Vagal Influence on Heart Rate in Hibernating Ground Squirrels

The heart rate of anesthetized golden mantled ground squirrels (Spermophilus lateralis) falls from 372±20 to 37±9beatsmin−1 during hibernation at 7°C body temperature. Heart rate in the hibernating animals often waxed and waned in a fashion that was not clearly linked to the breathing pattern. Similar observations have been made on unanesthetized ground squirrels. Under anesthesia, the effects of vagotomy were small in both euthermic and hibernating animals and led to a 6–8% increase in heart rate. Vagotomy also eliminated the cyclic fluctuations of heart rate in hibernating animals exhibiting this phenomenon. The post-vagotomy heart rate exhibited by these individuals suggested that both sympathetic excitation and parasympathetic depression were involved in producing these cyclic changes. Vagal stimulation reduced mean heart rate by at most 60–80% in euthermic and hibernating animals. The strength of the stimulus required to elicit a maximal response in the hibernating animals was 35–45% greater than that required in euthermic animals. Comparisons of mean heart rates obtained from euthermic and hibernating animals which were vagotomized, intact or stimulated to produce a maximum bradycardia produced temperature quotients of 2.21, 2.15 and 2.31, respectively. In this species, both resting vagal tone and the effects of vagal stimulation decrease in parallel with decreasing temperature over the range studied.