Vol. 57, Issue 6, 1182-1189, June 2000

A Kinetic Binding Study to Evaluate the Pharmacological Profile of a Specific Leukotriene C₄ Binding Site Not Coupled to Contraction in Human Lung Parenchyma

Saula Ravasi, Valérie Capra, Maurizio Mezzetti, Simonetta Nicosia, and G. Enrico Rovati

Laboratory of Molecular Pharmacology, Institute of Pharmacological Sciences, University of Milan, Milan, Italy (S.R., V.C., S.N., G.E.R.); and San Paolo Hospital, Department of Clinical Surgery, Milan, Italy (M.M.)

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 both ³H-LTC₄ and ³H-LTD₄ from 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.