Binding sites for 3H-LTC 4 in membranes from guinea pig ileal longitudinal muscle

Abstract

Leukotriene (LTC 4) is one of the components of Slow Reacting Substance of Anaphylaxis (SRS-A) and is a potent constrictor of guinea pig ilea. The contraction is likely to be a receptor-mediated process. Here we report the existence of specific binding sites for 3H-LTC 4 in a crude membrane preparation from guinea pig ileal longitudinal muscle. At 4°C in the presence of 20 mM Serine-borate, binding increases linearly with protein concentration, reaches equilibrium in 10 minutes, and is reversible upon addition of 3 × 10 −5M unlabelled LTC 4. The dissociation curve is consistent with the existence of more than one class of binding site. Ca ++ and Mg ++ greatly enhance the binding of 3H-LTC 4 at equilibrium. In the presence of 5mM CaCl 2 and MgCl 2 not only LTC 4 (IC 50 10 −7M), but also LTD 4 (albeit with much lower affinity, IC 50 = 6 × 10 −5M) and the SRS-A antagonist FPL 55712 (IC 50 = 10 −5M) can compete with 3H-LTC 4 for its binding sites. FPL 55712 only displaces 60–70% of the total amount bound, while LTC 4 displaces 90–95%. These studies indicate that multiple classes of binding sites exist for 3H-LTC 4 in guinea pig ileal longitudinal muscle, and that at least part of these binding sites might be related to the ability of LTC 4 to contract guinea pig ilea.