Developmental programming of cardiac anoxia tolerance in turtles relies on Ca²⁺ cycling by the sarcoplasmic reticulum

Oxygen deprivation during embryonic development can permanently remodel the vertebrate heart, leading to cardiovascular abnormalities in adulthood. While this phenomenon is mostly damaging, recent evidence suggests developmental hypoxia produces stress-tolerant phenotypes in some ectothermic vertebrates. Embryonic snapping turtles subjected to chronic hypoxia display improved cardiac anoxia tolerance after hatching, which is associated with altered Ca²⁺ homeostasis in heart cells (cardiomyocytes). Here we examined the possibility that changes in Ca²⁺ cycling, through the sarcoplasmic reticulum (SR), underlie the developmentally programmed cardiac phenotype of snapping turtles. We investigated this hypothesis by isolating cardiomyocytes from juvenile turtles that developed in either normoxia (21% O₂; “N21”) or chronic hypoxia (10% O₂; “H10”) and subjected the cells to anoxia/reoxygenation, either in the presence or absence of SR Ca2+-cycling inhibitors. We simultaneously measured cellular shortening, intracellular [Ca²⁺], and intracellular pH (pHi). Under normoxic conditions, N21 and H10 cardiomyocytes shortened equally, but H10 Ca²⁺ transients (Δ[Ca²⁺]i) were twofold smaller than N21 cells, and SR inhibition only decreased N21 shortening and Δ[Ca²⁺]i. Anoxia subsequently depressed shortening, Δ[Ca²⁺]i, and pHi in control N21 and H10 cardiomyocytes, yet H10 shortening and Δ[Ca2+]i recovered to pre-anoxic levels, partly due to enhanced myofilament Ca²⁺ sensitivity. SR blockade abolished the recovery by anoxic H10 cardiomyocytes, and potentiated decreases in shortening, Δ[Ca²⁺]i, and pHi. Our results are novel, by providing the first evidence of developmental programming of SR function and demonstrating that developmental hypoxia confers a long-lasting and superior anoxia-tolerant cardiac phenotype in snapping turtles, by enhancing myofilament Ca²⁺ sensitivity and modifying SR function.