Calcium Movement in Cardiac Mitochondria

Abstract

Existing theory suggests that mitochondria act as significant, dynamic buffers of cytosolic calcium ([Ca(2+)]i) in heart. These buffers can remove up to one-third of the Ca(2+) that enters the cytosol during the [Ca(2+)]i transients that underlie contractions. However, few quantitative experiments have been presented to test this hypothesis. Here, we investigate the influence of Ca(2+) movement across the inner mitochondrial membrane during both subcellular and global cellular cytosolic Ca(2+) signals (i.e., Ca(2+) sparks and [Ca(2+)]i transients, respectively) in isolated rat cardiomyocytes. By rapidly turning off the mitochondria using depolarization of the inner mitochondrial membrane potential (ΔΨm), the role of the mitochondria in buffering cytosolic Ca(2+) signals was investigated. We show here that rapid loss of ΔΨm leads to no significant changes in cytosolic Ca(2+) signals. Second, we make direct measurements of mitochondrial [Ca(2+)] ([Ca(2+)]m) using a mitochondrially targeted Ca(2+) probe (MityCam) and these data suggest that [Ca(2+)]m is near the [Ca(2+)]i level (∼100 nM) under quiescent conditions. These two findings indicate that although the mitochondrial matrix is fully buffer-capable under quiescent conditions, it does not function as a significant dynamic buffer during physiological Ca(2+) signaling. Finally, quantitative analysis using a computational model of mitochondrial Ca(2+) cycling suggests that mitochondrial Ca(2+) uptake would need to be at least ∼100-fold greater than the current estimates of Ca(2+) influx for mitochondria to influence measurably cytosolic [Ca(2+)] signals under physiological conditions. Combined, these experiments and computational investigations show that mitochondrial Ca(2+) uptake does not significantly alter cytosolic Ca(2+) signals under normal conditions and indicates that mitochondria do not act as important dynamic buffers of [Ca(2+)]i under physiological conditions in heart.

Mitochondrial calcium uptake in organ physiology: from molecular mechanism to animal models

“…Despite the physical proximity of the mitochondria to the SR compartment and their Ca 2+ -dependent role in ATP production, the ability of the mitochondria to serve as significant dynamic buffers of cytosolic Ca 2+ in the heart is still debated [13]. Furthermore, highly controversial is whether the fast cytosolic Ca 2+ transients in excitation-contraction coupling in beating cardiomyocytes are transmitted to the matrix compartment in a beat-to-beat fashion or in a slow integration pattern [43].…”

The mitochondrial Ca2+ uniporter complex

“…Mitochondria occupy over 75% of the cardiomyocyte cell volume. Nevertheless, it has been argued that mitochondrial Ca 2+ uptake, while sufficient to maintain oxidative phosphorylation at rates sufficient to supply ATP for proper cardiac function, is nevertheless sufficiently small that it does not shape the [Ca 2+ ] i signal experienced by much of the cytoplasm, including the contractile machinery (see [15, 16]). Whether this is true within microdomains associated with Ca 2+ release sites in close apposition to mitochondria is not as clear [17].…”

Mg2+ differentially regulates two modes of mitochondrial Ca2+ uptake in isolated cardiac mitochondria: implications for mitochondrial Ca2+ sequestration

“…Cardiac specific MCU knockout studies using a mouse model reveal that cardiac mitochondrial Ca 2+ is only essential during high stress conditions, but not for routine activities (Kwong et al 2015; Luongo et al 2015; Wu et al 2015). Whereas the role of mitochondria as a Ca 2+ sink against cytosolic Ca 2+ overload is well recognized (Szabadkai and Duchen 2008; Nicholls 2005; Rasola and Bernardi 2011), their role in shaping the cytosolic Ca 2+ transients during physiological conditions is still debated (Boyman et al 2014; Lu et al 2013). Moreover, the precise mechanisms and consequences by which mitochondria take up, extrude, and sequester cytosolic Ca 2+ remain obscure.…”

“…For example, recent studies show relatively little mitochondrial Ca 2+ uptake during physiological Ca 2+ transients (Boyman et al 2014; Lu et al 2013), which implies that in the physiological setting, MCU might play only a negligible role in shaping physiological Ca 2+ dynamics in the beating heart. Moreover, when mitochondria are loaded with sufficient Ca 2+ , changes in matrix free Ca 2+ are difficult to detect due to the higher buffering power associated with higher mitochondrial Ca 2+ loads (Bazil et al 2013; Blomeyer et al 2013).…”