Comparison of the physiological and morphological effects of cigarette smoke exposure at comparable weekly doses on Sprague-Dawley rats

A variety of dose x duration exposure regimens have been used in inhalation toxicity studies using rodents. We evaluated the effects of differences in smoke concentration and daily exposure duration under similar weekly cumulative exposures in rats to determine potential variation in type and severity of adverse effects in 13-week exposure studies. The weekly cumulative dosages were 2100 and 4200 μg wet total particle matter (WTPM)/L, and the daily exposure durations were 1 and 6 h. Weekly exposure duration was 5 or 7 days/week for groups exposed 1 h/day and 7 days/week for groups exposed 6 h/day. Recovery duration was 6 and 13 weeks. Mainstream smoke exposure suppressed body weight (BW) gain in both regimens. Lower dose groups exposed 1 h/day had a consistently greater of BW gain compared with corresponding 6 h/day groups. Respiratory rate, tidal volume, and minute volume (MV) were suppressed in a dose-dependent manner in both regimens. Higher MV in rats exposed for 6 h/day compared with rats exposed 1 h/day suggested that a lower concentration for longer duration resulted in a greater total inhaled mass (TIM) in rats exposed 6 h/day. Groups exposed for 6 h/day had lower blood carboxyhemoglobin and plasma nicotine levels than groups exposed 1 h/day, reflecting the lower carbon monoxide (CO) and WTPM concentrations in the 6 h/day groups. Data from examination of bronchoalveolar lavage fluids (BALF) and respiratory tract tissues indicated comparable effects between both regimens. Exposure-induced histopathological changes regressed similarly for both regimens after the recovery periods.