VASCULAR SMOOTH-MUSCLE CELLS METABOLIZE ENDOTHELIN-1 IN THE ABSENCE OF A FUNCTIONAL RECEPTOR

Endothelin-l (ET-1), a 21 amino acid vasoconstrictor peptide synthesiz ed by vascular endothelial cells, exerts powerful actions on the under lying smooth muscle cells. The receptor and signal transduction mechan isms for ET-1 have been well characterized in rat aortic A10 vascular smooth muscle cells (A10VSMC). This investigation has characterized th e internalization and metabolism of [I-125]ET-1 by A10VSMC. A10VSMC in ternalized [I-125]ET-1 rapidly in a receptor-mediated manner. However, inhibition of the binding/internalization had no effect on the metabo lism of [I-125]ET-I by these cells. Thus, the presence of excess unlab eled ET-I in the incubation, treatment of the cells with ET receptor a ntagonists, and homologous ligand-induced down-regulation of the ET-1 receptor all inhibited binding and internalization of [I-125]ET-1 by A 10VSMC, but not its metabolism. Furthermore, addition of excess unlabe led ET-1 to the incubations containing cells pretreated with the homol ogous ligand (receptor down-regulated cells) also failed to inhibit th e metabolism of [I-125]ET-1. Essentially similar characteristics of [I -125]ET-1 binding and metabolism were exhibited by primary cultures of smooth muscle cells derived from rat thoracic aorta. Such ability of the vascular smooth muscle cells to degrade ET-I, which is produced co nstitutively by the endothelial cells, presents a novel mechanism in t he regulation of its local and circulating concentration.