Competition of leukotrienes and ICI-198,615 for [3H]LTD4 binding sites in guinea pig lung membranes suggests the involvement of two LTD4 receptor subtypes.

Pharmacological analysis of the effects of leukotriene D4 (LTD4) antagonists on contraction of guinea pig airway smooth muscle to leukotrienes reveals the presence of two subtypes of the LTD4 receptor. This finding is, however, inconsistent with [3H]LTD4 equilibrium binding results, which show no evidence of a heterogeneity of pulmonary [3H]LTD4 binding sites. It is possible that LTD4 binds to the two receptor subsets with equal affinity, and the pharmacological difference between them lies in the relative ability of leukotriene (LT) agonists and antagonists to interact at the receptor sites. This study was, therefore, undertaken to determine the rank order of potency of LTs and ICI-198,615 in competing with [3H]LTD4 for their respective binding sites in guinea pig lung membranes. To determine precisely the inhibitory constant (Ki) of LTC4, we used the irreversible gamma-glutamyl transpeptidase inhibitor, acivicin (AT-125), to prevent LTC4 metabolism. Incubation of lung membranes with 5 mM AT-125 for 120 min at 25 degrees C resulted in greater than 98% recovery of LTC4. Unlike L-serine-borate complex, AT-125 failed to inhibit pulmonary [3H]LTD4 binding. These results suggest that AT-125 can be used in this study. Nonlinear least squares analysis of the results of LTD4/[3H]LTD4 or ICI-198,615/[3H]LTD4 competitive binding reveals that either LTD4 (Ki = 0.49 nM) or ICI-198,615 (Ki = 6.89 nM) interacts at a single homogenous population of [3H]LTD4 binding sites. However, the data of competitive binding results of LTC4 (in the presence of AT-125) or LTE4 are best fitted for its interaction with high- and low-affinity [3H]LTD4 binding sites, designated as LTD4 alpha and LTD4 beta sites, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)