Metabolic Modulator Effects of Inorganic Nitrate and Nitrite, and Therapeutic Potential in Patients with Chronic Heart Failure

Loudon, Brodie (2020) Metabolic Modulator Effects of Inorganic Nitrate and Nitrite, and Therapeutic Potential in Patients with Chronic Heart Failure. Doctoral thesis, University of East Anglia.

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Nitric oxide (NO) is a crucial signalling molecule within almost all aspects of human physiology. In the cardiovascular system, NO reduces blood pressure and vascular tone, prevents thrombus formation via platelet activation/aggregation, prevents inflammation by impairing leukocyte adherence to the endothelium, and prevents blood vessel thickening and pulmonary hypertension by impairing vascular smooth muscle cell proliferation. Inorganic nitrate (NO3) and nitrite (NO2) were historically considered to be inert byproducts of NO metabolism (i.e. oxidation within the tissues). The recently characterised nitrate-nitrite-NO pathway however, has revealed an important NO storage pool within the body that, importantly, is highly active in hypoxic tissues where typical mechanisms for NO production (i.e. via Larginine) are impaired.

In a series of studies we have sought to explore the effects of inorganic nitrate and nitrite on a less explored effect of NO in the cardiovascular system, cardiac metabolism. In a basic science study in mice supplemented with 7 days of oral inorganic nitrate or nitrite, we explore the effects of nitrite on the rate-limiting enzyme of glucose metabolism, pyruvate dehydrogenase (PDH), and on the formation of the oxidative dimer of protein kinase G-1α, a newly-characterised downstream transducer of reactive oxygen species and inflammation that may have beneficial effects acutely but detrimental effects if chronically activated. We also explore these effects in human cardiac biopsy samples obtained from patients undergoing coronary artery bypass surgery. Although nitrite increased PKG1α dimer content, this was not statistically significant, except for in patients with type 2 diabetes, a pro-inflammatory state, in whom there was a robust reduction in dimer formation. This is in-keeping with the known anti-inflammatory effects of nitrite. Nitrite also dephosphorylated PDH but did not increase its activity in mouse or human heart tissue.

Given the potential for nitrite to affect metabolism, either via NO or via a direct effect of its own, we also investigated whether nitrite could affect the metabolic profile of 786-0 renal cell carcinoma cells, which is unique to cancers and involves a switch away from oxidative phosphorylation towards aerobic glycolysis (termed the Warburg effect), and whether this might increase susceptibility to chemotherapy. Interestingly, nitrite slightly improved cancer cell survival at lower doses of chemotherapy, but did not have any effects on PDH or other metabolic proteins such as Akt or LDH, and may have also occurred via effects on redox signalling.

We also designed a large, randomised controlled clinical trial to investigate the effect of oral inorganic (sodium) nitrate (to increase plasma nitrite levels) on exercise capacity in patients with heart failure with reduced ejection fraction. Inflammation, NO deficiency, and reduced cardiac metabolism are key features of heart failure, and correction of these features with nitrite have the potential to improve exercise tolerance, as well as potential effects on cardiac function, serum biomarkers, quality of life score, serum glucose and insulin levels, and inflammatory markers. Unfortunately, the study was halted prematurely by the study Sponsor, with only 19 patients recruited from a target of 56. However, our results showed a fall in exercise tolerance in the group receiving the placebo medication, and a small increase in the nitrate arm, and the difference was statistically significant. The fall in the placebo group was likely due to disease progression and worsening heart failure given the better baseline function in this arm compared to the nitrate arm, and the change in echocardiographic data such as V100.

Finally, by analysing data previously obtained by others in patients with symptomatic non-obstructive hypertrophic cardiomyopathy (often considered a unique subphenotype of heart failure with preserved ejection fraction), we assessed whether impaired resting cardiac energetic status (measured via resting 31P cardiovascular magnetic resonance spectroscopy and dynamic change in vasculoventricular coupling with exercise) was associated with the dynamic diastolic dysfunction and impaired Frank-Starling mechanism that is largely responsible for the profound exercise intolerance in HCM. We demonstrate modest correlations between cardiac energetic impairment and exercise intolerance and vasculoventricular coupling in HCM, identifying energetic impairment as a key therapeutic target in HCM. Given the recent equivocal results of a definitive trial of nitrite in heart failure with preserved ejection fraction, and the results of our other studies, nitrite is unlikely to be a clinically useful treatment in these patients, however other metabolic modulators remain attractive.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Medicine and Health Sciences > Norwich Medical School
Depositing User: Chris White
Date Deposited: 01 Dec 2021 14:54
Last Modified: 18 Jan 2024 01:38


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