Platelet activating factors alters calcium homeostasis in cultured vascular endothelial cells.

Platelet activating factor (1-O-alkyl-2-acetyl-sn-glycerol-3-phosphorylcholine; PAF), a potent in vivo mediator of allergic and inflammatory reactions, induced a rapid (onset less than 30 s), concentration-dependent (threshold approximately 10(-11) M, half-maximal approximately 10(-10) M, maximal approximately 10(-8)-10(-7) M) efflux of 45Ca2+ from preloaded cultured bovine aortic endothelial cells (BAEC). In contrast, deacetylated and other PAF analogues were essentially ineffective. PAF (10(-7) M) was also shown to increase cytosolic free calcium (49 +/- 5%) in suspensions of quin 2 (calcium-sensitive fluorescent dye)-loaded BAEC. PAF-stimulated 45Ca2+ efflux was not blocked by aspirin treatment (100 or 500 microM, 30 min). In the absence of external calcium, PAF was still highly effective in stimulating unidirectional 45Ca2+ efflux, thus suggesting that PAF mobilized a sequestered pool of intracellular calcium. CV-3988, a PAF antagonist, inhibited PAF-stimulated 45Ca2+ efflux in a dose-dependent manner. Pretreatment of BAEC with PAF (10(-8) M, 15 min), but not with other PAF analogues, resulted in a decrease in subsequent PAF-stimulated 45Ca2+ efflux, thus suggesting an agonist-specific desensitization. PAF also stimulated a 30% net decrease in the equilibrium 45Ca2+ content of BAEC within 1 min, which gradually recovered to prestimulus levels in 10-15 min. PAF-stimulated 45Ca2+ efflux was also observed in endothelial cells cultured from human umbilical vein and baboon cephalic vein but not from cultured human dermal fibroblasts or bovine aortic smooth muscle. These studies provide direct evidence for agonist- and cell-specific effects of PAF on vascular endothelium.