Effects of Developmental Hypoxia on the Vertebrate Cardiovascular System.

Developmental hypoxia has profound and persistent effects on the vertebrate cardiovascular system, but the nature, magnitude, and long-term outcome of the hypoxic consequences are species specific. Here we aim to identify common and novel cardiovascular responses among vertebrates that encounter developmental hypoxia, and we discuss the possible medical and ecological implications.

Low production of mitochondrial reactive oxygen species after anoxia and reoxygenation in turtle hearts.

Extremely anoxia-tolerant animals, such as freshwater turtles, survive anoxia and reoxygenation without sustaining tissue damage to their hearts. In contrast, for mammals, the ischemia-reperfusion (IR) injury that leads to tissue damage during a heart attack is initiated by a burst of superoxide (O2·-) production from the mitochondrial respiratory chain upon reperfusion of ischemic tissue. Whether turtles avoid oxidative tissue damage because of an absence of mitochondrial superoxide production upon reoxygenation, or because the turtle heart is particularly protected against this damage, is unclear. Here, we investigated whether there was an increase in mitochondrial O2·- production upon the reoxygenation of anoxic red-eared slider turtle hearts in vivo and in vitro. This was done by measuring the production of H2O2, the dismutation product of O2·-, using the mitochondria-targeted mass-spectrometric probe in vivo MitoB, while in parallel assessing changes in the metabolites driving mitochondrial O2·- production, succinate, ATP and ADP levels during anoxia, and H2O2 consumption and production rates of isolated heart mitochondria. We found that there was no excess production of in vivo H2O2 during 1 h of reoxygenation in turtles after 3 h anoxia at room temperature, suggesting that turtle hearts most likely do not suffer oxidative injury after anoxia because their mitochondria produce no excess O2·- upon reoxygenation. Instead, our data support the conclusion that both the low levels of succinate accumulation and the maintenance of ADP levels in the anoxic turtle heart are key factors in preventing the surge of O2·- production upon reoxygenation.

The air-breathing Alaska blackfish (Dallia pectoralis) suppresses brain mitochondrial reactive oxygen species to survive cold hypoxic winters.

The Alaska blackfish (Dallia pectoralis) is the only air-breathing fish in the Arctic. In the summer, a modified esophagus allows the fish to extract oxygen from the air, but this behavior is not possible in the winter because of ice and snow cover. The lack of oxygen (hypoxia) and near freezing temperatures in winter is expected to severely compromise metabolism, and yet remarkably, overwintering Alaska blackfish remain active. To maintain energy balance in the brain and limit the accumulation of reactive oxygen species (ROS), we hypothesized that cold hypoxic conditions would trigger brain mitochondrial remodeling in the Alaska blackfish. To address this hypothesis, fish were acclimated to warm (15 °C) normoxia, cold (5 °C) normoxia or cold hypoxia (5 °C, 2.1-4.2 kPa; no air access) for 5-8 weeks. Mitochondrial respiration, ADP affinity and H202 production were measured at 10 °C in isolated brain homogenates with an Oroboros respirometer. Cold acclimation and chronic hypoxia had no effects on mitochondrial aerobic capacity or ADP affinity. However, cold acclimation in normoxia led to a suppression of brain mitochondrial H202 production, which persisted and became more pronounced in the cold hypoxic fish. Overall, our study suggests cold acclimation supresses ROS production in Alaska blackfish, which may protect the fish from oxidative stress when oxygen becomes limited during winter.

Maternal melatonin: Effective intervention against developmental programming of cardiovascular dysfunction in adult offspring of complicated pregnancy.

