via the NCE acting in the reverse mode.