Chronic hypercapnic incubation increases relative organ growth and reduces blood pressure of embryonic American alligator ( Alligator mississippiensis )

Developmental plasticity of the cardiovascular system in oviparous vertebrates: effects of chronic hypoxia and interactive stressors in the context of climate change

Animals at early life stages are generally more sensitive to environmental stress than adults. This is especially true of oviparous vertebrates that develop in variable environments with little or no parental care. These organisms regularly experience environmental fluctuations as part of their natural development, but climate change is increasing the frequency and intensity of these events. The developmental plasticity of oviparous vertebrates will therefore play a critical role in determining their future fitness and survival. In this Review, we discuss and compare the phenotypic consequences of chronic developmental hypoxia on the cardiovascular system of oviparous vertebrates. In particular, we focus on species-specific responses, critical windows, thresholds for responses and the interactive effects of other stressors, such as temperature and hypercapnia. Although important progress has been made, our Review identifies knowledge gaps that need to be addressed if we are to fully understand the impact of climate change on the developmental plasticity of the oviparous vertebrate cardiovascular system.

Changes in hemoglobin function and isoform expression during embryonic development in the American alligator, Alligator mississippiensis

In the developing embryos of egg-laying vertebrates, O₂ flux takes place across a fixed surface area of the eggshell and the chorioallantoic membrane. In the case of crocodilians, the developing embryo may experience a decrease in O₂ flux when the nest becomes hypoxic, which may cause compensatory adjustments in blood O₂ transport. However, whether the switch from embryonic to adult hemoglobin isoforms (isoHbs) plays some role in these adjustments is unknown. Here, we provide a detailed characterization of the developmental switch of isoHb synthesis in the American alligator, Alligator mississippiensis. We examined the in vitro functional properties and subunit composition of purified alligator isoHbs expressed during embryonic developmental stages in normoxia and hypoxia (10% O₂). We found distinct patterns of isoHb expression in alligator embryos at different stages of development, but these patterns were not affected by hypoxia. Specifically, alligator embryos expressed two main isoHbs: HbI, prevalent at early developmental stages, with a high O₂ affinity and high ATP sensitivity, and HbII, prevalent at later stages and identical to the adult protein, with a low O₂ affinity and high CO₂ sensitivity. These results indicate that whole blood O₂ affinity is mainly regulated by ATP in the early embryo and by CO₂ and bicarbonate from the late embryo until adult life, but the developmental regulation of isoHb expression is not affected by hypoxia exposure.

Cardiovascular responses to putative chemoreceptor stimulation of embryonic common snapping turtles (Chelydra serpentina) chronically incubated in hypoxia (10% O2)

Developmental hypoxia has been shown to result in significant changes in cardiovascular development of American alligators and common snapping turtles. These include similar effects on cardiac mass and aspects of cardiovascular function. However, given the distant phylogenetic relationship between crocodilians and chelonians, we hypothesized that snapping turtles would also exhibit differences in the effects of developmental hypoxia on cardiovascular regulation. This hypothesis was based in part on prior studies that documented differences in plasticity of vagal tone on the heart between alligators and snapping turtles incubated in hypoxic conditions. To test this hypothesis, we investigated how 10% O₂ exposure over final 80% of incubation altered the heart rate and blood pressure response to two chemical manipulations of the "chemoreflex" in common snapping turtles at 70% and 90% of incubation. NaCN injections produced a dose dependent bradycardia that was mediated by cholinergic receptor stimulation. This reflex was relatively unaffected by hypoxic incubation conditions in snapping turtle embryos. Injections of the 5-HT₃ agonist phenylbiguanide (PBG) caused a pronounced bradycardia that decreased in intensity at 90% of incubation in embryos from the normoxic group while the heart rate response was unchanged in the hypoxic group. This differs from the previously reported diminished heart rate response of embryonic alligators incubated in 10% O₂, suggesting plasticity in this chemoreflex response differs between the species. Our data also indicate the cardiovascular response is mediated by a secondary cholinergic receptor stimulation however the inability of ganglionic blockade to inhibit the PBG response leaves the location of the receptors antagonized by PBG in question in embryonic snapping turtles. Primarily, our findings refute the hypothesis that hypoxic incubation decreases the "chemoreflex' response of snapping turtle embryos.