Adopting an integrative approach, by combining studies of cardiovascular function with those at cellular and molecular levels, this study investigated whether maternal treatment with melatonin protects against programmed cardiovascular dysfunction in the offspring using an established rodent model of hypoxic pregnancy. Wistar rats were divided into normoxic (N) or hypoxic (H, 10% O2 ) pregnancy ± melatonin (M) treatment (5 µg·ml-1 .day-1 ) in the maternal drinking water. Hypoxia ± melatonin treatment was from day 15-20 of gestation (term is ca. 22 days). To control for possible effects of maternal hypoxia-induced reductions in maternal food intake, additional dams underwent pregnancy under normoxic conditions but were pair-fed (PF) to the daily amount consumed by hypoxic dams from day 15 of gestation. In one cohort of animals from each experimental group (N, NM, H, HM, PF, PFM), measurements were made at the end of gestation. In another, following delivery of the offspring, investigations were made at adulthood. In both fetal and adult offspring, fixed aorta and hearts were studied stereologically and frozen hearts were processed for molecular studies. In adult offspring, mesenteric vessels were isolated and vascular reactivity determined by in-vitro wire myography. Melatonin treatment during normoxic, hypoxic or pair-fed pregnancy elevated circulating plasma melatonin in the pregnant dam and fetus. Relative to normoxic pregnancy, hypoxic pregnancy increased fetal haematocrit, promoted asymmetric fetal growth restriction and resulted in accelerated postnatal catch-up growth. Whilst fetal offspring of hypoxic pregnancy showed aortic wall thickening, adult offspring of hypoxic pregnancy showed dilated cardiomyopathy. Similarly, whilst cardiac protein expression of eNOS was downregulated in the fetal heart, eNOS protein expression was elevated in the heart of adult offspring of hypoxic pregnancy. Adult offspring of hypoxic pregnancy further showed enhanced mesenteric vasoconstrictor reactivity to phenylephrine and the thromboxane mimetic U46619. The effects of hypoxic pregnancy on cardiovascular remodelling and function in the fetal and adult offspring were independent of hypoxia-induced reductions in maternal food intake. Conversely, the effects of hypoxic pregnancy on fetal and postanal growth were similar in pair-fed pregnancies. Whilst maternal treatment of normoxic or pair-fed pregnancies with melatonin on the offspring cardiovascular system was unremarkable, treatment of hypoxic pregnancies with melatonin in doses lower than those recommended for overcoming jet lag in humans enhanced fetal cardiac eNOS expression and prevented all alterations in cardiovascular structure and function in fetal and adult offspring. Therefore, the data support that melatonin is a potential therapeutic target for clinical intervention against developmental origins of cardiovascular dysfunction in pregnancy complicated by chronic fetal hypoxia.

Chronic developmental hypoxia alters mitochondrial oxidative capacity and reactive oxygen species production in the fetal rat heart in a sex-dependent manner.

Insufficient oxygen supply (hypoxia) during fetal development leads to cardiac remodeling and a predisposition to cardiovascular disease in later life. Previous work has shown hypoxia causes oxidative stress in the fetal heart and alters the activity and expression of mitochondrial proteins in a sex-dependent manner. However, the functional effects of these modifications on mitochondrial respiration remain unknown. Furthermore, while maternal antioxidant treatments are emerging as a promising new strategy to protect the hypoxic fetus, whether these treatments convey similar protection to cardiac mitochondria in the male or female fetus has not been investigated. Therefore, using an established rat model, we measured the sex-dependent effects of gestational hypoxia and maternal melatonin treatment on fetal cardiac mitochondrial respiration, reactive oxygen species (ROS) production, and lipid peroxidation. Pregnant Wistar rats were subjected to normoxia or hypoxia (13% oxygen) during gestational days (GDs) 6-20 (term ~22 days) with or without melatonin treatment (5 µg/ml in maternal drinking water). On GD 20, mitochondrial aerobic respiration and H2 O2 production were measured in fetal heart tissue, together with lipid peroxidation and citrate synthase (CS) activity. Gestational hypoxia reduced maternal body weight gain (p < .01) and increased placental weight (p < .05) but had no effect on fetal weight or litter size. Cardiac mitochondria from male but not female fetuses of hypoxic pregnancy had reduced respiratory capacity at Complex II (CII) (p < .05), and an increase in H2 O2 production/O2 consumption (p < .05) without any changes in lipid peroxidation. CS activity was also unchanged in both sexes. Despite maternal melatonin treatment increasing maternal and fetal plasma melatonin concentration (p < .001), melatonin treatment had no effect on any of the mitochondrial parameters investigated. To conclude, we show that gestational hypoxia leads to ROS generation from the mitochondrial electron transport chain and affects fetal cardiac mitochondrial respiration in a sex-dependent manner. We also show that maternal melatonin treatment had no effect on these relationships, which has implications for the development of future therapies for hypoxic pregnancies.

Developmental plasticity of cardiac anoxia-tolerance in juvenile common snapping turtles ( Chelydra serpentina).

