Investigation of The Effects of Inorganic Nitrate on Blood Pressure-dependent and -independent Cardiac Dysfunction

PhD ThesisIntroduction: Constitutive NO (synthesized by eNOS) is an endogenous vasodilator and independent modulator of cardiac hypertrophy. Hypertension accounts for approximately 50% of morbidity, alongside other risk factors such as ischaemic heart disease and obesity. Inorganic nitrate, through sequential reduction in vivo to NO, exerts potent BP lowering effects. However, its effects on cardiac hypertrophy/HF independent of BP lowering remain unknown. Thus, we investigated the effects of dietary nitrate in BP-dependent (ANGII) and independent (isoprenaline) models of cardiac dysfunction. Methods: C57Bl6 mice underwent minipump infusion of ANGII or isoprenaline and received 15 mM KNO3 or KCl drinking water. Echocardiographic, BP telemetry, organ bath, histological and molecular assessments were made. Furthermore, as some benefits of nitrate appeared dependent on xanthine oxidoreductase (XOR), Xdh knockout (KO) mice underwent cardiovascular phenotyping with and without ANGII infusion, to further investigate the influence of XOR in health and disease. Results: Inorganic nitrate attenuated ANGII-induced endothelial dysfunction, hypertension, left ventricular (LV) hypertrophy and fibrosis, and reduced H2O2 and superoxide generation, whilst increasing nitrite reductase capacity. Furthermore, nitrate prevented isoprenaline-induced decline in ejection fraction, reduced cardiac fibrosis and superoxide generation. Increased physiological BP, and trends towards increased LV hypertrophy and reduced ejection fraction were observed in Xdh Heterozygous KO (Het) mice compared to littermate controls. Despite higher baseline BPs, Xdh Hets were protected from ANGIIinduced hypertension and associated cardiac dysfunction. Conclusions: Inorganic nitrate improved cardiac remodelling both directly (by attenuating fibrosis and reducing superoxide generation) and indirectly (through BPlowering), demonstrating that dietary nitrate may offer a therapeutic option for BPdependent and -independent HF. Furthermore, XOR helped maintain physiological BP in wildtype mice (as a nitrite reductase), but upon ANGII infusion was detrimental (as a superoxide generator). Xdh Hets were protected from further BP increases and resulting cardiac dysfunction, highlighting the complexity of XOR’s role in health and disease