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 leu kotriene (LT)C-4 binding site we previously identified in human lung parenc hyma (HLP). We used a series of classic cysteinyl-LT (CysLT)(1) receptor an tagonists belonging to different chemical classes and the dual CysLT(1)-Cys LT(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, i ralukast, and BAY u9773 selectively competed only for H-3-LTD4 binding site s, whereas pobilukast, pranlukast, and CGP 57698 dissociated both H-3-LTC4 and H-3-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 (CysLT(1)) 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 contr act isolated HLP strips through the same CysLT1 receptor. The results of ki netic binding studies, coupled to a sophisticated data analysis, confirm ou r hypothesis that HLP membranes contain two cysteinyl-LT high-affinity bind ing sites with different pharmacological profiles. In functional studies, h owever, LTD4- and LTC4-induced contractions are mediated by the same CysLT( 1) receptor. In conclusion, the specific LTC4 high-affinity binding site ca nnot be classified as one of the officially recognized CysLT receptors, and it is not implicated in LTC4-induced HLP strip contractions.