Plasticity of cardiovascular function in snapping turtle embryos (Chelydra serpentina): chronic hypoxia alters autonomic regulation and gene expression

Abstract

Reptile embryos tolerate large decreases in the concentration of ambient oxygen. However, we do not fully understand the mechanisms that underlie embryonic cardiovascular short- or long-term responses to hypoxia in most species. We therefore measured cardiac growth and function in snapping turtle embryos incubated under normoxic (N21; 21% O₂) or chronic hypoxic conditions (H10; 10% O₂). We determined heart rate (fH) and mean arterial pressure (Pm) in acute normoxic (21% O₂) and acute hypoxic (10% O₂) conditions, as well as embryonic responses to cholinergic, adrenergic, and ganglionic pharmacological blockade. Compared with N21 embryos, chronic H10 embryos had smaller bodies and relatively larger hearts and were hypotensive, tachycardic, and following autonomic neural blockade showed reduced intrinsic fH at 90% of incubation. Unlike other reptile embryos, cholinergic and ganglionic receptor blockade both increased fH. β-Adrenergic receptor blockade with propranolol decreased fH, and α-adrenergic blockade with phentolamine decreased Pm. We also measured cardiac mRNA expression. Cholinergic tone was reduced in H10 embryos, but cholinergic receptor (Chrm2) mRNA levels were unchanged. However, expression of adrenergic receptor mRNA (Adrb1, Adra1a, Adra2c) and growth factor mRNA (Igf1, Igf2, Igf2r, Pdgfb) was lowered in H10 embryos. Hypoxia altered the balance between cholinergic receptors, α-adrenoreceptor and β-adrenoreceptor function, which was reflected in altered intrinsic fH and adrenergic receptor mRNA levels. This is the first study to link gene expression with morphological and cardioregulatory plasticity in a developing reptile embryo.

Reduced egg shell permeability affects embryonic development and hatchling traits in Lycodon rufozonatum and Pelodiscus sinensis

“…Snake embryos with parchment egg shells increased heart rate and therefore cardiac output in response to external regional hypoxia. This phenomenon of tachycardia in response to hypoxia has also been reported in other reptile species like the scincid lizard [Bassiana duperreyi (Gray, 1838)] (Du et al 2010b) and the snapping turtle (Chelydra serpentina Linnaeus, 1758) (Eme et al 2013). In contrast, hypoxia reduces the heart rate of American alligators [Alligator mississippiensis (Daudin, 1802)] (Crossley & Altimiras 2005a) and some species of birds (VanGolde et al 1997).…”

Embryonic hypoxia programmes postprandial cardiovascular function in adult common snapping turtles (Chelydra serpentina)

“…At ∼20% of development, i.e. ∼9-12 days post-laying (Yntema et al, 1968;Eme et al, 2013), 20 eggs from each clutch were randomly moved in vermiculite boxes to one of two 75 l Ziploc ® bags (SC Johnson, Racine, WI, USA) so that at least 20 eggs from each clutch was in each bag. Each bag was continuously flushed with either room air (21% O 2 ; normoxia) or 10% O 2 (hypoxia) at a rate of 2-3 l min −1 .…”

“…Chronic hypoxia during embryonic development has a profound effect on reptiles, changing the trajectory of phenotype maturation of multiple systems, with a pronounced impact on the cardiovascular system (Kam, 1993;Crossley et al, 2003;Crossley and Altimiras, 2005;Owerkowicz et al, 2009;Eme et al, 2011Eme et al, , 2013Eme et al, , 2014Enok et al, 2013;Tate et al, 2015Tate et al, , 2016Wearing et al, 2016). Over the past decade, our understanding of embryonic cardiovascular development in these species has improved markedly.…”

“…However, very few studies have determined lasting impacts of morphological and physiological phenotypic modifications on juveniles and adults (Owerkowicz et al, 2009;Galli et al, 2016;Wearing et al, 2016). Much of the work that has been conducted so far has illustrated that embryonic reductions in oxygen availability restrict body mass at hatching (Kam, 1993;Crossley and Altimiras, 2005;Owerkowicz et al, 2009;Eme et al, 2011Eme et al, , 2013Eme et al, , 2014Marks et al, 2013;Tate et al, 2015Tate et al, , 2016Crossley et al, 2017b), and this persists into the first years of posthatching life (Owerkowicz et al, 2009;Galli et al, 2016;Wearing et al, 2016). In addition, heart mass is relatively enlarged in embryonic reptiles chronically exposed to low oxygen (Kam, 1993;Crossley and Altimiras, 2005;Eme et al, 2013Eme et al, , 2014Marks et al, 2013;Tate et al, 2015Tate et al, , 2016, which again persists at least into the first years after hatching in American alligators (Owerkowicz et al, 2009;Galli et al, 2016), indicating that developmental hypoxia has a lasting impact on the cardiovascular phenotype of this species.…”

Differences in the timing of cardio-respiratory development determine whether marine gastropod embryos survive or die in hypoxia

“…There is good evidence that vertebrate cardiovascular organs can be remodelled (Crispo and Chapman, 2010;Blank and Burggren, 2014), and cellular and biochemical mechanisms underpinning cardio-respiratory function altered (Eme et al, 2013), when oxygen supply is limited. In vertebrates, such phenotypic alterations have been shown to occur early in development and have been expressed as changes in both the activity and the timing of development of cardio-respiratory function (e.g.…”