Both ETAand ETBEndothelin Receptors Mediate Contraction in the Isolated Rabbit Aorta

The endothelium of the rabbit aorta (in contrast to the endothelium of the rabbit mesenteric artery) did not possess an ETB receptor mediating vascular relaxation. The smooth muscle of the rabbit aorta had both ETA and ETB receptors both of which mediated a contractile response. The characteristics of the contractions of rabbit aorta to endothelin-1 (mixed ETA and ETB receptor agonists) and to Sarafotoxin 6c (selective ETB receptor agonist) were different. Endothelin-1 was a full agonist and the contractile response and the responses to this agonist showed little tachyphylaxis. Sarafotoxin 6c was a partial agonist and the responses of the rabbit aorta to this agonist exhibited substantial tachyphylaxis. The specific ETA receptor antagonist, BQ 123 inhibited responses of the rabbit aorta to endothelin-1 without affecting responses to sarafotoxin 6c. The non-specific endothelin receptor antagonist, PD 142893 inhibited responses of the rabbit aorta to both endothelin, and sarafotoxin 6c. It is concluded that the rabbit aorta has no ETB receptors on the endothelium mediating relaxation. It is also concluded that the smooth muscle of the rabbit aorta has both ETA and ETB receptors both of which mediate contraction.