Getting the jump on skeletal muscle disuse atrophy: preservation of contractile performance in aestivatingCyclorana alboguttata(Günther 1867)

Dramatic genome-wide reprogramming of mRNA in hypometabolic muscle

In response to seasonal droughts, the green striped burrowing frog Cyclorana alboguttata enters a reversible hypometabolic state called aestivation where heart rate and oxygen consumption can be reduced despite warm (>25C°) ambient temperatures. With a view to understanding molecular mechanisms we profiled aestivating versus control gastrocnemius muscle using mRNA sequencing. This indicated an extensive metabolic reprogramming, with nearly a quarter of the entire transcriptome (3996 of 16,960 mRNA) exhibiting a nominal >2-fold change. Consistent with a physiological adaptation to spare carbohydrate reserves, carbohydrate catabolism was systemically downregulated. A 630-fold downregulation of ENO3 encoding the enolase enzyme was most striking. The 590 frog orthologs of mRNA encoding the mitoproteome were, viewed as a population, significantly downregulated during aestivation, although not to the same extent as mRNA encoding carbohydrate catabolism. Prominent examples include members of the TCA cycle (IDH2), electron transport chain (NDUFA6), the ATP synthase complex (ATP5F1B) and ADP/ATP intracellular transport (SLC25A4). Moreover, mRNA derived from the mt genome itself (e.g. mt-ND1) were also downregulated. Most prominent among the upregulated mRNA are those encoding aspects of regulated proteolysis including the proteosome (e.g. PSME4L), peptidases (USP25), atrogins (FBXO32) and ubiquitination (VCP). Finally, we note the ∼5-fold upregulation of the mRNA EIFG3 that encodes part of the EIF4F complex. This possesses global control of protein synthesis. Given protein synthesis is repressed in aestivating frogs this indicates the skeletal musculature is poised for accelerated translation of mRNA upon emergence, supporting a strategy to rapidly restore function when the summer rains come.

Effects of hibernation on two important contractile tissues in tibetan frogs, Nanorana parkeri: a perspective from transcriptomics and metabolomics approaches

BACKGROUND

In response to seasonal cold and food shortage, the Xizang plateau frogs, Nanorana parkeri (Anura: Dicroglossidae), enter a reversible hypometabolic state where heart rate and oxygen consumption in skeletal muscle are strongly suppressed. However, the effect of winter hibernation on gene expression and metabolic profiling in these two tissues remains unknown. In the present study, we conducted transcriptomic and metabolomic analyses of heart and skeletal muscle from summer- and winter-collected N. parkeri to explore mechanisms involved in seasonal hibernation.

RESULTS

We identified 2407 differentially expressed genes (DEGs) in heart and 2938 DEGs in skeletal muscle. Enrichment analysis showed that shared DEGs in both tissues were enriched mainly in translation and metabolic processes. Of these, the expression of genes functionally categorized as "response to stress", "defense mechanisms", or "muscle contraction" were particularly associated with hibernation. Metabolomic analysis identified 24 and 22 differentially expressed metabolites (DEMs) in myocardium and skeletal muscle, respectively. In particular, pathway analysis showed that DEMs in myocardium were involved in the pentose phosphate pathway, glycerolipid metabolism, pyruvate metabolism, citrate cycle (TCA cycle), and glycolysis/gluconeogenesis. By contrast, DEMs in skeletal muscle were mainly involved in amino acid metabolism.

CONCLUSIONS

In summary, natural adaptations of myocardium and skeletal muscle in hibernating N. parkeri involved transcriptional alterations in translation, stress response, protective mechanisms, and muscle contraction processes as well as metabolic remodeling. This study provides new insights into the transcriptional and metabolic adjustments that aid winter survival of high-altitude frogs N. parkeri.

Can animals tune tissue mechanics in response to changing environments caused by anthropogenic impacts?

