HEPARIN AND PROTAMINE STIMULATE THE PRODUCTION OF NITRIC-OXIDE

Heparin has been shown to decrease total vascular resistance while pro tamine stimulates endothelium-dependent vasodilation. This study was u ndertaken to determine whether heparin and/or protamine could enhance endothelium-derived relaxing factor (EDRF), as determined by nitric ox ide (NO) production. Porcine carotid artery endothelial cells (PAECs) were seeded on multiwell plates, grown to confluence, and exposed to h eparin (1-20 U/ml) or protamine (50-200 mu g/ml) for 24 hours. With th e addition of the NO synthase inhibitor, N-G-monomethyl-L-arginine (NM MA), to heparin and/or protamine, the medium samples were collected in one hour. In a parallel clinical study, plasma samples were collected from patients undergoing cardiopulmonary bypass (CPB). The NO product ion was measured as reflected by the formation of nitrite (NO2-) and n itrate (NO3-), the stable end-metabolites of NO. NO production by PAEC s was significantly increased by heparin greater than or equal to 5 U/ ml or protamine greater than or equal to 50 mu g/ml in a concentration -dependent manner. The increase of NO production was prevented by the addition of NMMA. In CPB patients, plasma NO2-/NO3- concentration was significantly increased after heparin administration compared to the p reoperative value, at which time the mean plasma heparin level was 4.9 +/-0.5 U/ml. Following slow protamine infusion, there was no significa nt difference in plasma NO2-/NO3- concentration compared to preoperati ve value. In conclusion NO production increases following exposure of PAECs to heparin and/or protamine. In patients, NO concentration signi ficantly increased after heparin administration by IV bolus, but not w ith a slow infusion of protamine after CPB.