A kinetic binding study to evaluate the pharmacological profile of a specific leukotriene C(4) binding site not coupled to contraction in human lung parenchyma.

We report the identification of a novel pharmacological profile for the leukotriene (LT)C(4) 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 CysLT(1)-CysLT(2) 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 (3)H-LTD(4) binding sites, whereas pobilukast, pranlukast, and CGP 57698 dissociated both (3)H-LTC(4) and (3)H-LTD(4) from their binding sites. Thus, with binding studies, we have been able to identify a pharmacological profile for LTC(4) distinct from that of LTD(4) receptor (CysLT(1)) in HLP. On the contrary, in functional studies, all of the classic antagonists tested were able to revert both LTC(4)- and LTD(4)-induced contractions of isolated HLP strips. Thus, LTD(4) and LTC(4) contract isolated HLP strips through the same CysLT(1) 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, LTD(4)- and LTC(4)-induced contractions are mediated by the same CysLT(1) receptor. In conclusion, the specific LTC(4) high-affinity binding site cannot be classified as one of the officially recognized CysLT receptors, and it is not implicated in LTC(4)-induced HLP strip contractions.