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

We demonstrate unusual features of the intracellular processing of end othelin-1 (ET-1) and its receptor ET(A), the receptor subtype that med iates 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 endoc ytosis was measured both by immunofluorescence and by radioiodinated a ntibodies specific for ET(A). Second, we demonstrate that ET-1 remains intact for up to 2 h after endocytosis and, as judged by co-immunopre cipitation, internalized I-125-ET-1 remains bound to ET(A) receptors. We hypothesize that internalized ET-1, bound to ET(A) receptors, conti nues to activate a signal-transducing G protein, thus accounting for t he prolonged period of contraction induced in smooth muscle cells by a single administration of ET-1.