Seasonal variations in the rate and capacity of cardiac SR calcium accumulation in a hibernating species

α1-Adrenergic receptor regulates papillary muscle and aortic segment contractile function via modulation of store-operated Ca2+ entry in long-tailed ground squirrels Urocitellus undulatus

The effect of phenylephrine (PE) on right ventricle papillary muscle (PM) and aortic segment (AS) contractile activity was studied in long-tailed ground squirrels Urocitellus undulatus during summer activity, torpor and interbout active (IBA) periods in comparison to rat. We found that PE (10 μM) exerts positive inotropic effect on ground squirrel PM that was blocked by α1-AR inhibitor-prazosin. PE differently affected frequency dependence of PM contraction in ground squirrels and rats. PE significantly increased the force of PM contraction in summer and hibernating ground squirrels including both torpor and IBA predominantly at the range of low stimulation frequencies (0.003-0.1 Hz), while in rat PM it was evident only at high stimulation frequency range (0.2-1.0 Hz). Further, it was found that PE vasoconstrictor effect on AS contractility is significantly higher in ground squirrels of torpid state compared to IBA and summer periods. Overall vasoconstrictor effect of PE was significantly higher in AS of ground squirrels of all periods compared to rats. Positive inotropic effect of PE on PM along with its vasoconstrictor effect on AS of ground squirrels was not affected by pretreatment with inhibitors of L-type Ca2+ channels, or Na+/Ca2+ exchanger or Ca2+-ATPase but was completely blocked by an inhibitor of store-operated Ca2+ entry (SOCE)-2-APB, suggesting the involvement of SOCE in the mechanisms underlying PE action on ground squirrel cardiovascular system. Obtained results support an idea about the significant role of alpha1-AR in adaptive mechanisms critical for the maintaining of cardiovascular contractile function in long-tailed ground squirrel Urocitellus undulatus.

Agmatine modulates calcium handling in cardiomyocytes of hibernating ground squirrels through calcium-sensing receptor signaling

True hibernators are remarkable group of mammals whose hearts are resistant to such stressors as deep hypothermia, ischemia, arrhythmia. Capability of cardiac cells from hibernating species to effectively rule Ca²⁺ homeostasis during torpor is poorly studied. Better understanding of these mechanisms could allow to introduce new strategies for improvement the cardiac performance and may be useful for cardiovascular medicine. Here for the first time we have shown that the regulation of Ca²⁺ handling and thereby cardiomyocyte contractility by endogenous neurotransmitter agmatine occurs through the modulation of calcium-sensing receptor (CaSR). In isolated cardiocytes of hibernating ground squirrels generating stationary Ca²⁺ transients in the absence of actual myocellular excitation, low doses of this polyamine (up to 500 μM) induce the G_βγ-dependent activation of PI₃-kinase with subsequent stimulation of Akt-kinase and nitric oxide (NO) production by endothelial NO-synthase (eNOS). NO production abolishes Ca²⁺ oscillations in virtue of the enhancement of Ca²⁺ reuptake by sarco(endo)plasmic Ca²⁺ ATPase (SERCA). Simultaneously, the activation of phospholipase A₂ (PLA₂) and arachidonic-acid dependent Ca²⁺ entry occur providing replenishment of Ca²⁺ store. High concentrations of agmatine (> 2 mM) induce other CaSR-mediated pathways involving phospholipase C (PLC) pathway, the formation of inositoltriphosphate (IP₃) and diacylglicerol (DAG) followed by induction of their targets: IP₃ receptors and protein kinase C isoforms (PKC), respectively. Furthermore, it is also responsible for the stimulation of PLA₂ and elevation of intracellular calcium caused by arachidonic acid-regulated Ca²⁺-permeable (ARC) channels. Additionally, there is a potent store-operated Ca²⁺ entry (SOC) in cardiomyocyte. Negative (NPS 2143) and positive (R 568) allosteric modulators of CaSR recapitulate effects of low and high agmatine doses on Ca²⁺ handling and NO synthesis. These facts and the alteration of agmatine influence in response to an increase of extracellular Ca²⁺, which is the direct agonist of CaSR, may confirm the participation of CaSR in regulation of Ca²⁺ handling and excitability of cardiomyocytes by agmatine.

