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The increased afterload of pulmonary arterial hypertension (PAH) impairs right ventricular function, as the RV struggles to adapt to increased pressure with remodeling and fibrosis. RV failure is the primary cause of death in PAH, and a frequent cause of death in pulmonary hypertension secondary to pulmonary embolism, emphysema, pulmonary fibrosis, and left-heart failure. There is currently no RV-specific therapeutic to maintain function and prolong patient life. During PAH, cardiomyocytes upregulate cell-surface expression of the G αq /IP 3 -coupled thromboxane/prostanoid receptor (TPr), which is activated by multiple endogenous ligands increased in PAH, and leads to calcium influx. TPr activation is also pro-fibrotic in multiple cell types and models. Previous studies have shown reduced fibrosis following TPr antagonism in mouse models of short-term pulmonary hypertension; however this effect has been variable in long-term studies. To more precisely determine the functional effects of TPr activation in RV pressure overload, we used pulmonary arterial banding (PAB) for a direct, sustained increase in pressure, antagonism or deletion of the TPr, and murine cardiac MRI for simultaneous evaluation of the RV and LV. TPr antagonism in a long-term PAB model improved RV ejection fraction and restored LV volume and output by preventing septal bulging and LV eccentricity. This occurred with even short-term antagonism, and was associated with decreased fibrosis in the septum and RV insertion points. Similar results were seen following PAB of universal or cardiomyocyte-specific TPr knockout mice; in all cases, decreasing TPr activation during RV pressure overload reduced RV stiffness even when RV fibrosis remained unchanged. In long-term pressure overload, the improved compliance with antagonism normalized expression of Yap/Taz-associated genes. These results suggest that TPr activation contributes to cardiomyocyte mechanotransduction and deleterious remodeling in response to the RV pressure overload of pulmonary hypertension, and that antagonism of the TPr may preserve RV function to prolong life in PAH patients.

The increased afterload of pulmonary arterial hypertension (PAH) impairs right ventricular function and ultimately leads to failure, as the RV struggles to adapt to increased pressure with remodeling and fibrosis. During PAH, cardiomyocytes upregulate cell-surface expression of the G protein-coupled thromboxane/prostanoid receptor (TPr). Increased myofibroblast and immune cell populations may also contribute to the enhanced TPr expression seen in the PAH RV. 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. Preventing signaling through the TPr prevents RV fibrosis in murine models of PAH without affecting arterial pressure. Because infusion of TPr agonist can cause arrhythmia in anesthetized rabbits, and we have previously found that RV pressure overload causes sustained increases in end-diastolic calcium in RV cardiomyocytes that is blocked with TPr antagonist, we hypothesized that endogenous TPr activation can lead to conduction abnormalities in RV pressure overload. Here, we used pulmonary arterial banding (PAB) of female mice to induce fixed pressure overload of the RV. Sham-operated or PAB mice were treated with normal drinking water or water containing 25 mg/kg/day of the TPr antagonist ifetroban and were evaluated at 4 weeks past PAB. RV ejection fraction was similarly depressed in vehicle- and antagonist-treated mice, although spontaneous running, RV fibrosis, and RV relaxation time were improved in PAB mice given ifetroban. ECG abnormalities in PAB mice confirmed a prolonged relaxation and suggested delays in repolarization. These were abolished with TPr antagonism. PAB altered RV expression and localization of connexin-43 (Cx43) in vehicle-treated, but not ifetroban-treated mice. Cx43 derangement is associated with impaired cell-to-cell electrical conduction and impulse propagation. Compiled, our findings suggest that endogenous TPr activation produces alterations in RV calcium handling, signaling, and cell-cell junctions that contribute to early failure in pressure overload. Therapeutic TPr antagonism may prevent this deleterious remodeling and prolong survival in patients with PAH.

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.