Anthropogenic climate change and pollution are impacting environments across the globe. This Review summarises the potential impact of such anthropogenic effects on animal tissue mechanics, given the consequences for animal locomotor performance and behaviour. More specifically, in light of current literature, this Review focuses on evaluating the acute and chronic effects of temperature on the mechanical function of muscle tissues. For ectotherms, maximal muscle performance typically occurs at temperatures approximating the natural environment of the species. However, species vary in their ability to acclimate to chronic changes in temperature, which is likely to have longer-term effects on species range. Some species undergo periods of dormancy to avoid extreme temperature or drought. Whilst the skeletal muscle of such species generally appears to be adapted to minimise muscle atrophy and maintain performance for emergence from dormancy, the increased occurrence of extreme climatic conditions may reduce the survival of individuals in such environments. This Review also considers the likely impact of anthropogenic pollutants, such as hormones and heavy metals, on animal tissue mechanics, noting the relative paucity of literature directly investigating this key area. Future work needs to determine the direct effects of anthropogenic environmental changes on animal tissues and related changes in locomotor performance and behaviour, including accounting for currently unknown interactions between environmental factors, e.g. temperature and pollutants.

Sodium Handling and Interaction in Numerous Organs

Salt (NaCl) is a prerequisite for life. Excessive intake of salt, however, is said to increase disease risk, including hypertension, arteriosclerosis, heart failure, renal disease, stroke, and cancer. Therefore, considerable research has been expended on the mechanism of sodium handling based on the current concepts of sodium balance. The studies have necessarily relied on relatively short-term experiments and focused on extremes of salt intake in humans. Ultra-long-term salt balance has received far less attention. We performed long-term salt balance studies at intakes of 6, 9, and 12 g/day and found that although the kidney remains the long-term excretory gate, tissue and plasma sodium concentrations are not necessarily the same and that urinary salt excretion does not necessarily reflect total-body salt content. We found that to excrete salt, the body makes a great effort to conserve water, resulting in a natriuretic-ureotelic principle of salt excretion. Of note, renal sodium handling is characterized by osmolyte excretion with anti-parallel water reabsorption, a state-of-affairs that is achieved through the interaction of multiple organs. In this review, we discuss novel sodium and water balance concepts in reference to our ultra-long-term study. An important key to understanding body sodium metabolism is to focus on water conservation, a biological principle to protect from dehydration, since excess dietary salt excretion into the urine predisposes to renal water loss because of natriuresis. We believe that our research direction is relevant not only to salt balance but also to cardiovascular regulatory mechanisms.

Preparation for oxidative stress in Proceratophrys cristiceps (Anura, Odontophrynidae) naturally estivating in the Brazilian Caatinga

Preparation for oxidative stress (POS), i.e., the buildup of endogenous antioxidants during metabolic depression or low oxygen stress conditions, has been observed in at least 8 animal phyla under controlled conditions in laboratory. Despite the expected implications on the endurance to extreme environments and ecosystem occupation, the extent to which POS occurs in animals under natural conditions remains unexplored. Therefore, we took advantage of the natural history of the Brazilian Caatinga's frog Proceratophrys cristiceps to investigate the modulation of endogenous antioxidants and redox balance in their skeletal muscle and to verify if POS occurs under natural conditions. Expectedly, estivating frogs had low levels of the oxidative metabolism enzymes. Citrate synthase and isocitrate dehydrogenase activities were 36% and 25% lower than those in active frogs respectively. We found an overall upregulation of antioxidants in estivating P. cristiceps. Reduced glutathione concentration was 61% higher in estivating frogs than that in active animals. During estivation the activities of the hydroperoxide detoxification enzymes catalase, glutathione peroxidase, and glutathione H₂O₂-peroxidase were 48%, 57%, and 78% greater than those during the rainy season. Moreover, estivating frogs had a 47% lower ratio of disulfide to total glutathione levels than active frogs. Our findings confirm the occurrence of 'preparation for oxidative stress' in naturally estivating frogs and paves the way for further research on the redox biology of animals under natural settings. Such approach might reveal biochemical strategies under ecologically relevant scenarios.

Biomarker-based assessment of the muscle maintenance and energy status of anurans from an extremely seasonal semi-arid environment, the Brazilian Caatinga

