METABOLIC CORRELATES OF SELECTION FOR SWIM STRESS-INDUCED ANALGESIA IN LABORATORY MICE

Metabolic correlates of selection for swim stress-induced analgesia in laboratory mice. Am. J. Physiol. 273 (Regulatory Integrative Comp. Ph ysiol. 42): R337-R343, 1997.-The upper limits of metabolic rates and t he links between maximal and resting metabolic rates in vertebrates ha ve recently received a lot of attention, mainly due to their possible relationship to the evolution of endothermy. We measured peak metaboli c rates during 3 min swimming in 20 degrees C water ((V) over dot (O2s wim)), maximal metabolic rate ((V) over dot (O2max)) in -2.5 degrees C Helox, and basal metabolic rate (BMR) in two lines of mice selected f or high (HA) and low (LA) swim stress-induced analgesia (SSIA). We fou nd that exercise combined with heat loss used for producing SSIA also acted as a selection agent, resulting in a 15% HA/LA line difference i n (V) over dot (O2swim). Core body temperature of HA mice (characteriz ed by lower (V) over dot (O2swim)) was also on average 3.2 degrees C l ower than that of LA mice. Furthermore, (V) over dot (2max) of HA mice was lower than that of LA mice by 8% and accompanied by larger hypoth ermia. Thus mice with exceptionally high (or low) (V) over dot (O2max) tended to have exceptionally high (or low) (V) over dot (O2swim), res ulting in a positive correlation between (V) over dot (O2swim) and (V) over dot (O2max). All these suggest that selection for SSIA produced genetically correlated responses in both (V) over dot (O2swim) and (V) over dot (O2max). However, we did not observe HA/LA differences in BM R. Hence, changes in resting and maximum metabolic rates are not neces sarily correlated. We hypothesize that the lack of such a correlation was partially due to the modulation of metabolic responses by SSIA.