The mechanisms of lung fibrosis caused by bleomycin (BL) and other fibrogenic agents are not clearly understood. Our previous studies demonstrated that the platelet-activating factor (PAF) antagonist WEB2086 reduced lung fibrosis induced by BL and amiodarone in hamsters, suggesting a critical role for PAF and/or PAF receptors in this pathogenic process. In the present study, the PAF receptors in the lung and the functional activity of PAF receptors in the alveolar macrophages from BL (7.5 U/kg, intratracheally)-treated hamsters were investigated. The PAF receptor binding, measured by a [3H]WEB2086 binding assay in lung homogenates, was significantly increased at all times after BL treatment, compared with saline-treated control hamsters. At 3 days after BL treatment, the PAF receptor density (B(max) = 202.4 fmol/mg protein, with K(d) = 41 nM) was increased over control (B(max) = 116.9 fmol/mg protein, with K(d) = 45.3 nM). Most importantly, the functional activities of PAF receptors in alveolar macrophages, as determined by PAF-induced elevation of cytosolic Ca++ (both by mobilization of Ca++ stores and by Ca++ influx), were significantly higher in the BL-treated animals than in the saline control. The EC50 of PAF to increase internal Ca++ release was 5-fold less in BL-treated lungs than in control. The Ca++ signaling could not be stimulated by lyso-PAF (inactive PAF) but was inhibited by the PAF antagonists WEB2086 (at 100 nM) and L659,989, in a dose-dependent fashion, suggesting the involvement of specific receptors for PAF. The cells from BL-treated hamster lung required much higher concentrations of the antagonists, with increases in the IC50 values of 14-fold for WEB2086 and 63-fold for L659,989 over control. These results indicated that PAF receptors were functionally up-regulated in the lungs after BL treatment in vivo, and this may be an important mechanism, at least in part, for BL-induced lung injury. These findings also explain the antifibrotic effect of the PAF receptor antagonist WEB2086 in the BL-hamster model of lung fibrosis, as reported in our earlier paper.