LAUSR

Introduction: Heart failure (HF) remains a leading cause of death and indication for transplant in single ventricle congenital heart disease (SV). However, little is known regarding the molecular mechanisms leading to HF in SV. The purpose of this study was to characterize mitochondrial function in the myocardium of failing (SVHF) and non-failing (SVNF) SV patients compared to biventricular NF controls (BVNF). Furthermore, we investigated the effect of ex vivo treatment with the phosphodiesterase-5 inhibitor (PDE5i) sildenafil on mitochondrial function. Methods: Freshly explanted ventricular tissue was saponin permeabilized and mitochondrial oxygen consumption was measured sequentially throughout the electron transport system using SUbstrate-Inhibitor-Titration (“SUIT”) protocols and an Oroboros O2k high resolution respirometer. Permeabilized ventricular tissue was treated for 40 min with sildenafil [1μM] prior to measurement of oxygen consumption. A Western blot for PDE5 was performed in isolated mitochondrial proteins from SVHF subjects ± PDE5i. Results: Compared to BVNF (n=15) and SVNF (n=6), SVHF (n=8) hearts have decreased function of Complex I and Complex I and II (A, B), and decreased maximal respiration (C), all of which improve with acute ex vivo treatment with sildenafil in SVHF (SVHF+PDE5i, n=6). Importantly, mitochondrial function is impaired in BVNF+PDE5i (n=5) and SVNF+PDE5i hearts (n=5) (A-C, one-way Anova p<0.05). PDE5 protein is expressed in SVHF mitochondria, but expression is not affected by ex vivo PDE5i treatment (D). Conclusions: Our results indicate that mitochondrial function is impaired in SVHF, PDE5 protein is expressed in SVHF mitochondria, and PDE5i improves mitochondrial function in SVHF, but may be detrimental to mitochondrial function in SVNF and BVNF. Together these data suggest that mitochondrial PDE5 is a potential therapeutic target, but that indiscriminate use of PDE5i in SV patients may not be advisable.