For some species of ectothermic vertebrates, early exposure to hypoxia during embryonic development improves hypoxia-tolerance later in life. However, the cellular mechanisms underlying this phenomenon are largely unknown. Given that hypoxic survival is critically dependent on the maintenance of cardiac function, we tested the hypothesis that developmental hypoxia alters cardiomyocyte physiology in a manner that protects the heart from hypoxic stress. To test this hypothesis, we studied the common snapping turtle, which routinely experiences chronic developmental hypoxia and exploits hypoxic environments in adulthood. We isolated cardiomyocytes from juvenile turtles that embryonically developed in either normoxia (21% O2) or hypoxia (10% O2), and subjected them to simulated anoxia and reoxygenation, while simultaneously measuring intracellular Ca2+, pH and reactive oxygen species (ROS) production. Our results suggest developmental hypoxia improves cardiomyocyte anoxia-tolerance of juvenile turtles, which is supported by enhanced myofilament Ca2+-sensitivity and a superior ability to suppress ROS production. Maintenance of low ROS levels during anoxia might limit oxidative damage and a greater sensitivity to Ca2+ could provide a mechanism to maintain contractile force. Our study suggests developmental hypoxia has long-lasting effects on turtle cardiomyocyte function, which might prime their physiology for exploiting hypoxic environments.

Impacts of Deepwater Horizon Crude Oil on Mahi-Mahi (Coryphaena hippurus) Heart Cell Function.

Deepwater Horizon crude oil is comprised of polycyclic aromatic hydrocarbons that cause a number of cardiotoxic effects in marine fishes across all levels of biological organization and at different life stages. Although cardiotoxic impacts have been widely reported, the mechanisms underlying these impairments in adult fish remain understudied. In this study, we examined the impacts of crude oil on cardiomyocyte contractility and electrophysiological parameters in freshly isolated ventricular cardiomyocytes from adult mahi-mahi (Coryphaena hippurus). Cardiomyocytes directly exposed to oil exhibited reduced contractility over a range of environmentally relevant concentrations (2.8-12.9 µg l-1∑PAH). This reduction in contractility was most pronounced at higher stimulation frequencies, corresponding to the upper limits of previously measured in situ mahi heart rates. To better understand the mechanisms underlying impaired contractile function, electrophysiological studies were performed, which revealed oil exposure prolonged cardiomyocyte action potentials and disrupted potassium cycling (9.9-30.4 µg l-1∑PAH). This study is the first to measure cellular contractility in oil-exposed cardiomyocytes from a pelagic fish. Results from this study contribute to previously observed impairments to heart function and whole-animal exercise performance in mahi, underscoring the advantages of using an integrative approach in examining mechanisms of oil-induced cardiotoxicity in marine fish.

Developmental plasticity of mitochondrial function in American alligators, Alligator mississippiensis.

The effect of hypoxia on cellular metabolism is well documented in adult vertebrates, but information is entirely lacking for embryonic organisms. The effect of hypoxia on embryonic physiology is particularly interesting, as metabolic responses during development may have life-long consequences, due to developmental plasticity. To this end, we investigated the effects of chronic developmental hypoxia on cardiac mitochondrial function in embryonic and juvenile American alligators (Alligator mississippiensis). Alligator eggs were incubated in 21% or 10% oxygen from 20 to 90% of embryonic development. Embryos were either harvested at 90% development or allowed to hatch and then reared in 21% oxygen for 3 yr. Ventricular mitochondria were isolated from embryonic/juvenile alligator hearts. Mitochondrial respiration and enzymatic activities of electron transport chain complexes were measured with a microrespirometer and spectrophotometer, respectively. Developmental hypoxia induced growth restriction and increased relative heart mass, and this phenotype persisted into juvenile life. Embryonic mitochondrial function was not affected by developmental hypoxia, but at the juvenile life stage, animals from hypoxic incubations had lower levels of Leak respiration and higher respiratory control ratios, which is indicative of enhanced mitochondrial efficiency. Our results suggest developmental hypoxia can have life-long consequences for alligator morphology and metabolic function. Further investigations are necessary to reveal the adaptive significance of the enhanced mitochondrial efficiency in the hypoxic phenotype.

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.

Beating oxygen: chronic anoxia exposure reduces mitochondrial F1FO-ATPase activity in turtle (Trachemys scripta) heart.

