A mathematical model to quantify RYR Ca2+ leak and associated heat production in resting human skeletal muscle fibers

Cycling of Ca2+ between the sarcoplasmic reticulum (SR) and myoplasm is an important component of skeletal muscle resting metabolism. As part of this cycle, Ca2+ leaks from the SR into the myoplasm and is pumped back into the SR using ATP, which leads to the consumption of O2 and generation of heat. Ca2+ may leak through release channels or ryanodine receptors (RYRs). RYR Ca2+ leak can be monitored in a skinned fiber preparation in which leaked Ca2+ is pumped into the t-system and measured with a fluorescent dye. However, accurate quantification faces a number of hurdles. To overcome them, we developed a mathematical model of Ca2+ movement in these preparations. The model incorporated Ca2+ pumps that move Ca2+ from the myoplasm to the SR and from the junctional space (JS) to the t-system, Ca2+ buffering by EGTA in the JS and myoplasm and by buffers in the SR, and Ca2+ leaks from the SR into the JS and myoplasm and from the t-system into the myoplasm. The model accurately simulated Ca2+ uptake into the t-system, the relationship between myoplasmic [Ca2+] and steady-state t-system [Ca2+], and the effect of blocking RYR Ca2+ leak on t-system Ca2+ uptake. The magnitude of the leak through the RYRs would contribute ∼5% of the resting heat production of human muscle. In normal resting fibers, RYR Ca2+ leak makes a small contribution to resting metabolism. RYR-focused pathologies have the potential to increase RYR Ca2+ leak and the RYR leak component of resting metabolism.