Relationships between Respiratory Quotient and Metabolic Rate during Entry to and Arousal from Daily Torpor in Deer Mice (Peromyscus maniculatus)

Abstract

Retention of CO2 and respiratory acidosis have previously been reported to accompany entrance to hibernation, with acid-base and CO2 balance returning to normal upon arousal from hibernation. The present study indicates that these alterations are not limited to species which have the ability to undergo deep hibernation, but are also involved in another mammalian dormancy state, daily torpor. Deer mice (Peromyscus maniculatus) were used to examine the time course of changes in respiratory quotient (RQ), metabolic rate (MR), and body temperature (Tb) during daily torpor at an ambient temperature of 10° C For several hour sprior to daily torpor, RQ steadily declined from 0.92 to 0. 74, indicating a shift in primary metabolic substrate from carbohydrate to fat, while MR and Tb remained constant at 4.23 ± 0 44 mL O₂/g · h and 36.6° ± 0.9° C, respectively. Entrance to daily torpor involved a transitory decrease in RQ (0.74–0.63–0.73) that lasted less than 12 min and reflected CO2 retention and accompanying respiratory acidosis. This was subsequently followed by a rapid drop in MR and Tb. Respiratory quotient remained stable during steady-state torpor (0.74 ± 0.02). Metabolic rate and Tb in daily torpor were 1.12 ± 0.35 mL O2/g · h and 21.6° ± 1.6° C, respectively. A transitory increase in RQ (0.76–0.96–0.76) was associated with arousal, signifying release of the excess CO2, with MR and then Tb increasing after the RQ change. The order of change in the measured parameters suggests that adjustments in the CO2 content of the body contribute to the induction and termination of daily torpor in deer mice.