Abstract

We report the identification of a novel pharmacological profile for the leukotriene (LT)C₄ binding site we previously identified in human lung parenchyma (HLP). We used a series of classic cysteinyl-LT(CysLT)₁ receptor antagonists belonging to different chemical classes and the dual CysLT₁-CysLT₂ antagonist BAY u9773 for both binding and functional studies. Because the presence of (S)-decyl-glutathione interfered with cysteinyl-LT binding, with a kinetic protocol we avoided the use of this compound. By means of heterologous dissociation time courses, we demonstrated that zafirlukast, iralukast, and BAY u9773 selectively competed only for ³H-LTD₄ binding sites, whereas pobilukast, pranlukast, and CGP 57698 dissociated both³H-LTC₄ and ³H-LTD₄from their binding sites. Thus, with binding studies, we have been able to identify a pharmacological profile for LTC₄ distinct from that of LTD₄ receptor (CysLT₁) in HLP. On the contrary, in functional studies, all of the classic antagonists tested were able to revert both LTC₄- and LTD₄-induced contractions of isolated HLP strips. Thus, LTD₄ and LTC₄ contract isolated HLP strips through the same CysLT₁ receptor. The results of kinetic binding studies, coupled to a sophisticated data analysis, confirm our hypothesis that HLP membranes contain two cysteinyl-LT high-affinity binding sites with different pharmacological profiles. In functional studies, however, LTD₄- and LTC₄-induced contractions are mediated by the same CysLT₁ receptor. In conclusion, the specific LTC₄ high-affinity binding site cannot be classified as one of the officially recognized CysLT receptors, and it is not implicated in LTC₄-induced HLP strip contractions.