Strongly seasonal environments pose challenges for performance and survival of animals, especially when resource abundance seasonally fluctuates. We investigated the seasonal variation of key metabolic biomarkers in the muscles of males from three species (Rhinella jimi, R. granulosa and Pleurodema diplolister) of anurans from the drastically seasonal Brazilian semi-arid area, Caatinga. We examined the expression of proteins regulating energy turnover (AMP-activated protein kinase [AMPK] and protein kinase B [AKT]), protein synthesis and homeostasis (total and phosphorylated eukaryotic initiation factor 2α [eIF2α and p-eIF2α] and chaperone proteins [HSP 60, 70, and 90]) in muscles predominantly related to reproduction and locomotion. Cytochrome c oxidase (COX) activity was also assessed as an index of the muscle aerobic capacity. The expression pattern of metabolic biomarkers indicates that the maintenance of muscular function is regulated in a species-specific manner during the drastic seasonal variation. Rhinella jimi and R. granulosa that remain active during the drought appear to maintain muscles through more energy expensive pathways including elevated protein synthesis, while the aestivating P. diplolister employs energy conservation strategy suppressing protein synthesis, decreasing chaperone expression and increasing expression of AMPK. Two (P. diplolister and R. granulosa) of the three studied species activate cell survival pathways during the drought likely to prevent muscle atrophy, and all three studied species maintain the muscle aerobic capacity throughout the year, despite the resource limitation. These strategies are important considering the unpredictability of the reproductive event and high demand on muscular activity during the reproductive season in these amphibians. SUMMARY STATEMENT: We studied seasonal variation of key metabolic biomarkers in the muscles of anurans that experience drastic variation in environmental conditions and differ in seasonal activity patterns.

The likely effects of thermal climate change on vertebrate skeletal muscle mechanics with possible consequences for animal movement and behaviour

Climate change can involve alteration in the local temperature that an animal is exposed to, which in turn may affect skeletal muscle temperature. The underlying effects of temperature on the mechanical performance of skeletal muscle can affect organismal performance in key activities, such as locomotion and fitness-related behaviours, including prey capture and predator avoidance. The contractile performance of skeletal muscle is optimized within a specific thermal range. An increased muscle temperature can initially cause substantial improvements in force production, faster rates of force generation, relaxation, shortening, and production of power output. However, if muscle temperature becomes too high, then maximal force production and power output can decrease. Any deleterious effects of temperature change on muscle mechanics could be exacerbated by other climatic changes, such as drought, altered water, or airflow regimes that affect the environment the animal needs to move through. Many species will change their location on a daily, or even seasonal basis, to modulate the temperature that they are exposed to, thereby improving the mechanical performance of their muscle. Some species undergo seasonal acclimation to optimize muscle mechanics to longer-term changes in temperature or undergo dormancy to avoid extreme climatic conditions. As local climate alters, species either cope with the change, adapt, avoid extreme climate, move, or undergo localized extinction events. Given that such outcomes will be determined by organismal performance within the thermal environment, the effects of climate change on muscle mechanics could have a major impact on the ability of a population to survive in a particular location.

Climatic modulation of neurotransmitter release in amphibian neuromuscular junctions: role of dynorphin-A

Amphibian neuromuscular junctions (NMJs) become relatively more silent during the dry winter season in Australia. During the dry, calcium sensitivity is reduced, whereas calcium dependence remains unchanged. Endogenous opioid peptides play an important role in the regulation of the physiological functions of active and dormant vertebrates. Previous findings suggest that dynorphin-A is more potent than other opiates in decreasing evoked neurotransmission in amphibian NMJs. Dynorphin-A has been shown not to alter the amplitude or the frequency of miniature quantal neurotransmitter release. In the present study, we report that dynorphin-A exerted a more pronounced inhibitory effect on evoked neurotransmitter release during the dry (hibernating period) when compared with the wet (active period) season. Dynorphin-A increased the frequency and decreased the amplitude of miniature neurotransmitter release only at relatively high concentration during the dry season. In the present study, we propose that dynorphin-A suppresses evoked neurotransmitter release and thus contraction of skeletal muscles, while allowing subthreshold activation of the NMJ by miniature neurotransmission, thus preventing any significant neuromuscular remodeling. The inhibitory effect of dynorphin-A on evoked transmitter release is reduced by increasing the extracellular calcium concentration.

