Impact of chronic free hemoglobin infusion on skeletal muscle microvascular PO₂ dynamics in rats

Abstract

Patients with hemolytic diseases (i.e. sickle cell anemia) exhibit severely compromised exercise capacity and quality of life. This is likely due, at least in part, to chronic exposure to free hemoglobin (Hb) which results in a robust reduction in nitric oxide (NO) bioavailability and, particularly when combined with systemic hypoxia, enacts a proinflammatory cascade that causes vascular dysfunction and remodeling within the pulmonary circulation. Similar downstream effects may occur which could disrupt the QO2/VO2 relationship within skeletal muscle and thus impair tolerance to exercise. Therefore, the purpose of this investigation was to test the hypothesis that chronic exposure to free Hb would lower the microvascular PO2 (PO2mv, the pressure head that facilitates blood‐myocyte O2 flux) at rest and during muscle contractions with a greater deficit observed in rats exposed to hypoxia+Hb. Using programmable micropump technology, 8 young‐adult male Sprague‐Dawley rats were housed in normoxia (Hb+Nx, n=4) or hypoxia (Hb+Hx, n=4) while exposed to 60 mg/day Hb for 18 days while 4 rats were outfitted with dummy infusion pumps (SHAM, n=4) and were housed in normoxia. Following infusion, all rats performed a graded treadmill exercise test to fatigue for the determination of endurance capacity. PO2mv was then measured during 180s of electrically induced (1‐Hz) twitch spinotrapezius muscle contractions. There were no between group differences in exercise tolerance (average run time 16–18 min, p>0.05). While there were no significant differences in any PO2mv kinetic parameter, exposure to hypoxia and Hb resulted in a significantly greater resting baseline (SHAM: 28.3 ± 4, Hb+Nx: 31.5 ± 2, Hb+Hx: 42.8 ± 3 mmHg) and contracting steady‐state PO2 (SHAM: 20.70 ± 4, Hb+Nx: 26 ± 2, Hb:Hx: 36.3 ± 2 mmHg) when compared to SHAM and Hb+Nx conditions, while the contracting steady‐state PO2 was also significantly higher in Hb+Nx vs. SHAM (p<0.05). Blood hematocrit was significantly higher in Hb+Hx when compared to SHAM and Hb+Nx with no significant differences between SHAM and Hb+Nx (SHAM: 48.5 ± 3, Hb+Nx: 43.1 ± 3, Hb+Hx: 54.7 ± 2%, p<0.05). Contrary to our original hypothesis, combined exposure to free Hb and hypoxia resulted in a greater baseline and contracting steady‐state PO2mv while Hb alone raised the steady‐state PO2mv relative to SHAM operated rats. Given that free Hb results in oxidative stress and cellular damage, the elevated PO2mv in both Hb groups likely reflects an impaired ability to utilize oxygen (↓VO2) within the skeletal muscle. This is supported by the lack of differences in exercise tolerance in Hb vs. SHAM despite an increased Hct (i.e. oxygen carrying capacity) observed in the Hb+Hx rats and significantly greater steady‐state PO2mv in both Hb groups. These results carry robust implications for those suffering from hemolytic diseases and provides a potential mechanism by which chronic exposure to free Hb impacts exercise tolerance and quality of life.Support or Funding InformationFunded by NIH (R01HL125642) to DCI and Colorado Nutrition and Obesity Research Center (P30DK048520) award to SKF.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.