Transfer and tunneling of Ca2+ from sarcoplasmic reticulum to mitochondria in skeletal muscle

The role of mitochondrial Ca2+ transport in regulating intracellular Ca2+ signaling and mitochondrial enzymes involved in energy metabolism is widely recognized in many tissues. However, the ability of skeletal muscle mitochondria to sequester Ca2+ released from the sarcoplasmic reticulum (SR) during the muscle contraction-relaxation cycle is still disputed. To assess the functional cross-talk of Ca2+ between SR and mitochondria, we examined the mutual relationship connecting cytosolic and mitochondrial Ca2+ dynamics in permeabilized skeletal muscle fibers. Cytosolic and mitochondrial Ca2+ transients were recorded with digital photometry and confocal microscopy using fura-2 and mag-rhod-2, respectively. In the presence of 0.5 mM slow Ca2+ buffer (EGTA (ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid)), application of caffeine induced a synchronized increase in both cytosolic and mitochondrial [Ca2+]. 5 mM fast Ca2+ buffer (BAPTA (1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid)) nearly eliminated caffeine-induced increases in [Ca2+](c) but only partially decreased the amplitude of mitochondrial Ca2+ transients. Confocal imaging revealed that in EGTA, almost all mitochondria picked up Ca2+ released from the SR by caffeine, whereas only about 70% of mitochondria did so in BAPTA. Taken together, these results indicated that a subpopulation of mitochondria is in close functional and presumably structural proximity to the SR, giving rise to subcellular microdomains in which Ca2+ has preferential access to the juxtaposed organelles.