Prevention of muscle wasting and osteoporosis: the value of examining novel animal models

Bone mass and skeletal muscle mass are controlled by factors such as genetics, diet and nutrition, growth factors and mechanical stimuli. Whereas increased mechanical loading of the musculoskeletal system stimulates an increase in the mass and strength of skeletal muscle and bone, reduced mechanical loading and disuse rapidly promote a decrease in musculoskeletal mass, strength and ultimately performance (i.e. muscle atrophy and osteoporosis). In stark contrast to artificially immobilised laboratory mammals, animals that experience natural, prolonged bouts of disuse and reduced mechanical loading, such as hibernating mammals and aestivating frogs, consistently exhibit limited or no change in musculoskeletal performance. What factors modulate skeletal muscle and bone mass, and what physiological and molecular mechanisms protect against losses of muscle and bone during dormancy and following arousal? Understanding the events that occur in different organisms that undergo natural periods of prolonged disuse and suffer negligible musculoskeletal deterioration could not only reveal novel regulatory factors but also might lead to new therapeutic options. Here, we review recent work from a diverse array of species that has revealed novel information regarding physiological and molecular mechanisms that dormant animals may use to conserve musculoskeletal mass despite prolonged inactivity. By highlighting some of the differences and similarities in musculoskeletal biology between vertebrates that experience disparate modes of dormancy, it is hoped that this Review will stimulate new insights and ideas for future studies regarding the regulation of atrophy and osteoporosis in both natural and clinical models of muscle and bone disuse.

Integrative Physiology of Fasting

Extended bouts of fasting are ingrained in the ecology of many organisms, characterizing aspects of reproduction, development, hibernation, estivation, migration, and infrequent feeding habits. The challenge of long fasting episodes is the need to maintain physiological homeostasis while relying solely on endogenous resources. To meet that challenge, animals utilize an integrated repertoire of behavioral, physiological, and biochemical responses that reduce metabolic rates, maintain tissue structure and function, and thus enhance survival. We have synthesized in this review the integrative physiological, morphological, and biochemical responses, and their stages, that characterize natural fasting bouts. Underlying the capacity to survive extended fasts are behaviors and mechanisms that reduce metabolic expenditure and shift the dependency to lipid utilization. Hormonal regulation and immune capacity are altered by fasting; hormones that trigger digestion, elevate metabolism, and support immune performance become depressed, whereas hormones that enhance the utilization of endogenous substrates are elevated. The negative energy budget that accompanies fasting leads to the loss of body mass as fat stores are depleted and tissues undergo atrophy (i.e., loss of mass). Absolute rates of body mass loss scale allometrically among vertebrates. Tissues and organs vary in the degree of atrophy and downregulation of function, depending on the degree to which they are used during the fast. Fasting affects the population dynamics and activities of the gut microbiota, an interplay that impacts the host's fasting biology. Fasting-induced gene expression programs underlie the broad spectrum of integrated physiological mechanisms responsible for an animal's ability to survive long episodes of natural fasting.

Frogs and estivation: transcriptional insights into metabolism and cell survival in a natural model of extended muscle disuse

Green-striped burrowing frogs (Cyclorana alboguttata) survive in arid environments by burrowing underground and entering into a deep, prolonged metabolic depression known as estivation. Throughout estivation, C. alboguttata is immobilized within a cast-like cocoon of shed skin and ceases feeding and moving. Remarkably, these frogs exhibit very little muscle atrophy despite extended disuse and fasting. Little is known about the transcriptional regulation of estivation or associated mechanisms that may minimize degradative pathways of atrophy. To investigate transcriptional pathways associated with metabolic depression and maintenance of muscle function in estivating burrowing frogs, we assembled a skeletal muscle transcriptome using next-generation short read sequencing and compared gene expression patterns between active and 4 mo estivating C. alboguttata. This identified a complex suite of gene expression changes that occur in muscle during estivation and provides evidence that estivation in burrowing frogs involves transcriptional regulation of genes associated with cytoskeletal remodeling, avoidance of oxidative stress, energy metabolism, the cell stress response, and apoptotic signaling. In particular, the expression levels of genes encoding cell cycle and prosurvival proteins, such as serine/threonine-protein kinase Chk1, cell division protein kinase 2, survivin, and vesicular overexpressed in cancer prosurvival protein 1, were upregulated during estivation. These data suggest that estivating C. alboguttata are able to regulate the expression of genes in several major cellular pathways critical to the survival and viability of cells, thus preserving muscle function while avoiding the deleterious consequences often seen in laboratory models of muscle disuse.

