ADRB2 and hydrops fetalis: Perhaps the crucial role of presynaptic α2ARs in regulating NE release from cardiac ANS nerves stems from the fact that they are the only presynaptic ARs that can inhibit NE release; presynaptic βARs (of the β2AR subtype, mainly) are facilitatory autoreceptors enhancing NE release at sympathetic nerve terminals (Docherty, 2002), a phenomenon whose inhibition may contribute to the therapeutic benefit of β-blockers in HF (see below) (Figure 2).