CTRL and hydrops fetalis: In SAN cells, spontaneous action potentials are accompanied by [Ca2+]i transients in a 1:1 fashion in CTRL (Li et al., 1997; Rigg et al., 2000; Lakatta et al., 2003; Joung et al., 2009, 2010; van Borren et al., 2010) as well as in HF SAN cells (see Figure 7, below).