Both experimental and clinical studies have demonstrated that chronic myocardial infarction is associated with adverse remodeling of the left atrium, which in turn has an adverse impact on left atrial function. Left atrial dilatation is a powerful and independent predictor of mortality after myocardial infarction, but the mechanisms underlying the remodeling process are only poorly understood. Surprisingly, we have observed very high rates of cardiomyocyte cell cycle activity in the left atrium of infarcted mice. However, the role that cardiomyocyte cell cycle activity plays in mitigating or exacerbating structural and/or functional remodeling following myocardial infarction is not known. To directly address this question, we have generated a transgenic mouse model wherein infarct-induced left atrial cell cycle activity is silenced secondary to p16-Ink4a (p16) expression (p16 is a potent inhibitor of the proliferative kinase Cdk4 and transgenic expression in adult animals has previously been shown to be a potent cell cycle inhibitor). Here we propose to compare wild-type and p16 transgenic mice to determine the impact of atrial cardiomyocyte cell cycle activation on left atrial structural and functional remodeling following infarction. The experiments proposed in Aim 1 will determine the degree to which cardiomyocyte cell cycle activity impacts structural atrial remodeling following myocardial infarction. Left atrial structural analyses will be performed (a) at the tissue level to quantitate myocardial mass, cardiomyocyte number, fibrosis, immune cell infiltration, autonomic innervation and capillary density, (b) at the cardiomyocyte level to quantitate the extent of cell cycle progression and cellular hypertrophy as well as frequency of cardiomyocyte apoptosis, and (c) at the molecular level to quantitate transcriptome changes. The experiments proposed in Aim 2 will determine the degree to which cardiomyocyte cell cycle activity impacts atrial function following myocardial infarction. Atrial functional analyses will be performed (a) at whole organism level to quantitate echocardiographic and hemodynamic parameters, (b) at the intact heart level to quantitate calcium and voltage transient parameters in the left atrium (via optical mapping) and of cell clusters within the intact heart (via two-photon laser scanning microscopy), and (c) at the isolated cell level to quantitate fractional shortening and calcium handling kinetics. Importantly, cross-referencing the cell and molecular data from Aim 1 with the imaging and functional data from Aim 2 should enable us to better determine which cell cycle-mediated changes have a relevant impact on cardiac function. Such information may suggest interventions aimed at reversing compromised left atrial function in infarcted hearts.