AR and hydrops fetalis: Since the early 1990s, sympathetic nervous system activation and its downstream signaling in CMs (i.e., adrenoceptor (AR) signaling) has been well recognized as a main factor causing or exacerbating cardiac pathology, especially HF [44,45,46,47,48,49], in addition to its role as the primary regulator of E–C coupling under physiological conditions [50,51] (Figure 1).