The Mechanism Enabling Hibernation in Mammals

Some rodents including squirrels and hamsters undergo hibernation. During hibernation, body temperature drops to only a few degrees above ambient temperature. The suppression of whole-body energy expenditure is associated with regulated, but not passive, reduction of cellular metabolism. The heart retains the ability to beat constantly, although body temperature drops to less than 10 °C during hibernation. Cardiac myocytes of hibernating mammals are characterized by reduced Ca²⁺ entry into the cell membrane and a concomitant enhancement of Ca²⁺ release from and reuptake by the sarcoplasmic reticulum. These adaptive changes would help in preventing excessive Ca²⁺ entry and its overload and in maintaining the resting levels of intracellular Ca²⁺. Adaptive changes in gene expression in the heart prior to hibernation may be indispensable for acquiring cold resistance. In addition, protective effects of cold-shock proteins are thought to have an important role. We recently reported the unique expression pattern of cold-inducible RNA-binding protein (CIRP) in the hearts of hibernating hamsters. The CIRP mRNA is constitutively expressed in the heart of a nonhibernating euthermic hamster with several different forms probably due to alternative splicing. The short product contained the complete open reading frame for full-length CIRP, while the long product had inserted sequences containing a stop codon, suggesting production of a C-terminal deletion isoform of CIRP. In contrast to nonhibernating hamsters, only the short product was found in hibernating animals. Thus, these results indicate that CIRP expression in the hamster heart is regulated at the level of alternative splicing, which would permit a rapid increment of functional CIRP when entering hibernation. We will summarize the current understanding of the cold-resistant property of the heart in hibernating animals.

Ecophysiology of Omega Fatty Acids: A Lid for Every Jar

Omega fatty acids affect various physiological functions, such as locomotion, cardiac function, and thermogenesis. We highlight evidence from animal models that points to pathways by which specific omega fatty acids exert differential effects. We suggest that optimizing the omega fatty acid composition of tissues involves trade-offs between costs and benefits of specific fatty acids.

Gene expression changes controlling distinct adaptations in the heart and skeletal muscle of a hibernating mammal

Throughout the hibernation season, the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) experiences extreme fluctuations in heart rate, metabolism, oxygen consumption, and body temperature, along with prolonged fasting and immobility. These conditions necessitate different functional requirements for the heart, which maintains contractile function throughout hibernation, and the skeletal muscle, which remains largely inactive. The adaptations used to maintain these contractile organs under such variable conditions serves as a natural model to study a variety of medically relevant conditions including heart failure and disuse atrophy. To better understand how two different muscle tissues maintain function throughout the extreme fluctuations of hibernation we performed Illumina HiSeq 2000 sequencing of cDNAs to compare the transcriptome of heart and skeletal muscle across the circannual cycle. This analysis resulted in the identification of 1,076 and 1,466 differentially expressed genes in heart and skeletal muscle, respectively. In both heart and skeletal muscle we identified a distinct cold-tolerant mechanism utilizing peroxisomal metabolism to make use of elevated levels of unsaturated depot fats. The skeletal muscle transcriptome also shows an early increase in oxidative capacity necessary for the altered fuel utilization and increased oxygen demand of shivering. Expression of the fetal gene expression profile is used to maintain cardiac tissue, either through increasing myocyte size or proliferation of resident cardiomyocytes, while skeletal muscle function and mass are protected through transcriptional regulation of pathways involved in protein turnover. This study provides insight into how two functionally distinct muscles maintain function under the extreme conditions of mammalian hibernation.

The Sarcoplasmic Reticulum and the Evolution of the Vertebrate Heart

The sarcoplasmic reticulum (SR) is crucial for contraction and relaxation of the mammalian cardiomyocyte, but its role in other vertebrate classes is equivocal. Recent evidence suggests differences in SR function across species may have an underlying structural basis. Here, we discuss how SR recruitment relates to the structural organization of the cardiomyocyte to provide new insight into the evolution of cardiac design and function in vertebrates.

Membrane phospholipid fatty acid composition regulates cardiac SERCA activity in a hibernator, the Syrian hamster (Mesocricetus auratus)

Polyunsaturated fatty acids (PUFA) have strong effects on hibernation and daily torpor. Increased dietary uptake of PUFA of the n-6 class, particularly of Linoleic acid (LA, C18:2 n-6) lengthens torpor bout duration and enables animals to reach lower body temperatures (T_b) and metabolic rates. As previously hypothesized, this well-known influence of PUFA may be mediated via effects of the membrane fatty acid composition on sarcoplasmic reticulum (SR) Ca²⁺⁻ATPase 2a (SERCA) in the heart of hibernators. We tested the hypotheses that high proportions of n-6 PUFA in general, or specifically high proportions of LA (C18:2 n-6) in SR phospholipids (PL) should be associated with increased cardiac SERCA activity, and should allow animals to reach lower minimum T_b in torpor. We measured activity of SERCA from hearts of hibernating and non-hibernating Syrian hamsters (Mesocricetus auratus) in vitro at 35°C. Further, we determined the PL fatty acid composition of the SR membrane of these hearts. We found that SERCA activity strongly increased as the proportion of LA in SR PL increased but was negatively affected by the content of Docosahexaenoic acid (DHA; C22:6 n-3). SR PL from hibernating hamsters were characterized by high proportions of LA and low proportions of DHA. As a result, SERCA activity was significantly higher during entrance into torpor and in torpor compared to inter-bout arousal. Also, animals with increased SERCA activity reached lower T_b during torpor. Interestingly, a subgroup of hamsters which never entered torpor but remained euthermic throughout winter displayed a phenotype similar to animals in summer. This was characterized by lower proportions of LA and increased proportions of DHA in SR membranes, which is apparently incompatible with torpor. We conclude that the PUFA composition of SR membranes affects cardiac function via modulating SERCA activity, and hence determines the minimum T_b tolerated by hibernators.

