Endothelin 1 is not required for AV conduction tissue development. (A) Representative images of wild-type and endothelin 1 morphant fish. endothelin 1 morphants fail to develop structures derived from the pharyngeal arches, including the mandible. (B) endothelin 1 morphant fish develop normal AV conduction block on treatment with terfenadine at 48 hpf. Inset of contemporaneous recordings of atrial and ventricular contraction using intensity-time plots from regions over the respective chambers in an endothelin 1 morphant embryo demonstrating 1:1 AV conduction. (C) endothelin 1 morphant fish develop a normal AV conduction delay as seen in this calcium activation map of a single cardiac cycle at 48 hpf. Isochronal lines (50 mseconds) obtained by fluorescence microscopy are superimposed on a maximum intensity projection image.