Further receptor subclassifications have been proposed including suggestions that ETB could be subdivided into ETB1, present on endothelial cells, and ETB2 on smooth muscle cells, but there currently is no evidence that the receptors expressed by these two cell types can be distinguished pharmacologically.10,11 ET-receptor antagonists have not been successful in certain conditions such as heart failure,12 perhaps implying that ETs may mediate their actions via previously unsuspected receptors; however, this is unlikely. This evidence concerns the gene EDNRB and heart failure.