Does the left aorta provide proton-rich blood to the gut when crocodilians digest a meal?

Reptiles have the capacity to differentially perfuse the systemic and pulmonary vascular circuits via autonomic regulation of the heart and the vascular trees. While this aptitude is widely recognized, the role of 'shunting' as a homeostatic mechanism to match convective transport with tissue demand remains unknown. In crocodilians, it has been hypothesized that a pulmonary vascular bypass of systemic venous blood - a right-to-left (R-L) shunt - serves to deliver CO₂-rich blood with protons needed for gastric acid secretion during digestion. This hypothesis is partially based on the unique crocodilian vascular anatomy where a left aorta (LAo) arises from the right ventricle, and appears to preferentially supply the gastrointestinal system, whereas the right aorta emerges from the left ventricle. Recent theoretical considerations imply that a R-L shunt would have minuscule effects on P _CO2 , but direct measurements of blood gases in both the right and left aortae or both the right and left atria in fed animals have not been conducted. For this reason, we measured blood parameters including P _O2 , P _CO2 , pH_e and [HCO₃ ^-] in the right and left aortae and atria following ingestion of a gavage-fed standardized meal (5% body mass). Blood samples were taken at 3, 6, 12, 24, 36 and 48 h into the digestive period to encompass the period of maximal gastric acid secretion. At no point did P _CO2  or pH differ between the left and right aortae, whereas P _O2  was significantly lower in the left aorta at several time points during digestion. Our findings do not support the hypothesis that a R-L shunt serves to deliver CO₂ for the gastrointestinal system after feeding in crocodilians.

American alligator (Alligator mississippiensis) embryos tightly regulate intracellular pH during a severe acidosis

Crocodilian nests naturally experience high CO2 (hypercarbia), which leads to increased blood Pco2 and reduced blood pH (pHe) in embryos; their response to acid–base challenges is not known. During acute hypercarbia, snapping turtle embryos preferentially regulate tissue pH (pHi) against pHe reductions. This is proposed to be associated with CO2 tolerance in reptilian embryos and is not found in adults. In the present study, we investigated pH regulation in American alligator (Alligator mississippiensis (Daudin, 1802)) embryos exposed to 1 h of hypercarbia hypoxia (13 kPa Pco2, 9 kPa Po2). Hypercarbia hypoxia reduced pHe by 0.42 pH unit, while heart and brain pHi increased, with no change in the pHi of other tissues. The results indicate that American alligator embryos preferentially regulate pHi, similar to snapping turtle embryos, which represents a markedly different strategy of acid–base regulation than what is observed in adult reptiles. These findings suggest that preferential pHi regulation may be a strategy of acid–base regulation used by embryonic reptiles.

Periods of cardiovascular susceptibility to hypoxia in embryonic american alligators (Alligator mississippiensis)

During embryonic development, environmental perturbations can affect organisms' developing phenotype, a process known as developmental plasticity. Resulting phenotypic changes can occur during discrete, critical windows of development. Critical windows are periods when developing embryos are most susceptible to these perturbations. We have previously documented that hypoxia reduces embryo size and increases relative heart mass in American alligator, and this study identified critical windows when hypoxia altered morphological, cardiovascular function and cardiac gene expression of alligator embryos. We hypothesized that incubation in hypoxia (10% O2) would increase relative cardiac size due to cardiac enlargement rather than suppression of somatic growth. We exposed alligator embryos to hypoxia during discrete incubation periods to target windows where the embryonic phenotype is altered. Hypoxia affected heart growth between 20 and 40% of embryonic incubation, whereas somatic growth was affected between 70 and 90% of incubation. Arterial pressure was depressed by hypoxic exposure during 50-70% of incubation, whereas heart rate was depressed in embryos exposed to hypoxia during a period spanning 70-90% of incubation. Expression of Vegf and PdgfB was increased in certain hypoxia-exposed embryo treatment groups, and hypoxia toward the end of incubation altered β-adrenergic tone for arterial pressure and heart rate. It is well known that hypoxia exposure can alter embryonic development, and in the present study, we have identified brief, discrete windows that alter the morphology, cardiovascular physiology, and gene expression in embryonic American alligator.