Warm fish with cold hearts: thermal plasticity of excitation-contraction coupling in bluefin tuna

Bluefin tuna have a unique physiology. Elevated metabolic rates coupled with heat exchangers enable bluefin tunas to conserve heat in their locomotory muscle, viscera, eyes and brain, yet their hearts operate at ambient water temperature. This arrangement of a warm fish with a cold heart is unique among vertebrates and can result in a reduction in cardiac function in the cold despite the elevated metabolic demands of endothermic tissues. In this study, we used laser scanning confocal microscopy and electron microscopy to investigate how acute and chronic temperature change affects tuna cardiac function. We examined the temporal and spatial properties of the intracellular Ca2+ transient (Δ[Ca2+]i) in Pacific bluefin tuna (Thunnus orientalis) ventricular myocytes at the acclimation temperatures of 14°C and 24°C and at a common test temperature of 19°C. Acute (less than 5 min) warming and cooling accelerated and slowed the kinetics of Δ[Ca2+]i, indicating that temperature change limits cardiac myocyte performance. Importantly, we show that thermal acclimation offered partial compensation for these direct effects of temperature. Prolonged cold exposure (more than four weeks) increased the amplitude and kinetics of Δ[Ca2+]i by increasing intracellular Ca2+ cycling through the sarcoplasmic reticulum (SR). These functional findings are supported by electron microscopy, which revealed a greater volume fraction of ventricular SR in cold-acclimated tuna myocytes. The results indicate that SR function is crucial to the performance of the bluefin tuna heart in the cold. We suggest that SR Ca2+ cycling is the malleable unit of cellular Ca2+ flux, offering a mechanism for thermal plasticity in fish hearts. These findings have implications beyond endothermic fish and may help to delineate the key steps required to protect vertebrate cardiac function in the cold.

Recording of calcium transient and analysis of calcium removal mechanisms in cardiac myocytes from rats and ground squirrels

With confocal microscopy, we recorded calcium transients and analyzed calcium removal rate at different temperatures in cardiac myocytes from the rat, a non-hibernator, and the ground squirrel, a hibernator. The results showed a remarkable increase of the diastolic level of calcium transients in the rat but no detectable change in the ground squirrel. Calcium transient of the ground squirrel, compared with that of the rat at the same temperature, had a shorter duration and showed a faster calcium removal. As indicated by the pharmacological effect of cyclopiazonic acid, calcium uptake by sarcoplasmic reticulum (SR) was the major mechanism of calcium removal, and was faster in the ground squirrel than in the rat. Our results confirmed the essential role of SR in hypothermia-tolerant adaptation, and negated the importance of Na-Ca exchange. We postulated the possibility to improve hypothermia-tolerance of the cardiac tissue of non-hibernating mammals.

Temperature sensitivity of cardiac function in pelagic fishes with different vertical mobilities: yellowfin tuna (Thunnus albacares), bigeye tuna (Thunnus obesus), mahimahi (Coryphaena hippurus), and swordfish (Xiphias gladius)

We measured the temperature sensitivity, adrenergic sensitivity, and dependence on sarcoplasmic reticulum (SR) Ca(2+) of ventricular muscle from pelagic fishes with different vertical mobility patterns: bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares), and mahimahi (Coryphaena hippurus) and a single specimen from swordfish (Xiphias gladius). Ventricular muscle from the bigeye tuna and mahimahi exhibited a biphasic response to an acute decrease in temperature (from 26 degrees to 7 degrees C); twitch force and kinetic parameters initially increased and then declined. The magnitude of this response was larger in the bigeye tuna than in the mahimahi. Under steady state conditions at 26 degrees C, inhibition of SR Ca(2+) release and reuptake with ryanodine and thapsigargin decreased twitch force and kinetic parameters, respectively, in the bigeye tuna only. However, the initial inotropy associated with decreasing temperature was abolished by SR inhibition in both the bigeye tuna and the mahimahi. Application of adrenaline completely reversed the effects of ryanodine and thapsigargin, but this effect was diminished at cold temperatures. In the yellowfin tuna, temperature and SR inhibition had minor effects on twitch force and kinetics, while adrenaline significantly increased these parameters. Limited data suggest that swordfish ventricular muscle responds to acute temperature reduction, SR inhibition, and adrenergic stimulation in a manner similar to that of bigeye tuna ventricular muscle. In aggregate, our results show that the temperature sensitivity, SR dependence, and adrenergic sensitivity of pelagic fish hearts are species specific and that these differences reflect species-specific vertical mobility patterns.