The freshwater turtle Trachemys scripta can survive in the complete absence of O2 (anoxia) for periods lasting several months. In mammals, anoxia leads to mitochondrial dysfunction, which culminates in cellular necrosis and apoptosis. Despite the obvious clinical benefits of understanding anoxia tolerance, little is known about the effects of chronic oxygen deprivation on the function of turtle mitochondria. In this study, we compared mitochondrial function in hearts of T. scripta exposed to either normoxia or 2 weeks of complete anoxia at 5°C and during simulated acute anoxia/reoxygenation. Mitochondrial respiration, electron transport chain activities, enzyme activities, proton conductance and membrane potential were measured in permeabilised cardiac fibres and isolated mitochondria. Two weeks of anoxia exposure at 5°C resulted in an increase in lactate, and decreases in ATP, glycogen, pH and phosphocreatine in the heart. Mitochondrial proton conductance and membrane potential were similar between experimental groups, while aerobic capacity was dramatically reduced. The reduced aerobic capacity was the result of a severe downregulation of the F1FO-ATPase (Complex V), which we assessed as a decrease in enzyme activity. Furthermore, in stark contrast to mammalian paradigms, isolated turtle heart mitochondria endured 20 min of anoxia followed by reoxygenation without any impact on subsequent ADP-stimulated O2 consumption (State III respiration) or State IV respiration. Results from this study demonstrate that turtle mitochondria remodel in response to chronic anoxia exposure and a reduction in Complex V activity is a fundamental component of mitochondrial and cellular anoxia survival.

The air-breathing Alaska blackfish (Dallia pectoralis) remodels ventricular Ca2+ cycling with chronic hypoxic submergence to maintain ventricular contractility.

The Alaska blackfish (Dallia pectoralis) is a facultative air-breather endemic to northern latitudes where it remains active in winter under ice cover in cold hypoxic waters. To understand the changes in cellular Ca2+ cycling that allow the heart to function in cold hypoxic water, we acclimated Alaska blackfish to cold (5 °C) normoxia or cold hypoxia (2.1-4.2 kPa; no air access) for 5-8 weeks. We then assessed the impact of the acclimation conditions on intracellular Ca2+ transients (Δ[Ca2+]i) of isolated ventricular myocytes and contractile performance of isometrically-contracting ventricular strips. Measurements were obtained at various contractile frequencies (0.2-0.6 Hz) in normoxia, during acute exposure to hypoxia, and reoxygenation at 5 °C. The results show that hypoxia-acclimated Alaska blackfish compensate against the depressive effects of hypoxia on excitation-contraction coupling by remodelling cellular Δ[Ca2+]i to maintain ventricular contractility. When measured at 0.2 Hz in normoxia, hypoxia-acclimated ventricular myocytes had a 3.8-fold larger Δ[Ca2+]i peak amplitude with a 4.1-fold faster rate of rise, compared to normoxia-acclimated ventricular myocytes. At the tissue level, maximal developed force was 2.1-fold greater in preparations from hypoxia-acclimated animals. However, maximal attainable contraction frequencies in hypoxia were lower in hypoxia-acclimated myocytes and strips than preparations from normoxic animals. Moreover, the inability of hypoxia-acclimated ventricular myocytes and strips to contract at high frequency persisted upon reoxygenation. Overall, the findings indicate that hypoxia alters aspects of Alaska blackfish cardiac myocyte Ca2+ cycling, and that there may be consequences for heart rate elevation during hypoxia, which may impact cardiac output in vivo.

Temperature effects on Ca2+ cycling in scombrid cardiomyocytes: a phylogenetic comparison.

Specialisations in excitation-contraction coupling may have played an important role in the evolution of endothermy and high cardiac performance in scombrid fishes. We examined aspects of Ca(2+) handling in cardiomyocytes from Pacific bonito (Sarda chiliensis), Pacific mackerel (Scomber japonicus), yellowfin tuna (Thunnus albacares) and Pacific bluefin tuna (Thunnus orientalis). The whole-cell voltage-clamp technique was used to measure the temperature sensitivity of the L-type Ca(2+) channel current (I(Ca)), density, and steady-state and maximal sarcoplasmic reticulum (SR) Ca(2+) content (ssSR(load) and maxSR(load)). Current-voltage relations, peak I(Ca) density and charge density of I(Ca) were greatest in mackerel and yellowfin at all temperatures tested. I(Ca) density and kinetics were temperature sensitive in all species studied, and the magnitude of this response was not related to the thermal preference of the species. SR(load) was greater in atrial than in ventricular myocytes in the Pacific bluefin tuna, and in species that are more cold tolerant (bluefin tuna and mackerel). I(Ca) and SR(load) were particularly small in bonito, suggesting the Na(+)/Ca(2+) exchanger plays a more pivotal role in Ca(2+) entry into cardiomyocytes of this species. Our comparative approach reveals that the SR of cold-tolerant scombrid fishes has a greater capacity for Ca(2+) storage. This specialisation may contribute to the temperature tolerance and thermal niche expansion of the bluefin tuna and mackerel.

