Thermoregulatory and metabolic changes during fever in young and old rats

We injected old and young rats with lipopolysaccharide (LPS; 50 mug/kg ip) at two ambient temperatures (T-a; 21 and 31 degreesC). Young rats mounted equivalent fevers at both T(a)s [ peak body temperatures (T-b) of 38.3 and 38.7 degreesC, respectively]. The T-b of old rats was not different from baseline (37.3 degreesC) after LPS at T-a 21 degreesC, whereas, at 31 degreesC, their T-b rose to a mean peak of 38.4 degreesC. We also measured the associated thermoregulatory responses by use of calorimetry. At 21 degreesC, 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 degreesC, 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 degreesC is due mainly to the lowered metabolic rate.