Biology Reference
In-Depth Information
2012
). Bears shifted their activity patterns according to feeding times in spite of
a natural photoperiod, exhibiting a behavioral flexibility that could be neces-
sary to take advantage of unpredictable periods of food availability in order to
gain energy reserves for hibernation (
Ware et al., 2012
). Additionally, similar
to the previously mentioned findings in ground squirrel, leptin levels increased
in brown bears prior to hibernation onset (
Hissa, Hohtola, Tuomala-
Saramaki, Laine, & Kallio, 1998
).
4.2. Fuel switch
Generally, hibernating mammals do not use food consumption as a source of
energy, although there are some instances of food caching (
Gillis, Morrison,
Zazula, & Hik, 2005
). Some ground squirrel species will not eat any food
during hibernation, even if it is offered artificially in the lab (
Torke &
Twente, 1977
). Instead, hibernators rely on a metabolic switch from carbo-
hydrates to lipids by catabolizing their accumulated fat stores for energy.
The fuel switch is particularly important in the heart. In hibernation, the
heart beats at a much slower rate during torpor bouts: about 5 beats per
minute in the thirteen-lined ground squirrel (
Ictidomys tridecemlineatus
)
(
Hampton, Nelson, & Andrews, 2010
). Northern blot analysis showed that
two key genes involved in regulating metabolism were upregulated in the
heart during hibernation, pancreatic triacylglycerol lipase (
PTL
), which frees
fatty acids from triglycerides at low temperatures, and pyruvate dehydroge-
nase kinase isozyme 4 (
PDK4
), which prevents the conversion of pyruvate
to acetyl CoA, thus blocking glucose oxidation (
Andrews, Squire,
Bowen, & Rollins, 1998
). Protein analysis found that PDK4 and PTL pro-
tein were also upregulated in the heart during hibernation (
Buck, Squire, &
Andrews, 2002; Squire, Lowe, Bauer, & Andrews, 2003
), along with suc-
cinyl CoA-transferase, the rate limiting enzyme in the catabolism of ketone
bodies (
Russeth, Higgins, & Andrews, 2006
). This fuel switch is evident in
other tissues as well, including skeletal muscle, which is inactive during tor-
por but active during IBAs, and white adipose tissue, the primary fuel source
for the body (
Hampton et al., 2011
).
Further evidence of the fuel switch during hibernation comes from the
blood. Analysis of hibernator serum indicated that levels of a fat-derived
ketone,
D
-
b
-hydroxybutyrate (BHB), are elevated during torpor and is
the preferential fuel source of the heart and brain during torpor and arousal,
even in the presence of glucose (
Andrews, Russeth, Drewes, & Henry,
2009
). BHB is also elevated during hibernation in the liver (
Serkova,
