Influences of Experimental Air Pollution on Human Sympathetic Nerve Traffic and Norepinephrine Metabolism: A Double Blind, Randomized, Twofold Crossover Study

Abstract

Background Much of the evidence linking air pollutant exposure with changes in human cardiovascular autonomic regulation relied on epidemiological studies, exposure estimates, and indirect autonomic nervous system measurements. We tested the hypothesis that in healthy older individuals, experimental exposure to fine particles increases sympathetic nervous system activity and more so with addition of ozone. Material and Methods Eighteen healthy participants (age >50 years) completed all study visits and were included in the final analysis. Participants were exposed to clean air (‘placebo’), ultrafine particles (UFP, 50μg/m3), and combination of UFP+ozone (250 ppb) for 3 hours combined with intermittent bicycle ergometer training in a randomized, three‐period, cross‐over, double‐blind fashion. Three hours following exposure, respiration, ECG, blood pressure, and muscle sympathetic nerve activity (MSNA) were continuously recorded at supine rest, during deep breathing, and during a Valsalva maneuver. Venous blood samples for plasma catecholamine measurements were taken at baseline. Induced sputum was obtained at the end of each study day. Results Combined exposure to ozone and UFP but not UFP alone caused a significant increase in sputum neutrophils and circulating leucocytes. We did not detect significant effects on blood pressure or heart rate. Resting MSNA was 47±12 with clean air, 47±14 with UFP, and 45±14 bursts/min with UFP+ozone. Maximum MSNA during Valsalva phase IIb was similar. Yet, with UFP+ozone plasma norepinephrine concentrations were significantly increased with concomitant reduction in the dihydroxyphenylglycol (DHPG)/norepinephrine ratio. Respiratory sinus arrhythmia and Valsalva ratio were unaffected by experimental air pollution. Conclusion Exposure to ultrafine particles with or without ozone does not elicit clinically relevant changes in central sympathetic or parasympathetic activity in healthy older subjects. The paradoxical norepinephrine increase with UFP+ozone may be explained by reduced peripheral norepinephrine uptake and metabolism.