Non-receptor-mediated refractoriness in prostacyclin production by human endothelial cells in a continuous flow system is delayed by a low-molecular-weight plasma fraction devoid of reducing cofactors for peroxidase-catalysed reactions.

Using a microcarrier perfusion model, we have investigated the effects of a low-molecular-weight plasma fraction (mol. wt. less than 10,000) on the capacity of human umbilical vein endothelial cells (HUVECs) to synthesize and release prostacyclin (PGI2) during repeated stimulation with either calcium ionophore A23187 or arachidonic acid (AA). This plasma fraction (filtrate 10,000), which has recently been shown to contain most of the activity previously referred to as 'prostacyclin regulating plasma factor', delayed the exhaustion of the PGI2 generating capacity of the HUVECs during repeated stimulation with either inducer. No such effect was seen with whole plasma. The PGI2 regulatory activity of the filtrate was not significantly affected by removal of reducing cofactors for peroxidase-catalysed reduction of hydroperoxides, accumulation of which inhibits cyclooxygenase. In contrast, albumin added to the filtrate abolished its protective effect on endothelial PGI2 synthesis. We conclude that low-molecular-weight plasma constituents partially preserve the PGI2-generating capacity of human vascular endothelium during repeated stimulation. Under continuous flow conditions, as applied in our study, this effect appears not to be related to reducing cofactor activity for peroxidase-catalysed reactions. In whole plasma, this PGI2 regulatory activity may be partially masked by the presence of plasma proteins.