Dual Effect of Phosphate Transport on Mitochondrial Ca2+ Dynamics [Metabolism]

May 11th, 2015 by Wei, A.-C., Liu, T., O'Rourke, B.

The large inner membrane electrochemical driving force and restricted volume of the matrix confer unique constraints on mitochondrial ion transport. Cation uptake along with anion and water movement induces swelling if not compensated by other processes. For mitochondrial Ca2+ uptake, these include activation of countertransporters (Na+/Ca2+ exchanger and Na+/H+ exchanger) coupled to the proton gradient, ultimately maintained by the proton pumps of the respiratory chain, and Ca2+ binding to matrix buffers. Inorganic phosphate (Pi) is known to affect both the Ca2+ uptake rate and the buffering reaction, but the role of anion transport in determining mitochondrial Ca2+ dynamics is poorly understood. Here we simultaneously monitor extra- and intra-mitochondrial Ca2+ and mitochondrial membrane potential (ΔΨm) to examine the effects of anion transport on mitochondrial Ca2+ flux and buffering in Pi-depleted guinea pig cardiac mitochondria. Mitochondrial Ca2+ uptake proceeded slowly in the absence of Pi but matrix free Ca2+ ([Ca2+]mito) still rose to ~50 μM. Pi (0.001-1mM) accelerated Ca2+ uptake but decreased [Ca2+]mito by almost 50% while restoring ΔΨm. Pi-dependent effects on Ca2+ were blocked by inhibiting the phosphate carrier. Mitochondrial Ca2+ uptake rate was also increased by vanadate (Vi), acetate, ATP, or a non-hydrolyzable ATP analog (AMP-PNP), with differential effects on matrix Ca2+ buffering and ΔΨm recovery. Interestingly, ATP or AMP-PNP prevented the effects of Pi on Ca2+ uptake. The results show that anion transport imposes an upper limit on mitochondrial Ca2+ uptake and modifies the [Ca2+]mito response in a complex manner.