Previous clinical trials of PDE5A inhibitors, NO donors, sGC activators and neprilysin inhibitors have focused on acute vasodilatory effects. This focus is now shifting towards their potential direct myocardial effects, independent of afterload reduction. Nitroxyl donors, such as 1-nitrosocyclohexyl acetate, may have therapeutic benefits in heart failure due to the direct action of nitroxyl on myofilaments, sarco/endoplasmic reticulum Ca
2+-ATPase (SERCA2a) and ryanodine receptor 2 (RyR2) [
32], which all have overall positive inotropic and lusitropic effects on the heart. Whether nitroxyl donors are beneficial in treating HFpEF still needs to be elucidated. On the other hand, the NO donors increase diastolic cell length [
33], inducing earlier LV relaxation
in vitro. In line with this, in patients with chest pain but no evidence of coronary lesions, intracoronary infusion of sodium nitroprusside resulted in a slightly decreased LV peak systolic pressure and an acute increase of LV end-diastolic capacitance [
34,
35]. In theory, the described effects on LV compliance suggest that NO donors would be ideal in tackling LV stiffness in HFpEF patients. However, major limitations of the available NO donors have hindered progress in the field. Firstly, patients with heart failure develop a rapid tolerance phenomenon, also termed tachyphylaxis, leading to an impaired NO bioactivation that is related to increased vascular oxidative stress [
6,
12,
13]. Secondly, chronic administration of the NO donor isosorbide mononitrate was shown to increase oxidative stress and induce endothelial dysfunction by increasing the expression of endothelin [
36]. Finally, HFpEF patients are sensitive to preload reduction and are therefore more susceptible to haemodynamic instability and stroke volume reduction following vasodilation than HFrEF patients [
6,
12,
13]. NO donors remain important therapeutic options for vascular unloading and to decrease filling pressures in acutely decompensated patients, but are less applicable in chronic heart failure. In addition, NO signalling has anti-fibrotic, anti-hypertrophic and anti-adrenergic actions that oppose adverse cardiac remodelling, and thus may represent potential therapeutic strategies for HFpEF [
37‐
39]. Furthermore, tetrahydrobiopterin (BH4) and endothelial NOS activators could be useful in reducing diastolic dysfunction, since the deoxycorticosterone acetate (DOCA)-salt mouse model is associated with BH4 depletion and uncoupled NOS activity, but BH4 administration consistently reduced DOCA-salt-associated myosin binding protein C glutathionylation and diastolic dysfunction [
37‐
39].