Abstract

The aim of this study was to identify the role of chymase in the conversion of exogenously administered Big endothelin-1 in the mouse in vivo. Real-time polymerase chain reaction analysis detected mRNA of mucosal mast cell chymases 4 and 5, endothelin-converting enzyme 1a, and neutral endopeptidase 24.11 in pulmonary, cardiac, and aorta homogenates derived from C57BL/6J mice, with the latter tissue expressing the highest levels of both chymase isoforms. Furthermore, hydrolysis of a fluorogenic peptide substrate, Suc-Leu-Leu-Val-Tyr-7-amino-4-methylcoumarin, was sensitive to the chymase inhibitors Suc-Val-Pro-Phe^P(OPh)₂ (200 μM) and chymostatin [(S)-1-carboxy-2-phenylethyl]-carbamoyl-α-[2-iminohexahydro-4(S)-pyrimidyl]-(S)-Gly-X-Phe-al, where X can be the amino acid Leu, Val, or Ile) (100 μM) in supernatants extracted from the same tissue homogenates. In anesthetized mice, Big endothelin-1, endothelin-1 (1–31), and endothelin-1 triggered pressor responses (ED₅₀s, 0.67, 0.89, and 0.16 nmol/kg) that were all reduced or potentiated by selective endothelin ET_A or ET_B receptor antagonists, respectively, BQ-123 (cyclo[d-Asp-Pro-d-Val-Leu-d-Trp]) or BQ-788 (N-[N-[N-[(2,6-dimethyl-1-piperidinyl)carbonyl]-4-methyl-l-leucyl]-1-(methoxycarbonyl)-d-tryptophyl]-d-norleucine sodium salt), each at 1 mg/kg. The pressor responses to big endothelin-1 were significantly reduced by the neutral endopeptidase inhibitor thiorphan (dl-3-mercapto-2-benzylpropanoylglycine) (1 mg/kg) or the endothelin-converting enzyme inhibitor CGS 35066 [α-[(S)-(phosphonomethyl)amino]-3-dibenzofuranopropanoic acid] (0.1 mg/kg). In contrast, the responses to endothelin-1 (1–31) were abolished by thiorphan but unaffected by CGS 35066. In addition, Suc-Val-Pro-Phe^P(OPh)₂ (20–40 mg/kg) reduced, by more than 60%, the hemodynamic response to big endothelin-1 but not to endothelin-1 (1–31) and endothelin-1. Finally, intravenous administration of big endothelin-1 induced Suc-Val-Pro-Phe^P-(OPh)₂-sensitive increases in plasma-immunoreactive levels of endothelin-1 (1–31) and endothelin-1. The present study suggests that chymase plays a pivotal role in the conversion and cardiovascular properties of big endothelin-1 in vivo.