Protamine releases endothelium-derived relaxing factor from systemic arteries: A possible mechanism of hypotension during heparin neutralization

Background. When used to reverse the anticoagulant effect of heparin, protamine sulfate often causes vasodilation that can lead to systemic hypotension. Protamine is rich in the basic amino acid arginine, which is the precursor of endothelial cell synthesis of nitric oxide, and nitric oxide is the active component of endothelium-derived relaxing factor (EDRF). Methods and Results. To determine whether the hypotensive effect of protamine could be due to stimulated release of EDRF, we studied rings (4-5 mm) of canine coronary, femoral, and renal artery suspended in organ chambers containing physiological salt solution (37°C and 95% O₂-5% CO₂). Arterial rings with and without endothelium were contracted with prostaglandin F_2α (2×10^-6 M) and exposed to increasing concentrations of protamine (final organ bath concentration, 40-400 μg/ml). In arterial segments without endothelium, protamine caused only a modest decrease in tension. However, protamine induced concentration-dependent relaxation in all arterial segments with endothelium, which was significantly greater than in segments without endothelium (p<0.05). The endothelium-dependent relaxation induced by protamine was inhibited by N^G-monomethyl-L-arginine (L-NMMA) (10^- M), but L-NMMA had no effect on rings without endothelium. The action of L-NMMA could be reversed by L-arginine (10^-4 M) but not D-arginine (10^-4 M). Conclusions. This study demonstrates that protamine stimulates the release of EDRF from arterial endothelium, and that endothelium-dependent vasodilation may be an important cause of systemic hypotension during protamine infusion.