Cardiac Ca2+ Signals: From Local Elevations, A Matrix of Potential

University of Maryland in Baltimore. Molecular Medicine. Ph.D. 2018 ; The heart has frequent and robust elevations of cytosolic calcium ([Ca2+]i) that underlie each contraction. Ca2+ release originates from thousands of Ca2+ release units (CRUs) distributed throughout the cardiomyocyte which can generate localized Ca2+ elevations, Ca2+ sparks, that are synchronized during excitation-contraction (EC) coupling to produce the global [Ca2+]i transient. Positioned in close proximity (~ 100 nm) to these CRUs are the intramyofibrillar mitochondria (IFMs) which are briefly (10 - 20 ms) exposed to microdomains of high [Ca2+]i (1 - 10 μM) during Ca2+ release. Elevated [Ca2+]i coupled with the highly polarized inner mitochondrial membrane (IMM) potential (ΔΨm ≈ -180 mV) creates a powerful electrochemical driving force for Ca2+ uptake through the mitochondrial Ca2+ uniporter (MCU) complex. Low "physiological" mitochondrial matrix Ca2+ ([Ca2+]m) (~0.1 - 10 μM) is thought to regulate metabolism via oxidative phosphorylation, while "pathophysiological" [Ca2+]m overload (> 10 μM) leads to necrotic cell death. To date, the biophysical details surrounding the magnitude and regulation of mitochondrial Ca2+ uptake remain poorly understood, with the functional significance of [Ca2+]m signals providing further controversy. Three independent studies are provided in this thesis that look to improve our quantitative understanding of "local control" of [Ca2+]i signaling, the regulation and magnitude of [Ca2+]m signals, and the mechanism by which [Ca2+]m contributes to dynamic mitochondrial adenosine triphosphate (ATP) synthesis. The first study tests the hypothesis that "stable and synchronous release of local [Ca2+]i signals relies on physiological Ca2+ sensitivity of the ryanodine receptor". The second study tests the hypothesis that "mitochondrial Ca2+ uptake is under thermodynamic control to yield the small alterations in [Ca2+]m during EC coupling." The final study tests the hypothesis that "[Ca2+]m regulates ATP production ...