PROJECT SUMMARY Right ventricular (RV) failure is the primary cause of death in pulmonary arterial hypertension (PAH), and is a significant cause of morbidity and mortality in other forms of pulmonary hypertension. Therefore, treatment that would encourage successful RV adaptation to pressure overload would prolong patient life. Currently, no approved therapies exist that preserve RV function. The thromboxane/prostanoid receptor (TPr) is upregulated on RV cardiomyocytes in PAH patients, and isoprostanes, ligands of the TPr, are increased during cardiac strain and PAH due to increased oxidative stress. Activation of the TPr is generally pro-fibrotic, and we have previously found that pharmacologic inhibition of the TPr prevents RV fibrosis and improves function in a pulmonary artery banding (PAB) model of RV pressure overload. However, little is known about the mechanism by which this occurs. Based on our previous studies, the TPr antagonist ifetroban is slated for a clinical trial in systemic sclerosis patients with PAH; understanding the role of the TPr in the RV during PAH is critical to the clinical use of antagonist drugs as well as to determining their broader applicability to other diseases. Therefore, our two aims propose to test the hypothesis that TPr activation in cardiomyocytes drives the RV fibrotic response, and resulting decline in RV function during pressure overload, via calcium-mediated signaling. Using PAB to directly induce RV pressure overload, we will follow up our antagonist studies and determine whether fibrosis and the functional effects of RV pressure overload can be reversed via genetic or pharmacologic inhibition of TPr function. We will ascertain whether TPr activation in cardiomyocytes is sufficient for these effects with a cardiomyocyte-specific TPr knockout, compared with a global TPr knockout. We will also determine in a treatment study whether the pro-fibrotic and/or functional effects of TPr activation in this PAH model can be reversed with TPr antagonism. Our next goal is to determine the mechanism by which TPr activation causes fibrosis during pressure overload. Through PAB as well as an in vitro model of cardiomyocyte strain, we will delineate signaling events from the G-protein coupled TPr and test the hypothesis that sustained TPr-dependent calcium signaling in cardiomyocytes causes cellular dysfunction and alters their interaction with non-myocytes. We will examine how cardiomyocyte TPr activation affects surrounding fibroblasts, as well as any contribution from a fibroblast TPr. The intersection of fibrosis and functional changes in the heart are complex, and this study will also aid in the definition of signaling events from the TPr that contribute to fibrosis, compared with changes in contractile function, at both a cellular and whole heart level. The overall results of this study will delineate the role of the TPr in the RV during pressure overload, and determine the efficacy of a TPr antagonist as treatment supporting RV adaptation in PAH, as well as predict its use for other applications.