Master of Science
Staples, James F.
Hibernators experience changes in blood flow, similar to ischemia-reperfusion injury, which may lead to oxidative damage. I hypothesized that suppression of mitochondrial metabolism during hibernation protects against such damage. I compared oxidative damage and antioxidant capacity in tissues and isolated liver mitochondria among summer, torpid, and interbout euthermic thirteen-lined ground squirrels (TLGS). I found less oxidative tissue damage during summer than hibernation, but no mitochondrial differences. I also compared metabolic activity of isolated mitochondria before and after five minutes of anoxia, followed by reoxygenation, among hibernation states and rats. Anoxia-reoxygenation decreased state-three respiration (ST3) in all groups, with rat mitochondria affected the most, and torpor TLGS mitochondria affected the least. With rotenone inhibition, ST3 was less affected by anoxia-reoxygenation, suggesting ETS complex-I is a source of ROS production. An exogenous antioxidant (ascorbate) mitigated changes in ST3. My findings suggest that metabolic suppression offers some protection against oxidative damage during hibernation.
Summary for Lay Audience
Maintaining body temperature is metabolically expensive for mammals experiencing cold winters. Hibernation is one strategy of avoiding this high metabolic cost. In addition to lowered metabolic rates (less than 10 % of summer values), hibernators experience low levels of oxygen, but many can tolerate these conditions. They are therefore often used to study the effects of ischemia (lack of blood flow)-reperfusion (regain of blood flow) experiments, such as what occurs in strokes. Suppressed metabolism during hibernation may be protective against low oxygen levels and temperatures.
Our species of interest, the thirteen-lined ground squirrel, cycles between states of very low and very high metabolic rate throughout the hibernation season. During torpor, ground squirrels drastically suppress metabolic rate, lower heart rate, and decrease body temperature. Torpor lasts for approximately two weeks, before arousal (in approximately 5 h). They remain at these high body temperatures and metabolic rate for approximately 8 hours, and then gradually drop back into torpor. During these metabolic fluctuations, blood flow and oxygen supply and demand fluctuate, possibly increasing reactive oxygen species (ROS, sometimes referred to as free radicals) production.
I compared markers of oxidative damage and antioxidant capacity (ability to detoxify ROS) among torpor, aroused, and summer ground squirrels to determine if hibernation protects squirrels against ROS damage. I also looked at the functional consequences of anoxia-reoxygenation at the mitochondrial respiration level, to determine how mitochondrial respiration was affected by oxygen fluctuation.
I found that summer ground squirrels had less oxidative damage than torpid and aroused animals, but antioxidant capacity did not differ, suggesting damage was due to higher ROS levels during hibernation. I also found that mitochondrial respiration rates were lower following anoxia-reoxygenation but were least affected in torpor when there is the most suppression of mitochondrial metabolism. Finally, I found that the addition of an antioxidant protected mitochondria against the effects of anoxia-reoxygenation. In summary, my findings indicate that metabolic suppression during torpor offers protection against changes in oxygen supply during hibernation.
Duffy, Brynne, "Reactive oxygen species damage and consequences for mitochondrial function in the hibernating thirteen-lined ground squirrel, Ictidomys tridecemlineatus" (2020). Electronic Thesis and Dissertation Repository. 7467.
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