Ventilatory Sensitivity to Ambient CO2 at Different Hibernation Temperatures in Thirteen-Lined Ground Squirrels (Ictidomys tridecemlineatus)

Mammals entering hibernation undergo drastic reductions in metabolic rate and body temperature (Tb; to as low as ∼2% of euthermic metabolic rate and 1°C to −2°C). Although ventilation (V˙E) is also greatly reduced in hibernating ground squirrels, their relative ventilatory response (%ΔV˙E) to increases in inspired CO2 (∼400% increase to 7% CO2) dwarfs that of euthermic squirrels (∼60% increase). On the basis of data from earlier studies on hypothermic animals, we hypothesized that this switch in apparent ventilatory sensitivity was the result of the change in state (from euthermic to hibernating) and not due to the change in core Tb. Thus, we used whole-body plethysmography to assess the hypercapnic ventilatory response (HCVR) in thirteen-lined ground squirrels in steady-state hibernation at 20°C, 15°C, 10°C, 7°C, and 5°C. With the transition into hibernation as Tb fell, the breathing pattern became irregular and then episodic. Total V˙E and the oxygen consumption rate (V˙O2) decreased progressively as Tb fell. Hibernating squirrels with a core Tb of 20°C increased V˙E by 150% from normocapnic levels when given 7% CO2 to breathe, while squirrels with a Tb of 7°C increased V˙E by 650% when exposed to the same inspired CO2. When Tb was cooled from 7°C to 5°C, however, the increase in the HCVR fell to 450% and was associated with a rise in V˙O2 and total V˙E. These results reveal progressive changes in breathing pattern and the HCVR with decreasing Tb and suggest that the effects of hibernation state may be Tb dependent. V˙E did not fall in proportion to metabolic rate, and the HCVR increased progressively in both absolute terms and relative terms until a Tb of 7°C, both of which potentially constrain the extent of the metabolic suppression.