Species differences in PAF receptor binding in the lungs between hamster and guinea pig

Platelet-activating factor (PAF) receptor in normal Golden Syrian hamster lung was characterized using radioligand binding studies and compared with guinea pig lung PAF receptor. [³H]WEB2086, a potent and specific PAF antagonist, was used as a radioligand for equilibrium binding, kinetic studies, competitive binding in receptor preparation (0–110 000 g fraction of lung homogenate) from hamster and guinea pig lungs. Binding of [³H]WEB 2086 to the receptor preparation was saturable, reversible and specific in both hamster and guinea pig lungs. Scatchard plot analysis of equilibrium binding data indicates a single binding site in hamster lung with the equilibrium dissociation constant (K_D) of 66.1 ± 36.7 nM (n = 4) and maximal binding (B_max) of 135.4 ± 63.1 fmol/mg, but two binding sites in guinea pig lung with a high affinity site (K_D = 1.7 ± 0.6 nM; B_max = 48.6 ± 2.6 fmol/mg) and a low affinity site (K_D = 83.8 ± 32 nM; B_max = 480.8 ± 158 fmol/mg). The heterogeneity of [³H]WEB2086 binding to guinea pig lung but not to hamster lung was also confirmed by dissociation kinetic studies, in which biphasic dissociation kinetic was shown in guinea pig and monophasic kinetic in hamster lung. Although the specific [³H]WEB 2086 binding to lungs of both species was displaced by PAF-C18 and antagonists L659989 and CL 184005 in a dose-dependent manner and not by lyso-PAF (a biologically inactive form of PAF), the potencies of the competitive inhibition were significantly different between the two species. The relative potencies ranked WEB2086 ≈ L659989 > PAF > CL184005 in hamster lung, whereas in guinea pig lung the potencies ranked PAF > WEB2086 ≈ L659 989 ≈ CL184005. The present study demonstrates for the first time the existence of PAF receptor in the hamster lung and its binding characteristics different from guinea pig lung suggest the possible existence of different PAF receptor subtypes in hamster lung.