Cardiovascular effects of the sirtuin and urocortin systems in humans
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Background Cardiovascular disease continues to remain a leading cause of morbidity and mortality in both developing and developed worlds. The sirtuin and urocortin systems are novel hormone systems in humans with an emerging role in cardiovascular physiology and pathophysiology. Through a series of studies, this thesis examines the cardiovascular effects of SRT2104 (a novel small molecule SIRT1 activator) in otherwise healthy cigarette smokers and in patients with type 2 diabetes mellitus, and of urocortins 2 and 3 in healthy volunteers and in patients with heart failure. Methods Twenty-four otherwise healthy cigarette smokers and 15 subjects with stable type 2 diabetes participated in a randomised, double blind, placebo controlled, crossover trial and received 28 days of oral SRT2104 (2.0 g/day) or matched placebo. Plasma SRT2104 concentrations, serum lipid profile, plasma fibrinolytic factors, markers of platelet and monocyte activation and pulse wave analysis and velocity were measured at baseline and the end of each treatment period together with an assessment of forearm blood flow during intra-arterial bradykinin, acetylcholine and sodium nitroprusside infusions. The pharmacodynamic profile of urocortins 2 and 3 were assessed in 18 healthy male volunteers recruited into a series of randomised, double blind, placebo controlled, crossover studies. Bilateral forearm venous occlusion plethysmography was performed during incremental intra-arterial infusions of urocortin 2 (3.6-120 pmol/min), urocortin 3 (1.2-36 nmol/min) and substance P (2-8 pmol/min) in the presence or absence of inhibitors of cyclooxygenase (aspirin), cytochrome P450 metabolites of arachidonic acid (fluconazole) and nitric oxide synthase (L-NG-monomethyl-arginine (L-NMMA)). Finally, 12 patients with stable heart failure (New York Heart Association (NYHA) II-IV) and 10 age- and sex-matched healthy volunteers were recruited to attend once each. Bilateral forearm arterial blood flow was measured using forearm venous occlusion plethysmography during incremental intra-arterial infusions of urocortin 2 (3.6-36 pmol/min), urocortin 3 (360-3600 pmol/min) and substance P (2-8 pmol/min). Results SRT2104 was safe and well tolerated in otherwise healthy cigarette smokers and subjects with type 2 diabetes mellitus. There were no significant differences in fibrinolytic or blood flow parameters between placebo and SRT2014. Treatment with SRT2104 was associated with a significant reduction in augmentation pressure (P=0.0273) and a trend towards improvement in the augmentation index (AIx) and corrected augmentation index (0.10>P>0.05 for both) without significant changes in pulse wave velocity (PWV) and time to wave reflection (Tr) (P>0.05). Administration of SRT2104 had a favourable effect on lipid profile in otherwise healthy cigarette smokers in comparison to placebo. Urocortins 2 and 3 evoked arterial vasodilatation (P<0.0001) without tachyphylaxis but with a slow onset and offset of action. Inhibition of nitric oxide synthase with L-NMMA reduced vasodilatation to substance P and urocortin 2 (P≤0.001 for both) but had little effect on urocortin 3 (P>0.05). Neither aspirin nor fluconazole affected vasodilatation induced by any of the infusions (P>0.05 for all). In the presence of all three inhibitors, urocortin 2- and urocortin 3-induced vasodilatation were attenuated (P<0.001 for all) to a greater extent than with L-NMMA alone (P≤0.005). The vasodilatory effects of urocortins 2 and 3 were preserved in patients with heart failure. Conclusion Activation of SIRT1 through SRT2104 improved lipid profile but did not produce demonstrable differences in vascular or platelet function with some effect on measures of arterial stiffness. Urocortins 2 and 3 appear to be potent arterial vasodilators whose vasomotor responses remained preserved in patients with heart failure and were at least partly mediated via the endothelium. Both hormone systems hold potential in their role in cardiovascular disease in man but require further studies to help translate findings of this thesis to clinical practice.