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Changes in ventilatory responses within and between hibernation bouts in Spermophilus Lateralis the golden-mantled ground squirrel Harris, Michael B.

Abstract

For most hibernating mammals, the hibernation season is composed of bouts of hibernation of various lengths punctuated by short periods of arousal. Hibernation depth fluctuates during each bout, and possibly over the season, even when ambient and body temperatures remain constant. This suggests that physiological changes associated with hibernation may also vary in a similar fashion. Investigations of central control of ventilation, however, have assumed that the changes seen in ventilatory responses during hibernation are consistent at any given body temperature throughout the hibernation season. To test whether this assumption is valid, the hypercapnic ventilatory response of hibernating golden mantled ground squirrels was characterized in individual animals throughout bouts of hibernation at different times in the hibernation season. The overall level of ventilation produced by an animal is normally expressed as the total amount of air moved in and out of the lung per minute (minute ventilation, VE). This variable is a function of the volume of each breath (tidal volume, VT) and the number of breaths taken per minute (respiratory frequency, Rf). It was found that the hypercapnic ventilatory response during hibernation consisted almost exclusively of increases in Rf as levels of ambient CO₂ were increased, while VT changed very little. Thus, it was determined that the overall ventilatory response could be adequately described by changes in breathing frequency alone. The hypercapnic ventilatory response was complex and could be characterized by the changes occuring in ventilation over four specific ranges (regions) of inspired CO₂. In the range from 0 to 2 % CO₂, Rf appeared independent of changes in inspired CO₂. In the range from 2 to 6 % CO₂, Rf increased with increasing levels of inspired CO₂. In the third range, beginning at approximately 6 % CO₂, Rf remained constant or decreased slightly as levels of inspired CO₂ increased further. The fourth range began at a level of inspired CO₂ which not only promoted a further increase in VE but also prompted the animals to arouse from hibernation (7 to 13 %). No significant inter or intra-bout variations were observed in any aspect of the hypercapnic ventilatory response. Although no significant variation was found in the level of CO₂ which prompted arousal (arousal threshold) between bouts of hibernation, arousal thresholds did appear to increase from early to mid season and decrease from mid to late season. This was similar to the pattern of variation observed in the length of hibernation bouts during the season. It is possible that changes in both arousal threshold and bout length are associated with changes in hibernation depth and that there is a regular pattern of seasonal variation in hibernation depth. These data indicate, however, that ventilatory sensitivity is exclusively a function of body temperature, regardless of possible temperature-independent changes in hibernation depth.

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