ENDOTHELIN-INDUCED ENDOCYTOSIS OF CELL-SURFACE ET(A) RECEPTORS - ENDOTHELIN REMAINS INTACT AND BOUND TO THE ET(A) RECEPTOR

We demonstrate unusual features of the intracellular processing of endothelin-1 (ET-1) and its receptor ET(A), the receptor subtype that mediates contraction of vascular smooth muscle cells. First, we show that in stably transfected CHO cells expressing ET(A), binding of an ET-1 ligand induces rapid endocytosis of cell surface ET(A). Receptor endocytosis was measured both by immunofluorescence and by radioiodinated antibodies specific for ET(A). Second, we demonstrate that ET-1 remains intact for up to 2 h after endocytosis and, as judged by co-immunoprecipitation, internalized I-125-ET-1 remains bound to ET(A) receptors. We hypothesize that internalized ET-1, bound to ET(A) receptors, continues to activate a signal-transducing G protein, thus accounting for the prolonged period of contraction induced in smooth muscle cells by a single administration of ET-1.