br Materials and methods br Results
Materials and methods
Discussion In this study, we show that adaptive modifications of Ca2+ cycling through the hypertrophic response to chronic high blood pressure overload play a critical role during early development of MitoPY1 failure with nearly-normal ejection fraction (HFpEF). Four weeks after AAB, rat hearts were characterized by an increase of LV mass and concentric LV hypertrophy, a preserved systolic function, and other criteria used for preclinical models of HFpEF including elevated blood pressure, lung congestion, impaired active relaxation, impaired passive filling, enlarged left atrium, and cardiomyocyte hypertrophy . We relied on all of these criteria to ensure investigations on a homogenous population of animals. This approach provided unique access to cardiac cellular and molecular functions not accessible in human patients. The AAB animals presented stiffer cardiomyocytes with stronger contraction and increased Ca2+ transient. Ca2+ cycling was characterized by SR Ca2+ leak through RyR2, impaired Ca2+ extrusion through NCX, and increased PLN/SERCA and pPLN/PLN ratios. The latter protein change may aid in the compensation of the higher PLN/SERCA ratio to rescue SR-Ca2+ re-uptake by SERCA2a. We were able to reproduce clinical facets of HFpEF at a defined time point in the AAB rat model. LV hypertrophy is frequently associated with global diastolic dysfunction and HFpEF in experimental and clinical studies . LV hypertrophy and diastolic dysfunction are common cardiac complications of hypertension, which has stimulated the use of experimental models based on pressure overload [4,5,, , ]. Here, chronic pressure overload promoted early functional and structural cardiac features consistent with HFpEF-like characteristics. Four weeks post-banding, despite only moderate changes in EF, the hearts presented a reduced cardiac performance index (reflected in the increased MPI index) with diastolic dysfunction. The hearts had higher LV filling pressure (estimated by the E/e' ratio), prolonged isovolumic relaxation time (IVRT), and reduced early mitral annulus velocity in line with a rise of LV wall stiffness and impaired active relaxation. These results are consistent with a recent publication on the same model but at a later time point . AAB rats also presented structural, surrogate markers of HFpEF such as concentric LV remodeling and hypertrophy, LA enlargement, and pulmonary edema [20,, , ]. The heterogeneity of interstitial fibrosis among AAB hearts was in line with various degrees of interstitial fibrosis and cardiomyocyte hypertrophy evidenced in HFpEF patients [14,19,50,51]. Fibrosis may develop later, suggesting that early signs of HFpEF both in vivo and at the cellular level precede fibrosis. These results, taken together, supported this animal model and our selective approach as a robust method for cellular investigations of HFpEF. Myocardial remodeling in HFpEF differs from that of HFrEF driven primarily by cardiomyocyte death . Although diastolic dysfunction in HFpEF has multiple origins at non-cardiomyocyte levels , cardiac cells are the main effector of the contraction/relaxation cycling involved in proper heart pump function. A major finding in our study was evidence that AAB LV cardiomyocytes had a different functional phenotype than controls and HFrEF, particularly in regards to excitation-contraction coupling. The electrical phenotype of the cardiomyocytes was quasi normal, contrasting with the marked AP plateau prolongation and blunting of repolarizing potassium currents IK, in particular Ito, reported in compensated hypertrophy and/or end-stage failing human, canine and rat hearts [, , , ,54,55]. The positive inotropy of the cardiomyocytes also contrasted sharply with the depressed contraction in HFrEF due mainly to reduced Ca2+ transient, reflecting lower SR Ca2+ content. This is due to depressed SR Ca2+ uptake, resulting from blunted SERCA2a activity (reduced SERCA2a protein and dephosphorylated PLN), and SR Ca2+ leak through RyR2 channels [, , ]. Despite opposite functional impacts on cell contraction and Ca2+ transient, some of the Ca2+ cycling defects seen in HF were however identified in our study. They involved delayed Ca2+ transient decay, augmented diastolic Ca2+, irregular Ca2+ transients during pacing, Ca2+ leakage through RyR2 and aberrant spontaneous diastolic Ca2+ waves. Although these effects collectively contribute to SR Ca2+ depletion and cause defective cardiac excitation-contraction coupling in HFrEF, this was not the case in our model.