Acid-base state and intermittent breathing in the torpid bat, Eptesicus fuscus

White-nose syndrome initiates a cascade of physiologic disturbances in the hibernating bat host

Background

The physiological effects of white-nose syndrome (WNS) in hibernating bats and ultimate causes of mortality from infection with Pseudogymnoascus (formerly Geomyces) destructans are not fully understood. Increased frequency of arousal from torpor described among hibernating bats with late-stage WNS is thought to accelerate depletion of fat reserves, but the physiological mechanisms that lead to these alterations in hibernation behavior have not been elucidated. We used the doubly labeled water (DLW) method and clinical chemistry to evaluate energy use, body composition changes, and blood chemistry perturbations in hibernating little brown bats (Myotis lucifugus) experimentally infected with P. destructans to better understand the physiological processes that underlie mortality from WNS.

Results

These data indicated that fat energy utilization, as demonstrated by changes in body composition, was two-fold higher for bats with WNS compared to negative controls. These differences were apparent in early stages of infection when torpor-arousal patterns were equivalent between infected and non-infected animals, suggesting that P. destructans has complex physiological impacts on its host prior to onset of clinical signs indicative of late-stage infections. Additionally, bats with mild to moderate skin lesions associated with early-stage WNS demonstrated a chronic respiratory acidosis characterized by significantly elevated dissolved carbon dioxide, acidemia, and elevated bicarbonate. Potassium concentrations were also significantly higher among infected bats, but sodium, chloride, and other hydration parameters were equivalent to controls.

Conclusions

Integrating these novel findings on the physiological changes that occur in early-stage WNS with those previously documented in late-stage infections, we propose a multi-stage disease progression model that mechanistically describes the pathologic and physiologic effects underlying mortality of WNS in hibernating bats. This model identifies testable hypotheses for better understanding this disease, knowledge that will be critical for defining effective disease mitigation strategies aimed at reducing morbidity and mortality that results from WNS.

Electronic supplementary material

The online version of this article (doi:10.1186/s12899-014-0010-4) contains supplementary material, which is available to authorized users.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Natural hypometabolism during hibernation and daily torpor in mammals

Daily torpor and hibernation are the most powerful measures of endotherms to reduce their energy expenditure. During entrance into these torpid states metabolic rate is suppressed to a fraction of euthermic metabolism, paralleled by reductions in ventilation and heart rate. Body temperature gradually decreases towards the level of ambient temperature. In deep torpor body temperature as well as metabolic rate are controlled at a hypothermic and hypometabolic level. Torpid states are terminated by an arousal where metabolic rate spontaneously returns to normal levels again and euthermic body temperature is established by a burst of heat production. In recent years some of the cellular mechanisms which contribute to hypometabolism have been disclosed. Transcription, translation, as well as protein synthesis are largely suppressed. Cell proliferation in highly proliferating epithelia like the intestine is suspended. ATP production from glucose is reduced and lipids serve as the major substrate for remaining energy requirements. All these changes are rapidly reverted to normometabolism during arousal. Hibernation and daily torpor are found in small mammals inhabiting temperate as well as tropical climates. It indicates that this behaviour is not primarily aimed for cold defense, instead points to a general role of hypometabolism, as a measure to cope with a timely limited or seasonal bottleneck of energy supply.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

In vivo blood oxygen binding in hypoxic lesser spear-nosed bats: relationship to control of breathing

The hypoxic ventilatory threshold of many mammals correlates with their hemoglobin-oxygen affinity (P50). Yet, in some small mammals ventilation actually declines, rather than increases, with exposure to decreasing PaO2; their air convection requirement (V(E)/V(O2)), however, is elevated in hypoxia. We propose that the threshold of the hypoxic V(E)/V(O2) of small mammals coincides with the inflection ('knee') of their in vivo O2 equilibrium curve (O2EC). In vivo blood gas and pH data were obtained from normoxic and hypoxic lesser-spear nosed bats, Phyllostomus discolor; in vitro blood O2EC were also generated for normoxic bats at 32 and 37 degrees C and at three P(CO2)'s. The hypoxic V(E)/V(O2) threshold of P. discolor occurs at PaO2 = 39 Torr; the corresponding in vivo O2 saturation is 0.70, approximating the inflection of the O2EC. This animal has a high blood O2 affinity (P50 = 27.5 Torr at pH 7.40 and 37 degrees C; P50 = 30.8 Torr at in vivo pH of 7.31 and TB of 37.4 degrees C). As PaO2 is reduced, a pronounced hypoxia-induced respiratory alkalosis and hypothermia help maintain SaO2 near the O2EC inflection (0.64-0.70 S(O2)).

Apneic oxygen uptake in the torpid pocket mouse Perognathus parvus

The apneas of many torpid mammals can persist longer than estimated O2 stores allow. This suggests that some O2 is acquired during these apneas by either cutaneous uptake or by a nonventilatory flux down an open airway (tracheal flux). Previous experiments confirmed apneic O2 uptake in the bat Eptesicus fuscus with the conclusion that the uptake most likely occurred by tracheal flux. However, the bat's large cutaneous wing area remained a potential source of cutaneous O2 uptake, leaving uncertainty regarding the mechanism of O2 uptake, particularly in regard to some evidence suggesting that small mammals might be obligated to maintain a closed glottis during apnea. This study sought experimental confirmation of passive O2 uptake in the pocket mouse Perognathus parvus, torpid at a body temperature of 10 degrees C, body mass 16.0 +/- 0.6 g (N = 9). Ventilation bouts lasted 1.49 +/- 0.06 min, whereas apneas lasted 4.51 +/- 0.14 min, despite estimated O2 stores able to support apneas of only 1.0 min. The maximum predicted cutaneous O2 uptake was 0.67 mumol O2/h, whereas the theoretically calculated tracheal flux was 20.2 mumol O2/h. This theoretical rate of tracheal flux compared favorably to the measured plateau apneic O2 uptake rate of 16.7 mumol O2/h. However, the diffusional component of tracheal flux was 3.6 times greater than predicted, indicating an important contribution from cardiogenic mixing. Overall, apneic O2 uptake provided 10.2% of the mouse's total O2 uptake. We conclude that passive tracheal flux is the most likely mechanism by which this animal acquires O2 during apnea.

Ventilatory response to hypoxia and hypercapnia in the torpid bat, Eptesicus fuscus

Ventilatory pattern and ventilatory responses to hypercapnia and hypoxia were investigated in torpid big brown bats at body temperatures of 5, 10, 20, 30 and 37 degrees C. The pattern of breathing at temperatures below 30 degrees C was intermittent, consisting of rhythmic breathing bouts separated by apneic periods with occasional sporadic, non-rhythmic breathing episodes. Overall ventilation (Ve) was matched consistently to overall oxygen consumption (MO2) over the entire range of temperatures with a mean air convection requirement (Ve/MO2) of 1.28 L/mmol. However, calculating the air convection requirement using only oxygen uptake acquired during ventilation yielded an ectotherm-like temperature relationship. Ventilation was stimulated at all temperatures by either increased inspired CO2 or decreased inspired O2. At 20 degrees C, graded hypercapnic stimulation increased the duration of the rhythmic bouts and decreased the duration of apneas until at high CO2 (greater than 3%) breathing was continuous. Hypoxic stimulation below about 7% O2 increased ventilation by selectively increasing the non-rhythmic ventilations and decreasing rhythmic bouts.