Replicative DNA synthesis in tissues of the rat exposed to aged and diluted sidestream smoke

Male Sprague-Dawley rats were exposed to aged and diluted sidestream smoke (ADSS) from Kentucky 1R4F reference cigarettes for 6 h/day, 5' days/wk, for a 13-wk period. Exposure concentrations were 0, 0.1, 1, and 10 mg ADSS/m(3). Exposures were conducted in whole-body inhalation chambers. Rats were held in nose-only exposure tubes for the 6-h exposures to minimize pelt deposition and subsequent ingestion of ADSS. Groups of 10 rats from each exposure group were killed after 5, 28, and 90 d of exposure to examine the rates of replicative DNA synthesis; 6 rats from each exposure group were kept for a 90-day recovery period after termination of exposures to examine replicative DNA synthesis rates. Three days prior to each scheduled necropsy, an osmotic pump containing 5-bromo-2'-deoxyuridine (BrdU) was implanted subcutaneously. After necropsy, tissues were processed for examination of BrdU-containing cells at several sites. Incorporation of BrdU was assessed either by counting the number of labeled cells along a length of an epithelial surface or by counting the number of labeled cells in an area of tissue. Tissues examined were from the nasal cavity, ventral larynx, and trachea, in addition to bronchial, bronchiolar, and alveolar regions of the lung. Endocardium, myocardium, epicardium, and aortic smooth muscle sites were also examined. Increased replicative DNA synthesis occurred in some sites of the respiratory tract at the 5-day timepoint at the mid or high exposure concentrations, although at 28 and 90 days, these effects had diminished in intensity or were not present, indicating adaptation to the ADSS exposure. The only tissues with elevated rates of replicative DNA synthesis at 90 days were the cuboidal and respiratory epithelium at the most rostral portion of the nasal cavity at the highest exposure concentration. Increased rates of replicative DNA synthesis were not noted in heart tissues or lung alveolar epithelium at any concentration at any time point. Examination of rats killed after the end of the 90-day recovery period indicated that the increase in replicative DNA synthesis was not sustained after termination of exposures. The no observed effect level (NOEL) for increased replicative DNA synthesis after subchronic exposure to ADSS in the rat is greater than 1 mg ADSS/m(3).