Transgenic Mice Used to Test Nitric Oxide as Therapy for Sickle Cell Disease. Martinez-Ruiz et al. (page 1113)

Although carbon monoxide and cyanate have been used to increase the affinity of sickle hemoglobin for oxygen (thus reducing sickling) in vitro , the therapy is too toxic to be used in vivo . Because low-dose inhaled nitric oxide (NO) has been shown to increase the oxygen affinity of sickle hemoglobin erythrocytes in vitro  and in vivo , Martinez-Ruiz et al.  studied the effects of NO breathing at various doses and time regimens in the presence of severe hypoxia (6% oxygen) using the transgenic SAD mouse model.

Fifty-nine SAD mice were divided into two groups and exposed to either hypoxia or no hypoxia plus NO gas (in doses from 9 to 60 parts per million [ppm]). Their survival was recorded for up to 1 h or until death. Tail blood samples were collected before and after hypoxia to study erythrocyte structure. The percent of erythrocytes that were sickled, deformed, or normal were compared between prehypoxic and posthypoxic samples with and without NO exposure. Oxygen hemoglobin dissociation curves were determined in an additional five mice before and after 60 minutes’ exposure to 20 ppm NO breathing at 21% oxygen.

The SAD mice had improved survival rates when they breathed 20 ppm NO gas in air for 30 min before and during severe hypoxic gas exposure. The beneficial, protective effects of NO seemed to be rapid and dose dependent. Pretreatment alone or breathing lower doses of NO were not protective. Changes in hemoglobin SAD oxygen affinity were not detected with NO breathing, and methemoglobinemia levels were low in all mice. Multiple mechanisms and sites, such as hemoglobin interactions and vascular and blood cell components, may be involved in the beneficial effects of inhaled NO. Further studies may delineate and define mechanisms and illuminate the possible role of NO as a therapeutic intervention for sickle cell disease.