E155 Why don’t bears lose muscle mass after hibernating for 3 months?

What are some human diseases that might benefit from understanding the physiology of hibernation?

stroke, osteoporosis, Parkinson’s disease and Alzheimer’s

How are bears different from other hibernators?

• Hardly lose any heat at all in their winter dens, but their metabolism still slows 

• Complete shutdown and resetting the parameter of their daily lives 

• Shut down genes involved with the breakdown of bone (can be used to prevent osteoporosis) 

• Stay insulated by their fur and fat 

• Bears decide when to shut down and when to recover (driven by food supply and temperature)

 Describe the changes in body temperature during the winter for ground squirrels.

• The body temperature dramatically decreases and peaks for a short amount of time at relatively consistent intervals 

• Arctic ground squirrels overwintering in frozen soils must defend themselves against large thermal gradients (or differences between ambient and body temperatures) while torpid using nonshivering thermogenesis (thermogenic torpor)

How is climate change affecting the start and end of the hibernation (torpor) period?

• Climate change and global warming decreases the torpor period 

• Bears are hibernating for less time because the climate is warmer

According to the discussion, what is a potential benefit of climate change for ground squirrels?

Reduced intensity and/or duration of thermogenesis caused by warmer conditions would allow arctic ground squirrels to conserve energy and potentially increase winter survival

According to the discussion, what is a potential problem with climate change for ground squirrels?

Climate change could shorten the hibernation season and increase the number of days that arctic ground squirrels are active above ground, increasing mortality rates because of increased exposure to predators 

In the section called “Morphological changes during hibernation”, how are changes in muscle mass in hibernators different from what happens in disease or atrophy? What muscle fiber type appears to be upregulated during hibernation?

• Losses of muscle mass, protein, and fiber size are typically small during hibernation 

• Hibernators prevent alterations to muscle fiber type ratio that typically accompany inactivity; greater proportion of fast glycolytic muscle fibers 

• Marginal loss of oxidative type I fibers and a retention of fiber ratio throughout the winter 

• Moderate increase in type I slow oxidative fiber ratio during hibernation 

• Hibernators increase type I fiber ratio by 25% relative to fiber ratios during the summer active period

In the section called “Skeletal muscle contractile performance” what happens to muscle fiber strength during hibernation?

There is little loss in muscle strength over the hibernation period (average muscle strength is only reduced by 12.6%) 

In the section called “PI3K/Akt/mTOR”, what happens to protein synthesis and degradation during hibernation? 

• Decrease in protein synthesis and protein degradation during individual torpor bouts 

• Net balance of protein synthesis and degradation 

• Bats have low rates of both during torpor, but rates comparable to summer levels during arousal bouts

In the section called “Myostatin”, what is a potential benefit of downregulating myostatin?

• Myostatin: a transforming growth factor that prevents activation of Akt and reduces mTOR-induced protein synthesis 

• Downregulation during hibernation can preserve muscle mass

how does PGC-1α affect the production of slow oxidative muscle fibers (Type I)?

determines muscle fiber-type plasticity by enhancing the gene expression characteristic of slow oxidative fibers 

• Endurance training increases PGC-1α and expression of slow oxidative muscle fibers 

• Inactivity enhances the expression of fast glycolytic muscle fibers 

• Fasting and cold exposure increase expression due to increase in free fatty acids; active PPARS drives expression of slower, more oxidative muscle fibers 

how does Ca2+ affect the production of slow oxidative muscle fibers (Type I)?

increase in muscle activity with shivering increases Ca2+ and the other pathways

how does AMPK affect the production of slow oxidative muscle fibers (Type I)?

relative expression increases during hibernation

How long do arctic squirrels hibernate?

8 months 

What do they do while hibernating?

• Decrease in HR 

• Decrease in respiration

how cold is the arctic squirrel burrow

-3 C

how do ectotherms maintain stability without generating heat

conduction = lay in burrow, posture

radiation = sun

brumation = low VO2 (cooling)

evaporation

hibernators are deliberately hypothermic

hibernation = body temps significant drop then increase daily torpor = decreased metabolic rate to save energy

bats and hummingbirds are

endothermic homeotherms

bats and hummingbirds in torpor

enter torpor at night

• temp decreases during sleep and torpor

body and ambient temp of bats

body temperature = ambient temperature

VO2 and ambient temperature of bats

energy expenditure/VO2 is greatest at extremely low and high temps

• low VO2 at optimal, moderate temps

waking up costs a lot of energy. what is the advantage of arctic ground squirrels waking up every 10-14 days?

restroom; protect neurons

potential mechanisms for bear hibernation

reduce oxidative stress and downregulate atrophy pathways

• increasing pathways that maintain muscle

• decreasing pathways for muscle atrophy

Pl3K/Akt/mTOR pathway for cell survival

limiting apoptosis (cell death)

altering mRNA translation, cell cycle entry, angiogenesis