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This study was designed to examine further the role of platelet-activating factor (PAF) in asthma, comparing leukotriene B4 (LTB4) release, 5-lipoxygenase activity and intracellular calcium levels ([Ca2+]i) in macrophages. LTB4 and other lipoxygenase metabolites in macrophages in bronchoalveolar lavage fluids obtained from 23 asthmatic patients and 20 control subjects were measured by reverse-phase high-performance liquid chromatography. [Ca2+]i was monitored using the fluorescent probe fura-2. The basal LTB4 release of resting macrophages was not different between groups (0.02+/-0.01 versus 0.05+/-0.02 ng x 10(-6) cells). When stimulated with calcium ionophore A23187 (2.5 microM), however, macrophages from asthmatic patients released more LTB4 than cells from control subjects (30.2+/-3.4 versus 13.7+/-2.1 ng x 10(-6) cells). Although PAF alone did not alter LTB4 release, it enhanced the response to subsequent A23187 stimulation. This effect was noted following short treatment (i.e., 5 min) at concentrations of &gt; or =1.0 microM PAF, with the maximal effect noted after treatment with 5.0 microM PAF + 2.5 microM A23187 (105.1+/-6.7 versus 15.3+/-2.6 ng x 10(-6) cells). Treatment of macrophages with PAF also increased 5-lipoxygenase activity and [Ca2+]i more in cytosols from asthmatic patients than in cytosols from control subjects. These findings support a role of intracellular calcium in the activation of 5-lipoxygenase which, in turn, augments the release of leukotriene B4. Because levels of platelet-activating factor may be increased in the lung during asthma and can increase the subsequent release of a chemotactic mediator leukotriene B4, from macrophages, these findings suggest that platelet-activating factor may prime the constitutive cells of the lung to augment inflammatory effects important in the pathogenesis of asthma.

Platelet-activating factor increases leukotriene B4 release in stimulated alveolar macrophages from asthmatic patients