Question: 

Heart failure with preserved ejection fraction (HFpEF) is a major cause of morbidity and mortality. Key pathophysiological alterations in HFpEF include increased left ventricular (LV) stiffness and abnormal relaxation. We aimed to compare alterations in phosphorylation and function of myofilament proteins in experimental HFpEF vs. normal myocardium under two different conditions of tissue procurement: beating-heart biopsy and postmortem tissue sampling.

Methods: 

LV tissue samples were procured (beating-heart biopsy: n=7/group; postmortem tissue: n=8/group) from normal dogs (CTRL) or old dogs made hypertensive by renal wrapping (OHT), causing LV hypertrophy with worsening diastolic function (HFpEF). Expression and phosphorylation of myofilament proteins was assessed, along with myofilament calcium sensitivity (pCa₅₀) and passive tension (F_passive) of isolated permeabilized cardiomyocytes.

Results: 

Postmortem tissue and biopsy samples showed similar alterations in OHT vs. CTRL, including increased F_passive, phospho-PEVK-titin, and phospho-PKCa expression, but decreased titin N2BA:N2B isoform ratio and reduced phosphorylation of all-titin, cMyBPC, cTnT, all-cTnI, and cTnI (S23/24). Differences appeared in terms of increased pCa₅₀ in OHT myocytes from beating-heart biopsies, but lowered pCa₅₀ in OHT myocytes from postmortem tissue, explainable by a reduction in cMLC2 phosphorylation in OHT only in beating-heart biopsies.

Conclusions: 

This patient-mimicking HFpEF model shows titin stiffening owing to isoform shift and altered phosphorylation, all contributing to elevated LV stiffness as a common feature of HFpEF. Many regulatory myofilament proteins are hypo-phosphorylated in this model, suggesting contractile dysregulation could be an early step in HFpEF. Many, but not all, remodeling events seen in postmortem tissue appeared to be genuinely disease-related.