Small hibernating mammals show regular oscillations in their heart rate and body temperature throughout the winter. Long periods of torpor are abruptly interrupted by arousals with heart rates that rapidly increase from 5 beats per minute to over 400 beats per minute and body temperatures that increase by approximately 30°C, only to drop back into the hypothermic torpid state within hours. Surgically implanted transmitters were used to obtain high resolution electrocardiogram and body temperature data from hibernating thirteen-lined ground squirrels (Spermophilus tridecemlineatus). These data were used to construct a model of the circulatory system to gain greater understanding of these rapid and extreme changes in physiology. Our model provides estimates of metabolic rates during the torpor-arousal cycles in different model compartments which would be difficult to measure directly. In the compartment that models the more metabolically active tissues and organs (heart, brain, liver, and brown adipose tissue) the peak metabolic rate occurs at a core body temperature of 19°C approximately midway through an arousal. The peak metabolic rate of the active tissues is 9 times the normothermic rate after the arousal is complete. For the overall metabolic rate in all tissues, the peak to resting ratio is 5. This value is high for a rodent, which provides evidence for the hypothesis that the arousal from torpor is limited by the capabilities of the cardiovascular system.
- Mathematical model
- heart rate
- ground squirrel
- Copyright © 2010, American Journal of Physiology - Regulatory, Integrative and Comparative Physiology