Comparative shell buffering properties correlate with anoxia tolerance in freshwater turtles

Special Challenges in PET Imaging of Ectothermic Vertebrates

The bulk of biomedical positron emission tomography (PET)-scanning experiments are performed on mammals (ie, rodents, pigs, and dogs), and the technique is only infrequently applied to answer research questions in ectothermic vertebrates such as fish, amphibians, and reptiles. Nevertheless, many unique and interesting physiological characteristics in these ectothermic vertebrates could be addressed in detail through PET. The low metabolic rate of ectothermic animals, however, may compromise the validity of physiological and biochemical parameters derived from the images created by PET and other scanning modalities. Here, we review some of the considerations that should be taken into account when PET scanning fish, amphibians, and reptiles. We present specific results from our own experiments, many of which remain previously unpublished, and we draw on examples from the literature. We conclude that knowledge on the natural history and physiology of the species studied and an understanding of the limitations of the PET scanning techniques are necessary to avoid the design of faulty experiments and erroneous conclusions.

Diagnostic Use of Lactate in Exotic Animals

Monitoring blood lactate concentrations with a handheld, point-of-care (POC) meter is an efficient and inexpensive method of monitoring critically ill or anesthetized exotic patients. Serial monitoring of lactate allows early recognition of hypoperfusion, allowing for prompt implementation of resuscitative efforts. Reference ranges for exotic animals are currently sparse and often gathered from field studies of wild animals. In the absence of reference ranges, extrapolations can be made regarding mammals and birds, but may be more difficult in reptiles and amphibians.

Dynamic regulation of histone H3 lysine (K) acetylation and deacetylation during prolonged oxygen deprivation in a champion anaerobe

Trachemys scripta elegans can survive up to three months of absolute anoxia at 3 °C and recover with minimal cellular damage. Red-eared sliders employ various physiological and biochemical adaptations to survive anoxia with metabolic rate depression (MRD) being the most prominent adaptation. MRD is mediated by epigenetic, transcriptional, post-transcriptional, and post-translational mechanisms aimed at shutting down cellular processes that are not needed for anoxia survival, while reprioritizing ATP towards cell processes that are vital for anaerobiosis. Histone acetylation/deacetylation are epigenetic modifications that maintain a proper balance between permissive chromatin and restricted chromatin, yet very little is known about protein regulation and enzymatic activity of the writers and erasers of acetylation during natural anoxia tolerance. As such, this study explored the interplay between transcriptional activators, histone acetyltransferases (HATs), and transcriptional repressors, sirtuins (SIRTs), along with three prominent acetyl-lysine (K) moieties of histone H3 in the liver of red-eared sliders. Western immunoblotting was used to measure acetylation levels of H3-K14, H3-K18, and H3-K56, as well as protein levels of histone H3-total, HATs, and nuclear SIRTs in the liver in response to 5 h and 20 h anoxia. Global and nuclear enzymatic activity of HATs and enzymatic activity of nuclear SIRTs were also measured. Overall, a strong suppression of HATs-mediated H3 acetylation and SIRT-mediated deacetylation was evident in the liver of red-eared sliders that could play an important role in ATP conservation as part of the overall reduction in metabolic rate.

Surviving anoxia: the maintenance of energy production and tissue integrity during anoxia and reoxygenation

The development of anoxia within tissues represents a significant challenge to most animals because of the decreased capacity for aerobic ATP production, the associated loss of essential cellular functions and the potential for detrimental tissue oxidation upon reoxygenation. Despite these challenges, there are many animals from multiple phyla that routinely experience anoxia and can fully recover. In this Review, we integrate knowledge gained from studies of anoxia-tolerant species across many animal taxa. We primarily focus on strategies used to reduce energy requirements, minimize the consequences of anaerobic ATP production and reduce the adverse effects of reactive oxygen species, which are responsible for tissue damage with reoxygenation. We aim to identify common strategies, as well as novel solutions, to the challenges of anoxia exposure. This Review chronologically examines the challenges faced by animals as they enter anoxia, as they attempt to maintain physiological function during prolonged anoxic exposure and, finally, as they emerge from anoxia. The capacity of animals to survive anoxia is also considered in relation to the increasing prevalence of anoxic zones within marine and freshwater environments, and the need to understand what limits survival.