Cardiac survival in anoxia-tolerant vertebrates: An electrophysiological perspective

Certain vertebrates, such as freshwater turtles of the genus Chrysemys and Trachemys and crucian carp (Carassius carassius), have anoxia-tolerant hearts that continue to function throughout prolonged periods of anoxia (up to many months) due to successful balancing of cellular ATP supply and demand. In the present review, we summarize the current and limited understanding of the cellular mechanisms underlying this cardiac anoxia tolerance. What emerges is that cold temperature substantially modifies cardiac electrophysiology to precondition the heart for winter anoxia. Intrinsic heart rate is slowed and density of sarcolemmal ion currents substantially modified to alter cardiac action potential (AP) characteristics. These changes depress cardiac activity and reduce the energetic costs associated with ion pumping. In contrast, anoxia per se results in limited changes to cardiac AP shape or ion current densities in turtle and crucian carp, suggesting that anoxic modifications of cardiac electrophysiology to reduce ATP demand are not extensive. Additionally, as knowledge of cellular physiology in non-mammalian vertebrates is still in its infancy, we briefly discuss the cellular defense mechanisms towards the acidosis that accompanies anoxia as well as mammalian cardiac models of hypoxia/ischemia tolerance. By examining if fundamental cellular mechanisms have been conserved during the evolution of anoxia tolerance we hope to have provided a framework for the design of future experiments investigating cardiac cellular mechanisms of anoxia survival.

Endogenous circannual clock and HP complex in a hibernation control system

Hibernation in mammals is a mysterious biological phenomenon that appears on a seasonal basis for surviving a potentially lethal low body temperature (Tb) near 0 degrees C and protecting organisms from various diseases and harmful events during hibernation. The exact mechanism by which such a unique ability is seasonally developed is still unknown. On the basis of our previous finding that the source of calcium ions for excitation-contraction coupling in myocardium of chipmunks, a rodent hibernator, is seasonally modulated for hibernation, the liver-derived hibernation-specific protein (HP) complex was discovered. Recently, the HP complex was identified as a promising candidate hormone that carries a hibernation signal to the brain independently of Tb and environmental changes for developing a capacity for tolerating low Tb. This finding will promote new approaches to understanding biological hibernation systems, including a circannual clock and its signaling pathway between the brain and the periphery. A new definition of hibernation and a possible model of a hibernation control system are proposed.

Central A1-receptor activation associated with onset of torpor protects the heart against low temperature in the Syrian hamster

Body temperature drops dramatically during hibernation, but the heart retains the ability to contract and is resistant to induction of arrhythmia. Although adaptive changes in the heart prior to hibernation may be involved in the cold-resistant property, it remains unclear whether these changes are sufficient for maintaining cardiac pulsatility under an extreme hypothermic condition. We forcibly induced hypothermia in Syrian hamsters by pentobarbital anesthesia combined with cooling of the animals. This allows reproduction of a hypothermic condition in the absence of possible hibernation-specific reactions. Unlike hypothermia in natural hibernation, the forced induction of hypothermia caused atrioventricular block. Furthermore, J-waves, which are typically observed during hypothermia in nonhibernators, were recorded on an ECG. The origin of the J-wave seemed to be related to irreversible injury of the myocardium, because J-waves remained after recovery of body temperature. An abnormal ECG was also found when hypothermia was induced in hamsters that were well adapted to a cold and darkened environment or hamsters that had already experienced hibernation. These results suggest that acclimatization prior to hibernation does not have a crucial effect at least on acquisition of cardiac resistance to low temperature. In contrast, an abnormal ECG was not observed in the case of hypothermia induced by central administration of an adenosine A1-receptor agonist and subsequent cooling, confirming the importance of the adenosine system for inducing hibernation. Our results suggest that some specific mechanisms, which may be driven by a central adenosine system, operate for maintaining the proper cardiac pulsatility under extreme hypothermia.

