Arterial Blood Pressure Responses to Cell-free Hemoglobin Solutions and the Reaction with Nitric Oxide

Abstract

Changes in mean arterial pressure were monitored in rats following 50% isovolemic exchange transfusion with solutions of chemically modified hemoglobins. Blood pressure responses fall into three categories: 1) an immediate and sustained increase, 2) an immediate yet transient increase, or 3) no significant change either during or subsequent to exchange transfusion. The reactivities of these hemoglobins with nitric monoxide ( ⅐ NO) were measured to test the hypothesis that different blood pressure responses to these solutions result from differences in ⅐ NO scavenging reactions. All hemoglobins studied exhibited a value of 30 M ؊1 s ؊1 for both ⅐ NO bimolecular association rate constants and the rate constants for ⅐ NO-induced oxidation in vitro. Only the ⅐ NO dissociation rate constants and, thus, the equilibrium dissociation constants varied. Values of equilibrium dissociation constants ranged from 2 to 14 pM and varied inversely with vasopressor response. Hemoglobin solutions that exhibited either transient or no significant increase in blood pressure showed tighter ⅐ NO binding affinities than hemoglobin solutions that exhibited sustained increases. These results suggest that blood pressure increases observed upon exchange transfusion with cell-free hemoglobin solutions can not be the result of ⅐ NO scavenging reactions at the heme, but rather must be due to alternative physiologic mechanisms.Control of blood pressure and resistance to blood flow is achieved by a dynamic constriction and relaxation of smooth muscle tissue which surrounds all blood vessels except capillaries. Vascular smooth muscle tension is continually adjusted by a complex system that causes either vasoconstriction or vasodilation, depending on metabolic need (1). Research performed over the last decade has established that endotheliumderived nitric oxide ( ⅐ NO) 1 can cause vasodilation. ⅐ NO is produced by endothelial cells that lie between the intravascular space and the surrounding smooth muscle. Among the findings was the demonstration that ⅐ NO donors (e.g. nitroprusside, nitroglycerin) lead to vasorelaxation through activation of guanylate cyclase, whereas inhibitors of ⅐ NO synthesis (e.g. N Gmonomethyl-L-arginine) or scavengers (e.g. hemoglobin) cause vasoconstriction (for reviews, see Refs. 2 and 3).Since cell-free hemoglobin is being developed as a red cell substitute (4), reactions between hemoglobin and ⅐ NO are of potential importance in maintenance of microvascular blood flow and O 2 delivery. Despite the wide variation that exists in the physical properties (O 2 affinity, molecular mass, and solution properties) of different cell-free hemoglobins, it appears vasoconstriction is a feature common to many hemoglobin solutions (for reviews, see Refs. 2, 3, and 5). It is tempting to conclude that ⅐ NO scavenging is the principal, if not sole mechanism for vasoconstriction associated with cell-free hemoglobin. However, it is well established that multiple factors contribute to the physiological control of vascular smooth muscle to...

Allosteric effects on oxidative and nitrosative reactions of cell‐free hemoglobins

“…NO-scavenging by reaction of NO with oxy Hb is essentially instantaneous under physiological conditions, although differences in the rate constant for this process have been documented among HBOCs (39). Hbs within red blood cells show much less NO-scavenging than cell-free Hbs (12), and next generation HBOCs may simulate this packaging effect.…”

Encapsulation of Concentrated Hemoglobin Solution in Phospholipid Vesicles Retards the Reaction with NO, but Not CO, by Intracellular Diffusion Barrier

“…In this experiment, we added no allosteric effector, such as an organic phosphate, because we intended to compare the Hb solution and HbV with different [Hb] in at the same O 2 affinity (P 50 , oxygen partial pressure at which Hb is half-saturated). A linear relationship exists between P 50 and NO affinity of Hb (38). In the present study, P 50 was regulated solely using Cl Ϫ and phosphate of PBS (35).…”

Control of Nitric Oxide Dynamics by Guanylate Cyclase in Its Activated State

“…An in vivo study monitoring arterial pressure in rat (38) suggests that the blood pressure increase following transfusion of hemoglobin solution is not due to NO scavenging. Furthermore, the deactivation occurs faster in whole cell (13) or in cytosol (14) than it does for the isolated sGC when free NO is trapped by HbO 2 .…”