The permeability transition pore as a Ca(2+) release channel: New answers to an old question

Mitochondria possess a sophisticated array of Ca(2+) transport systems reflecting their key role in physiological Ca(2+) homeostasis. With the exception of most yeast strains, energized organelles are endowed with a very fast and efficient mechanism for Ca(2+) uptake, the ruthenium red (RR)-sensitive mitochondrial Ca(2+) uniporter (MCU); and one main mechanism for Ca(2+) release, the RR-insensitive 3Na(+)–Ca(2+) antiporter. An additional mechanism for Ca(2+) release is provided by a Na(+) and RR-insensitive release mechanism, the putative 3H(+)–Ca(2+) antiporter. A potential kinetic imbalance is present, however, because the V(max) of the MCU is of the order of 1400 nmol Ca(2+) mg(−1) protein min(−1) while the combined V(max) of the efflux pathways is about 20 nmol Ca(2+) mg(−1) protein min(−1). This arrangement exposes mitochondria to the hazards of Ca(2+) overload when the rate of Ca(2+) uptake exceeds that of the combined efflux pathways, e.g. for sharp increases of cytosolic [Ca(2+)]. In this short review we discuss the hypothesis that transient opening of the Ca(2+)-dependent permeability transition pore may provide mitocondria with a fast Ca(2+) release channel preventing Ca(2+) overload. We also address the relevance of a mitochondrial Ca(2+) release channel recently discovered in Drosophila melanogaster, which possesses intermediate features between the permeability transition pore of yeast and mammals.