Endothelin (16-21): biphasic effect and no desensitization on the guinea-pig isolated ileum.

1. In the guinea-pig ileum the C-terminal hexapeptide of the endothelins, endothelin (16-21), induced a biphasic effect (relaxation followed by contraction) qualitatively similar to that seen in the responses to endothelins 1 and 3. Both components of the response were concentration-dependent in the range studied (2-100 microM). 2. The response induced by endothelin (16-21) was inhibited in low-sodium (80 mM) medium. 3. Repeated administration of endothelin (16-21) induced no desensitization of the preparation, contrasting with the tachyphylaxis induced by endothelin-1 and endothelin-3 in the guinea-pig ileum. 4. Tissues rendered tachyphylatic to endothelin-1 or endothelin-3 responded normally to endothelin (16-21). 5. The results suggest that the C-terminal tail of the endothelins contains the message for the biphasic response, whereas the N-terminal domain may be responsible for the strong binding to the receptor and for the tachyphylactic properties of endothelin-1 and endothelin-3, in the guinea-pig isolated ileum. However, the possibility that endothelin (16-21) may be acting on a site other than the endothelin receptor cannot be ruled out.