Perspectives on Metabolic Suppression during Mammalian Hibernation and Daily Torpor

Abstract

During hibernation and daily torpor, metabolic rate (MR) can be maximally reduced to only 1 or 12%, respectively, of the minimum euthermic levels. The physiological mechanisms causing these metabolic reductions remain unclear and are under debate. Earlier observations based on high ratio of (MR_cuthermia / MR_hibernation) and high corresponding Q₁₀ values (>3), suggest temperatureindependent mechanisms such as tissue acidification through retention of respiratory CO2 may be involved in actively inhibiting metabolism beyond the effect of low body temperature (Tb) alone. This, however, has been challenged due to inherent problems in defining the proper MR_cuthermia for comparison and the proper execution of Q₁₀ calculation. The proposition that a reduced thermoconductance during torpor could account for the reduced metabolism and that the (Tb-Ta) gradient ultimately determines the level of metabolism during torpor have also been challenged. Since the thermoregulatory set-point (Tset) is progressively lowered during entry into torpor and remains low during torpor, and hibernators and daily heterotherms are capable of strong thermogenic efforts even at low Tb the prevailing level of MR during torpor likely reflects the thermogenic magnitude (or its suppression) of the error signal (Thypo-Tset) and the Arrhenius effect on MR. To date, however, no direct evidence has been provided to argue for or against any of the above conjectures. We have proposed a different experimental approach which could allow direct comparison of minimum MR at the same Tb during hibernation and induced hypothermia. This hopefully will resolve if active inhibition of MR is indeed a state-dependent characteristic of torpor and hibernation.