In vivo monitoring of Ca2+ uptake into mitochondria of mouse skeletal muscle during contraction

Although the importance of mitochondria in pathophysiology has become increasingly evident, it remains unclear whether these organelles play a role in Ca2+ handling by skeletal muscle. This undefined situation is mainly due to technical limitations in measuring Ca2+ transients reliably during the contraction-relaxation cycle. Using two-photon microscopy and genetically expressed "cameleon" Ca2+ sensors, we developed a robust system that enables the measurement of both cytoplasmic and mitochondrial Ca2+ transients in vivo. We show here for the first time that, in vivo and under highly physiological conditions, mitochondria in mammalian skeletal muscle take up Ca2+ during contraction induced by motor nerve stimulation and rapidly release it during relaxation. The mitochondrial Ca2+ increase is delayed by a few milliseconds compared with the cytosolic Ca2+ rise and occurs both during a single twitch and upon tetanic contraction.