Atrial myocyte cell biology will be examined in isolated single cells in vitro and mice in vivo to characterize quantitatively how chemo-mechanical signaling works in health and disease. This signaling pathway is activated by changes in myocyte shape as happens when the atria fill with blood, and myocytes stretch, during diastolic filling. Using extremely high temporal and spatial resolution imaging the PIs will examine how chemo-mechanical signaling contributes to subcellular changes in Ca2+, excitation-contraction coupling to influence both electrical and Ca2+ instability. Preliminary results suggest that newly identified large axial tubules in atrial myocytes (discovered by the PIs) along with Ca2+ release "super-hubs" play a role in a unique Ca2+ signaling system found in atrial myocytes. Furthermore, the mechano-chemo X-ROS pathway discovered by the PIs in ventricular myocytes is likely to have a special role to play in atrial myocytes. This signaling pathway links the mechanics of cellular stretch, transmitted through microtubules, to the generation of local subcellular reactive oxygen species (ROS) that likely target multiple Ca2+ signaling proteins such as CaMKII and RyR2. Preliminary results suggest this X-ROS signaling is very active in atrial myocytes and may be linked to the novel structures described by the PIs. The proposed work will identify quantitatively the contributions of the special structures, X-ROS signaling and chemo-mechanical signaling to the normal physiology of atrial myocytes and the contributions to the development of atrial fibrillation (AF). Two very different mouse models of AF will be used along with specific transgenic mice to quantitatively characterize Ca2+ signaling and cellular electrophysiology in atrial myocytes and determine how chemo-mechanical signaling contributes to cellular physiology and pathophysiology. This investigation will provide critically important new information on how atrial myocytes work and fail in health and disease. The likely new discoveries produced by the proposed work will broaden our understanding of atrial cell biology and lay the foundation for innovative, effective and novel therapies for atrial dysfunction and AF.