Thermoregulatory and metabolic changes during fever in young and old rats

Department of Psychology and Program in Neuroscience, University of Delaware, Newark, Delaware 19716-2590 Submitted 1 May 2003 ; accepted in final form 25 July 2003 We injected old and young rats with lipopolysaccharide (LPS; 50 µg/kg ip) at two ambient temperatures ( T a ; 21 and 31°C). Young rats mounted equivalent fevers at both T a s [peak body temperatures ( T b ) of 38.3 and 38.7°C, respectively]. The T b of old rats was not different from baseline (37.3°C) after LPS at T a 21°C, whereas, at 31°C, their T b rose to a mean peak of 38.4°C. We also measured the associated thermoregulatory responses by use of calorimetry. At 21°C, young rats developed a fever by increasing both O 2 consumption and heat conservation. Old rats did not become febrile, and O 2 consumption fell by 15%. Heat loss was the same in old and young rats. At 31°C, young and old rats developed similar fevers with similar increases in heat production and conservation. Our results suggest that the lack of LPS fever in old rats at 21°C is due mainly to the lowered metabolic rate. aging; heat production; heat loss; ambient temperature; lipopolysaccharide Address for reprint requests and other correspondence: E. Satinoff, Dept. of Psychology, Univ. of Delaware, Newark, DE 19716 (E-mail: satinoff{at}udel.edu ).