Effects of polyunsaturated fatty acids on hibernation and torpor: a review and hypothesis

Polyunsaturated fatty acids (PUFAs) can have strong effects on hibernation and daily torpor in mammals. High dietary PUFA contents were found to increase proneness for torpor, decrease body temperatures, prolong torpor bout duration, and attenuate hibernation mass loss. The mechanism by which PUFAs enhance torpor and hibernation is unknown, however. On the basis of a review of the literature, and on reexamining our own data on alpine marmots, we propose that effects on hibernation are not due to PUFAs in general, but to shifts in the ratio of n-6 PUFAs to n-3 PUFAs in membrane phospholipids. Specifically, high ratios of n-6 to n-3 PUFAs increase the activity of the Ca2+-Mg2+pump in the sarcoplasmic reticulum of the heart (SERCA) and counteract Q10effects on SERCA activity at low tissue temperatures. Therefore, high n-6 to n-3 PUFA ratios in cardiac myocyte membranes appear to protect the hibernating heart from arrhythmia, which in hypothermic nonhibernators is caused by massive increases in cytosolic Ca2+. The resulting reduced risk of cardiac arrest during hypothermia may explain why increased dietary uptake of n-6 PUFAs, but not of n-3 PUFAs, can strongly enhance the propensity for hibernation, and allows heterotherms to reach lower body temperatures, with associated increased energy savings. Therefore, at least for herbivorous hibernators, such as marmots, linoleic acid (C18:2 n-6)—the dietary source of all n-6 PUFAs—appears to represent a crucial and limited resource in natural environments.

The seasonal peculiarities of force-frequency relationships in active ground squirrel Spermophilus undulatus ventricle

The plasticity of calcium homeostasis is of crucial importance for the unique ability of the hibernators' heart to function under conditions of body temperature changing from 37 degrees C to near freezing point. However, the precise mechanism of calcium homeostasis regulation in these animals is largely unknown. Force-frequency relationship, as an indicator of participation of various sources of calcium (external and intracellular) in the activation of contraction, and post-rest potentiation as an index of the capacity of sarcoplasmic reticulum (intracellular calcium source) to store and release Ca(2+), were studied to analyse the role of different calcium-transporting systems in seasonal and temperature-induced changes in isometric twitch force of ground squirrel papillary muscles. The obtained results revealed significant functional differences during the annual cycle, which are indicative of an increased role of the sarcoplasmic reticulum in regulation of contractility in animals in transition to the hibernation period. Also, how myocardium during the hibernation period copes functionally with acute decreases in temperature was investigated.

Advances in molecular biology of hibernation in mammals

Mammalian hibernation is characterized by profound reductions in metabolism, oxygen consumption and heart rate. As a result, the animal enters a state of suspended animation where core body temperatures can plummet as low as -2.9 degrees C. Not only can hibernating mammals survive these physiological extremes, but they also return to a normothermic state of activity without reperfusion injury or other ill effects. This review examines recent findings on the genes, proteins and small molecules that control the induction and maintenance of hibernation in mammals. The molecular events involved with remodeling metabolism, inducing hypothermia and maintaining organ function are discussed and considered with respect to analogous processes in non-hibernating mammals such as mice and humans. The advent of sequenced genomes from three distantly related hibernators, a bat, hedgehog and ground squirrel, provides additional opportunities for molecular biologists to explore the mechanistic aspects of this biological adaptation in greater detail.

Digital transcriptome analysis indicates adaptive mechanisms in the heart of a hibernating mammal

Survival of near-freezing body temperatures and reduced blood flow during hibernation is likely the result of changes in the expression of specific genes. In this study, we described a comprehensive survey of mRNAs in the heart of the thirteen-lined ground squirrel ( Spermophilus tridecemlineatus) before and during hibernation. The heart was chosen for this study because it is a contractile organ that must continue to work despite body temperatures of 5°C and the lack of food for periods of 5–6 mo. We used a digital gene expression assay involving high-throughput sequencing of directional cDNA libraries from hearts of active and hibernating ground squirrels to determine the identity and frequency of 3,532 expressed sequence tags (ESTs). Statistical analysis of the active and hibernating heart expression profile indicated the differential regulation of 48 genes based on a P ≤ 0.03 threshold. Several of the differentially expressed genes identified in this screen encode proteins that likely account for uninterrupted cardiac function during hibernation, including those involved in metabolism, contractility, Ca2+handling, and low-temperature catalysis. A sampling of genes showing higher expression during hibernation includes phosphofructokinase, pancreatic triacylglycerol lipase, pyruvate dehydrogenase kinase 4 (PDK4), aldolase A, sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a), titin, and four-and-a-half LIM domains protein 2 (FHL2). Genes showing reduced levels of expression during hibernation include cyclin-dependent kinase 2-associated protein 1 (CDK2AP1), troponin C, phospholamban, Ca2+/calmodulin-dependent protein kinase II (CaMKII), calmodulin, and four subunits of cytochrome c oxidase.

