Studies on the regulation of endothelial nitric oxide synthase in endothelial dysfunction

Abstract

Ischaemic heart disease and cerebrovascular disease are the leading causes of morbidity and mortality in the world. The underlying progression of the disease is linked to a reduction in the bioavailability of nitric oxide. One factor contributing to this is an increase in the production of superoxide radicals. A combination of increased oxidative stress, inappropriate lipid metabolism and cell death sets the stage for what will subsequently develop into atherosclerosis. The process of atherogenesis can slow down if patients at risk are identi ed early, receive the necessary pharmacological treatment and change to a healthier lifestyle. The aim of the following studies was to identify whether the uptake, synthesis and recycling of tetrahydrobiopterin (BH4), the essential co-factor of endothelial nitric oxide synthase (eNOS), could in uence oxidative stress in human vasculature. We also sought to elucidate whether endothelin-1 (ET-1), a potent vasoconstrictor, played an important role in oxidative stress in human vasculature and the potential mechanisms underlying this in uence.

In Study I, 49 patients with coronary artery disease took part in a placebo-controlled clinical trial with the aim of determining the mechanisms of exogenous BH4 in relation to vascular function. Oral BH4 treatment signi cantly elevates the levels of BH4 in blood, but this effect is limited by the rapid systemic oxidation of exogenous BH4. The ratio of reduced to oxidised biopterins in blood and vascular tissue is unchanged by exogenous BH4 treatment, resulting in no net effect on vascular superoxide production or endothelial function.

In Study II, the aim was to explore the regulation of endogenous BH4 and subsequent effects on endothelial function in patients with coronary artery disease. In three clinical models and one in vitro model, involving 465 subjects, we observed that an inability to increase vascular BH4 synthesis leads to signi cant impairment of endothelial function.

In Study III, the aim was to explore the role of ET-1 in endothelial dysfunction, speci cally with regard to superoxide production. ET-1 increases superoxide production in human coronary artery bypass grafts via a receptor-driven mechanism involving, the largest contributor of superoxide in the vascular wall, nicotine amide dinucleotide phosphate (NADPH) oxidase.

In Study IV, I applied what I had learned from Study I and Study II and sought to further delineate whether endothelin-1 in uences biopterin homeostasis in both human and animal tissues. ET-1 did not have any effect on BH4 in human coronary artery bypass grafts or resistance arteries, endothelial cells and mice with an over-expression of ET-1 in the endothelium (ET-transgenic mice).