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 ¹²⁵I-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.