Developmental aspects of cardiac Ca2+ signaling: interplay between RyR- and IP3R-gated Ca2+ stores

Janowski E, Berrios M, Cleemann L, Morad M. Developmental aspects of cardiac Ca2+ signaling: interplay between RyR- and IP3R-gated Ca2+ stores. Am J Physiol Heart Circ Physiol 298: H1939-H1950, 2010. First published March 19, 2010; doi:10.1152/ajpheart.00607.2009.-The dominant mode of intracellular Ca2+ release in adult mammalian heart is gated by ryanodine receptors (RyRs), but it is less clear whether inositol 1,4,5-trisphosphate (IP3)-gated Ca2+ release channels (IP(3)Rs), which are important during embryogenesis, play a significant role during early postnatal development. To address this question, we measured confocal two-dimensional Ca2+ dependent fluorescence images in acutely isolated neonatal (days 1 to 2) and juvenile (days 8-10) rat cardiomyocytes, either voltage-clamped or permeabilized, where rapid exchange of solution could be used to selectively activate the two types of Ca2+ release channel. Targeting RyRs with caffeine produced large and rapid Ca2+ signals throughout the cells. Application of ATP and endothelin-1 to voltage-clamped, or IP3 to permeabilized, cells produced smaller and slower Ca2+ signals that were most prominent in subsarcolemmal regions and were suppressed by either the IP3R-blocker 2-aminoethoxydiphenylborate or replacement of the biologically active form of IP3 with its L-stereoisomer. Such IP3R-gated Ca2+ releases were amplified by Ca2+-induced Ca2+ release (CICR) via RyRs since they were also reduced by compounds that block the RyRs (tetracaine) or deplete the Ca2+ pools they gate (caffeine, ryanodine). Spatial analysis revealed both subsarcolemmal and perinuclear origins for the IP3-mediated Ca2+ release events RyR- and IP3R-gated Ca2+ signals had larger magnitudes in juvenile than in neonatal cardiomyocytes. Ca2+ signaling was generally quite similar in atrial and ventricular cardiomyocytes but showed divergent development of IP3-mediated regulation in juveniles. Our data suggest that an intermediate stage of Ca2+ signaling may be present in developing cardiomyocytes, where, in addition to RyR-gated Ca2+ pools, IP3-gated Ca2+ release is sufficiently large in magnitude and duration to trigger or contribute to activation of CICR and cardiac contraction.