Electronic cigarette (e-cigarette) usage in the USA has drastically increased in the past 5 years due to age restrictions on conventional cigarettes, aggressive marketing and a perception that e-cigarettes are a healthy alternative. E-cigarettes contain nicotine, water, glycerol, propylene glycol and optional flavouring. On inhalation, the device heats the ingredients into a vapour [1]. While tobacco cigarette smoke is known to cause deleterious effects on the cardiovascular system, angiogenesis and skin capillary perfusion by causing direct injury to blood vessel walls, increased platelet aggregation, microvascular thrombosis [2-4] and inflammation [5], the consequences of e-cigarette vapour exposure on the lung are still largely unexplored [6, 7]. Recently, LERNER et al. [8] reported that vapours produced by e-cigarettes and e-cigarette fluids with flavourings induced toxicity, oxidative stress and inflammatory response in human bronchial airway epithelial cells (H292) and fetal lung fibroblasts (HFL1) as well as mouse lung. GARCIA-ARCOS et al. [9] showed that the aerosolised nicotine-containing e-cigarette fluid increased airway hyperreactivity, distal airspace enlargement, mucin production, and cytokine and protease expression in mice, implying potential dangers of nicotine inhalation during e-cigarette use. The inflammatory response to e-cigarette use involved increased neutrophil activation and mucus production [10], and decreased mucociliary clearance [11]. In human embryonic and mouse neural stem cells, human pulmonary fibroblasts [12], and skin and lung cells [13], cytotoxicity of e-cigarette vapour was correlated with the number and concentration of chemicals used to flavour the fluids. We recently showed in the skin flap survival model in vivo that nicotine-containing e-cigarette vapour is just as harmful to the microcirculation as tobacco cigarette smoke [4]. In the present study, we examined whether long-term exposure to e-cigarette vapour or nicotine produce the same damaging effect on lung structure and vasculature as tobacco smoke in a rat model in vivo. 6-week-old, male Sprague Dawley rats (Envigo Laboratories, Denver, CO, USA) were divided into four groups of eight animals per group and exposed for 5 weeks as follows. 1) Room air. 2) Subcutaneous injections of (−)-nicotine ditartrate (Sigma Aldrich, St Louis, MO, USA) 2 mg•kg −1 twice daily; the amount of nicotine for injections was based on that known from previous studies to produce stable plasma nicotine levels of approximately 25 ng•mL −1 , which is compatible with plasma levels in habitual smokers [14, 15]. 3) Blu E-cigs (Classic Tobacco Flavour (Blu, Charlotte, NC, USA), containing 12 mg•mL −1 nicotine) vapour produced in a TE-2E e-cigarette smoking machine (Teague Enterprises, Davis, CA, USA); the coil temperature of the e-cigarettes was within the normal range usually used by vapers (200-250°C). Rats in this group were exposed to 48 mg nicotine per day [4]. Our experimental design, by subjecting rats to e-cigarette vapour, is a major imp...
