Ozone toxicity in the rat. II. Modeling changes due to ambient temperatures and duration

Previous studies involving exposures to xenobiotic agents have demonstrated decreases in physiological parameters such as heart rate (HR) and core body temperature (Tco) and have shown that these toxic responses are modulated by changes in ambient temperature (Ta). We recently published the results of a study in which male Fischer 344 rats were implanted with radiotelemetry transmitters that permitted continuous monitoring of HR, Tco, and motor activity. These animals were divided into nine treatment groups (n = 4-5/group) composed of combinations of one of three O3-exposure regimens [0.0 parts/million (ppm) O3 x 24 h/day x 5 days; 0.5 ppm O3 x 6 h/day x 5 days; or 0.5 ppm O3 x 23 h/day x 5 days] at one of three Ta values (10,22, or 34 degrees C). We now report on statistical approaches for the modeling and analyses of these data. The models utilized were dependent on the treatment combinations. Circadian rhythms of Tco during air control periods were fit by cosine models. Overall effects of O3 in the 6- and 23-h exposure groups were best modeled by modified damped-sine and one-compartment models, respectively, for both HR and Tco. These results demonstrate improved methods for the evaluation of biorhythmicity.