Impairment of Endothelial Function by Cigarette Smoke Is Not Caused by a Specific Smoke Constituent, but by Vagal Input From the Airway

Abstract

Background: Exposure to tobacco or marijuana smoke, or e-cigarette aerosols, causes vascular endothelial dysfunction in humans and rats. We aimed to determine what constituent, or class of constituents, of smoke is responsible for endothelial functional impairment.

Methods: We investigated several smoke constituents that we hypothesized to mediate this effect by exposing rats and measuring arterial flow-mediated dilation (FMD) pre- and post-exposure. We measured FMD before and after inhalation of sidestream smoke from research cigarettes containing normal and reduced nicotine level with and without menthol, as well as 2 of the main aldehyde gases found in both smoke and e-cigarette aerosol (acrolein and acetaldehyde), and inert carbon nanoparticles.

Results: FMD was reduced by all 4 kinds of research cigarettes, with extent of reduction ranging from 20% to 46% depending on the cigarette type. While nicotine was not required for the impairment, higher nicotine levels in smoke were associated with a greater percent reduction of FMD (41.1±4.5% reduction versus 19.2±9.5%; P=0.047). Lower menthol levels were also associated with a greater percent reduction of FMD (18.5±9.8% versus 40.5±4.8%; P=0.048). Inhalation of acrolein or acetaldehyde gases at smoke-relevant concentrations impaired FMD by roughly 50% (P=0.001). However, inhalation of inert carbon nanoparticles at smoke-relevant concentrations with no gas phase also impaired FMD by a comparable amount (P<0.001). Bilateral cervical vagotomy blocked the impairment of FMD by tobacco smoke.

Conclusions: There is no single constituent or class of constituents responsible for acute impairment of endothelial function by smoke; rather, we propose that acute endothelial dysfunction by disparate inhaled products is caused by vagus nerve signaling initiated by airway irritation.

Keywords: acetaldehyde; acrolein; carbon; menthol; nanoparticles.

Figure 1.

A. Impairment of FMD after 10 minute exposure to sidestream smoke from conventional and reduced-nicotine cigarettes with and without menthol addition. Each colored line shows FMD of an individual rat measured before and after exposure. See Supplemental Figure S2 for separate presentation of groups exposed to smoke levels of ≈600 and ≈200 μg/m³ RSP. B. Independent reduction in the extent of FMD impairment with decreased nicotine levels or addition of menthol. Each colored dot is the group mean of one of the four smoke exposure groups. Error bars are SD.

Figure 2.

Impairment of FMD after a 10-minute exposure to acrolein, acetaldehyde, or sidestream smoke from Marlboro Red cigarettes. Each colored line represents one rat; black bars show group means.

Figure 3.

A. Impairment of FMD after a 10-minute exposure to carbon nanoparticles or sidestream cigarette smoke. The smoke group showed significant impairment even without removing the one outlier. B. Impairment of FMD after a 10-minute exposure to carbon nanoparticles was recovered 40 minutes post exposure. Each colored line represents one rat.

Figure 4.

Bilateral cervical vagotomy prevented impairment of FMD by cigarette smoke. Anesthetized rats underwent bilateral cervical vagotomy or sham operation before exposure to 10 min of sidestream smoke. Average starting concentration of respirable suspended particles (RSP) were 790±18 μg/m³ for the vagotomy group and 808±16 μg/m³ for sham-operation group (see Supplemental Figure S3 for decreasing RSP levels over the exposure period. Vagotomy group consisted of 4 male and 5 female (n=9); sham-operation group consisted of 4 male and 4 female (n=8). “Pre-op” = pre-operation, “post-op” = post-operation, “post-SHS” = post-sidestream smoke. Data are mean±SD. (A) Changes in FMD (see Supplemental Figure S4 for breakdown by gender). (B,C) Changes in heart rate and respiratory rate. (D) Serum nicotine at 20 min post-end-of-exposure in subsequent control experiment. Group sizes for panel C are smaller than those in A,B because we did not measure respiratory rate for the first several rats. Group sizes in panel D are the same as those in panel C by coincidence.