Thermoregulatory and metabolic changes during fever in young and old rats

Abstract

We injected old and young rats with lipopolysaccharide (LPS; 50 μg/kg ip) at two ambient temperatures ( Ta; 21 and 31°C). Young rats mounted equivalent fevers at both Tas [peak body temperatures ( Tb) of 38.3 and 38.7°C, respectively]. The Tbof old rats was not different from baseline (37.3°C) after LPS at Ta21°C, whereas, at 31°C, their Tbrose 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 O2consumption and heat conservation. Old rats did not become febrile, and O2consumption 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.