Endothelin degradation by vascular smooth muscle cells

The mechanism(s) of degradation of the potent vasoconstrictor endothelin-1 (ET-1) by rat vascular smooth muscle A-10 cells, which possess the ET, receptor subtype, was investigated by incubating [¹²⁵I]ET-1 (0.1 nM) with cells for 0-4 h at 37°C in the presence and absence of lysosomal enzyme inhibitors, NH₄Cl and chloroquine, and a neutral endopeptidase inhibitor, phosphoramidon. The assay buffer and cell extracts were analyzed by reverse-phase HPLC, and the radioactivity in the fractions was measured. In the absence of inhibitors, most of the radioactivity in the medium was in the form of [¹²⁵I]Tyr after a 4 h incubation. When [¹²⁵I]ET-I was incubated with A-10 cells at 4°C, six radiolabeled peaks, including some [¹²⁵I]Tyr and about 30% of the original [¹²⁵I]ET-1, were present in the medium. In the presence of 5 μM chloroquine there was no [[¹²⁵I]Tyr peak in the medium, indicating that internalization and putative lysosomal degradation of ET-1 were blocked. NH₄Cl (50 and 100 mM) also reduced the amount of [¹²⁵I]Tyr formed. The presence of ET-1 fragments indicated that, in addition to lysosomal degradation, some of the ligand is metabolized by enzymes located on the cell membrane; we demonstrated, however, that secreted proteases from A-10 cells are not involved in the degradation of ET-1. The neutral endopeptidase inhibitor, phosphoramidon, did not completely inhibit the metabolism of [¹²⁵I]ET-1 to [¹²⁵I]Tyr. These results establish that various cell-associated enzymes are capable of degrading ET-1 in A-10 cells. Moreover, analysis of the cell lysates indicated the presence of a relatively stable pool of ET-1-occupied receptors or compartmentalized ET-1, protected from cell proteases, which may contribute to the potent contractility of ET-1.