CLOCK and hydrops fetalis: The slope of late diastolic [Ca2+]i elevation, associated with localized Ca2+ releases (LCRs) or Ca2+ sparks (Bogdanov et al., 2006; Maltsev et al., 2006; Joung et al., 2009, 2010; van Borren et al., 2010), was unaffected under basal conditions, suggesting that the Ca2+ clock is not the primary cause of the slower intrinsic cycle length in HF SAN cells.