MOLECULAR PHARMAcoLoGY, 37:60-64 Post Soluble Binding to the Leukotriene D4 Receptor from

Guinea pig lung membranes were extracted with 1 % digitonin and yielded a preparation that contained soluble leukotriene D4 (LID4) receptor. Specific binding of the high affinity radiolabeled receptor antagonist [3H]ICI-1 9861 5 to the soluble LTD4 receptor was time dependent and reversible. The dissociation constant (Kd) and the density (Bm,,) of [3H]ICl-1 9861 5 binding to the soluble LTD4 receptor was 0.2 ± 0.08 nM and 380 ± 40 fmol/ mg of protein, respectively. Radioligand competition studies showed several classes of structurally diverse, functionally de-fined, receptor antagonists competed with [3H]lCl-1 9861 5 bind-ing to the soluble receptor. The rank order of potency and specificity of these antagonists in binding to the soluble receptor were equivalent to those determined from the membrane-bound receptor binding assay and from the smooth muscle contraction assay. Binding of LTD4 to the soluble receptor was observed, i