SOLUBLE SACCHARIDES BLOCK THE INHIBITION OF AGONIST-INDUCED HUMAN PLATELET-AGGREGATION OBSERVED AFTER IN-VITRO INCUBATION OF HUMAN PLATELET-RICH PLASMA WITH PORCINE AORTIC ENDOTHELIAL-CELLS

Platelet aggregation is a prominent feature in the hyperacute process of vascularized allografts and xenografts. In a study of extracorporea l connection of pig kidneys to the blood circulation of human voluntee rs, we observed in one case considerable destruction of human platelet s in the pig kidney without signs of hyperacute rejection or microthro mbi formation. In the present study, we have investigated the agonist- induced aggregation of human platelets in mixtures with porcine aortic endothelial cells (PAEC). In vitro incubation of human platelet-rich plasma (PRP) with PAEC inhibited platelet aggregation induced by ADP, collagen and arachidonic acid in a time-dependent manner and partially inhibited adrenalin-induced aggregation. Aggregation of the human pla telets could not be induced by high concentrations of ADP (20 mu M) to overcome the inhibition capacity of the PAEC. The PAEC inhibiting eff ect could be transferred by the supernatants of PAEC/PRP and PAEC/PPP incubation mixtures. Preincubation of the PAEC with aspirin, but not w ith N-G-methyl-L-Arg, reduced the aggregation inhibitory effect. Contr ol experiments mixing human umbilical vein endothelial cells (HUVEC) a nd human PRP or mixing porcine PRP and PAEC did not elicit any inhibit ion of ADP-induced platelet aggregation. The aggregation inhibition ef fect could partially be blocked by preincubation of PRP with soluble G al alpha 1-3Gal, Gal alpha 1-3 beta 1-4GlcNAc, lactose, galactose, and glucose, but not by lactosamine,galactosamine, or glucosamine. The Ga l alpha 1-3Gal disaccharide was most effective in blocking aggregation inhibition, and to a similar extent as its ability to block the human anti-pig lymphocytotoxicity reaction. In conclusion, the data indicat e that PAEC, upon stimulation by human anti-pig xenoantibodies in a no ndynamic system, inhibits agonist-induced human platelet aggregation, and that this effect is probably at least partially caused by prostacy clin released from the PAEC.