ADRB1 and hydrops fetalis: Of note, the differences between the two predominant cardiac βARs, i.e., β1AR & β2AR, in terms of their signaling properties, might take a quite different shape and have a much bigger bearing on pathophysiologic implications in the setting of human HF: for instance, and as discussed in more detail in subsequent sections, β1AR is selectively downregulated (i.e., functional receptor number reduced) in human HF, thus shifting the above mentioned stoichiometry of β1AR:β2AR toward 50:50 in the failing heart from ~75:~20% in the normal, healthy heart (Bristow et al., 1982, 1986).