The effect of temperature on isolated perfused hearts of heterothermic marsupials

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.

Cool running: locomotor performance at low body temperature in mammals

Mammalian torpor saves enormous amounts of energy, but a widely assumed cost of torpor is immobility and therefore vulnerability to predators. Contrary to this assumption, some small marsupial mammals in the wild move while torpid at low body temperatures to basking sites, thereby minimizing energy expenditure during arousal. Hence, we quantified how mammalian locomotor performance is affected by body temperature. The three small marsupial species tested, known to use torpor and basking in the wild, could move while torpid at body temperatures as low as 14.8-17.9°C. Speed was a sigmoid function of body temperature, but body temperature effects on running speed were greater than those in an ectothermic lizard used for comparison. We provide the first quantitative data of movement at low body temperature in mammals, which have survival implications for wild heterothermic mammals, as directional movement at low body temperature permits both basking and predator avoidance.

Cardiovascular changes during daily torpor in the laboratory mouse

The laboratory mouse is a facultative daily heterotherm in that it experiences bouts of torpor under caloric restriction. Mice are the most frequently studied laboratory mammal, and often, genetically modified mice are used to investigate many physiological functions related to weight loss and caloric intake. As such, research documenting the cardiovascular changes during fasting-induced torpor in mice is warranted. In the current study, C57BL/6 mice were implanted either with EKG/temperature telemeters or blood pressure telemeters. Upon fasting and exposure to an ambient temperature (T(a)) of 19 degrees C, mice entered torpor bouts as assessed by core body temperature (T(b)). Core T(b) fell from 36.6 +/- 0.2 degrees C to a minimum of 25.9 +/- 0.9 degrees C during the fast, with a concomitant fall in heart rate from 607 +/- 12 beats per minute (bpm) to a minimum of 158 +/- 20 bpm. Below a core T(b) of 31 degrees C, heart rate fell exponentially with T(b), and the Q(10) was 2.61 +/- 0.18. Further, mice implanted with blood pressure telemeters exhibited similar heart rate and activity profiles as those implanted with EKG/temperature telemeters, and the fall in heart rate and core T(b) during entrance into torpor was paralleled by a fall in blood pressure. The minimum systolic, mean, and diastolic blood pressures of torpid mice were 62.3 +/- 10.2, 51.9 +/- 9.2, 41.0 +/- 7.5 mmHg, respectively. Torpid mice had a significantly lower heart rate (25-35%) than when euthermic at mean arterial pressures from 75 to 100 mmHg, suggesting that total peripheral resistance is elevated during torpor. These data provide new and significant insight into the cardiovascular adjustments that occur in torpid mice.

Application of mesenchymal stem cell-derived cardiomyocytes as bio-pacemakers: current status and problems to be solved

Bone marrow mesenchymal stem cells (CMG cells) are multipotent and can be induced by 5-azacytidine to differentiate into cardiomyocytes. We characterized the electrophysiological properties of these cardiomyocytes and investigated their potential for use as transplantable bio-pacemakers. After differentiation, action potentials in spontaneously beating cardiomyocytes were initially sinus node-like, but subsequently became ventricular cardiomyocyte-like. RT-PCR established that ion channels mediating I(K1) and I(Kr) were expressed before differentiation. After differentiation, ion channels underlying ICa,L and If were expressed first, followed by ion channels mediating I(to) and I(K,ATP). Differentiated CMG cells expressed beta-adrenergic receptors and increased their beat rate in response to isoproterenol. CMG cardiomyocytes were purified using GFP fluorescence and transplanted into the free walls of the left ventricles of mice. The transplanted cardiomyocytes survived and connected to surrounding recipient cardiomyocytes via intercalated discs. Although further innovation is required, the present findings provide evidence of the potential for bone marrow-derived cardiomyocytes to be used as bio-pacemakers.

Hibernator Citellus undulatus maintains safe cardiac conduction and is protected against tachyarrhythmias during extreme hypothermia: possible role of Cx43 and Cx45 up-regulation

BACKGROUND

Most mammals experience cardiac arrest during hypothermia. In contrast, hibernators remain in sinus rhythm even at body temperatures of 0 degrees C.

OBJECTIVES

The purpose of this study was to quantify electrical activity and connexin expression in the heart of hibernating Siberian ground squirrel Citellus undulatus.

METHODS

Optical imaging and microelectrode recordings were conducted in Langendorff-perfused hearts and isolated papillary muscles of summer active (SA, n = 19), winter hibernating (WH, n = 21), interbout arousal (IBA, n = 12), and winter active (WA, n = 3) ground squirrels and rabbits (n = 14) at temperatures from +37 degrees C to +3 degrees C.

