Abstract

Pulmonary hypertension is increasingly recognized in sickle cell disease (SCD) as a strong risk factor for early mortality. The finding of pulmonary hypertension in other hemolytic anemias suggests that the mechanism is linked to hemolysis and/or thrombosis. Pathophysiologic roles of nitric oxide (NO) consumption and recurrent lung injury have been considered. Transgenic mice expressing exclusively human sickle hemoglobin (sickle mice)(Pastzy 1997) are well established models of severe hemolytic anemia and ischemic organ damage in SCD, and provide the opportunity to examine mechanisms of pulmonary hypertension with invasive studies.

Hypotheses: Pulmonary hypertension will spontaneously occur in sickle mice but not age-matched colony controls, and severity will increase as the mice grow older.

Methods: Male sickle mice were compared with age-matched hemizygotes from the same colony. Mice had cardiac catheterization for baseline hemodynamics, then challenges to assess pulmonary vascular responsiveness. A pathologist made blinded assessments of the pulmonary histology.

Results: Cardiac catheterization showed pulmonary hypertension in all sickle mice, and blunted pulmonary vasodilation to all NO donor compounds as well as authentic NO gas. Computed tomography in vivo detected pulmonary vascular congestion. Older sickle mice had modestly increased vessel wall thickness and vascular congestion but no thrombi by histology. Older mice also appear to be in right heart failure. Sickle mouse lungs had decreased eNOS activity (measured by L-arginine to citrulline turnover) and loss of active eNOS dimer (measured by western blotting). Sickle mouse plasma had high NO consumption, consistent with increased NO scavenging by free hemoglobin released by steady state hemolysis.

mean & SDhemizygote control (5 mo & 13 mo)5 mo sickle13 mo sickle
Pulmonary Arterial Pressure (torr) 9.4 (0.7) 18.2 (0.5) 14.8 (0.3) 
Pulmonary Vascular Resistance 0.37 (0.6) 0.80 (0.07) 0.75 (0.04) 
Cardiac Output (ml/min) 14.2 (2) 17.1 (2) 12.2 (2) 
Vasodilation to NO & NO donors, or bradykinin (endothelium-dependent) normal blunted none 
Vasodilation to CGRP (NO-independent and endothelium-independent) normal normal blunted 
Hypoxic vasoconstriction (10%O2) normal enhanced enhanced 
mean & SDhemizygote control (5 mo & 13 mo)5 mo sickle13 mo sickle
Pulmonary Arterial Pressure (torr) 9.4 (0.7) 18.2 (0.5) 14.8 (0.3) 
Pulmonary Vascular Resistance 0.37 (0.6) 0.80 (0.07) 0.75 (0.04) 
Cardiac Output (ml/min) 14.2 (2) 17.1 (2) 12.2 (2) 
Vasodilation to NO & NO donors, or bradykinin (endothelium-dependent) normal blunted none 
Vasodilation to CGRP (NO-independent and endothelium-independent) normal normal blunted 
Hypoxic vasoconstriction (10%O2) normal enhanced enhanced 

Discussion: This is one of the few descriptions of spontaneous pulmonary hypertension in an animal, and implicates low NO bioavailability mediated by NO resistance/scavenging. Interestingly, pulmonary thromboembolism was not observed. Combined effects of NO scavenging and the loss of active eNOS dimer may explain paradoxical blunted responses to NO donor agents, blunted responses to NO synthase inhibition, and arginine supplementation observed in patients with SCD, despite increased NO synthase protein expression. It is also likely that aberrant superoxide formation from uncoupled monomeric NO synthase contributes to vascular NO scavenging. In conclusion, pulmonary hypertension, associated with a vasoconstrictor phenotype and low NO bioavailability, develops early in the sickle cell transgenic mouse.

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