Mechanism of action of platelet-activating factor on guinea-pig lung parenchyma strips

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Abstract
The contribution of thromboxane A2 to platelet-activating factor (PAF)induced contraction of guinea-pig lung parenchyma strips (GPLPS) was investigated using an experimental design that allowed us to record the contractions of the tissues in parallel with the determination of thromboxane B2 (TXB2) levels in the organ baths by enzyme immunoassay. It was found that the first injection of PAF induced the contraction of GPLPS and the release of TXB2. Following subsequent additions of PAF to the same tissue, the contractile response was abolished but TXB2 levels were not significantly reduced. Pretreatment of the tissue with the thromboxane synthetase inhibitor OKY-046 (3.5, 170, and 350 μM) strongly inhibited the release of TXB2 but had no effect on the contraction of the tissues induced by PAF. The mechanism of PAF-induced contraction of GPLPS was further investigated using several drugs that interfere with arachidonic acid metabolism. It was found that pretreatment of the tissues with the cyclooxygenase and thromboxane synthetase inhibitors indomethacin (2.8, 28, and 56 μM) and OKY-046 (170 μM) or with the thromboxane antagonist SKF-88046 (1.25 and 12.5 μM) had no significant effect on the contractile response to PAF. The compound L-655,240 (2.5, 25, and 50 μM), which acts simultaneously as an antagonist of thromboxane and inhibitor of lipoxygenase, significantly reduced GPLPS contractions induced by PAF. Another lipoxygenase inhibitor, nordihydroguaiaretic acid (33 μM), and the inhibitor of both pathways of arachidonic acid metabolism, BW775c (110 μM), both reduced PAF-induced contractions of GPLPS. We conclude that although PAF induces release of thromboxane from GPLPS, this mediator does not contribute significantly to the myotropic activity of PAF, which seems to be mediated by products of the lipoxygenase pathway.