The Long-Term Effects of Developmental Hypoxia on Cardiac Mitochondrial Function in Snapping Turtles.

It is well established that adult vertebrates acclimatizing to hypoxic environments undergo mitochondrial remodeling to enhance oxygen delivery, maintain ATP, and limit oxidative stress. However, many vertebrates also encounter oxygen deprivation during embryonic development. The effects of developmental hypoxia on mitochondrial function are likely to be more profound, because environmental stress during early life can permanently alter cellular physiology and morphology. To this end, we investigated the long-term effects of developmental hypoxia on mitochondrial function in a species that regularly encounters hypoxia during development-the common snapping turtle (Chelydra serpentina). Turtle eggs were incubated in 21% or 10% oxygen from 20% of embryonic development until hatching, and both cohorts were subsequently reared in 21% oxygen for 8 months. Ventricular mitochondria were isolated, and mitochondrial respiration and reactive oxygen species (ROS) production were measured with a microrespirometer. Compared to normoxic controls, juvenile turtles from hypoxic incubations had lower Leak respiration, higher P:O ratios, and reduced rates of ROS production. Interestingly, these same attributes occur in adult vertebrates that acclimatize to hypoxia. We speculate that these adjustments might improve mitochondrial hypoxia tolerance, which would be beneficial for turtles during breath-hold diving and overwintering in anoxic environments.

Sex-dependent effects of developmental hypoxia on cardiac mitochondria from adult murine offspring.

Insufficient oxygen supply (hypoxia) during fetal and embryonic development can lead to latent phenotypical changes in the adult cardiovascular system, including altered cardiac function and increased susceptibility to ischemia reperfusion injury. While the cellular mechanisms underlying this phenomenon are largely unknown, several studies have pointed towards metabolic disturbances in the heart of offspring from hypoxic pregnancies. To this end, we investigated mitochondrial function in the offspring of a mouse model of prenatal hypoxia. Pregnant C57 mice were subjected to either normoxia (21%) or hypoxia (14%) during gestational days 6-18. Offspring were reared in normoxia for up to 8 months and mitochondrial biology was assessed with electron microscopy (ultrastructure), spectrophotometry (enzymatic activity of electron transport chain complexes), microrespirometry (oxidative phosphorylation and H202 production) and Western Blot (protein expression). Our data showed that male adult offspring from hypoxic pregnancies possessed mitochondria with increased H202 production and lower respiratory capacity that was associated with reduced protein expression of complex I, II and IV. In contrast, females from hypoxic pregnancies had a higher respiratory capacity and lower H202 production that was associated with increased enzymatic activity of complex IV. From these results, we speculate that early exposure to hypoxia has long term, sex-dependent effects on cardiac metabolic function, which may have implications for cardiovascular health and disease in adulthood.

Prolonged phenanthrene exposure reduces cardiac function but fails to mount a significant oxidative stress response in the signal crayfish (Pacifastacus leniusculus).

Crustaceans are important ecosystem bio-indicators but their response to pollutants such as polyaromatic hydrocarbons (PAHs) remains understudied, particularly in freshwater habitats. Here we investigated the effect of phenanthrene (at 0.5, 1.0 and 1.5 mg L-1), a 3-ringed PAH associated with petroleum-based aquatic pollution on survival, in vivo and in situ cardiac performance, the oxidative stress response and the tissue burden in the signal crayfish (Pacifastacus leniusculus). Non-invasive sensors were used to monitor heart rate during exposure. Phenanthrene reduced maximum attainable heart rate in the latter half (days 8-15) of the exposure period but had no impact on routine heart rate. At the end of the 15-day exposure period, the electrical activity of the semi-isolated in situ crayfish heart was assessed and significant prolongation of the QT interval of the electrocardiogram was observed. Enzyme pathways associated with oxidative stress (superoxide dismutase and total oxyradical scavenging capacity) were also assessed after 15 days of phenanthrene exposure in gill, hepatopancreas and skeletal muscle; the results suggest limited induction of protective antioxidant pathways. Lastly, we report that 15 days exposure caused a dose-dependent increase in phenanthrene in hepatopancreas and heart tissues which was associated with reduced survivability. To our knowledge, this study is the first to provide such a thorough understanding of the impact of phenanthrene on a crustacean.

