Apneic oxygen uptake in the torpid pocket mouse Perognathus parvus

The apneas of many torpid mammals can persist longer than estimated O2 stores allow. This suggests that some O2 is acquired during these apneas by either cutaneous uptake or by a nonventilatory flux down an open airway (tracheal flux). Previous experiments confirmed apneic O2 uptake in the bat Eptesicus fuscus with the conclusion that the uptake most likely occurred by tracheal flux. However, the bat's large cutaneous wing area remained a potential source of cutaneous O2 uptake, leaving uncertainty regarding the mechanism of O2 uptake, particularly in regard to some evidence suggesting that small mammals might be obligated to maintain a closed glottis during apnea. This study sought experimental confirmation of passive O2 uptake in the pocket mouse Perognathus parvus, torpid at a body temperature of 10 degrees C, body mass 16.0 +/- 0.6 g (N = 9). Ventilation bouts lasted 1.49 +/- 0.06 min, whereas apneas lasted 4.51 +/- 0.14 min, despite estimated O2 stores able to support apneas of only 1.0 min. The maximum predicted cutaneous O2 uptake was 0.67 mumol O2/h, whereas the theoretically calculated tracheal flux was 20.2 mumol O2/h. This theoretical rate of tracheal flux compared favorably to the measured plateau apneic O2 uptake rate of 16.7 mumol O2/h. However, the diffusional component of tracheal flux was 3.6 times greater than predicted, indicating an important contribution from cardiogenic mixing. Overall, apneic O2 uptake provided 10.2% of the mouse's total O2 uptake. We conclude that passive tracheal flux is the most likely mechanism by which this animal acquires O2 during apnea.