INVOLVEMENT OF A PHOSPHORAMIDON-SENSITIVE ENDOPEPTIDASE IN THE PROCESSING OF BIG ENDOTHELIN-1 IN THE GUINEA-PIG

In anaesthetized and ventilated guinea-pigs, i.v. injection of 1 nmol/kg big endothelin-1 (big ET-1) did not evoke significant changes in pulmonary inflation pressure (PIP) and mean arterial blood pressure (MBP), whereas injection of the same dose of endothelin-1 (ET-1) induced marked and rapid bronchoconstrictor and pressor responses. Administered at the dose of 10 nmol/kg, big ET-1 provoked significant increases in PIP and MBP, which developed slowly and were long-lasting as compared to those evoked by ET-1. When big ET-1 was incubated for 45 min at 37-degrees-C with alpha-chymotrypsin (2 mU/nmol) or pepsin (1-mu-g/nmol) and then injected into guinea-pigs at the dose of 1 nmol/kg, marked bronchoconstrictor and pressor responses were observed, with kinetics similar to those noted after administration of the same dose of ET-1. The magnitude of the alpha-chymotrypsin- or pepsin-treated big ET-1 responses was similar to that induced by ET-1, incubated or not with the enzymes. Injected i.v. at the dose of 5 mg/kg, 5 min before the challenge, phosphoramidon almost totally inhibited the bronchoconstrictor and pressor responses induced by 10 nmol/kg big ET-1, whereas thiorphan (5 mg/kg) partially reduced the increase in PIP and exerted a minimal effect on the changes in MBP. Administered at the dose of 20 mg/kg per os, 1 h before i.v. administration of 10 nmol/kg big ET-1, enalapril maleate and captopril did not significantly alter the bronchoconstriction and the hypertensive response evoked by the peptide. The present results indicate that compared to ET-1, big ET-1 exhibits minimal direct bronchoconstrictor and pressor activities in the guinea-pig and has to be converted by proteolytic enzymes for full expression of its activity. The effects observed after administration of 10 nmol/kg big ET-1, as well as the inhibition of the bronchoconstrictor and pressor responses after treatment of the guinea-pigs with phosphoramidon, suggest that the peptide is metabolized in vivo into an active form and that neutral endopeptidase, or a metallopeptidase related to it, is involved in this process.