Involvement of platelet cyclic GMP but not cyclic AMP suppression in leukocyte‐dependent platelet adhesion to endothelial cells induced by platelet‐activating factor in vitro

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Abstract
1 Incubation of endothelial cells with platelets in the absence or the presence of PAF (10 μm) markedly increased platelet cyclic AMP levels, which were significantly decreased by indomethacin (3 μm). Co-incubation of endothelial cells and platelets with polymorphonuclear leukocytes (PMNs) did not change the platelet cyclic AMP levels. 2 Incubation of endothelial cells with platelets in the absence of PAF increased platelet cyclic GMP levels, which were increased 3.5 fold by PAF. These cyclic GMP levels were significantly decreased by NG-nitro-L-arginine (100 μm), and completely by methylene blue (10 μm). When endothelial cells and platelets were co-incubated with PMNs, the cyclic GMP level in the cell mixture was 42.5 and 65.3% lower than that in endothelial cells and platelets without and with PAF stimulation, respectively. 3 PAF induced platelet adhesion to endothelial cells only when PMNs were present. Methylene blue dose-dependently potentiated the PMN-dependent platelet adhesion induced by PAF, although it had no effect in the absence of PMNs. 4 Sodium nitroprusside and 8-bromo cyclic GMP but not dibutyryl cyclic AMP significantly, although partially, inhibited the platelet adhesion. Inhibition of cyclic GMP-specific phosphodiesterase by zaprinast slightly inhibited the PMN-induced platelet adhesion and potentiated the inhibitory effect of 8-bromo cyclic GMP, while these drugs markedly inhibited the adhesion of platelet aggregates induced by PMN sonicates. 5 These results suggest that the impairment by activated PMNs of EDRF-induced platelet cyclic GMP formation is involved in part in the mechanism of PMN-dependent platelet adhesion to endothelial cells induced by PAF in vitro. The precise mechanism still remains to be clarified.