Photoperiod-dependent modulation of cardiac excitation contraction coupling in the Siberian hamster

In mammals, changes in photoperiod regulate a diverse array of physiological and behavioral processes, an example of which in the Siberian hamster (Phodopus sungorus) is the expression of bouts of daily torpor following prolonged exposure to a short photoperiod. During torpor, body temperature drops dramatically; however, unlike in nonhibernating or nontorpid species, the myocardium retains the ability to contract and is resistant to the development of arrhythmias. In the present study, we sought to determine whether exposure to a short photoperiod results in alterations to cardiac excitation-contraction coupling, thus potentially enabling the heart to survive periods of low temperature during torpor. Experiments were performed on single ventricular myocytes freshly isolated from the hearts of Siberian hamsters that had been exposed to either 12 wk of short-day lengths (SD) or 12 wk of long-day lengths (LD). In SD-acclimated animals, the amplitude of the systolic Ca(2+) transient was increased (e.g., from 142 +/- 17 nmol/l in LD to 229 +/- 31 nmol/l in SD at 4 Hz; P < 0.001). The increased Ca(2+) transient amplitude in the SD-acclimated animals was not associated with any change in the shape or duration of the action potential. However, sarcoplasmic reticulum Ca(2+) content measured after current-clamp stimulation was increased in the SD-acclimated animals (at 4 Hz, 110 +/- 5 vs. 141 +/- 15 mumol/l, P < 0.05). We propose that short photoperiods reprogram the function of the cardiac sarcoplasmic reticulum, resulting in an increased Ca(2+) content, and that this may be a necessary precursor for maintenance of cardiac function during winter torpor.

Insights into cardioprotection obtained from study of cellular Ca2+ handling in myocardium of true hibernating mammals

Mammalian hibernators exhibit remarkable resistance to low body temperature, whereas non-hibernating (NHB) mammals develop ventricular dysfunction and arrhythmias. To investigate this adaptive change, we compared contractile and electrophysiological properties of left ventricular myocytes isolated from hibernating (HB) woodchucks (Marmota monax) and control NHB woodchucks. The major findings of this study were the following: 1) the action potential duration in HB myocytes was significantly shorter than in NHB myocytes, but the amplitude of peak contraction was unchanged; 2) HB myocytes had a 33% decreased L-type Ca2+ current (I(Ca)) density and twofold faster I(Ca) inactivation but no change in the current-voltage relationship; 3) there were no changes in the density of inward rectifier K+ current, transient outward K+ current, or Na+/Ca2+ exchange current, but HB myocytes had increased sarcoplasmic reticulum Ca2+ content as estimated from caffeine-induced Na+/Ca2+ exchange current values; 4) expression of the L-type Ca2+ channel alpha(1C)-subunit was decreased by 30% in HB hearts; and 5) mRNA and protein levels of sarco(endo)plasmic reticulum Ca2+-ATPase 2a (SERCA2a), phospholamban, and the Na+/Ca2+ exchanger showed a pattern that is consistent with functional measurements: SERCA2a was increased and phospholamban was decreased in HB relative to NHB hearts with no change in the Na+/Ca2+ exchanger. Thus reduced Ca2+ channel density and faster I(Ca) inactivation coupled to enhanced sarcoplasmic reticulum Ca2+ release may underlie shorter action potentials with sustained contractility in HB hearts. These changes may account for natural resistance to Ca2+ overload-related ventricular dysfunction and point to an important cardioprotective mechanism during true hibernation.

Temperature dependence of the Ca2+-ATPase (SERCA2) in the ventricles of tuna and mackerel

Recent physiological studies on the cardiovascular performance of tunas suggest that the elevated heart rates of these fish may rely on increased use of intracellular sarcoplasmic reticulum (SR) Ca2+stores. In this study, we compare the cellular cardiac performance in endothermic tunas (bluefin, albacore, yellowfin) and their ectothermic sister taxa (mackerel) in response to acute temperature change. The cardiac sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2) plays a major role during cardiac excitation-contraction (E-C) coupling, transporting Ca2+from the cytosol into the lumen of the SR and thus promoting the relaxation of the muscle. Measurements of oxalate-supported Ca2+uptake in SR-enriched ventricular vesicles indicated that tunas were capable of sustaining a rate of Ca2+uptake that was significantly higher than the mackerel. Among tunas, the cold-tolerant bluefin had the highest rates of SR Ca2+uptake and ATPase activity. The differences among Ca2+uptake and ATP hydrolysis rates do not seem to result from intrinsic differences between the SERCA2 present in the different tunas, as shown by their similar temperature sensitivities and similar values for activation energy. Western blots reveal that increased SERCA2 protein content is associated with the higher Ca2+uptake and ATPase activities seen in bluefin ventricles compared with albacore, yellowfin, and mackerel. We hypothesize that a key step in the evolution of high heart rate and high metabolic rate in tunas is increased activity of the SERCA2 enzyme. We also suggest that high levels of SERCA2 in bluefin tuna hearts may be important for retaining cardiac function at cold temperatures.

