Exposure to vehicular pollutants is associated with exacerbation of both cardiovascular and respiratory diseases

Exposure to vehicular pollutants is associated with exacerbation of both cardiovascular and respiratory diseases (Ibald-Mulli et al. 2002; Peters et al. 2004; Pope 2000; Timonen et al. 2006; Wichmann et al. 2000). Diesel engine exhaust (DE) is an important contributor to urban air pollution (Brunekreef and Holgate 2002; Laden et al. 2000; Pope et al. 1999). Although the exact components of DE responsible for its effects have yet to be defined, both short-term and chronic DE exposure are associated with arrhythmias and adverse cardiac events (Dockery et al. 2005; Peters et al. 2000; Rich et al. 2005) and cause endothelial dysfunction that diminishes vasodilator response in systemic arteries (Hansen et al. 2007). Multiple pathways regulate endothelium-dependent vasodilatation, including the release of nitric oxide (NO) and prostacyclin from the nitric oxide synthase (NOS) and cyclooxygenase (COX) pathways, respectively. Additionally, activation of small (SK) and intermediate (IK) Ca2+-dependent potassium channels can result in the hyperpolarization and ultimately the relaxation of vascular smooth muscle. However, short-term inhalation of dilute DE has been shown to inhibit forearm vasodilation to both acetylcholine (ACh) and the NO donor, sodium nitroprusside, in healthy volunteers (Mills et al. 2005; Tornqvist et al. 2007), suggesting that DE exposure may specifically affect signaling downstream of NO. Endothelial NOS (eNOS) is a homodimeric protein that generates NO from the conversion of l-arginine to l-citrulline. NO inhibits platelet aggregation, leukocyte adherence, and vascular smooth muscle proliferation to regulate vascular homeostasis (Alheid et al. 1987; Kubes et al. 1991; Nunokawa and Tanaka 1992). Synthesized NO diffuses into adjacent vascular smooth muscle, where it activates soluble guanylate cyclase to reduce intracellular Ca2+ concentration and decrease vascular tone, leading to vasodilation. The eNOS cofactor 5,6,7,8-tetrahydrobiopterin (BH4) is required for the production of NO, possibly by stabilizing the physical and/or electro chemical coupling of the NOS dimer, and is generated from guanosine-5 ́triphosphate (GTP) through a de novo pathway or recycled from the oxidized form of BH4, 7,8-dihydrobiopterin (BH2) by dihydrofolate reductase (DHFR) (Sugiyama et al. 2009). DE exposure increases oxidative stress within the vasculature (Miller et al. 2009), which can potentially increase the oxidation of BH4 to BH2, thereby limiting the bioavailability of this essential cofactor. Depletion of BH4 or increases in BH2 electrochemically uncouple eNOS, resulting in the generation of superoxide radical (O2) rather than NO (Crabtree et al. 2008; Sugiyama et al. 2009). In a cascade fashion, peroxy nitrite (ONOO–) production from the reaction of O2 and NO not only depletes bioavailable NO, but can also oxidize BH4 to catalyze further eNOS uncoupling. We have previously reported that vasoconstriction is augmented in systemic arteries from DE-exposed rodents and that the augmented constriction is endothelium dependent and can be reversed with NOS inhibition (Campen et al. 2005; Cherng et al. 2009; Knuckles et al. 2008). The aim of this study was to evaluate agonist-mediated vasodilation in coronary arteries from healthy rats exposed to DE. We hypothesized that DE-induced reactive oxygen species (ROS) uncouple NOS to diminish NO-dependent vasodilatation in coronary arteries.