Pharmacological characterization of a receptor for platelet-activating factor on guinea pig peritoneal macrophages using [3H]apafant, a selective and competitive platelet-activating factor antagonist: evidence that the noncompetitive behavior of apafant in functional studies relates to slow kinetics of dissociation.

In this paper we report the characterization of a receptor for platelet-activating factor (PAF) on guinea pig peritoneal macrophages, using a radiolabeled hydrophilic PAF antagonist, [3H] apafant. [3H]Apafant bound to intact macrophages in a concentration-dependent manner that was specific, saturable, reversible, and inhibited competitively by C18-PAF (1-O-octadecyl-2-O-acetyl-sn-glyceryl- 3-phosphocholine). Scatchard transformation and Hill analysis of these data revealed that [3H]apafant identified a homogeneous population of noninteracting sites with a pKd of 8.22 nM and a Bmax of 31,600 sites/cell. The rate at which [3H]apafant associated with (Kon = 2.9 x 10(6) M-1.min-1) and dissociated from (Koff = 0.043 min-1) intact macrophages was slow, with t1/2 values of 15 and 50 min, respectively; the kinetically derived pKd was 8.3. In competition studies C18-PAF inhibited in a biphasic manner the binding of [3H]apafant to intact macrophages, which could be resolved into high (pKi = 8.27; 60%) and low (pKi = 6.06; 40%) affinity components. In macrophage membranes, the affinity of C18-PAF (pKi = 8.48) determined from competition studies with [3H]apafant was significantly reduced (pKi = 6.95) by guanosine-5'-O-(3-thio)triphosphate, whereas the mean slope of the inhibition curves was increased from 0.470 to 0.700. Functionally, C18-PAF (10 nM to 10 microM) evoked concentration-dependent .O2- generation that was biphasic in nature. Pretreatment of macrophages with apafant antagonized in a noncompetitive manner the first phase of C18-PAF (< 100 nM)-induced respiratory burst, whereas the second component (> 1 microM C18-PAF) of this response was unaffected. It is concluded that guinea pig peritoneal macrophages express receptors for PAF for which apafant has high affinity. The biphasic competition curves obtained with C18-PAF in binding experiments and the effect of guanosine-5'-O-(3-thio)triphosphate are consistent with the hypothesis that these apafant-sensitive PAF receptors are coupled to guanine nucleotide-binding proteins and can exist in at least two guanine nucleotide-regulated conformational states. It is also suggested that the noncompetitive antagonism of PAF-induced .O2- generation by apafant may be a consequence of the slow rate at which this antagonist dissociates from PAF receptors on intact macrophages.