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.

Adaptive mechanisms of intracellular calcium homeostasis in mammalian hibernators

Intracellular Ca2+ homeostasis is a prerequisite for a healthy cell life. While cells from some mammals may suffer dysregulation of intracellular Ca2+ levels under certain deleterious and stressful conditions, including hypothermia and ischemia, cells from mammalian hibernators exhibit a remarkable ability to maintain a homeostatic intracellular Ca2+ environment. Compared with cells from non-hibernators, hibernator cells are characterized by downregulation of the activity of Ca2+ channels in the cell membrane, which helps to prevent excessive Ca2+ entry. Concomitantly, sequestration of Ca2+ by intracellular Ca2+ stores, especially the sarcoplasmic/endoplasmic reticulum, is enhanced to keep the resting levels of intracellular Ca2+ stable. An increase in stored Ca2+ in heart cells during hibernation ensures that the levels of Ca2+messenger are sufficient for forceful cell contraction under conditions of hypothermia. Maintenance of Na+ gradients, viaNa+—Ca2+ exchangers, is also important in the Ca2+ homeostasis of hibernator cells. Understanding the adaptive mechanisms of Ca2+ regulation in hibernating mammals may suggest new strategies to protect nonhibernator cells, including those of humans, from Ca2+-induced dysfunction.

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.

Contractile properties of atrial and ventricular myocardium of the heart of rainbow trout oncorhynchus mykiss: effects of thermal acclimation

Atrial and ventricular myocardium perform different tasks in the pumping work of the vertebrate heart, which are reflected in their contractile properties. Although atrial contraction is assumed to have an important role in the function of fish heart, the contractile properties of atrial and ventricular myocardium have not been directly compared in any fish species. The objective of this study was to clarify any contractile differences in the heart of teleost fish and, in particular, to elucidate the contribution of myofibrillar ATPase and intracellular Ca(2+) stores to the characteristics of atrial and ventricular contraction. Experiments were conducted on thermally acclimated rainbow trout Oncorhynchus mykiss to determine whether the effects of temperature adaptation are the same in atrial and ventricular tissue. It was shown that the rate of isometric contraction is much faster in atrial than in ventricular tissue of the fish heart and that acclimation to cold increases the rate of contraction in both cardiac compartments. The rapid contraction kinetics of the atrial tissue were associated with higher myofibrillar ATPase activity and faster Ca(2+) uptake rate of the sarcoplasmic reticulum (SR) compared with ventricular tissue. Similarly, the faster kinetics of contraction following cold acclimation could be attributed to enhancement of the myofibrillar and/or SR function. The atrio-ventricular and temperature-induced differences were also expressed in the recovery of force from inactivation, i.e. in the mechanical restitution. The refractory period and the rate constant of force restitution were shorter in atrial than in ventricular muscle tissue. Similar differences also existed between the tissues of cold-acclimated (CA, 4 degrees C) and warm-acclimated (WA, 17 degrees C) fish. The fast recovery of force from inactivation in the heart of the CA trout was, at least in part, due to more active SR. Furthermore, it was shown that the force of atrial contraction in the CA trout is sensitive to ryanodine (10 (μ)mol l(−)(1)), a Ca(2+)-release channel blocker of SR, at physiological body temperature (4 degrees C) and at a physiological pacing rate (0.6 Hz). This finding indicates that the Ca(2+) stores of SR contribute to activation of cardiac contraction in the fish heart, and that the SR of fish heart is able to retain its Ca(2+) load at low body temperatures, i.e. the Ca(2+)release channels of SR are not leaky in the cold. The present data show that in the atrial tissue of CA trout, the SR directly contributes to the cytosolic Ca(2+) and that in the atrium and ventricle of CA trout, the SR significantly accelerates the recovery of contractility from inactivation. The fast recovery from inactivation allows relatively high heart rates and therefore adequate cardiac outputs at low environmental temperatures for the cold-active rainbow trout.

Medical significance of cardiovascular function in hibernating mammals

1. The hibernating mammals inherited a stable cardiovascular function as a result of adaptation to extreme external and internal environments during hibernation. 2. The cardiovascular function of a hibernator is characterized by three distinct features: (i) tolerance of hypothermia; (ii) resistance to dysrhythmia; and (iii) endurance of hypoxia. All these have clinical relevance in cardiovascular disorders in humans. 3. We suggest that the hibernating mammal can serve as a useful animal model for research into the cardiovascular system.

