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

Janowski E, Berríos M, Cleemann L, Morad M. Developmental aspects of cardiac Ca signaling: interplay between RyRand IP3R-gated Ca 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 Ca release in adult mammalian heart is gated by ryanodine receptors (RyRs), but it is less clear whether inositol 1,4,5-trisphosphate (IP3)-gated Ca release channels (IP3Rs), which are important during embryogenesis, play a significant role during early postnatal development. To address this question, we measured confocal two-dimensional Ca 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 Ca release channel. Targeting RyRs with caffeine produced large and rapid Ca signals throughout the cells. Application of ATP and endothelin-1 to voltage-clamped, or IP3 to permeabilized, cells produced smaller and slower Ca 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 Lstereoisomer. Such IP3R-gated Ca releases were amplified by Ca -induced Ca release (CICR) via RyRs since they were also reduced by compounds that block the RyRs (tetracaine) or deplete the Ca pools they gate (caffeine, ryanodine). Spatial analysis revealed both subsarcolemmal and perinuclear origins for the IP3-mediated Ca release events RyRand IP3R-gated Ca signals had larger magnitudes in juvenile than in neonatal cardiomyocytes. Ca 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 Ca signaling may be present in developing cardiomyocytes, where, in addition to RyR-gated Ca pools, IP3-gated Ca release is sufficiently large in magnitude and duration to trigger or contribute to activation of CICR and cardiac contraction.