Ca²⁺ handling remodeling and STIM1L/Orai1/TRPC1/TRPC4 upregulation in monocrotaline-induced right ventricular...

Right ventricular (RV) function is the most important prognostic factor for pulmonary arterial hypertension (PAH) patients. The progressive increase of pulmonary vascular resistance induces RV hypertrophy (RVH) and at term RV failure (RVF). However, the molecular mechanisms of RVH and RVF remain understudied. In this study, we gained insights into cytosolic Ca²⁺ signaling remodeling in ventricular cardiomyocytes during the pathogenesis of severe pulmonary hypertension (PH) induced in rats by monocrotaline (MCT) exposure, and we further identified molecular candidates responsible for this Ca²⁺ remodeling.

After PH induction, hypertrophied RV myocytes presented longer action potential duration, higher and faster [Ca²⁺]_i transients and increased sarcoplasmic reticulum (SR) Ca²⁺ content, whereas no changes in these parameters were detected in left ventricular (LV) myocytes. These modifications were associated with increased P-Ser¹⁶-phospholamban pentamer expression without altering SERCA2a (Sarco/Endoplasmic Reticulum Ca²⁺-ATPase) pump abundance. Moreover, after PH induction, Ca²⁺ sparks frequency were higher in hypertrophied RV cells, while total RyR2 (Ryanodine Receptor) expression and phosphorylation were unaffected. Together with cellular hypertrophy, the T-tubules network was disorganized. Hypertrophied RV cardiomyocytes from MCT-exposed rats showed decreased expression of classical STIM1 (Stromal Interaction molecule) associated with increased expression of muscle-specific STIM1 Long isoform, glycosylated-Orai1 channel form, and TRPC1 and TRPC4 channels, which was correlated with an enhanced Ca²⁺-release-activated Ca²⁺ (CRAC)-like current. Pharmacological inhibition of TRPCs/Orai1 channels in hypertrophied RV cardiomyocytes normalized [Ca²⁺]_i transients amplitude, the SR Ca²⁺ content and cell contractility to control levels. Finally, we showed that most of these changes did not appear in LV cardiomyocytes.

These new findings demonstrate RV-specific cellular Ca²⁺ cycling remodeling in PH rats with maladaptive RVH and that the STIM1L/Orai1/TRPC1/C4-dependent Ca²⁺ current participates in this Ca²⁺ remodeling in RVH secondary to PH.