Acetyl-CoA carboxylase control of fatty acid oxidation in hearts from hibernating Richardson's ground squirrels

Although mammalian hibernators rely on stored body fat as a source of energy, direct measurement of energy substrate preference in heart tissue during hibernation, as well as potential mechanisms controlling fatty acid oxidation has not been examined. In order to determine whether an increase in fatty acid utilization occurs during hibernation, glucose and palmitate oxidation were measured in isolated working hearts from hibernating and non-hibernating Richardson's ground Squirrels. Hearts were perfused at either 37 degrees or 5 degrees C with perfusate containing 11 mM [U-14C]glucose and 1.2 mM [9,10-3H]palmitate, which allowed for direct measurement of both glucose oxidation (14CO2 production) and fatty acid oxidation (3H2O production). The contribution of fatty acid oxidation as a source of citric acid cycle acetyl-CoA was significantly greater in hearts from hibernating animals, compared to hearts from non-hibernating animals. Since acetyl-CoA carboxylase (ACC) regulates cardiac fatty acid oxidation (producing malonyl-CoA, a potent inhibitor of mitochondrial fatty acid uptake), we measured the activity and expression of ACC in these hearts. ACC activity was significantly decreased in hibernating ground squirrels, regardless of whether ACC was assayed at 37 degrees or 5 degrees C. This decrease in activity could not be explained by a change in the activity of 5'AMP-activated protein kinase, which can phosphorylate and inhibit ACC. Rather, the expression of the 280 kDa isoform of ACC (which predominates in cardiac muscle) was decreased in hearts from hibernating squirrel hearts. This suggests that a down regulation of ACC expression occurs as an adaptation for the increased utilization of fatty acid in hearts of hibernating ground squirrels.

Dependence of myocardial hypothermia tolerance on sources of activator calcium

To determine the relationship between cardiac hypothermia tolerance and the sources of activator calcium, we selectively modified either the sarcolemmal calcium permeability by nifedipine or the sarcoplasmic reticulum function by caffeine in papillary muscles from both the rat, as a cold sensitive model, and the ground squirrel, Citellus dauricus, a deep hibernator. Both force-interval relationship and cooling performance were investigated. At 25 degrees C, the slope of the force-interval curve of the ground squirrel was nearly double that of the rat. At shorter test intervals 0.5 muM nifedipine moved the curve down with little effect at longer intervals, and the curve slope increased. Caffeine (1 mM) decreased the peak force and eliminated its dependence upon test interval. When the temperature was lowered, rat preparations showed a marked increase of resting tension and aftercontraction between 7 and 12 degrees C and became inexcitable. In contrast, they maintained contractility down to a few degrees above 0 degrees C without aftercontraction and increased resting tension in the ground squirrel. In the rat nifedipine shortened the contractions, prevented the increase of resting tension, and minimized aftercontractions, with little improvement of contractility. Caffeine prolonged the contractions, caused a striking increase of resting tension and aftercontractions, and finally disabled the contractility at about 5-10 degrees C, even in the ground squirrel. We conclude that depressed calcium influx helps to prevent hypothermic calcium overload of the cardiac cells. Good function of the sarcoplasmic reticulum is essential for tolerance of hypothermia by cardiac cells. A suggestion that may improve the hypothermic tolerance of the myocardium from nonhibernators is postulated.

Sarcoplasmic reticulum Ca(2+)-ATPase and ryanodine receptor in cold-acclimated ducklings and thermogenesis

In birds, prolonged cold exposure induces the development of a nonshivering thermogenesis (NST) of muscular origin. NST is characterized by an increased heat production, which may be achieved by an increased ATP-dependent cycling of Ca2+ between the sarcoplasmic reticulum (SR) and cytosolic compartments in muscle. In this study, the effects of prolonged cold exposure on SR function were assessed by determining the contents of the SR Ca(2+)-ATPase and Ca2+ release channel (ryanodine receptor) in the gastrocnemius muscle of ducklings (Cairina moschata) kept at thermoneutrality (25 degrees C) or cold acclimated (4 degrees C, 5 wk). Measurement of oxalate-supported 45Ca2+ uptake by whole muscle homogenates revealed that the SR Ca(2+)-ATPase activity, and fraction of vesicles containing a ryanodine-sensitive Ca2+ release channel were increased by 30-50% in response to prolonged cold exposure. Sodium dodecyl sulfate-polyacrylamide gel and immunoblot analysis, 45Ca2+ uptake, Ca(2+)-ATPase activity and [3H]ryanodine binding measurements with unfractionated and "heavy" SR membrane fractions also indicated an elevated Ca(2+)-ATPase and Ca2+ release channel content in cold-acclimated ducklings. These results showed that the contents of two components directly involved in Ca2+ cycling by the SR are increased by cold acclimation, and we suggest that this is related to NST.