Daily variation of thermoregulatory costs in laboratory mice selected for high and low basal metabolic rate

Most mammals are known to have clear circadian rhythms of body temperature (Tb) and metabolic rate. Large parts of the rhythms correspond to the oscillation of nonshivering thermogenesis (NST), dependent on visceral organ mass, and, affected by mass of brown adipose tissue (BAT). I tested whether: (1) a different levels of BMR result in respective changes of Tb values and the magnitude of daily RMR oscillations both within and below thermoneutrality; (2) the amplitude of daily variation of RMR depends on ambient temperature (Ta). I studied: (1) daily variation of body temperature at Ta of 23 °C, and (2) the rate of resting metabolism (RMR) within and below thermoneutrality at the time of minimum and increasing Tb (minimum and maximum NST capacity), in two lines of laboratory mice subjected to divergent, artificial selection toward high (HBMR) and low (LBMR) basal rate of metabolism (BMR). All mice had a clear circadian rhythm of Tb with minimum of 36.4±0.2 °C at 7:00 and maximum of 37.8±0.2 °C at 21:00. Their RMR measured below thermoneutrality exhibited significant daily variation, with the maximum between 16:00 and 19:00, when Tb was rising. Within thermoneutral zone (TNZ) I found between-line, but not between-time, differences in RMR. All between-line differences in RMR could be explained by the magnitude of BMR. I did not find any between-line differences of RMR value in temperatures below thermoneutrality. The amplitude of daily variation of RMR measured below TNZ depended neither on the Ta value nor on level of BMR (or visceral organs).