Dynamics of Calcium Sparks and Calcium Leak in the Heart

AbstractWe present what we believe to be a new mathematical model of Ca2+ leak from the sarcoplasmic reticulum (SR) in the heart. To our knowledge, it is the first to incorporate a realistic number of Ca2+-release units, each containing a cluster of stochastically gating Ca2+ channels (RyRs), whose biophysical properties (e.g., Ca2+ sensitivity and allosteric interactions) are informed by the latest molecular investigations. This realistic model allows for the detailed characterization of RyR Ca2+-release properties, and shows how this balances reuptake by the SR Ca2+ pump. Simulations reveal that SR Ca2+ leak consists of brief but frequent single RyR openings (∼3000 cell−1 s−1) that are likely to be experimentally undetectable, and are, therefore, “invisible”. We also observe that these single RyR openings can recruit additional RyRs to open, due to elevated local (Ca2+), and occasionally lead to the generation of Ca2+ sparks (∼130 cell−1 s−1). Furthermore, this physiological formulation of “invisible” leak allows for the removal of the ad hoc, non-RyR mediated Ca2+ leak terms present in prior models. Finally, our model shows how Ca2+ sparks can be robustly triggered and terminated under both normal and pathological conditions. Together, these discoveries profoundly influence how we interpret and understand diverse experimental and clinical results from both normal and diseased hearts