VAGAL INFLUENCE ON HEART-RATE IN HIBERNATING GROUND-SQUIRRELS

The heart rate of anesthetized golden mantled ground squirrels (Spermo philus lateralis) falls from 372 +/- 20 to 37 +/- 9 beats min-1 during hibernation at 7-degrees-C body temperature. Heart rate in the hibern ating animals often waxed and waned in a fashion that was not clearly linked to the breathing pattern. Similar observations have been made o n unanesthetized ground squirrels. Under anesthesia, the effects of va gotomy were small in both euthermic and hibernating animals and led to a 6-8 % increase in heart rate. Vagotomy also eliminated the cyclic f luctuations of heart rate in hibernating animals exhibiting this pheno menon. The post-vagotomy heart rate exhibited by these individuals sug gested that both sympathetic excitation and parasympathetic depression were involved in producing these cyclic changes. Vagal stimulation re duced mean heart rate by at most 60-80 % in euthermic and hibernating animals. The strength of the stimulus required to elicit a maximal res ponse in the hibernating animals was 35-45 % greater than that require d in euthermic animals. Comparisons of mean heart rates obtained from euthermic and hibernating animals which were vagotomized, intact or st imulated to produce a maximum bradycardia produced temperature quotien ts of 2.21, 2.15 and 2.3 1, respectively. In this species, both restin g vagal tone and the effects of vagal stimulation decrease in parallel with decreasing temperature over the range studied.