Crucial role of the sarcoplasmic reticulum in the developmental regulation of Ca ²⁺ transients and contraction in cardiomyocytes derived from embryonic stem cells

<jats:p> In adult myocardium, excitation‐contraction coupling is critically regulated by sarcoplasmic reticulum (SR) Ca <jats:sup>2+</jats:sup> release via type 2 ryanodine receptor (RyR2), but generally, it is believed that SR‐function is rudimentary in the fetal heart and in embryonic stem (ES) cell‐derived cardiomyocytes (ESCMs), a possible source for cell replacement therapies. This study used wild‐type (RyR2 <jats:sup>+/+</jats:sup> ) and RyR2 null (RyR2 <jats:sup>−/−</jats:sup> ) ESCMs as an in vitro model of cardiomyogenesis, together with pharmacological approaches and expression profiles of genes relevant for SR function, to elucidate the functional importance of RyR2 and SR on the regulation of Ca <jats:sup>2+</jats:sup> transients and contraction during early cardiomyocyte development. During differentiation of RyR2 <jats:sup>+/+</jats:sup> ESCMs, SR function developed progressively with increased basal cytosolic free Ca <jats:sup>2+</jats:sup> concentration ([Ca <jats:sup>2+</jats:sup> ]i), enhanced frequency and amplitude, and decreased duration of Ca <jats:sup>2+</jats:sup> transients that were inhibited by ryanodine and thapsigargin. These functional traits correlated with SR Ca <jats:sup>2+</jats:sup> load and the expression of RyR2, SERCA2a, and phospholamban. RyR2 <jats:sup>−/−</jats:sup> ESCMs, comparatively, demonstrated a significantly prolonged time‐to‐peak and reduced frequency of Ca <jats:sup>2+</jats:sup> transients and contractions. β‐adrenergic stimulation of RyR2 <jats:sup>+/+</jats:sup> ESCMs increased the frequency and amplitude of Ca <jats:sup>2+</jats:sup> transients with differentiation but was much weaker in RyR2 <jats:sup>−/−</jats:sup> ESCMs. We conclude that functional SR and control of RyR2‐mediated SR Ca <jats:sup>2+</jats:sup> release directly contribute to the spontaneous and β‐adrenergic receptor‐stimulated contraction of ESCMs, even at very immature stages of development. </jats:p>