Decreased hydrogen peroxide production and mitochondrial respiration in skeletal muscle but not cardiac muscle of the green-striped burrowing frog, a natural model of muscle disuse

Suppression of disuse-induced muscle atrophy has been associated with altered mitochondrial reactive oxygen species (ROS) production in mammals. However, despite extended hindlimb immobility aestivating animals exhibit little skeletal muscle atrophy compared with artificially-immobilised mammalian models. Therefore, we studied mitochondrial respiration and ROS (H2O2) production in permeabilised muscle fibres of the green-striped burrowing frog, Cyclorana alboguttata. Mitochondrial respiration within saponin-permeabilised skeletal and cardiac muscle fibres was measured concurrently with ROS production using high-resolution respirometry coupled to custom-made fluorometers. After four months of aestivation, C. alboguttata had significantly depressed whole body metabolism by approximately 70% relative to control (active) frogs, and mitochondrial respiration in saponin-permeabilised skeletal muscle fibres decreased by almost 50% both in the absence of ADP and during oxidative phosphorylation. Mitochondrial ROS production showed up to an 88% depression in aestivating skeletal muscle when malate, succinate and pyruvate were present at concentrations likely reflecting those in vivo. The percentage ROS released per O2 molecule consumed was also approximately 94 % less at these concentrations indicating an intrinsic difference in ROS production capacities during aestivation. We also examined mitochondrial respiration and ROS production in permeabilised cardiac muscle fibres and found that aestivating frogs maintained respiratory flux and ROS production at control levels. These results show that aestivating C. alboguttata has the capacity to independently regulate mitochondrial function in skeletal and cardiac muscles. Furthermore, this work indicates that ROS production can be suppressed in the disused skeletal muscle of aestivating frogs, which may in turn protect against potential oxidative damage and preserve skeletal muscle structure during aestivation and following arousal.

Aestivation: signaling and hypometabolism

Aestivation is a survival strategy used by many vertebrates and invertebrates to endure arid environmental conditions. Key features of aestivation include strong metabolic rate suppression, strategies to retain body water, conservation of energy and body fuel reserves, altered nitrogen metabolism, and mechanisms to preserve and stabilize organs, cells and macromolecules over many weeks or months of dormancy. Cell signaling is crucial to achieving both a hypometabolic state and reorganizing multiple metabolic pathways to optimize long-term viability during aestivation. This commentary examines the current knowledge about cell signaling pathways that participate in regulating aestivation, including signaling cascades mediated by the AMP-activated kinase, Akt, ERK, and FoxO1.

Warmer is better: thermal sensitivity of both maximal and sustained power output in the iliotibialis muscle isolated from adult Xenopus tropicalis

Environmental temperature varies temporally and spatially and may consequently affect organismal function in complex ways. Effects of temperature are often most pertinent on locomotor performance traits of ectothermic animals. Given the importance of locomotion to mobility and dispersion, variability in temperature may therefore affect the current and future distribution of species. Many previous studies have demonstrated that burst muscle performance changes with temperature. However, less is known about the effects of temperature on sustained skeletal muscle performance. The iliotibialis muscle was isolated from eight male Xenopus tropicalis individuals and subjected to in vitro isometric and work-loop studies at test temperatures of 15, 24, 30 and 32°C. Work-loop power output (average power per cycle) was maximised at each temperature by altering stimulation and strain parameters. A series of 10 work loops was also delivered at each test temperature to quantify endurance performance. Warmer test temperatures tended to increase twitch stress (force normalised to muscle cross-sectional area) and significantly increased tetanic stress. Increased temperature significantly reduced twitch and tetanus activation and relaxation times. Increased temperature also significantly increased both burst muscle power output (cycle average) and sustained (endurance) performance during work loop studies. The increase in burst power output between 15 and 24°C yielded a high Q10 value of 6.86. Recent studies have demonstrated that the negative effects of inorganic phosphate accumulation during prolonged skeletal muscle performance are reduced with increased temperature, possibly explaining the increases in endurance found with increased test temperature in the present study.

