The permeability transition pore as a Ca2+ release channel: New answers to an old question

Mitochondria possess a sophisticated array of Ca2+ transport systems reflecting their key role in physiological Ca2+ homeostasis. With the exception of most yeast strains, energized organelles are endowed with a very fast and efficient mechanism for Ca2+ uptake, the ruthenium red (RR)-sensitive mitochondrial Ca2+ uniporter (MCU); and one main mechanism for Ca2+ release, the RR-insensitive 3Na+-Ca2+ antiporter. An additional mechanism for Ca2+ release is provided by a Na+ and RR-insensitive release mechanism, the putative 3H+-Ca2+ antiporter. A potential kinetic imbalance is present, however, because the Vmax of the MCU is of the order of 1,400 nmol Ca2+ x mg-1 protein x min-1 while the combined Vmax of the efflux pathways is about 20 nmol Ca2+ x mg-1 protein x min-1. This arrangement exposes mitochondria to the hazards of Ca2+ overload when the rate of Ca2+ uptake exceeds that of the combined efflux pathways, e.g. for sharp increases of cytosolic [Ca2+]. In this short review we discuss the hypothesis that transient opening of the Ca2+-dependent permeability transition pore may provide mitocondria with a fast Ca2+ release channel preventing Ca2+ overload. We also address the relevance of a mitochondrial Ca2+ release channel recently discovered in Drosophila melanogaster, which possesses intermediate features between the permeability transition pore of yeast and mammals.