Regulation of Mitochondrial Ca2+ Dynamics by Inorganic Phosphate

Abstract

The mitochondrial Ca2+ uniporter (mCU) is the main influx pathway for the uptake of mitochondria Ca2+ and plays an important role in regulating energy production as well as cell life and death. In our previous study, we reported two modes of the mCU (mode1 and mode 2) that have different properties with respect to Ca2+ affinity (mode1 > mode 2) and Ru360 sensitivity (mode 1 < mode 2). This study further investigates the role of concomitant inorganic phosphate (Pi) transport on mitochondrial uptake of Ca2+ mediated by the mCU in isolated cardiac mitochondria loaded with mitochondrial and extra-mitochondrial Ca2+ fluorescent dyes (Fura-FF and Calcium Green-5N). In Pi-depleted mitochondria, the maximal Ca2+ uptake rate is shown here to be limited by extramitochondrial Pi. The mitochondrial Ca2+ uptake rate was accelerated in a concentration-dependent manner by phosphate concentrations ranging from 0.01 mM to a maximum at ∼0.1 mM. The effect of Pi on mode 2 Ca2+ uptake was largely inhibited in the presence of the mitochondrial phosphate carrier inhibitor N-ethylmaleimide; however, mode 1 uptake was still observed, i.e., bulk Ca2+ uptake through mCU mode 2 was more Pi- dependent than mode 1. These experiments demonstrate another distinction between mCU modes 1 and 2 and contribute to an understanding of their possible physiological roles in mitochondrial function as either a signal for regulating energetics or as a Ca2+ sink.