Recombinant human hemoglobin inhibits both constitutive and cytokine-induced nitric oxide-mediated relaxation of rabbit isolated aortic rings

A genetically engineered recombinant human hemoglobin (rHb1.1) was recently developed for use as a blood substitute (Nature 1992;356:258-60). Like other mammalian hemoglobin (Hb) molecules, it might bind and antagonize the actions of nitric oxide (NO). We used an isolated rabbit aortic ring preparation to examine the ability of rHb1.1 to inhibit acetylcholine (ACh)- and interleukin-1 beta (IL-1 beta)-induced reductions of vasoconstrictor responses to the alpha-adrenoceptor agonist phenylephrine (PE). rHb1.1 (0.04-4.4 microM) rapidly and reversibly inhibited, in a concentration-dependent manner, both ACh- and IL-1 beta-induced decreases in PE contractile responses. These inhibitory effects of rHb1.1 were non-competitive and were equipotent to those of purified, cell-free human Hb (p.hHb). These two forms of soluble Hb were at least 10 times more potent than Hb in erythrocytes (red blood cells: RBC-Hb). Both NG-nitro-L-arginine (10 microM) a NO synthase inhibitor, and LY-83583 (10 microM), a guanylyl cyclase inhibitor, mimicked the effects of rHb1.1. The inhibitory effects of rHb1.1 were not shared by either human serum albumin (HSA 44 microM), which combines with but does not deactivate NO, or cytochrome C (44 microM), a heme-containing protein that does not bind NO; neither were they reversed by L-arginine (L-ARG) (1 mM), the presumed NO precursor. These and other results suggest that the chemical antagonism of NO is likely to be the mechanism by which rHb1.1 and other Hbs inhibit ACh- and IL-1 beta-induced decreases in the response to PE in rabbit aortic rings.