END-TIDAL GAS-COMPOSITION IS NOT CORRELATED WITH EPISODIC BREATHING IN HIBERNATING GROUND-SQUIRRELS

During hibernation, the breathing pattern of the golden-mantled ground squirrel (Spermophilus lateralis) consists of short episodes of breat hing separated by pauses ranging in length from less than 1 min to gre ater than 30 min. Computerized tomography scans of two hibernating ani mals during apnea indicated that the glottis was closed and that apnei c oxygenation could not occur. Analysis of end-tidal gas composition, indicative of arterial blood gas composition, revealed no clear thresh olds in gas composition for initiating or terminating episodes of brea thing. Over the course of a breathing episode, however, O-2 extraction fell exponentially, while CO2 excretion fell in a Linear fashion. The breathing episode terminated at the point where O-2 extraction asympt oted, suggesting that the length of the episode was just sufficient to repay the O-2 debt that accumulated during the preceding period of ap nea. The oxidative cost of the events associated with breathing episod es in these animals was calculated to be approximately 90% of the tota l metabolic rate during hibernation. This suggests that metabolic rate is not constant during hibernation but varies in a cyclic fashion ass ociated with the breathing pattern. It further suggests that although the levels of blood gases play a key role in establishing the total le vel of ventilation, the cyclic Variations in their composition, associ ated with periods of apnea and eupnea, do not by themselves initiate o r terminate breathing episodes.