Developmental programming of DNA methylation and gene expression patterns is associated with extreme cardiovascular tolerance to anoxia in the common snapping turtle.

BACKGROUND: Environmental fluctuation during embryonic and fetal development can permanently alter an organism's morphology, physiology, and behaviour. This phenomenon, known as developmental plasticity, is particularly relevant to reptiles that develop in subterranean nests with variable oxygen tensions. Previous work has shown hypoxia permanently alters the cardiovascular system of snapping turtles and may improve cardiac anoxia tolerance later in life. The mechanisms driving this process are unknown but may involve epigenetic regulation of gene expression via DNA methylation. To test this hypothesis, we assessed in situ cardiac performance during 2 h of acute anoxia in juvenile turtles previously exposed to normoxia (21% oxygen) or hypoxia (10% oxygen) during embryogenesis. Next, we analysed DNA methylation and gene expression patterns in turtles from the same cohorts using whole genome bisulfite sequencing, which represents the first high-resolution investigation of DNA methylation patterns in any reptilian species. RESULTS: Genome-wide correlations between CpG and CpG island methylation and gene expression patterns in the snapping turtle were consistent with patterns observed in mammals. As hypothesized, developmental hypoxia increased juvenile turtle cardiac anoxia tolerance and programmed DNA methylation and gene expression patterns. Programmed differences in expression of genes such as SCN5A may account for differences in heart rate, while genes such as TNNT2 and TPM3 may underlie differences in calcium sensitivity and contractility of cardiomyocytes and cardiac inotropy. Finally, we identified putative transcription factor-binding sites in promoters and in differentially methylated CpG islands that suggest a model linking programming of DNA methylation during embryogenesis to differential gene expression and cardiovascular physiology later in life. Binding sites for hypoxia inducible factors (HIF1A, ARNT, and EPAS1) and key transcription factors activated by MAPK and BMP signaling (RREB1 and SMAD4) are implicated. CONCLUSIONS: Our data strongly suggests that DNA methylation plays a conserved role in the regulation of gene expression in reptiles. We also show that embryonic hypoxia programs DNA methylation and gene expression patterns and that these changes are associated with enhanced cardiac anoxia tolerance later in life. Programming of cardiac anoxia tolerance has major ecological implications for snapping turtles, because these animals regularly exploit anoxic environments throughout their lifespan.

The cellular force-frequency response in ventricular myocytes from the varanid lizard, Varanus exanthematicus.

To investigate the cellular mechanisms underlying the negative force-frequency relationship (FFR) in the ventricle of the varanid lizard, Varanus exanthematicus, we measured sarcomere and cell shortening, intracellular Ca(2+) ([Ca(2+)](i)), action potentials (APs), and K(+) currents in isolated ventricular myocytes. Experiments were conducted between 0.2 and 1.0 Hz, which spans the physiological range of in vivo heart rates at 20-22 degrees C for this species. As stimulation frequency increased, diastolic length, percent change in sarcomere length, and relaxation time all decreased significantly. Shortening velocity was unaffected. These changes corresponded to a faster rate of rise of [Ca(2+)](i), a decrease in [Ca(2+)](i) transient amplitude, and a seven-fold increase in diastolic [Ca(2+)](i). The time constant for the decay of the Ca(2+) transient (tau) decreased at higher frequencies, indicating a frequency-dependent acceleration of relaxation (FDAR) but then reached a plateau at moderate frequencies and did not change above 0.5 Hz. The rate of rise of the AP was unaffected, but the AP duration (APD) decreased with increasing frequency. Peak depolarization tended to decrease, but it was only significant at 1.0 Hz. The decrease in APD was not due to frequency-dependent changes in the delayed inward rectifier (I(Kr)) or the transient outward (I(to)) current, as neither appeared to be present in varanid ventricular myocytes. Our results suggest that a negative FFR relationship in varanid lizard ventricle is caused by decreased amplitude of the Ca(2+) transient coupled with an increase in diastolic Ca(2+), which leads to incomplete relaxation between beats at high frequencies. This coincides with shortened APD at higher frequencies.