Palaeophysiology of pH regulation in tetrapods

The involvement of mineralized tissues in acid-base homeostasis was likely important in the evolution of terrestrial vertebrates. Extant reptiles encounter hypercapnia when submerged in water, but early tetrapods may have experienced hypercapnia on land due to their inefficient mode of lung ventilation (likely buccal pumping, as in extant amphibians). Extant amphibians rely on cutaneous carbon dioxide elimination on land, but early tetrapods were considerably larger forms, with an unfavourable surface area to volume ratio for such activity, and evidence of a thick integument. Consequently, they would have been at risk of acidosis on land, while many of them retained internal gills and would not have had a problem eliminating carbon dioxide in water. In extant tetrapods, dermal bone can function to buffer the blood during acidosis by releasing calcium and magnesium carbonates. This review explores the possible mechanisms of acid-base regulation in tetrapod evolution, focusing on heavily armoured, basal tetrapods of the Permo-Carboniferous, especially the physiological challenges associated with the transition to air-breathing, body size and the adoption of active lifestyles. We also consider the possible functions of dermal armour in later tetrapods, such as Triassic archosaurs, inferring palaeophysiology from both fossil record evidence and phylogenetic patterns, and propose a new hypothesis relating the archosaurian origins of the four-chambered heart and high systemic blood pressures to the perfusion of the osteoderms. This article is part of the theme issue 'Vertebrate palaeophysiology'.

Dynamic regulation of six histone H3 lysine (K) methyltransferases in response to prolonged anoxia exposure in a freshwater turtle

The importance of histone lysine methylation is well established in health, disease, early development, aging, and cancer. However, the potential role of histone H3 methylation in regulating gene expression in response to extended periods of oxygen deprivation (anoxia) in a natural, anoxia-tolerant model system is underexplored. Red-eared sliders (Trachemys scripta elegans) can tolerate and survive three months of absolute anoxia and recover without incurring detrimental cellular damage, mainly by reducing the overall metabolic rate by 90% when compared to normoxia. Stringent regulation of gene expression is a vital aspect of metabolic rate depression in red-eared sliders, and as such we examined the anoxia-responsive regulation of histone lysine methylation in the liver during 5 h and 20 h anoxia exposure. Interestingly, this is the first study to illustrate the existence of histone lysine methyltransferases (HKMTs) and corresponding histone H3 lysine methylation levels in the liver of anoxia-tolerant red-eared sliders. In brief, H3K4me1, a histone mark associated with active transcription, and two corresponding histone lysine methyltransferases that modify H3K4me1 site, significantly increased in response to anoxia. On the contrary, H3K27me1, another transcriptionally active histone mark, significantly decreased during 20 h anoxia, and a transcriptionally repressive histone mark, H3K9me3, and the corresponding KMTs, similarly increased during 20 h anoxia. Overall, the results suggest a dynamic regulation of histone H3 lysine methylation in the liver of red-eared sliders that could theoretically aid in the selective upregulation of genes that are necessary for anoxia survival, while globally suppressing others to conserve energy.

Changes in the material properties of the shell during simulated aquatic hibernation in the anoxia-tolerant painted turtle

Western painted turtles (Chrysemys picta bellii) tolerate anoxic submergence longer than any other tetrapod, surviving more than 170 days at 3°C. This ability is due, in part, to the shell and skeleton simultaneously releasing calcium and magnesium carbonates, and sequestering lactate and H+ to prevent lethal decreases in body fluid pH. We evaluated the effects of anoxic submergence at 3°C on various material properties of painted turtle bone after 60, 130, and 167-170 days, and compared them to normoxic turtles held at the same temperature for the same time periods. To assess changes in the mechanical properties, beams (4×25 mm) were milled from the plastron and broken in a three-point flexural test. Bone mineral density, CO2 concentration (a measure of total bone HCO3−/CO32-), and elemental composition were measured using microCT, HCO3−/CO32- titration, and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Tissue mineral density of the sampled bone beams were not significantly altered by 167-170 days of aquatic overwintering in anoxic or normoxic water, but bone CO2 and Mg were depleted in anoxic compared normoxic turtles. At this time point, the plastron beams from anoxic turtles yielded at stresses that were significantly smaller and strains significantly greater than the plastron beams of normoxic turtles. When data from anoxic and normoxic turtles were pooled, plastron beams had a diminished elastic modulus after 167-170 days compared to control turtles sampled on Day 1, indicating an effect of prolonged housing of the turtles in 3°C water without access to basking sites. There were no changes in the mechanical properties of the plastron beams at any of the earlier time points in either group. We conclude that anoxic hibernation can weaken the painted turtle's plastron, but likely only after durations that exceed what it might naturally experience. The duration of aquatic overwintering, regardless of oxygenation state, is likely to be an important factor determining the mechanical properties of the turtle shell during spring emergence.

Lactate metabolism in anoxic turtles: an integrative review

Painted turtles can accumulate lactic acid to extremely high concentrations during long-term anoxic submergence, with plasma lactate exceeding 200 mmol l(-1). The aims of this review are twofold: (1) To summarize aspects of lactate metabolism in anoxic turtles that have not been reviewed previously and (2) To identify gaps in our knowledge of turtle lactate metabolism by comparing it with lactate metabolism during and after exercise in other vertebrates. The topics reviewed include analyses of lactate's fate during recovery, the effects of temperature on lactate accumulation and clearance, the interaction of activity and recovery metabolism, fuel utilization during recovery, stress hormone responses during and following anoxia, and cellular lactate transport mechanisms. An analysis of lactate metabolism in anoxic turtles in the context of the 'lactate shuttle' hypothesis is also presented.