Contribution of Mitochondrial Calcium Uptake to Intracellular Calcium Homeostasis Studied with Organelle-Targetted Probes in Skeletal Muscle Fibers

Organelle-targetted Cameleons provide a useful tool to quantify variations in local free calcium concentration. We studied murine FDB single fibers with a probe targetted to sarcoplasmic reticulum (SR) and a probe targetted to mitochondria and assuming organelle volumes available in the literature (Luff & Atwood, J Cell Biol 1971). In wild type C57 mice, SR free calcium concentration was found to be 570 uM and, assuming a bound/free calcium ratio of 60/1 (Murphy et al, J Physiol 2009) and a volume of 5%, this corresponded to 1.7 mmole/fiber liter. Following similar approach the amount released was 0.4 and 0.6 mmole/fiber liter in a 2 s - 60Hz tetanus and, respectively, after 20 mM caffeine administration. Resting mitochondrial calcium concentration was 140 nM and, assuming a ratio of bound to free of 100/1 and a volume of 5%, this corresponded to a 1.2 umol/fiber liter. According to this line of reasoning, the increase in mitochondrial calcium content was 1.32 and 1.4 umole/fiber liter in a 2 s - 60Hz tetanus and, respectively, after 20 mM caffeine administration. The results showed that the fraction of calcium released by SR and taken up by mitochondria is below 1 % of the calcium release, with minimal impact on cytosolic calcium level, but likely sufficient to activate metabolic mitochondrial response. Supported by Telethon Grant GGP 13 213.