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Changes to Metabolism and Cell Physiology that Enable Mammalian Hibernation

  • Narrative Student Review
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Springer Science Reviews

Abstract

Heterothermy is a widespread, adaptive strategy used by many species of bird and mammal to conserve energy during periods of energetic deficit, the expression of which varies greatly depending on the species and environment. A temporary, reversible reduction in metabolic rate and body temperature (i.e., torpor) is an adaptive response used by many species of birds and mammals to conserve energy during periods of resource scarcity. Long-term employment of torpor (i.e., hibernation) is a seasonally expressed phenotype, the genetic and regulated pathways of which can be found throughout all mammal lineages, including hibernators and nonhibernators alike. In mammals, adaptations that allow for hibernation can be classified as those involved in preparation for hibernation, metabolic reduction, continued cellular function and protection, and arousal. Key physiological changes involve seasonal regulation of metabolic hormones, a shift to largely using endogenous fuel sources (i.e., increased lipolysis), global down regulation of protein transcription by posttranslational modification and microRNA save for the increased production of a small number of protective proteins, shifts in membrane composition, and thermogenesis by brown adipose tissue. There is some evidence of cold acclimations in nonhibernators, such as during fetal development, but responses are limited and cursory, and eventually cellular damage occurs. Therefore, it appears that a complete suite of adaptations to metabolism, vital physiological functions, and thermogenic mechanisms is required for the successful expression of the hibernation phenotype.

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Abbreviations

ATP:

Adenosine triphosphate

BAT:

Brown adipose tissue

MR:

Metabolic rate

PUFA:

Polyunsaturated fatty acid

T a :

Ambient temperature

T b :

Body temperature

T set :

Hypothalamic body temperature setpoint

WAT:

White adipose tissue

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Correspondence to Brandon J. Klug.

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Glossary

Daily torpor

Short-term employment of torpor characterized by bouts typically lasting less than 24 h with body temperature (T b) often remaining some degrees above ambient temperature (T a).

Endothermy

Regulation of T b using a high rate of metabolism to produce endogenous heat.

Euthermy

Maintenance of T b at a relatively high temperature conducive to normal biological function.

Heterothermy

The diel or seasonal pattern of temperature regulation where T b varies outside the normal euthermic range

Hibernation

Extended employment of torpor characterized by bouts typically lasting days to weeks with T b dropping to near T a. Obligate hibernation lasts for months during predictable periods of energy imbalance (the “hibernation season”) and consists of multiple bouts interspersed with regular arousals. Conversely, facultative hibernation typically occurs during ephemeral energy crises outside of any predictable season

Hypothermia

An uncontrolled state of reduced T b (i.e., heat loss exceeds capacity for heat production)

Torpor

A temporary and controlled reduction in metabolic rate (MR) and body temperature (T b) followed by a return to euthermy using an endogenous source of heat. Torpor is often employed as an adaptive response to conserve energy during periods of resource scarcity

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Klug, B.J., Brigham, R.M. Changes to Metabolism and Cell Physiology that Enable Mammalian Hibernation. Springer Science Reviews 3, 39–56 (2015). https://doi.org/10.1007/s40362-015-0030-x

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