Influence of elevated temperature on metabolism during aestivation: implications for muscle disuse atrophy

Reactive oxygen species (ROS), produced commensurate with aerobic metabolic rate, contribute to muscle disuse atrophy (MDA) in immobilised animals by damaging myoskeletal protein and lipids. Aestivating frogs appear to avoid MDA in part by substantially suppressing metabolic rate. However, as ectotherms, metabolic rate is sensitive to environmental temperature, and the high ambient temperatures that may be experienced by frogs during aestivation could in fact promote MDA. In this study, we investigated the effect of temperature on the metabolic rate of the aestivating frog Cyclorana alboguttata and its skeletal muscles in order to determine their likely susceptibility to MDA. Compared with non-aestivating frogs, a significant decrease in metabolic rate was recorded for aestivating frogs at 20, 24 and 30°C. At 30°C, however, the metabolic rate of aestivating frogs was significantly higher, approximately double that of frogs aestivating at 20 or 24°C, and the magnitude of the metabolic depression was significantly reduced at 30°C compared with that at 20°C. Temperature effects were also observed at the tissue level. At 24 and 30°C the metabolic rate of all muscles from aestivating frogs was significantly depressed compared with that of muscles from non-aestivating frogs. However, during aestivation at 30°C the metabolic rates of gastrocnemius, sartorius and cruralis were significantly elevated compared with those from frogs aestivating at 24°C. Our data show that the metabolism of C. alboguttata and its skeletal muscles is elevated at higher temperatures during aestivation and that the capacity of the whole animal to actively depress metabolism is impaired at 30°C.

Daily torpor reduces mass and changes stress and power output of soleus and EDL muscles in the Djungarian hamster, Phodopus sungorus

Djungarian hamsters (Phodopus sungorus) undergo bouts of daily torpor in response to reduced photoperiod. Metabolic rate, body temperature and energy cost are reduced during torpor. The present study exposed Djungarian hamsters to two different photoperiod regimes at a room temperature of 19–21°C: long photoperiod (control, 16 h:8 h light:dark, N=8) and short photoperiod (torpor, 8 h:16 h light:dark, N=8). After 14 weeks, muscle mechanics were analyzed in each group, examining both extensor digitorum longus (EDL) muscle and soleus muscle from each individual. Control hamsters had significantly greater body mass (43%), EDL mass (24%), EDL length (9%) and soleus mass (48%) than the torpor hamsters. However, there were no significant differences between control and torpor groups in forearm length or soleus muscle length. There were no significant differences in either muscle between control and torpor hamsters in maximum twitch stress (force per unit area), tetanus force generation or relaxation times. Maximum soleus tetanic stress was 43% greater (P=0.039) and soleus work loop power output (P<0.001) was higher in torpor than in control hamsters. Maximum EDL tetanic stress was 26% greater in control than in torpor hamsters (P=0.046), but there was no significant effect on EDL power output (P=0.38). Rate of fatigue was not affected by torpor in either soleus or EDL muscles (P>0.43). Overall, extended use of daily torpor had no effect on the rate at which stress or work was produced in soleus and EDL muscles in Djungarian hamsters; however, torpor did increase the stress and power produced by the soleus.

Skeletal muscle atrophy occurs slowly and selectively during prolonged aestivation in Cyclorana alboguttata (Günther 1867)

We investigated the effect of prolonged immobilisation of six and nine months duration on the morphology and antioxidant biochemistry of skeletal muscles in the amphibian aestivator Cyclorana alboguttata. We hypothesised that, in the event of atrophy occurring during aestivation,larger jumping muscles were more likely to be preserved over smaller non-jumping muscles. Whole muscle mass (g), muscle cross-sectional area (CSA)(μm2), water content (%) and myofibre number (per mm2) remained unchanged in the cruralis muscle after six to nine months of aestivation; however, myofibre area (μm2) was significantly reduced. Whole muscle mass, water content, myofibre number and myofibre CSA remained unchanged in the gastrocnemius muscle after six to nine months of aestivation. However, iliofibularis dry muscle mass, whole muscle CSA and myofibre CSA was significantly reduced during aestivation. Similarly,sartorius dry muscle mass, water content and whole muscle CSA was significantly reduced during aestivation. Endogenous antioxidants were maintained at control levels throughout aestivation in all four muscles. The results suggest changes to muscle morphology during aestivation may occur when lipid reserves have been depleted and protein becomes the primary fuel substrate for preserving basal metabolic processes. Muscle atrophy as a result of this protein catabolism may be correlated with locomotor function, with smaller non-jumping muscles preferentially used as a protein source during fasting over larger jumping muscles. Higher levels of endogenous antioxidants in the jumping muscles may confer a protective advantage against oxidative damage during aestivation; however, it is not clear whether they play a role during aestivation or upon resumption of normal metabolic activity.