EFFECTS OF HEPARIN ON THE INHIBITORY-ACTION OF PROTAMINE ON ENDOTHELIUM-MEDIATED VASORELAXATION

The precise mechamism(s) of inhibitory action of protamine on endothelium-mediated vasorelaxation has not been fully elucidated. In addition, no information is available regarding the effects of a heparin-protamine complex on the endothelium-mediated relaxation. Employing isometric tension recording methods, we studied the effects of heparin, an anionic substance, on the protamine-induced inhibition of acetylcholine (ACh)-induced vasorelaxation in isolated rabbit small mesenteric artery. Protamine (greater than or equal to 50 mu g/ml) inhibited ACh (0.03-10 mu M)-induced relaxation under a norepinephrine (10 mu M)-stimulated condition (P<0.05). The ACh relaxation, even 20 min after washout of protamine (150 mu g/ml), was still significantly inhibited as compared to the control (before protamine) ACh relaxation, and further, it was not significantly different from the ACh relaxation maximally inhibited in the presence of protamine. Preapplication of heparin (700 U/ml) almost abolished the protamine inhibition (50 and 150 mu g/ml) of the ACh relaxation. However, heparin (700 U/ml), applied on washout of protamine (150 mu g/ml), had no effect on the prolonged protamine inhibition. In conclusion, a heparin-protamine complex had no direct effect on the endothelium-mediated relaxation, and the inhibitory action of protamine on the endothelium-mediated relaxation might be due to its polycationic property. The prolongation of protamine inhibition and the lack of effects of heparin on the prolonged protamine inhibition may suggest a toxic effect of protamine on the endothelium.