We report that in heart cells, physiologic stretch rapidly activates reduced-form nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) to produce reactive oxygen species (ROS) in a process dependent on microtubules (X-ROS signaling). ROS production occurs in the sarcolemmal and t-tubule membranes where NOX2 is located and sensitizes nearby ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). This triggers a burst of Ca²⁺ sparks, the elementary Ca²⁺ release events in heart. Although this stretch-dependent “tuning” of RyRs increases Ca²⁺ signaling sensitivity in healthy cardiomyocytes, in disease it enables Ca²⁺ sparks to trigger arrhythmogenic Ca²⁺ waves. In the mouse model of Duchenne muscular dystrophy, hyperactive X-ROS signaling contributes to cardiomyopathy through aberrant Ca²⁺ release from the SR. X-ROS signaling thus provides a mechanistic explanation for the mechanotransduction of Ca²⁺ release in the heart and offers fresh therapeutic possibilities.

Calcium (Ca ²⁺ ) uptake into the mitochondrial matrix is critically important to cellular function. As a regulator of matrix Ca ²⁺ levels, this flux influences energy production and can initiate cell death. If large, this flux could potentially alter intracellular Ca ²⁺ ([Ca ²⁺ ] _i ) signals. Despite years of study, fundamental disagreements on the extent and speed of mitochondrial Ca ²⁺ uptake still exist. Here, we review and quantitatively analyze mitochondrial Ca ²⁺ uptake fluxes from different tissues and interpret the results with respect to the recently proposed mitochondrial Ca ²⁺ uniporter (MCU) candidate. This quantitative analysis yields four clear results: ( i ) under physiological conditions, Ca ²⁺ influx into the mitochondria via the MCU is small relative to other cytosolic Ca ²⁺ extrusion pathways; ( ii ) single MCU conductance is ∼6–7 pS (105 mM [Ca ²⁺ ]), and MCU flux appears to be modulated by [Ca ²⁺ ] _i , suggesting Ca ²⁺ regulation of MCU open probability ( P _O ); ( iii ) in the heart, two features are clear: the number of MCU channels per mitochondrion can be calculated, and MCU probability is low under normal conditions; and ( iv ) in skeletal muscle and liver cells, uptake per mitochondrion varies in magnitude but total uptake per cell still appears to be modest. Based on our analysis of available quantitative data, we conclude that although Ca ²⁺ critically regulates mitochondrial function, the mitochondria do not act as a significant dynamic buffer of cytosolic Ca ²⁺ under physiological conditions. Nevertheless, with prolonged (superphysiological) elevations of [Ca ²⁺ ] _i , mitochondrial Ca ²⁺ uptake can increase 10- to 1,000-fold and begin to shape [Ca ²⁺ ] _i dynamics.

The canonical atrial myocyte (AM) is characterized by sparse transverse tubule (TT) invaginations and slow intracellular Ca²⁺ propagation but exhibits rapid contractile activation that is susceptible to loss of function during hypertrophic remodeling. Here, we have identified a membrane structure and Ca²⁺-signaling complex that may enhance the speed of atrial contraction independently of phospholamban regulation. This axial couplon was observed in human and mouse atria and is composed of voluminous axial tubules (ATs) with extensive junctions to the sarcoplasmic reticulum (SR) that include ryanodine receptor 2 (RyR2) clusters. In mouse AM, AT structures triggered Ca²⁺ release from the SR approximately 2 times faster at the AM center than at the surface. Rapid Ca²⁺ release correlated with colocalization of highly phosphorylated RyR2 clusters at AT-SR junctions and earlier, more rapid shortening of central sarcomeres. In contrast, mice expressing phosphorylation-incompetent RyR2 displayed depressed AM sarcomere shortening and reduced in vivo atrial contractile function. Moreover, left atrial hypertrophy led to AT proliferation, with a marked increase in the highly phosphorylated RyR2-pS2808 cluster fraction, thereby maintaining cytosolic Ca²⁺ signaling despite decreases in RyR2 cluster density and RyR2 protein expression. AT couplon “super-hubs” thus underlie faster excitation-contraction coupling in health as well as hypertrophic compensatory adaptation and represent a structural and metabolic mechanism that may contribute to contractile dysfunction and arrhythmias.