Background: HFpEF accounts for more than 50% of all cases of HF and is closely associated with co-morbidities. Evidence suggests that co-morbidities common to HFpEF promote a systemic inflammatory state that contributes to endothelial dysfunction, reactive oxygen species production, nitrosative stress, depressed nitric oxide (NO) bioavailability and thereby downregulated cGMP-dependent protein kinase G (PKG). As oxidative stress and inflammation affect HFpEF pathophysiology by modulation of LV stiffness at least partly by changing the myocardial cGMP-PKG pathway, we therefore assessed specific treatment strategies via this pathway in an attempt to develop tailored HFpEF therapy.

Methods: Endothelial function, cardiomyocyte function, pro-inflammatory cytokines, oxidation stress level, protein expression and phosphorylation were measured in human HFpEF samples and in ZSF1 obese rat model (HFpEF).  Chronic (2 weeks) of Phosphodiesterase 9A (PDE9A) inhibitor PF04447943 (3 mg/kg) or vehicle was administered to 18-week old HFpEF rats by daily oral gavage. At age 20 weeks, HFpEF rats developed LV diastolic dysfunction, which was assessed by transthoracic echocardiography and vasodilation was examined in isolated and pressurized small coronary arteries using videomicroscopy.

Results:  HFpEF patients and HFpEF rats showed significantly lower coronary artery vasodilation compared to control groups. Pro-inflammatory cytokines such as Intercellular and vascular cell adhesion molecule-1, interleukin 6 and tumor necrosis factor-α were increased in HFpEF patients and HFpEF rats compared to control groups. Oxidative stress parameters, including 3-Nitrotyrosine, lipid peroxidase and hydrogen peroxidase and proinflammatroy cytokines were significantly higher in HFpEF patients and rats compared to control groups. All these changes were associated with reduced NO, soluble guanylyl cyclase, and PKG activity. Patients and animals showed reduced cardiomyocyte Ca²⁺ sensitivity, max tension and a higher passive stiffness (F_passive). Phosphorylation of myofilament proteins such as myosin light chain, troponin I, myosin binding protein C, and Titin was significantly lower in HFpEF compared to controls. Many other proteins in HFpEF patients were dysregulated compared to control groups. In HFpEF rats LV relaxation, coronary artery dilation to acetylcholine (Ach) and Sodium nitroprusside (SNP) were significantly reduced in the 20-week old vehicle treated HFpEF rats compared to controls. PDEA9A inhibitor treated rats showed improved LV relaxation, and coronary artery dilations to ACh and SNP.  Ex vivo exposure of coronary arteries to the PDE9A inhibitor (18 nmol/L) for 30 minutes augmented vasodilation to Ach and SNP. In addition, PDE9A inhibitor improved cardiomyocyte function, via increased phosphorylation of myofilament proteins. These improvements were associated with reduced inflammation and oxidative stress after treatment with PDE9 inhibitor.

Conclusion: Increased inflammation and oxidative stress in HFpEF caused endothelial and cardiomyocyte dysfunction. Chronic stimulation of the cGMP/PKG signalling pathway with PDE9A inhibition decreased LV passive stiffness and improved global LV performance via improved endothelial function. The effect of PDE9A treatment on the whole signalling cascade, from the upstream pathway of inflammation/oxidative stress to the downstream targets is still being investigated.