Metabolic adjustments during daily torpor in the Djungarian hamster

Abstract

Djungarian hamsters ( Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.

Daily torpor reduces mass and changes stress and power output of soleus and EDL muscles in the Djungarian hamster, Phodopus sungorus

“…Hamsters of similar age and size were split into two different photoperiod regimes that are commonly used for this species (Heldmaier et al, 1999;Atgié et al, 2009;Braulke et al, 2010): control (summer; N8), 16h light (05:00-21:00h), 8h dark; and torpor (winter; N8), 8h light (11:00-19:00h), 16h dark. Room temperature was maintained between 19 and 21°C and animals were without access to a running wheel.…”

“…In this study we used Djungarian hamsters to analyze the effects of prolonged exposure to short photoperiod conditions (which lead to extended use of daily torpor) (Heldmaier et al, 1999) to determine whether: (1) prolonged use of daily torpor affects the mechanical performance of skeletal muscle during maximal (sprint) and sustained (endurance) type activities as assessed using the work loop technique; and (2) responses to daily torpor differ between relatively fast twitch (extensor digitorum longus) and relatively slow twitch (soleus) skeletal muscles. This study represents the first such application of the work loop technique to a mammalian species that uses torpor/hibernation.…”

The effects of hibernation on the contractile and biochemical properties of skeletal muscles in the thirteen-lined ground squirrel, Ictidomys tridecemlineatus

“…However, in the present study glycogen levels were not significantly different, in gastrocnemius muscle, between summer and torpid thirteen-lined ground squirrels. During torpor, mammals primarily mobilise fatty acids for fuel (Heldmaier et al, 1999;Buck and Barnes, 2000;Carey et al, 2003). Therefore, glycogen stores may be kept constant throughout hibernation or may be reserved for more metabolically demanding time points such as during interbout arousal with its shivering thermogenesis.…”

A simple molecular mathematical model of mammalian hibernation

“…In hibernating golden mantled ground squirrels, Brooks and Storey observed a 96% reduction in cardiac PDH activity during hibernation (Brooks and Storey, 1992). In the hearts of Djungarian hamsters, Heldmaier et al (1999) showed that inactivation of PDH closely correlates with metabolic rate reduction seen during daily torpor. Inactivation of PDH by this post-translational modification impedes carbohydrate oxidation by blocking the conversion of pyruvate to acetyl CoA thus leading to metabolic rate reduction and a reliance on stored lipids as the primary source of fuel.…”