Pulmonary ventilation and cardiac activity in hibernating and arousing golden-mantled ground squirrels (Spermophilus lateralis)

No effect of season on the electrocardiogram of long-eared bats (Nyctophilus gouldi) during torpor

Heterothermic animals regularly undergo profound alterations of cardiac function associated with torpor. These animals have specialised tissues capable of withstanding fluctuations in body temperature > 30 °C without adverse effects. In particular, the hearts of heterotherms are able to resist fibrillation and discontinuity of the cardiac conduction system common in homeotherms during hypothermia. To investigate the patterns of cardiac conduction in small insectivorous bats which enter torpor year round, I simultaneously measured ECG and subcutaneous temperature (T_sub) of 21 Nyctophilus gouldi (11 g) during torpor at a range of ambient temperatures (T_a 1-28 °C). During torpor cardiac conduction slowed in a temperature dependent manner, primarily via prolongation along the atrioventricular pathway (PR interval). A close coupling of depolarisation and repolarisation was retained in torpid bats, with no isoelectric ST segment visible until animals reached T_sub <6 °C. There was little change in ventricular repolarisation (JT interval) with decreasing T_sub, or between rest and torpor at mild T_a. Bats retained a more rapid rate of ventricular conduction and repolarisation during torpor relative to other hibernators. Throughout all recordings across seasons (> 2500 h), there was no difference in ECG morphology or heart rate during torpor, and no manifestations of significant conduction blocks or ventricular tachyarrhythmias were observed. My results demonstrate the capacity of bat hearts to withstand extreme fluctuations in rate and temperature throughout the year without detrimental arrhythmogenesis. I suggest that this conduction reserve may be related to flight and the daily extremes in metabolism experienced by these animals, and warrants further investigation of cardiac electrophysiology in other flying hibernators.

Cardiorespiratory and metabolic reactions during entrance into torpor in dormice, Glis glis

Dormice voluntarily enter torpor at ambient temperatures ranging between 0-28 degrees C. This study describes heart rate, ventilation frequency, O2-consumption (defined as metabolic rate), CO2-production and body temperature during entrance into torpor. Their temporal relationship was analysed during the time course of metabolic depression at different ambient temperatures. Body temperature and heart rate were measured in unrestrained dormice with implanted transmitter. Ventilation frequency was monitored by total body plethysmography or infrared video monitoring. To compare entries into torpor at different T(a) these periods were distinguished into four different phases: the resting phase prior to torpor, the phase of pre-torpor adjustments, the reduction phase and the phase of steady state torpor. In the pre-torpor phase, dormice increased their ventilation, metabolic rate and heart rate, indicating that the torpid state is initiated by an enhanced metabolic activity for about an hour. This was followed by a rapid reduction of ventilation, metabolism and heart rate, which reached their minimum values long before body temperature completed its decline. The results of the present study show that the entrance into torpor is caused by an active respiratory, cardiac and metabolic depression.

Tissue and plasma peptidase activity is altered during hypothermic hibernation in the 13 lined ground squirrel (Spermophilus tridecemlineatus)

Adult 13 lined ground squirrels were monitored for entry into a state of hypothermic hibernation or arousal in a cold room on a photoperiod LD 2:22. Once animals developed predictable hibernation patterns, animals were killed at the mid point of hypothermic hibernation or arousal for determination of plasma and tissue angiotensin-1-converting enzyme (Kininase II) activity. Enzyme was extracted from plasma, lung, kidney, liver, forebrain and brainstem and assayed in vitro. During hypothermic hibernation enzyme activity is significantly decreased in all tissues examined. These data suggest that the activity of tissue and plasma peptidases are altered during the cyclic torporous periods characteristics of hibernation in this species.

