Nitrite and S-nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation: Relationship to Basal and Exercise Blood Flow Responses to Hypoxia

Background—The mechanisms underlying red blood cell (RBC)-mediated hypoxic vasodilation remain controversial with separate roles for nitrite (NO2-) and S-nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide (NO) bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous (a-v) concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O2 tension and blood flow.

Methods—Ten healthy participants (5 male, 5 female) aged 24 (mean) ± 4 (SD) years old were randomly assigned to a normoxic (21% O2) and hypoxic (10% O2) trial with measurements performed at rest and following 30 min of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery, internal jugular and femoral veins with plasma and RBC NO metabolites measured by tri-iodide reductive chemiluminescence. Cerebral and femoral venous blood flow were determined by transcranial Doppler ultrasound (CBF) and constant infusion thermodilution (FBF) with net exchange calculated via the Fick principle.

Results—Hypoxia was associated with a mild increase in both CBF and FBF (P < 0.05 vs. normoxia) with further more pronounced increases observed in FBF during exercise (P < 0.05 vs. rest) in proportion to the reduction in RBC oxygenation (r = 0.680 to 0.769, P < 0.001). Plasma NO2- gradients reflecting consumption (a > v, P < 0.05) were accompanied by RBC iron nitrosylHb formation (a > v, P < 0.05) at rest in normoxia, during hypoxia (P < 0.05 vs. normoxia) and especially during exercise (P< 0.05 vs. rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic and acidotic femoral circulation (P < 0.05 vs. cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S-nitrosothiols (P > 0.05).

Conclusions—These findings suggest that hypoxia, and to a far greater extent exercise, independently promote a-v delivery gradients of intravascular NO with deoxyHb-mediated NO2- reduction identified as the dominant mechanism underlying hypoxic vasodilation.