Hydralazine in heart failure: a study of the mechanism of action in human blood vessels

Rocchiccioli, John Paul (2015) Hydralazine in heart failure: a study of the mechanism of action in human blood vessels. MD thesis, University of Glasgow.

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Hydralazine is a vasodilator that has been in clinical use for nearly six decades. Despite this, the mechanism of its action in human blood vessels is uncertain. Understanding how hydralazine works may have importance for the better treatment of heart failure and other cardiovascular diseases. In the first Vasodilator Heart Failure trial, hydralazine was shown, in combination with oral nitrates, to reduce mortality in patients with heart failure, treated at a time when the benefits of ACE inhibitors, beta-blockers and mineralocorticoid receptor antagonists were not known. As the combination of hydralazine and isosorbide dinitrate was subsequently shown to be less effective than an ACE inhibitor in the second Vasodilator Heart Failure trial, it was little used. Recently, however, the same combination was shown to reduce mortality and morbidity in the African-American Heart Failure Trial. Crucially, in this trial, the patients were already treated with the best currently available drug therapy. Though the patients studied were self-designated African-Americans, it is widely believed that the incremental benefits of the combination of hydralazine and isosorbide dinitrate are as likely to be obtained in other patients.

While the vasodilator action of nitrates is well understood, a better understanding of the action of hydralazine (and its interaction with nitrates) could lead to the development of more effective and/or better-tolerated drugs. Nitrate therapy is limited by the development of pharmacological tolerance, possibly secondary to the increased production of reactive oxygen species. Hydralazine co-treatment has been shown to prolong the vasodilator effect of nitrates in animal models and clinical studies, although the mechanism of this protection in humans is uncertain. There are many postulated mechanisms of the vasodilator action of hydralazine, based upon studies carried out - mostly in animals - or animal tissues. Hydralazine reduces contractile responses to a number of vasoconstrictors, and this effect appears greater in arteries than in veins. The most (though not entirely) consistent findings are those suggesting that hydralazine leads to the activation of guanylate cyclase. This action to increase intracellular cGMP, could explain the favourable clinical benefits of its combination with oral nitrates.

Hydralazine may affect a number of other vascular enzymes. These include key regulators of vascular superoxide production such as NAD(P)H oxidases. These systems are regulated in vivo and ex vivo by angiotensin-II and aldosterone, and are believed to be pivotal in the development of endothelial dysfunction, a key pathophysiological abnormality in heart failure. Renin-angiotensin system activation and oxidative stress are important (and inter-related) pathophysiological processes in heart failure and other cardiovascular problems. There is experimental evidence that hydralazine may inhibit these vascular and mitochondrial oxidases, and may also act as a radical scavenger, thus helping restore the balance between NO and superoxide in endothelial dysfunction. Inhibition of superoxide production may also help prevent nitrate tolerance: this may be critical in permitting therapeutic synergy between hydralazine and nitrates. However, the evidence emanating from different animal species is contradictory. Surprisingly, the antioxidant effect of hydralazine has never been directly characterised in human blood vessels.

In this thesis I investigated the action of hydralazine in human blood vessels. To make this project clinically relevant, I characterised the actions of hydralazine in arteries and veins of various calibre (saphenous vein and internal mammary artery taken at the time of coronary artery bypass surgery and subcutaneous resistance arteries dissected from gluteal biopsies), from patients with low ejection fraction heart failure secondary to coronary artery disease. I also investigated the purported ability of hydralazine to reduce vascular superoxide production. 40 patients undergoing elective coronary artery bypass surgery were recruited for large vessel studies and 20 patients underwent gluteal biopsy, which yielded subcutaneous resistance arteries. Vascular reactivity was assessed using organ bath techniques and wire myography with the construction of cumulative concentration response curves. Production of vascular superoxide was measured using lucigenin chemiluminescence.

Summary of results:

1. There was no direct vasodilator effect of hydralazine at therapeutic concentrations (<1 µmol/L). This suggests that the favourable benefits of hydralazine are not simply dependent on direct vasodilatation.

2. There was a modest – but not statistically significant – interaction between hydralazine and endothelium-dependent vasodilatation using carbachol. This is consistent with a trend of potential biological relevance. There was a similarly modest interaction with organic nitrates. These data are consistent with theories that the therapeutic benefits of hydralazine may be partly explained by improved endothelium-dependent vasodilatation and that the interaction with organic nitrates in vivo is not simply dependent on augmented vasodilatation.

3. Hydralazine reduced basal superoxide production in both internal mammary artery [1.09 ± 0.14 nmol/mg/min vs. 0.77 ± 0.16 nmol/mg/min (P=0.026) controls and pre-treated vessels respectively] and saphenous veins [0.77 ± 0.08 nmol/mg/min vs. 0.68 ± 0.08 nmol/mg/min (P=0.018) controls and pre-treated vessels respectively]. A dose-response in superoxide production in saphenous vein (which were more readily available for experimentation) was also evident.

4. Hydralazine significantly inhibited angiotensin-II mediated superoxide production in internal mammary arteries [1.68 ± 0.434 nmol/mg/min vs. 0.843 ± 0.144 nmol/mg/min (P=0.032) controls and pre-treated vessels respectively]. Angiotensin II plays a key role in the pathophysiology of heart failure, with pleotropic effects including increased vascular superoxide production through stimulation of NAD(P)H oxidase. Attenuation of angiotensin-II stimulated superoxide production by hydralazine could mechanistically be through interaction with the NAD(P)H oxidase enzyme group; supporting the best available animal data suggesting that hydralazine prevents nitrate tolerance through modulation of this enzyme group.

Appropriate recognition must be had to the limitations innate in this work and recognise that all protocols were ex vivo and, as such, none could accurately reflect the complex phenotype recognised in chronic heart failure. The relatively small sample sizes in the study protocols must also be given recognition; however, my group - and others - have published, scientifically meaningful results utilising similar sample sizes. Future developments ought to include larger scale bench and in vivo studies of hydralazine and organic nitrate interaction with particular emphasis on assessing endothelium-dependent vasodilatation. In my studies hydralazine functionally reduced vascular superoxide production; future studies will evaluate this mechanistically with particular emphasis on the NAD(P)H oxidase system.

Item Type: Thesis (MD)
Qualification Level: Doctoral
Keywords: Heart failure, vascular biology, oxidative stress, pharmacology
Subjects: R Medicine > R Medicine (General)
Colleges/Schools: College of Medical Veterinary and Life Sciences > School of Cardiovascular & Metabolic Health
Supervisor's Name: McMurray, Professor John JV, Dominiczak, Professor Anna F and Delles, Professor Christian
Date of Award: 2015
Depositing User: Dr John Paul Rocchiccioli
Unique ID: glathesis:2015-5887
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Jan 2015 16:11
Last Modified: 18 May 2015 15:50
URI: https://theses.gla.ac.uk/id/eprint/5887

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