Draft Genome of the Common Snapping Turtle, Chelydra serpentina, a Model for Phenotypic Plasticity in Reptiles.

Turtles are iconic reptiles that inhabit a range of ecosystems from oceans to deserts and climates from the tropics to northern temperate regions. Yet, we have little understanding of the genetic adaptations that allow turtles to survive and reproduce in such diverse environments. Common snapping turtles, Chelydra serpentina, are an ideal model species for studying adaptation to climate because they are widely distributed from tropical to northern temperate zones in North America. They are also easy to maintain and breed in captivity and produce large clutch sizes, which makes them amenable to quantitative genetic and molecular genetic studies of traits like temperature-dependent sex determination. We therefore established a captive breeding colony and sequenced DNA from one female using both short and long reads. After trimming and filtering, we had 209.51Gb of Illumina reads, 25.72Gb of PacBio reads, and 21.72 Gb of Nanopore reads. The assembled genome was 2.258 Gb in size and had 13,224 scaffolds with an N50 of 5.59Mb. The longest scaffold was 27.24Mb. BUSCO analysis revealed 97.4% of core vertebrate genes in the genome. We identified 3.27 million SNPs in the reference turtle, which indicates a relatively high level of individual heterozygosity. We assembled the transcriptome using RNA-Seq data and used gene prediction software to produce 22,812 models of protein coding genes. The quality and contiguity of the snapping turtle genome is similar to or better than most published reptile genomes. The genome and genetic variants identified here provide a foundation for future studies of adaptation to climate.

Ca2+ cycling in cardiomyocytes from a high-performance reptile, the varanid lizard (Varanus exanthematicus).

The varanid lizard possesses one of the largest aerobic capacities among reptiles with maximum rates of oxygen consumption that are twice that of other lizards of comparable sizes at the same temperature. To support this aerobic capacity, the varanid heart possesses morphological adaptations that allow the generation of high heart rates and blood pressures. Specializations in excitation-contraction coupling may also contribute to the varanids superior cardiovascular performance. Therefore, we investigated the electrophysiological properties of the l-type Ca(2+) channel and the Na(+)/Ca(2+) exchanger (NCX) and the contribution of the sarcoplasmic reticulum to the intracellular Ca(2+) transient (Delta[Ca(2+)](i)) in varanid lizard ventricular myocytes. Additionally, we used confocal microscopy to visualize myocytes and make morphological measurements. Lizard ventricular myocytes were found to be spindle-shaped, lack T-tubules, and were approximately 190 microm in length and 5-7 microm in width and depth. Cardiomyocytes had a small cell volume ( approximately 2 pL), leading to a large surface area-to-volume ratio (18.5), typical of ectothermic vertebrates. The voltage sensitivity of the l-type Ca(2+) channel current (I(Ca)), steady-state activation and inactivation curves, and the time taken for recovery from inactivation were also similar to those measured in other reptiles and teleosts. However, transsarcolemmal Ca(2+) influx via reverse mode Na(+)/Ca(2+) exchange current was fourfold higher than most other ectotherms. Moreover, pharmacological inhibition of the sarcoplasmic reticulum led to a 40% reduction in the Delta[Ca(2+)](i) amplitude, and slowed the time course of decay. In aggregate, our results suggest varanids have an enhanced capacity to transport Ca(2+) through the Na(+)/Ca(2+) exchanger, and sarcoplasmic reticulum suggesting specializations in excitation-contraction coupling may provide a means to support high cardiovascular performance.

A Novel Cardiotoxic Mechanism for a Pervasive Global Pollutant.

The Deepwater Horizon disaster drew global attention to the toxicity of crude oil and the potential for adverse health effects amongst marine life and spill responders in the northern Gulf of Mexico. The blowout released complex mixtures of polycyclic aromatic hydrocarbons (PAHs) into critical pelagic spawning habitats for tunas, billfishes, and other ecologically important top predators. Crude oil disrupts cardiac function and has been associated with heart malformations in developing fish. However, the precise identity of cardiotoxic PAHs, and the mechanisms underlying contractile dysfunction are not known. Here we show that phenanthrene, a PAH with a benzene 3-ring structure, is the key moiety disrupting the physiology of heart muscle cells. Phenanthrene is a ubiquitous pollutant in water and air, and the cellular targets for this compound are highly conserved across vertebrates. Our findings therefore suggest that phenanthrene may be a major worldwide cause of vertebrate cardiac dysfunction.