Binding of radiolabeled high affinity antagonist to leukotriene D4 receptor in guinea pig lung membranes: interconversion of agonist-receptor binding affinity states

Abstract

The kinetic parameters and the pharmacological specificity of a high affinity leukotriene D4 (LTD4) receptor antagonist, ICI-198615, binding to guinea pig lung membranes were characterized. Binding of [3H]ICI-198615 to the membranes was rapid and displaceable with excess ICI-198615. The specific binding of [3H] ICI-198615 was dependent upon the concentration of membrane protein. Monovalent cations (Na+, Li+, and Cs+), divalent cations (Mg2+, Ca2+, and Mn2+), and guanine nucleotides did not significantly affect the specific binding of [3H]ICI-198615 to guinea pig lung LTD4 receptors. The specific binding of [3H]ICI-198615 to guinea pig lung membranes was saturable and the equilibrium saturation binding was best approximated by a single-site model. The dissociation constant (KD) and the density (Bmax) were 0.08 +/- 0.04 nM and 1030 +/- 180 fmol/mg, respectively. In competition studies, LTD4, stereoisomer (5R,6S)-LTD4, and leukotriene E4 (LTE4) competed with [3H]ICI-198615 binding to specific sites, with a stereoselectivity and a rank order of potency equivalent to those described in [3H]LTD4 binding studies and in functional studies. LTD4 and LTE4 displaced maximally 70 and 40%, respectively, of the [3H]ICI-198615 specific binding component defined by ICI-198615. Several LTD4 receptor antagonists (ICI-198615, WY-48252, WY-49511, FPL-55712, and LY-171883) displaced [3H]ICI-198615 specific binding, with a rank order of potency equivalent to that described in the guinea pig tracheal smooth muscle contraction system. The leukotriene structure-like receptor antagonists, e.g., SK&F 104353 and SK&F 104373, also competed with the [3H]ICI-198615 specific binding, with binding affinities comparable to those expected from the functional studies. However, SK&F 104353 and SK&F 104373 displaced maximally 70% of the specific binding component of [3H]ICI-198615, equivalent to that displaced by LTD4. Guanosine-5'-3-thiotriphosphate (GTP gamma S), EDTA, and Na+ shifted the LTD4 displacement curve to the right, indicating that these agents regulated the binding of LTD4 to the receptor. In the absence of GTP gamma S or cations, the LTD4 displacement curve was heterogeneous. The LTD4 displacement curve was resolved into a higher affinity component (KDH = 0.5 +/- 0.2 nM; percentage of receptor density at high affinity = 24 +/- 3%) and a low affinity component (KDL = 60 +/- 7 nM; percentage of receptor density at low affinity = 76 +/- 3%).(ABSTRACT TRUNCATED AT 400 WORDS)