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INHALATION TOXICITY OF POTASSIUM OCTATITANATE FIBERS (TISMO) IN RATS FOLLOWING 13 WEEKS OF AEROSOL EXPOSURE

One hundred and forty male and 140 female rats were divided into 1 control and 3 test groups of 35 rats each, per sex, and exposed by whole-body inhalation to test compound at target concentrations of 0, 1 mg/m3 (1700 fibers/cm3, 123 WHO fibers/cm3), 10 mg/m3 (5900 fibers/cm3, 952 WHO fibers/cm3), and 100 mg/m3 (112,700 fibers/cm3, 7440 WHO fibers/cm3) for 6 h/day, 5 days/wk for 13 wk. Ten rats from each group were killed after 13 wk of exposure and 13 wk of recovery, respectively, for histopathological evaluation. The other 15 rats from each group were killed to study lung clearance after 91 days of exposure, and approximately 1.5 and 3 mo of recovery following the end of the 13 wk of exposure. The mean fiber length of the chamber atmosphere was 2.8, 2.7, and 2.8 µm, while the mean fiber width was 0.48, 0.48, and 0.45 µm for the 1-, 10-, and 100-mg/m3 chambers, respectively. In the 1-mg/m3 (123 WHO fibers/cm3) exposure group, inhaled particles were mostly retained in a few fiberladen alveolar macrophages (AMs) within the alveoli adjacent to alveolar ducts without any adverse tissue response throughout 13 wk of exposure and following 13 wk of recovery. This exposure concentration was considered to be a no-observable-adverse-effect level (NOAEL), since the alveoli containing fiber-laden AMs preserved normal structure. After 13 wk of exposure to 10 mg/m3 (952 WHO fibers/cm3), fiber-laden AMs were mainly retained at the alveoli adjacent to the alveolar ducts. Infrequently, slight fibrotic thickening was observed in the alveolar ducts and adjoining alveoli, with proliferating fibroblasts and hyperplastic Type II pneumocytes, and microgranulomas. Occasionally, trace amounts of collagenous material were deposited in the thickened alveolar ducts and adjoining alveolar walls. In addition, minimal alveolar bronchiolarization was occasionally found in the alveoli adjacent to the terminal bronchioles. The peribronchial lymphoid tissue and thymic lymph nodes contained migrated fiber-laden AMs. After 13 wk of recovery, fiber-laden AMs had mostly disappeared from alveoli located in the peripheral acini, but they localized in the alveolar ducts region, suggesting there was active lung clearance of fibers by the AMs via airways. Thickened alveolar ducts and adjacent alveoli were decreased in thickness, a reversible change manifested by reduction of proliferating Type II pneumocytes and fibroblasts. Collagenized fibrosis was slightly more pronounced in the thickened alveolar ducts and adjoining alveoli. The lung response following 13 wk of exposure to 100 mg/m3 (7440 WHO fibers/cm3) and after 13 wk of recovery was similar to those findings of the 952 WHO fibers/cm3 group but more pronounced, demonstrating a clear concentration-related response. Alveolar ducts and adjoining alveolar walls in the central acini were irregularly thickened with more frequent evidence of minimal collagenized fibrosis. The lung burden and clearance of fibers were estimated by measuring the total content of titanium (Ti) in the lungs, but high variability of Ti content in control and exposed groups prevented meaningful lung clearance analysis.