Abstract 824: Thromboxane/Prostanoid Receptor Activation Increases Calpain-Mediated Proteolysis and Alters Calcium Handling and Fibrosis Following Right Ventricular Pressure Overload

In pulmonary arterial hypertension (PAH), the right ventricle undergoes remodeling and fibrosis as it struggles to adapt to the increased pressure overload. RV dysfunction and failure is the primary cause of death in PAH patients. The G protein-coupled thromboxane/prostanoid receptor (TPr) is expressed in vascular smooth muscle, myofibroblasts, and immune cells, and is upregulated in cardiomyocytes following PAH. Activation of the cardiomyocyte TPr increases intracellular calcium via G αq /IP 3 ; activation of the receptor in other cells leads to fibrosis and vasoconstriction. The TPr is activated by isoprostane as well as thromboxane, which suggests that the receptor could contribute to deleterious remodeling during cardiac stress. Our previous studies demonstrate that TPr antagonism prevents RV fibrosis and dilatation in murine models of PAH, without affecting pressures. Because the TPr can signal through Gq, we hypothesized that its activation in PAH causes changes in cardiomyocyte calcium-handling proteins which contribute to remodeling and failure. In this study, we used pulmonary arterial banding (PAB) to induce fixed pressure overload of the RV. Mice were treated for 4 weeks past PAB with normal drinking water or water containing 25 mg/kg/day of the TPr antagonist ifetroban, and either underwent pressure-volume catheterization and whole RV evaluation, or cardiomyocytes were isolated for calcium handling and protein. PAB caused an increase in cardiomyocyte resting (end-diastolic) intracellular calcium, which was ameliorated in mice given TPr antagonist. The increased intracellular calcium following PAB was associated with increased activity of the calcium-activated protease calpain, also blocked with TPr antagonism. There was no decrease in caffeine-mediated release of calcium from the sarcoplasmic reticulum (SR) at 4 weeks past PAB, and phosphorylation of phospholamban was increased, suggesting compensation to drive calcium into the SR. Our findings suggest that TPr activation produces alterations in RV calcium handling, signaling, and calpain activity that contribute to deleterious remodeling and early failure in pressure overload. Therapeutic TPr antagonism may help preserve RV function in patients with PAH.