Metabolic correlates of selection for swim stress-induced analgesia in laboratory mice

M. Konarzewski, B. Sadowski and I. Jozwik Institute of Biology, University of Warsaw, Branch in Bialystok, Poland. The upper limits of metabolic rates and the links between maximal and resting metabolic rates in vertebrates have recently received a lot of attention, mainly due to their possible relationship to the evolution of endothermy. We measured peak metabolic rates during 3 min swimming in 20 degrees C water (Vo2swim), maximal metabolic rate (Vo2max) in -2.5 degrees C Helox, and basal metabolic rate (BMR) in two lines of mice selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA). We found that exercise combined with heat loss used for producing SSIA also acted as a selection agent, resulting in a 15% HA/LA line difference in Vo2swim. Core body temperature of HA mice (characterized by lower Vo2swim) was also on average 3.2 degrees C lower than that of LA mice. Furthermore, Vo2max of HA mice was lower than that of LA mice by 8% and accompanied by larger hypothermia. Thus mice with exceptionally high (or low) Vo2max tended to have exceptionally high (or low) Vo2swim, resulting in a positive correlation between Vo2swim and Vo2max. All these suggest that selection for SSIA produced genetically correlated responses in both Vo2swim and Vo2max. However, we did not observe HA/LA differences in BMR. Hence, changes in resting and maximum metabolic rates are not necessarily correlated. We hypothesize that the lack of such a correlation was partially due to the modulation of metabolic responses by SSIA.