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

Ilan M Ruhr,Heather Mccourty,Afaf Bajjig,Dane A Crossley,Gina L J Galli,Holly A Shiels

doi:10.1098/rspb.2019.1072

Abstract

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.

Highlights

Developmental plasticity refers to the process whereby genetically similar individuals display substantially different phenotypes, depending on the environmental conditions that they experience during early life [1]
We investigated the correlation between external Ca2þ concentration and cardiomyocyte length, as a measure of myofilament Ca2þ-sensitivity, defined here as the relationship between the concentration of free Ca2þ ions available to bind to troponin C and the amount of force generated by the cardiomyocyte
While the N21 and H10 turtles did not differ in body or heart mass, there were distinct differences in the size and shape of their ventricular cardiomyocytes

Summary

Introduction

Developmental plasticity refers to the process whereby genetically similar individuals display substantially different phenotypes, depending on the environmental conditions that they experience during early life [1]. Cardiomyocytes were placed in the recording chamber (at room temperature), perfused with normoxic saline and stimulated to contract at a frequency of 0.2 Hz, which is within the normal physiological range of heart-rates in common snapping turtles [21]. Myofilament Ca2þ-sensitivity was determined by perfusing cardiomyocytes with normoxic saline, with increasing levels of extracellular [Ca2þ] ([Ca2þ]e), from 1 to 5 mM ( pH 7.72, at room temperature), while simultaneously measuring relaxed cell length, according to a protocol by Wisløff et al [22]. A further GLM was used, followed by Sidak post hoc tests, to reveal the effect of the developmental O2 and experimental period (normoxia, anoxia or reoxygenation) on cardiomyocyte shortening (the dependent variable), while controlling for D[Ca2þ]i (the covariate). All data are presented as means +s.e.m. and considered significantly different, when p 0.05

Results

A A pHi

Discussion

H10 N H10 A H10 R

Perspectives