EVIDENCE FOR SIGNALING BY BIG ENDOTHELIN-1 VIA CONVERSION TO ENDOTHELIN-1 IN PULMONARY-ARTERY SMOOTH-MUSCLE CELLS

In this study, signalling by big endothelin-1 (big ET-1) and a role for the phosphoramidon-sensitive endothelin converting enzyme in this signalling was investigated in smooth muscle cells cultured from bovine pulmonary artery. In whole cells, both ET-1 and big ET-1 stimulated inositol phosphate accumulation in a time- and concentration-dependent manner. The endothelin ET-A receptor antagonist (BQ 123, 1 mu M) significantly inhibited stimulation by ET-1 (10 nM) or big ET-1 (100 nM). When cells were pretreated with 100 mu M phosphoramidon, the stimulation by big ET-I, but not ET-1 was abolished. In separate experiments, when cells were incubated with exogenous big ET-1, a time-dependent phosphoramidon-sensitive conversion to ET-1 was detected by radioimmunoassay. These results are consistent with the presence of a phosphoramidon-sensitive endothelin converting enzyme on the surface of bovine pulmonary artery smooth muscle cells, which may play a role in regulating signalling by circulating big ET-1.