Breathing pattern and metabolic rate responses of rats exposed to ozone.

Breathing pattern (frequency and tidal volume), minute ventilation (VE), oxygen consumption (MO2), and rectal temperature (TR) were measured from rats exposed to 0.8, 0.6, 0.4, and 0.2 ppm O3 to determine the relation between breathing pattern responses to O3 and metabolic rate. In 0.8 ppm O3, rapid-shallow breathing began at 60 min exposure while VE and MO2 declined beginning at 40 min. In comparison to clean air exposed animals, rats (n = 8) during the third hour of 0.8 ppm O3 exposure had a 27% increase in frequency, 35% decline in tidal volume, 20% decrease in VE, 24% decrease in MO2, and 1.3 degrees C decrease in TR. At lower O3 concentrations, responses were diminished in magnitude, and in rats exposed to 0.2 ppm O3, only MO2 was significantly decreased, irritant-induced depression of VE did not imply a state of hypoventilation or hypoxia because ventilation equivalent for O2 (VE/MO2) did not decline during O3 exposures. Body temperature and metabolic rate depression have not been observed during the development of rapid-shallow breathing in dogs or humans exposed to these low levels of O3, and the present observations in rats may reflect a more labile thermoregulatory physiology among rodents. Ventilatory and metabolic rate depression in response to irritant inhalation can be an effective pulmonary defense in rodents and other heterothermic mammals.