Mitochondrial Ca2+-induced Ca2+ release mediated by the Ca2+ uniporter

We have reported that a population of chromaffin cell mitochondria takes up large amounts of Ca2+ during cell stimulation. The present study focuses o n the pathways for mitochondrial Ca2+ efflux. Treatment with protonophores before cell stimulation abolished mitochondrial Ca2+ uptake and increased t he cytosolic [Ca2+] ([Ca2+](c)) peak induced by the stimulus. Instead, when protonophores were added after cell stimulation, they did not modify [Ca2](c) kinetics and inhibited Ca2+ release from Ca2+-loaded mitochondria. Thi s effect was due to inhibition of mitochondrial Na+/Ca2+ exchange, because blocking this system with CGP37157 produced no further effect. Increasing e xtramitochondrial [Ca2+](c) triggered fast Ca2+ release from these depolari zed Ca2+-loaded mitochondria, both in intact or permeabilized cells. These effects of protonophores were mimicked by valinomycin, but not by nigericin . The observed mitochondrial Ca2+- induced Ca2+ release response was insens itive to cyclosporin A and CGP37157 but fully blocked by ruthenium red, sug gesting that it may be mediated by reversal of the Ca2+ uniporter. This nov el kind of mitochondrial Ca2+-induced Ca2+ release might contribute to Ca2 clearance from mitochondria that become depolarized during Ca2+ overload.