Cardiovascular effects of diesel exhaust : mechanistic and interventional studies

Abstract

Background: Air pollution is associated with negative health effects. Exposure to combustion-derived particulate matter (PM) air pollution has been related to increased incidence of cardiovascular and respiratory morbidity and mortality, specifically in susceptible populations. Ambient particles, with a diameter of less than 2.5 mm, have been suggested to be the strongest contributor to these health effects. Diesel exhaust (DE) is a major source of small combustion-derived PM air pollution world wide.  In healthy volunteers, exposure to DE, has been associated with airway inflammation and impaired vasomotor function and endogenous fibrinolysis. The aims of this thesis were to further elucidate the underlying mechanisms to the reported cardiovascular effects following exposure to DE, with specific focus on endothelin-1 (ET-1). Additionally, the vascular effects of the major gaseous component of DE, nitrogen dioxide (NO2), were assessed together with the impact of an exhaust particle trap to reduce the observed negative vascular effects after DE exposure. Methods: In all studies healthy, non-smoking male volunteers were included and exposed for one hour during intermittent exercise in a randomised double-blind crossover fashion. In studies I-III, subjects were exposed to DE at a particulate matter concentration of approximately 300 μg/m3 and filtered air, on two different occasions. In study V an additional exposure was employed, during which DE was filtered through an exhaust particle trap. In study IV subjects were exposed to nitrogen dioxide (NO2) at 4 ppm or filtered air. In study I, thrombus formation and platelet activation were assessed using the Badimon ex vivo perfusion chamber and flow cytometry. Study II comprised the determination of arterial stiffness including pulse wave analysis and velocity. In studies III-V, vascular assessment was performed using venous occlusion plethysmography. In studies IV and V, the vascular responses to intra-arterially infused endothelial-dependent and endothelial-independent vasodilatators were registered. In study III, vascular responses to intra-arterial infusion of Endothelin-1 (ET-1) and ET-1-receptor antagonists were assessed. Venous occlusion phlethysmography was in all cases performed 4-6 hours following exposures. Blood samples for markers of inflammation, coagulation and platelet activation were collected before and throughout the study periods in studies III and V. Results: Exposure to DE increased ex vivo thrombus formation and arterial stiffness, in terms of augmentation index. DE inhalation impaired vasomotor function and endogenous fibrinolysis. The exhaust particle trap reduced the particle concentration by 98% and abolished the effects on vasomotor function, endogenous fibrinolysis and ex vivo thrombus formation. Plasma concentrations of ET-1 and its precursor big-ET-1 were unchanged following exposure. Dual endothelial receptor antagonism caused similar vasodilatation after both exposures, although vasodilatation to the endothelin-A receptor alone was blunted after DE exposure. ET-1 infusion induced vasoconstriction only following DE exposure. Exposure to nitrogen dioxide did not affect vascular function. Conclusion: Inhalation of diesel exhaust in young healthy men impaired important and complementary aspects of vascular function in humans; regulation of vascular tone and endogenous fibrinolysis as well as increased ex vivo thrombus formation. The use of an exhaust particle trap significantly reduced particle emissions and abolished the DE-induced vascular and prothrombotic effects. The adverse vascular effects following DE exposure do not appear to be directly mediated through the endothelin system. Neither is NO2 suggested to be a major arbiter of the DE-induced cardiovascular responses. Arterial stiffness is a non-invasive and easily accessible method and could thus be employed to address vascular function in larger field studies. Taken together, this thesis has given further knowledge about the mechanisms underlying the DE-induced vascular effects