High affinity receptor binding of platelet-activating factor in rat peritoneal polymorphonuclear leukocytes

Abstract

Rat peritoneal polymorphonuclear leukocytes (PMNs) showed a single class of high affinity binding sites for platelet-activating factor (PAF) with an equilibrium dissociation constant (K D) of 4.74 (±2.59) nM. Each cell contained 2.79 (±1.40) × l0 4 receptors. In the isolated membranes at pH 7.0 in 10 mM MgCl 2, 10 mM Tris and 0.25% bovine serum albumin, the K D value was 0.61(±0.1) nM, which is roughly identical to the K D values reported previously for various membrane systems under identical ionic conditions. The receptors were highly specific for PAF. Several receptor antagonists that are reported to inhibit the binding of [ 3H]PAF and the PAF-induced function in platelets could fully displace the binding. The biologically inactive enantiomer (enantio-C 16-PAF), a PAF analog, azido-PAF, and an indene derivative of the PAF receptor antagonist, L-651,142, had different potencies to inhibit [ 3H]PAF binding to rat and human PMN membranes. L-652,731, a tetrahydrofuran analog of the PAF receptor antagonist was about 10 times more potent to inhibit the binding in rat liver tissues than in rat PMNs. These results suggest that PAF receptors on human and rat PMNs are not identical and that PAF receptor subtypes may exist in rat liver and PMNs.