Fetal and Neonatal Development of Ca²⁺ Transients and Functional Sarcoplasmic Reticulum in Beating Mouse Hearts

Abstract

Background: It is generally accepted that Ca²⁺-induced Ca²⁺ release is not the predominant mechanism during embryonic stages. Most studies have been conducted either on primary cultures or acutely isolated cells, in which an apparent reduction of ryanodine receptor density and alterations in the cell shape have been reported. The aim of the present study was to investigate developmental changes in Ca²⁺ transients using whole hearts of mouse embryos and neonates. Methods and Results: Fluo-3 fluorescence signals from stimulated whole hearts were detected using a photomultiplier and stored as Ca²⁺ transients. The upstroke and decay of Ca²⁺ transients became more rapid from the late embryonic stages to the neonatal stage. After thapsigargin application (an inhibitor of the sarcoplasmic Ca²⁺-ATPase [SERCA]), time to 50% relaxation (T₅₀) of Ca²⁺ transients was significantly prolonged. There were no significant changes in T₅₀ after Ru360 application (an inhibitor of mitochondrial Ca²⁺ uniporter). The rate of increase in the amplitude of Ca²⁺ transients after caffeine application became larger during developmental stages. Conclusions: Ca²⁺ homeostasis developmentally changes from a slow rise and decay of Ca²⁺ transients to rapid kinetics after the mid-embryonic stage. SERCA began to contribute significantly to Ca²⁺ homeostasis at early embryonic stages and sarcoplasmic reticulum Ca²⁺ contents increased from embryonic to neonatal stages, whereas mitochondrial Ca²⁺ uptake did not contribute to Ca²⁺ transients on a beat-to-beat basis.