Effects of low temperature on breathing pattern and ventilatory responses during hibernation in the golden-mantled ground squirrel
During entrance into hibernation in golden-mantled ground squirrels (Callospermophilus lateralis), ventilation decreases as metabolic rate and body temperature fall. Two patterns of respiration occur during deep hibernation. At 7 °C body temperature (Tb), a breathing pattern characterized by episodes of multiple breaths (20.6 ± 1.9 breaths/episode) separated by long apneas or nonventilatory periods (Tnvp) (mean = 11.1 ± 1.2 min) occurs, while at 4 °C Tb, a pattern in which breaths are evenly distributed and separated by a relatively short Tnvp (0.5 ± 0.05 min) occurs. Squirrels exhibiting each pattern have similar metabolic rates and levels of total ventilation (0.2 and 0.23 ml O2/hr/kg and 0.11 and 0.16 ml air/min/kg, respectively). Squirrels at 7 °C Tb exhibit a significant hypoxic ventilatory response, while squirrels at 4 °C Tb do not respond to hypoxia at any level of O2 tested. Squirrels at both temperatures exhibit a significant hypercapnic ventilatory response, but the response is significantly reduced in the 4 °C Tb squirrels. Carotid body denervation has little effect on the breathing patterns or on the hypercapnic ventilatory responses. It does reduce the magnitude and threshold for the hypoxic ventilatory response. Taken together the data suggest that (1) the fundamental rhythm generator remains functional at low temperatures; (2) the hypercapnic ventilatory response arises from central chemoreceptors that remain functional at very low temperatures; (3) the hypoxic ventilatory response arises from both carotid body and aortic chemoreceptors that are silenced at lower temperatures; and (4) there is a strong correlation between breathing pattern and chemosensitivity.