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

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% O-2) or chronic hypoxic conditions (H10; 10% O-2). We determined heart rate (f(H)) and mean arterial pressure (P-m) in acute normoxic (21% O-2) and acute hypoxic (10% O-2) 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 f(H) at 90% of incubation. Unlike other reptile embryos, cholinergic and ganglionic receptor blockade both increased f(H). beta-Adrenergic receptor blockade with propranolol decreased f(H), and alpha-adrenergic blockade with phentolamine decreased P-m. 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, alpha-adrenoreceptor and beta-adrenoreceptor function, which was reflected in altered intrinsic f(H) and adrenergic receptor mRNA levels. This is the first study to link gene expression with morphological and cardioregulatory plasticity in a developing reptile embryo.