Morning NO₂ exposure sensitizes hypertensive rats to the cardiovascular effects of same day O₃ exposure in the afternoon

Abstract

Context: Within urban air sheds, specific ambient air pollutants typically peak at predictable times throughout the day. For example, in environments dominated by mobile sources, peak nitrogen dioxide (NO₂) levels coincide with morning and afternoon rush hours, while peak levels of ozone (O₃), occur in the afternoon.

Objective: Given that exposure to a single pollutant might sensitize the cardiopulmonary system to the effects of a subsequent exposure to a second pollutant, we hypothesized that a morning exposure to NO₂ will exaggerate the cardiovascular effects of an afternoon O₃ exposure in rats.

Materials and methods: Rats were divided into four groups that were each exposed for 3 h in the morning (m) and 3 h in the afternoon (a) on the same day: (1) m-Air/a-Air, (2) m-Air/a-O₃ (0.3 ppm), (3) m-NO₂ (0.5 ppm)/a-Air and (4) m-NO₂/a-O₃. Implanted telemetry devices recorded blood pressure and electrocardiographic data. Sensitivity to the arrhythmogenic agent aconitine was measured in a separate cohort.

Results: Only m-NO₂/a-O₃-exposed rats had significant changes in electrophysiological, mechanical and autonomic parameters. These included decreased heart rate and increased PR and QT_c intervals and increased heart rate variability, suggesting increased parasympathetic tone. In addition, only m-NO₂/a-O₃ exposure decreased systolic and diastolic blood pressures and increased pulse pressure and QA interval, suggesting decreased cardiac contractility.

Discussion and conclusion: The findings indicate that initial exposure to NO₂ sensitized rats to the cardiovascular effects of O₃ and may provide insight into the epidemiological data linking adverse cardiovascular outcomes with exposures to low concentrations of O₃.

• Air pollution, blood pressure
• cardiovascular
• electrocardiogram
• nitrogen dioxide
• ozone
• sequential exposure

Acknowledgements

The authors would like to thank Judy Richards of the U.S. EPA for her excellent technical assistance, and Drs. M. Ian Gilmour, Michael Hughes, Yongho Kim, Robert Devlin, and Ronald Hines, also of the U.S. EPA, for their thorough review of this manuscript.

Declaration of interest

The authors report no conflicts of interest.

This paper has been reviewed and approved for release by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency. Approval does not signify that the contents necessarily reflect the views and policies of the U.S. EPA, nor does mention of trade names or commercial products constitute endorsement or recommendation for use.