METABOLIC RATE AS A FUNCTION OF AGE IN BROWN NORWAY AND LONG‐EVANS RATS

Abstract

Brown Norway (BN) rats are commonly used in aging studies but relatively little is known on their metabolic rate (MR) as with aging. There is considerable disagreement in this field with some studies indicating a decrease and others showing an increase in MR with advanced age. Understanding how MR varies with age is critical for the development of pharmacokinetic models of environmental toxicants in aged animals. Male rats of the BN and Long‐Evans (LE) strain were housed unrestrained inside a direct calorimeter maintained at 22 °C for three hr while dry and evaporative heat loss were monitored. The sum of dry and evaporative heat loss was averaged over the final hour in the calorimeter to calculate MR (W/kg). MR was assessed in rats ranging from 1 to 23 months in BN rats (N=34) and 1 to 12 months in LE rats (N=18). MR of both strains was ~25 W/kg at 1 mo and then dropped rapidly with aging and growth. After reaching an age of 4 mo, MR of both strains reached an asymptote. As age increased beyond 4 mo, MR of the BN and LE strains was approximately 7 and 5 W/kg, respectively. There was no indication of an increase with MR as rats approached senescence. Body weight of the LE strain at maturity was approximately double that of the BN strain. While body weight has a marked effect on MR, there were strain specific differences in MR that were independent of body weight. This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.