ADRB2 and hydrops fetalis: Interestingly, due to chronic stimulation by CAs, cardiac β-adrenergic receptor (β-AR) responsiveness is altered in both HF and aging, and treatments (e.g., β-blockers and physical activity) that improve their signaling can ameliorate cardiac performance and outcomes during HF, particularly in older aged patients (Baxter et al., 2002; Leosco et al., 2007; Femminella et al., 2013; Rengo et al., 2013; Ferrara et al., 2014).