Hearts from subjects with different ages have different Ca2+ signaling. Release of Ca2+ from intracellular stores in response to an action potential initiates cardiac contraction. Both depolarizationstimulated and spontaneous Ca2+ releases, Ca2+ transients and Ca2+ sparks, demonstrate the main events of excitation-contraction coupling (ECC). Global increase in free Ca2+ concentration ([Ca 2+]i) consists of summation of Ca2+ release events in cardiomyocytes. Since the Ca2+ flux induced by Ca 2+ sparks reports a summation of ryanodine-sensitive Ca2+ release channels (RyR2s)'s behavior in a spark cluster, evaluation of the properties of Ca2+ sparks and Ca2+ transients may provide insight into the role of RyR2s on altered heart function between 3-month-old (young adult) and 6-month-old (mature adult) rats. Basal [Ca2+] i and Ca2+ sparks frequency were significantly higher in mature adult rats compared to those of young adults. Moreover, amplitudes of Ca 2+ sparks and Ca2+ transients were significantly smaller in mature adults than those of young adults with longer time courses. A smaller L-type Ca2+ current density and decreased SR Ca2+ load was observed in mature adult rats. In addition, RyR2s were markedly hyperphosphorylated, and phosphorylation levels of PKA and CaMKII were higher in heart from mature adults compared to those of young adults, whereas their SERCA protein levels were similar. Our data demonstrate that hearts from rats with different ages have different Ca2+ signaling including hyperphosphorylation of RyR2s and higher basal [Ca2+]i together with increased oxidized protein-thiols in mature adult rats compared to those of young adults, which play important roles in ECC. Finally, we report that ECC efficiency changes with age during maturation, partially related with an increased cellular oxidation level leading to reduced free proteinthiols in cardiomyocytes. © University of Navarra 2011.