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

Abstract

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 (${\dot{\text{V}}}$o₂), CO₂ excretion rates, body temperature, and hence the RER and ACR (air convection ratio, ventilation/${\dot{\text{V}}}$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 T_b, the HCVR was lower during entrance compared to arousal producing a significant hysteresis. The HCVR, however, was the same at any given ${\dot{\text{V}}}$o₂ during entrance and arousal. These data suggest that both the changes in ${\dot{\text{V}}}$o₂ and in the HCVR are associated with changes in central regulation of the effector limbs establishing steady-state hibernation.