Interactions of heparin with alpha-2-adrenoceptors

The interactions of the anticoagulant Heparin with the alpha-2-adrenoceptor in rat brain cortex membranes were investigated. Binding experiments with 3H-Clonidine were performed in both the absence and presence of Heparin. 1 uM Na-Heparin caused a significant decrease in the maximal number of binding sites (Bmax) from 129.4 fmol/mg protein to 93.7 fmol/mg protein with an associated decrease in affinity (KD = 0.79 pM vs. KD = 1.53 pM) of these binding sites. Addition of Na+-Heparin to 3H-Clonidine (3.1 nM) labelled membranes inhibited 50% of specific 3H-Clonidine binding (IC50) at a concentration of 0.95 uM. Based on our findings we conclude that the simultaneous long term administration of Na-Heparin and the antihypertensive agonist Clonidine should be regarded under consideration of the inhibitory effect of Na-Heparin to the alpha-2-adrenoceptors.