RESULTS

All studied SA and WH hearts maintained spontaneous sinus rhythm, safe propagation through the entire conduction system, and normal pattern of ventricular excitation at all temperatures. However, three of the seven IBA and all rabbit hearts lost excitability at 10 degrees C +/- 1 degrees C and 12 degrees C +/- 1 degrees C, respectively. In WH, SA, and IBA ground squirrels, temperature reduction from 37 degrees C to 3 degrees C resulted in a 10-fold slowing of ventricular conduction velocity and increased excitation threshold. At any temperature, WH ventricles had faster conduction velocity and lower excitation threshold compared with SA and IBA. Immunolabeling demonstrated that connexin43 (Cx43) was significantly up-regulated in WH and WA compared with SA myocardium: Cx43 area density was 12.4 +/- 1.3, 15.0 +/- 3.0 and 8.6 +/- 1.1 microm(2)/1,000 microm(2), respectively. Moreover, Cx45 was expressed in the WH but not in the SA or WA ventricles.

CONCLUSION

Hibernator Citellus undulatus has evolved to maintain safe conduction at extreme hypothermia via up-regulation of Cx43 and Cx45 in order to protect the heart against arrhythmia associated with hypothermia.

Making human nasal cilia beat in the cold: a real time assay for cell signalling

Human nasal epithelium must adapt to cold climates, and yet, in vitro, human ciliary beat frequency (CBF) is zero at 4 degrees C. Similarly, hibernating mammals do not die of pneumonia despite a core body temperature as low as 6 degrees C, implying that cilia continue to beat. Here, we show that protein kinase C (PKC) and Ca(2+)/calmodulin-dependent kinase II (CaMKII) regulate the profile of human nasal CBF in response to rising temperature from 4 degrees C. Onset of ciliary beat was at 10 degrees C in Medium 199, 7 degrees C in the presence of the PKC activator phorbol 12-myristate 13-acetate (PMA), the calcium ionophore ionomycin, or the CAMKII blocker myristoylated autocamtide-2 related inhibitory peptide (MACI), and 6 degrees C for the myristoylated peptide PKC inhibitor EGF-R Fragment 651-658 (MyrPKCI). During cell warming to 32 degrees C, the thermal profile was sigmoid in all solutions except those containing MACI+PMA. Surprisingly, cilia continued to beat despite 4 degrees C and were significantly more responsive to rising temperature with either MACI+PMA, or MACI+MyrPKCI. Our data suggest that CaMKII and PKC regulate the thermal slope and profile of CBF in vitro, and that when these protein kinases are manipulated, cilia can continue to beat despite hypothermia. These findings may relate to adaptive responses to cold climates.

Invited review: molecular adaptations in mammalian hibernators: unique adaptations or generalized responses?

Hibernators are unique among mammals in their ability to attain, withstand, and reverse low body temperatures. Hibernators repeatedly cycle between body temperatures near zero during torpor and 37 degrees C during euthermy. How do these mammals maintain cardiac function, cell integrity, blood fluidity, and energetic balance during their prolonged periods at low body temperature and avoid damage when they rewarm? Hibernation is often considered an example of a unique adaptation for low-temperature function in mammals. Although such adaptation is apparent at the level of whole animal physiology, it is surprisingly difficult to demonstrate clear examples of adaptations at the cellular and biochemical levels that improve function in the cold and are unique to hibernators. Instead of adaptation for improved function in the cold, the key molecular adaptations of hibernation may be to exploit the cold to depress most aspects of biochemical function and then rewarm without damage to restore optimal function of all systems. These capabilities are likely due to novel regulation of biochemical pathways shared by all mammals, including humans.

Influence of polyunsaturated and saturated dietary lipids on adipose tissue, brain and mitochondrial membrane fatty acid composition of a mammalian hibernator

Dietary lipid composition profoundly influences the hibernation pattern of the chipmunk Eutamias amoenus. The object of the present study was to investigate whether these physiological changes following feeding of saturated and unsaturated lipids were associated by compositional changes of fatty acids of tissues and membranes. Animals were fed with rodent chow (control diet), rodent chow with 10% sunflower seed oil (unsaturated diet) and rodent chow with 10% sheep fat (saturated diet). Diet-induced changes in the fatty acid composition of depot fat and brain total lipids and of mitochondrial phospholipids were determined. The fatty acid unsaturation index was lower in animals on saturated diet than in animals on unsaturated diet (depot fat 86.1 vs. 145.9; heart mitochondria 207.6 vs. 247.1; liver mitochondria 148.4 vs. 173.5). Pronounced differences between dietary groups were also observed in n-3 or n-6 fatty acids or their ratios of depot fat, brain and liver mitochondria. Generally, the diet-induced differences in tissue and membrane fatty acid composition in E. amoenus were more pronounced than those observed previously in non-hibernating species. Selective feeding and incorporation of high amounts of unsaturated fatty acids into tissues and cell membranes may be an important preparation for hibernation of E. amoenus which lowers its body temperature during torpor to about 0 degrees C.