Episodic ozone exposure in adult and senescent Brown Norway rats: acute and delayed effect on heart rate, core temperature and motor activity

Setting exposure standards for environmental pollutants may consider the aged as a susceptible population but the few published studies assessing susceptibility of the aged to air pollutants are inconsistent. Episodic ozone (O-3) is more reflective of potential exposures occurring in human populations and could be more harmful to the aged. This study used radiotelemetry to monitor heart rate (HR), core temperature (T-c) and motor activity (MA) in adult (9-12 months) and senescent (20-24 months) male, Brown Norway rats exposed to episodic O-3 (6 h/day of 1 ppm O-3 for 2 consecutive days/week for 13 weeks). Acute O-3 initially led to marked drops in HR and T-c. As exposures progressed each week, there was diminution in the hypothermic and bradycardic effects of O-3. Senescent rats were less affected than adults. Acute responses were exacerbated on the second day of O-3 exposure with adults exhibiting greater sensitivity. During recovery following 2 d of O-3, adult and senescent rats exhibited an elevated Tc and HR during the day but not at night, an effect that persisted for at least 48 h after O-3 exposure. MA was elevated in adults but not senescent rats during recovery from O-3. Overall, acute effects of O-3, including reductions in HR and T-c, were attenuated in senescent rats. Autonomic responses during recovery, included an elevation in T-c with a pattern akin to that of a fever and rise in HR that were independent of age. An attenuated inflammatory response to O-3 in senescent rats may explain the relatively heightened physiological response to O-3 in younger rats.