Changes in CO2 sensitivity during entrance into, and arousal from hibernation in Ictidomys tridecemlineatus.

At the onset of entrance into hibernation in many mammals, there is a reduction in the respiratory exchange ratio (RER) thought to result in a retention of CO2 that contributes to the ensuing metabolic suppression. In steady-state hibernation, the relative hypercapnic ventilatory response (HCVR; the % change in ventilation to CO2 exposure) is elevated. These two observations, paradoxically, suggest a transient decrease in CO2 sensitivity at the onset of entrance into hibernation, allowing the retention of CO2, then a subsequent increase in CO2 sensitivity giving rise to the elevated HCVR in steady-state hibernation. We examined the time course of the changes in ventilation, O2 consumption rates ([Formula: see text]o2), CO2 excretion rates, body temperature, and hence the RER and ACR (air convection ratio, ventilation/[Formula: see text]o2) and the HCVR throughout entrance and arousal into and out of hibernation in 13-lined ground squirrels to confirm this. We observed a significant drop (entrance) and rise (arousal) in the RER produced by hypo- and hyperventilation, respectively. CO2 chemo-sensitivity while the RER was reduced on entrance was blunted and rose late in entrance. On arousal, CO2 chemo-sensitivity was elevated when the RER was elevated and fell immediately after RER returned to normal values. At any given Tb, the HCVR was lower during entrance compared to arousal producing a significant hysteresis. The HCVR, however, was the same at any given [Formula: see text]o2 during entrance and arousal. These data suggest that both the changes in [Formula: see text]o2 and in the HCVR are associated with changes in central regulation of the effector limbs establishing steady-state hibernation.