Cardiovascular effects of environmental tobacco smoke and benzo[a]pyrene exposure in rats
Gentner, Nicole Joy
Smoking and environmental tobacco smoke (ETS) exposure are major risk factors for cardiovascular disease (CVD), although the exact components and pathophysiological mechanisms responsible for this association remain unclear. Polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene (BaP), are ubiquitous environmental contaminants that form during organic material combustion and are thus found in cigarette smoke, vehicle exhaust particles, and air pollution. We hypothesize that PAHs are key agents responsible for mediating the cigarette smoke effects in the cardiovascular system, including increased oxidative stress, inflammation, and arterial stiffness. Arterial stiffness is a powerful, independent predictor of cardiovascular risk and is regulated, in part, by vasoactive mediators derived from the endothelium. The first objective of this project was to determine whether pulse wave dP/dt collected from radiotelemetry-implanted rats is a reliable indicator of changes in arterial stiffness following administration of vasoactive drugs or acute ETS exposure. Anaesthetized rats were administered a single dose of saline (vehicle control), acetylcholine, norepinephrine, and N(G)-nitro-L-arginine methyl ester (L-NAME) via the tail vein, allowing a washout period between injections. Acetylcholine decreased and norepinephrine increased dP/dt compared to saline vehicle. Injection of the nitric oxide (NO) synthase inhibitor L-NAME decreased plasma nitrate/nitrite (NOx), but transiently increased dP/dt. For the ETS experiment, rats were exposed for one hour to sham, low dose ETS, or high dose ETS. Exposure to ETS did not significantly alter dP/dt or plasma endothelin-1 (ET-1) levels, but increased plasma NOx levels at the high ETS exposure and increased plasma nitrotyrosine levels in both ETS groups. In conclusion, acute changes in NO production via acetylcholine or L-NAME alter the arterial pulse wave dP/dt consistently with the predicted changes in arterial stiffness. Although acute ETS appears to biologically inactivate NO, a concomitant increase in NO production at the high ETS exposure may explain why ETS did not acutely alter dP/dt. The second objective of this project was to compare the effects of subchronic ETS and BaP exposure on circadian blood pressure patterns, arterial stiffness, and possible sources of oxidative stress in radiotelemetry-implanted rats. Pulse wave dP/dt was used as an indicator of arterial stiffness, and was compared to both structural (wall thickness) and functional (NO production and bioactivity, ET-1 levels) features of the arterial wall. In addition, histology of lung, heart, and liver were examined as well as pulmonary and hepatic detoxifying enzyme activity (cytochrome P450 – specifically CYP1A1). Daily ETS exposure for 28 days altered the circadian pattern of heart rate and blood pressure in rats, with a loss in the normal dipping pattern of blood pressure during sleep. Subchronic ETS exposure also increased dP/dt in the absence of any structural modifications in the arterial wall. Although NO production and ET-1 levels were not altered by ETS, there was increased biological inactivation of NO via peroxynitrite production (as indicated by increased plasma nitrotyrosine levels). Thus, vascular stiffness and failure of blood pressure to dip precede structural changes in rats exposed to ETS for 28 days. Exposure to ETS also caused increased number of lung neutrophils as well as increased CYP1A1 activity in lung microsomes. Since ETS-induced increases in arterial stiffness occurred as early as day 7, radiotelemetry-implanted rats were exposed daily to intranasal BaP for 7 days. Similar to ETS, BaP exposure altered circadian blood pressure patterns and reduced blood pressure dipping during sleep. Thus, in support of part of our hypothesis, the PAH component of cigarette smoke may be responsible for the ETS-induced increase in blood pressure and the loss of dipping pattern during sleep. Increased neutrophil recruitment was observed in the lungs of both ETS- and BaP-exposed rats, suggesting that lung inflammatory reactions may be involved in the disruption of circadian blood pressure rhythms. Unlike ETS however, BaP exposure did not significantly alter pulse wave dP/dt, endothelial function, or lung CYP1A1 activity. Thus, contrary to our hypothesis, the reduction in NO bioactivity and increased arterial stiffness caused by ETS cannot be explained by BaP at the dose and length of the exposure in the current study. Production of reactive metabolites in the lung following ETS exposure may be responsible, at least in part, for the increases in oxidative stress in the vasculature, leading to reduced NO bioactivity and increased arterial stiffness. Oxidative stress caused by BaP exposure may have been insufficient to reduce NO bioactivity in the peripheral vasculature. Therefore arterial stiffness was not increased and factors other than NO may be responsible for the increase in blood pressure observed with ETS and BaP exposure.
DegreeMaster of Science (M.Sc.)
Copyright DateMarch 2010
environmental tobacco smoke