Anne-Marie Koop

2 43 Also expression of SERCA2a, phospholamban and NCX was comparable between groups and not correlated to deterioration of diastolic function. In contrast, passive chamber properties (Eed, EDP) revealed progressive stiffening of the RV. Interstitial fibrosis contributes to myocardial stiffening in heart failure. However, we found interstitial fibrosis to be less increased in the PAB-rats with clinical RVF and deteriorated diastolic dysfunction, as compared to those without clinical RVF and less diastolic dysfunction. The pro-fibrotic signaling (TGF β 1, osteopontin) appeared similarly activated in both groups, suggesting a higher degradation of collagen in the PAB-rats with RVF. In other words, the increased myocardial stiffening in PAB-rats with clinical RVF cannot be explained by increased interstitial fibrosis. In other species (e.g. rabbits) fibrosis may play a more prominent role. 28,29 Stiffening of sarcomeres might be another explanation for the deterioration of diastolic function. We observed an increased expression of the more compliant N2Ba titin isoform in PAB+CF, which may be an adaptive response. However, titin compliance also depends on phosphorylation, which is mediated by PKG-1. 24 We did not measure titin phosphorylation, but PKG-1 activity was not increased in PAB+CF, which may (partly) explain the adverse change in ventricular stiffness. The key to effective treatment of RVF may not be found in interventions preserving RV diastolic function, for instance by increasing titin phosphorylation status. Clinical RVF is associated with a hypoxia-prone cellular environment In this PABmodel, the development of clinical signs of RVFwas associatedwith a state of hypoxia-prone cellular environment and increased intrinsic protective response to oxidative stress. Pathological hypertrophy, characterized by activation of e.g. the calcineurin-NFAT signaling system 30 was similarly present in both PAB-groups. However, capillary-myocyte ratio was reduced in PAB-rats with clinical RVF. This finding is in line with previous studies suggesting that insufficient capillary formation to supply the hypertrophic myocardium contributes to the development clinical RVF. 8 Myocyte hypoxia may also lead to RV failure via increased oxidative stress, 31 which has been linked to cardiac stiffness. 32 We found circumstantial evidence of increased oxidative stress (increased heme oxygenase-1 mRNA expression; a powerful anti- oxidant enzyme in heart failure, 33 in PAB-rats with clinical RVF, but not in those without clinical RVF. Obviously, besides HO-1, multiple pathways (both related and unrelated to oxidative stress) are contributing to the formation and degradation of fibrosis in RV failure, many of which show significantly changed expression in the transcriptome array. However, the upregulation of HO-1 is intriguing, especially because the direction of expression change is opposite to that in the Sugen-hypoxia model of RVF, in which the stress to induce the model itself may cause oxidative stress. 34