BLOCKADE OF ENDOTHELIUM-DEPENDENT AND GLYCERYL TRINITRATE-INDUCED RELAXATION OF RABBIT AORTA BY CERTAIN FERROUS HEMOPROTEINS

Abstract

Hg inhibits endothelium-dependent and glyceryl trinitrate-induced relaxation in the rabbit aorta. The effects of other ferrous and ferric hemoproteins on endothelium-dependent and glyceryl trinitrate-induced relaxation to determine whether they also share the inhibitory properties of Hb. Of the 2 ferrous hemoproteins tested, myoglobin (1-10 .mu.M) abolished the endothelium-dependent relaxation induced by acetycholine and produced a concentration-dependent reduction in the endothelium-independent relaxation induced by glyceryl trinitrate, in a manner similar to that reported previously for Hb, but reduced cytochrome c was completely ineffective. The ferric hemoproteins methemoglobin (10 .mu.M) and metmyoglobin (40 .mu.M) produced only a slight inhibition of acetylcholine-induced relaxation. Methemoglobin (10 .mu.M) also blocked only slightly the endothelium-dependent relaxation induced by the ionophore A23187 and had no effect on glyceryl trinitrate-induced relaxation. The inhibitory effect of these hemoproteins were reflected in their respective effects on the stimulation of cGMP levels; thus, myoglobin (10 .mu.M) inhibited the endothelium-dependent rise in cGMP content induced by acetylcholine, as was found previously for Hb, but methemoglobin (10 .MU.M) was much less effective. The effectiveness of Hb and myoglobin and the ineffectiveness of reduced cytochrome c in blocking the relaxations induced by acetylcholine and glyceryl trinitrate might suggest that only ferrous hemoproteins with ligand binding sites are inhibitory. Hb and myoglobin are known to bind avidly, at their ligand binding sites, nitric oxide, the active principle released by glyceryl trinitrate, and it is possible that the endothelium-derived relaxing factor might also be inactivated by being bound to these 2 hemoproteins in a similar manner. The active hemoproteins apparently produce their effects by somehow inhibiting the stimulation of guanylate cyclase, possibly by interfering with the interaction of cyclase stimulants with an endogenous ferrous hemoprotein linked to the enzyme.