In vivo versus in vitro airway surface liquid nicotine levels following cigarette smoke exposure

Whole cigarette smoke (WCS) is composed of approximately 5% particulates and 95% vapors by weight and is difficult to reproduce quantitatively in the laboratory, where typically, routine in vitro application of smoke normally only utilizes the particulate phase. In this study, we used a system for exposing epithelial cells cultured at an air-liquid interface to WCS. We hypothesized that the use of WSC in vitro was more relevant to what is seen in vivo than methods of cigarette smoke application that only use a small fraction of WCS i.e., aqueous extract or cigarette smoke condensate (CSC)]. To test this hypothesis, we compared nicotine and cotinine concentrations (measured by mass spectrometry) in the airway surface liquid (ASL) of human primary bronchial epithelial cultures (HBECs) exposed to serial dilutions of WCS to the concentrations found in induced sputum of human subjects who had recently smoked a cigarette; this was also compared to the concentrations found after an exposure to a concentration of CSC commonly used in vitro. When measured by mass spectrometry, nicotine levels were not significantly different in induced sputum versus the ASL of HBECs exposed in vitro to a 1:30 exposure of WCS. However, HBECs that had been exposed to CSC returned significantly lower concentrations of ASL nicotine. These results suggest that nicotine is a good dosimetry marker of WCS exposure and provides direct evidence that the use of WCS is more relevant than the use of CSC for in vitro systems.