ADRB2 and hydrops fetalis: In particular, β-AR population of the non-failing heart is composed of β 1 and β 2, in a ratio of 8:2; however, in both ageing and HF, the proportion of β 1 subtypes decreases due to mRNA down-regulation, while levels of β 2-AR remain stable, thus achieving a 1:1 ratio of β 1- and β 2-ARs (Lohse et al., 2003).