We have reported previously that hemoglobin inhibits endothelium-dependent and glyceryl trinitrate-induced relaxation in the rabbit aorta. In this study we have examined 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 hemoglobin. Of the two ferrous hemoproteins tested, myoglobin (1-10 microM) abolished the endothelium-dependent relaxation induced by acetylcholine and produced a concentration-dependent reduction in the endothelium-independent relaxation induced by glyceryl trinitrate, in a manner similar to that reported previously for hemoglobin, but reduced cytochrome C was completely ineffective. The ferric hemoproteins methemoglobin (10 microM) and metmyoglobin (40 microM) produced only a slight inhibition of acetylcholine-induced relaxation. Methemoglobin (10 microM) also blocked only slightly the endothelium-dependent relaxation induced by the ionophore A23187 and had no effect on glyceryl trinitrate-induced relaxation. The inhibitory effects of these hemoproteins were reflected in their respective effects on the stimulation of cyclic GMP levels; thus, myoglobin (10 microM) inhibited the endothelium-dependent rise in cyclic GMP content induced by acetylcholine, as was found previously for hemoglobin, but methemoglobin (10 microM) was much less effective. The effectiveness of hemoglobin 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.(ABSTRACT TRUNCATED AT 250 WORDS)