A comparison of the heart rate at different ambient temperatures during long-term hibernation in the garden dormouse, Eliomys quercinus L

Heart rate in hibernating garden dormice, Eliomys quercinus, was studied by means of permanently implanted electrodes; ambient temperatures (TA's) were maintained at 0, 4, 6.5, and 9 degrees C during the 6-month test period in each winter study. The animals were kept under constant conditions in darkness and without food or water. Heart rate remained at a low level during deep hibernation at all TA's studied. There were no differences in midwinter values between the TA's of 6.5 and 9 degrees C: the means were 9-12 beats/min during apnea. Heart rate thus differs from other hibernation parameters studied simultaneously, which were strongly TA dependent. However, the optimal TA of 4 degrees C could be distinguished and heart rate was significantly lower, 8-10 beats/min. At 0 degree C the values were slightly higher: 12-13 beats/min. The TA of 0 degree C was exceptional for all parameters studied. At the beginning of the hibernation season was a transition period with elevated heart rate values. Respiratory-related heart-rate changes appeared during periodic respiration, heart rate being significantly higher during respiratory periods at all TA's. At 0, 6.5, and 9 degrees C tachycardia occurred also during apnea, very close to the respiratory period. There are responses that are comparable to hypoxic environmental conditions during hibernation, diving, and pregnancy and under high-altitude conditions. Parallel adaptations appear in heart rate and respiration, i.e., bradycardia and periodic respiration. In conclusion, heart-rate values were low during deep hibernation, and compared with other parameters measured at different TA's heart rate is maintained inside narrow limits during deep hibernation.

Reduction of metabolism during hibernation and daily torpor in mammals and birds: temperature effect or physiological inhibition?

The present study addresses the controversy of whether the reduction in energy metabolism during torpor in endotherms is strictly a physical effect of temperature (Q10) or whether it involves an additional metabolic inhibition. Basal metabolic rates (BMR; measured as oxygen consumption, VO2), metabolic rates during torpor, and the corresponding body temperatures (Tb) in 68 mammalian and avian species were assembled from the literature (n = 58) or determined in the present study (n = 10). The Q10 for change in VO2 between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasing Tb from 30 to less than 10 degrees C in hibernators (species that show prolonged torpor). In daily heterotherms (species that show shallow, daily torpor) the Q10 remained at a constant value of 2.2 as Tb decreased. In hibernators with a Tb less than 10 degrees C, the Q10 was inversely related to body mass. The increase of mass-specific metabolic rate with decreasing body mass, observed during normothermia (BMR), was not observed during torpor in hibernators and the slope relating metabolic rate and mass was almost zero. In daily heterotherms, which had a smaller Q10 than the hibernators, no inverse relationship between the Q10 and body mass was observed, and consequently the metabolic rate during torpor at the same Tb was greater than that of hibernators. These findings show that the reduction in metabolism during torpor of daily heterotherms and large hibernators can be explained largely by temperature effects, whereas a metabolic inhibition in addition to temperature effects may be used by small hibernators to reduce energy expenditure during torpor.

Effects of carbon dioxide on preoptic thermosensitive neurons in vitro

Effects of carbon dioxide (CO2) on the firing rates of preoptic thermosensitive neurons were examined in rat brain slice preparations. The perfusing medium was saturated with gas mixtures consisting of 90% O2 and one of various concentrations (5%, 6.3%, 7.5%, and 10%) of CO2 balanced with N2. The medium containing 5% CO2 was used as control. Most preoptic neurons were inhibited during application of a high CO2 medium. An excitatory effect of CO2 on a small number of neurons was also significant, although this was weak and transient compared to the inhibitory effect. Thermosensitivities of the neurons did not correlate with their CO2 sensitivities. The influence of CO2 tended to be more evident at higher temperatures. We conclude that the direct effect of CO2 on PO thermosensitive neurons as well as on thermally insensitive neurons is mainly inhibitory.

Effects of carbon dioxide inhalation on preoptic thermosensitive neurons

The effects of inspired CO2 on preoptic thermosensitive neurons were studied in urethanized rats. Most of the neurons changed their activities diversely while the rat breathed 4%, 7%, and 10% CO2 gas mixtures. Half of the neurons increased activity during CO2 inhalation, but activity was not necessarily intensified by elevating CO2 concentration. Thermosensitive neurons tended to increase activity more than thermally insensitive neurons. The effect of CO2 on sensitivity of thermosensitive neurons was also examined by regression of neuronal activity on preoptic temperature. The slopes of the regression lines during CO2 inhalation did not differ significantly from those during air inhalation in either warm-sensitive or cold-sensitive neurons, but CO2 did elevate the intercepts in most instances (P less than 0.01). However, if P less than 0.05 is accepted as a significance level, the slopes of the regression lines for warm-sensitive neurons tended to decrease during CO2 inhalation (9/39 pairs). The present results indicate that preoptic thermosensitive neurons generally increase their activities and modify their thermosensitivities during CO2 inhalation.