Investigating the role of nitrite in the cardiovascular system

Worrall, Sophie (2019). Investigating the role of nitrite in the cardiovascular system. University of Birmingham. M.Sc.

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Abstract

Introduction and aims: Nitric oxide (NO) bioavailability and responsiveness decrease with age, leading to a rise in platelet activation and aggregation, however very little evidence exists on the changes to platelet function in the elderly. Platelet and vascular responses to NO are also impaired in patients with heart failure with reduced ejection fraction (HFrEF) when compared to healthy volunteers, due to scavenging of NO with subsequent reduction to soluble guanylate cyclase (sGC), thus limiting the therapeutic potential of NO donors. “Platelet NO resistance” is widely recognised in HFrEF, but the existence of the phenomenon in heart failure with preserved ejection fraction (HFpEF) remains to be elucidated. Many pharmacotherapeutic agents utilised in HFrEF are ineffective in HFpEF, thus the discovery of a novel agent that circumvents “platelet NO resistance” is desirable. Previous studies have shown nitrite to inhibit platelet aggregation in healthy volunteers, however the underlying mechanism(s) of the anti-aggregatory effects of nitrite remain unclear. We aimed to investigate platelet function in the elderly, the potential for nitrite to be used as an anti-platelet agent in patients with HFpEF, and the role of NO/sGC/ALDH2 in nitrite-mediated platelet inhibition.

Methods and results: Platelet responses to nitrite and the NO donor sodium nitroprusside (SNP) were compared in: 1) young vs old healthy volunteers, 2) age-matched old healthy volunteers, heart failure with preserved ejection fraction with chronic atrial fibrillation (HFpEF-AF) patients and chronic atrial fibrillation (CAF) patients, and 3) ALDH2 WT vs KO mice. Nitrite-mediated platelet inhibition was assessed in the presence of NO scavengers/an sGC inhibitor, whilst vasodilator-stimulated phosphoprotein (VASP) phosphorylation was measured using Western blotting. Platelet responses to the sGC stimulator BAY 41-2272 were also assessed in the presence of nitrite/SNP. Nitrite and SNP triggered concentration-dependent attenuation of platelet aggregation in healthy volunteers and CAF pateints. A diminished response to SNP was observed in washed platelets from HFpEF-AF patients, whilst the anti-aggregatory effects of nitrite were not impaired. Nitrite also activated sGC independently of NO, phosphorylated VASP and exhibited synergistic activity with BAY-41-2272 in human platelets, whilst required ALDH2 to inhibit platelet aggregation in mice.

Conclusion: We demonstrate for the first time that the phenomenon of “platelet NO resistance” exists in HFpEF-AF, whilst also revealing that high concentration nitrite is able to circumvent “platelet NO resistance” in washed platelets independently of NO. We also show that platelet function is maintained in the elderly population, whilst revealing the involvement of ALDH2 in nitrite-mediated platelet inhibition in mice.

Type of Work:

Thesis (Masters by Research > M.Sc.)

Award Type:

Masters by Research > M.Sc.

Supervisor(s):