Receptor externalization determines sustained contractile responses to endothelin-1 in the rat aorta

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Abstract
The role of receptor internalization and recycling in the vasoconstrictor action of endothelin-1 (ET-1) is investigated using a combination of biochemical and physiological experiments. The binding of 125I-ET-1 to cultured aortic myocytes is first defined. Binding is rapidly followed by an internalization of the peptide. Part of the receptor sites then slowly reappears at the cell surface via a cycloheximide-insensitive mechanism. Evidence that externalizing receptors are functional and can trigger contractions is presented. Finally, the actions of cyclo[D-Trp-D-Asp-Pro-D-Val-Leu] (BQ-123), an antagonist of ETA receptors, are investigated. BQ-123 prevents 125I-ET-1 binding to aortic myocytes (dissociation constant, 10 nM). It prevents the constricting action of ET-1 but not that of angiotensin II. BQ-123 also relaxes almost completely aortic strips that have been precontracted by ET-1 irrespective of the time of its addition. It is concluded that a recycling of internalized ET-1 receptors occurs in ET-1-treated aortic myocytes. This process amplifies the action of the peptide and is probably responsible for the unique contractile action of ET-1.

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