Brain-derived erythropoietin protects from intermittent hypoxia-induced cardiorespiratory dysfunction and oxidative stress in mice

Abstract

Study objectives: Based on the fact that erythropoietin (Epo) administration in rodents protects against spatial learning and cognitive deficits induced by chronic intermittent hypoxia (CIH)-mediated oxidative damage, here we tested the hypothesis that Epo in the brain protects against cardiorespiratory disorders and oxidative stress induced by CIH in adult mice.

Methods: Adult control and transgenic mice overexpressing Epo in the brain only (Tg21) were exposed to CIH (21%-10% O2-10 cycles/hour-8 hours/day-7 days) or room air. After CIH exposure, we used the tail cuff method to measure arterial pressure, and whole-body plethysmography to assess the frequency of apneic episodes at rest, minute ventilation, and ventilatory responses to hypoxia and hypercapnia. Finally, the activity of pro-oxidant (XO-xanthine oxidase, and NADPH) and antioxidant (super oxide dismutase) enzymes was evaluated in the cerebral cortex and brainstem.

Results: Exposure of control mice to CIH significantly increased the heart rate and arterial pressure, the number of apneic events, and the ventilatory response to hypoxia and hypercapnia. Furthermore, CIH increased the ratio of pro-oxidant to antioxidant enzymes in cortex and brainstem tissues. Both physiological and molecular changes induced by CIH were prevented in transgenic Tg21 mice.

Conclusions: We conclude that the neuroprotective effect of Epo prevents oxidative damage in the brain and cardiorespiratory disorders induced by CIH. Considering that Epo is used in clinics to treat chronic kidney disease and stroke, our data show convincing evidence suggesting that Epo may be a promising alternative drug to treat sleep-disorder breathing.

Figure 1.

Control-CIH mice show increased heart rate and arterial pressure. Values obtained for heart rate and arterial pressure were evaluated in control mice exposed to room air (Control-RA), control mice exposed to chronic intermittent hypoxia (control-CIH), and Tg21 mice exposed to chronic intermittent hypoxia (Tg21-CIH). (A) Heart rate, (B) MAP, (C) DAP, (D) SAP. *p < 0.05; ***p < 0.001.

Figure 2.

Ventilatory and metabolic parameters during sleep are not affected by CIH. Ventilatory and metabolic parameters were evaluated in all animal groups during sleep. (A) Minute-ventilation (V_E), (B) respiratory frequency (Rf), (C) tidal volume (Vt), (D) oxygen consumption, (E) carbon dioxide production, (F) respiratory rate, (G) V_E/VO₂, and (H) V_E/VCO₂.

Figure 3.

The number of apneic events is increased in control-CIH, but not in Tg21-CIH animals. Apneic episodes in adult female mice during 1 hr of sleep. (A) Total apneic events, (B) spontaneous apneic events, (C) post-sigh apneic events. *p < 0.05; ** p < 0.01.

Figure 4.

The CIH-mediated increase in HVR was prevented in Tg21 mice. Basal ventilation, HVR, and HcVR were evaluated after exposure to CIH (or RA in control animals). (A) Minute-ventilation (V_E), (B) respiratory frequency (Rf), (C) tidal volume (Vt) under conditions of normoxia (Nx), hypoxia (HVR), and hypercapnia (HcVR). Significantly different from the corresponding normoxic level: *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001; significantly different between treatment groups: ^#p < 0.05, ^##p < 0.01.

Figure 5.

Cytosolic XO activity in the cortex and brainstem. CIH mediated an increase in cytosolic XO activity in the cortex (A), but this increase was prevented in Tg21 mice. (B) XO activity in brainstem samples was undetectable. **p < 0.01.

Figure 6.

Cytosolic NOX activity in the cortex and brainstem. In the cortex (A), CIH mediated an increase in cytosolic XO activity. This increase was prevented in Tg21 mice. (B) No significant changes in brainstem NOX activity were observed between groups. **p < 0.01.

Figure 7.

SOD activity in the cortex and brainstem. CIH mediated an increase in SOD activity in the cortex (A) and brainstem (B). These increases were prevented in Tg21 mice. **p < 0.01.

Figure 8.

Ratio of NOX to SOD activity. CIH mediated an increased NOX/SOD ratio in the cortex (A) and brainstem (B), which was prevented in transgenic Tg21 animals. *p < 0.05, **p < 0.01.