Patients with obstructive sleep apnea (OSA) are at increased risk of cardiometabolic diseases. The gold standard treatment for this condition is continuous positive airway pressure (CPAP), which unfortunately does little to mitigate the complications of OSA. Laboratory research in OSA is hampered by the lack of a suitable animal model. We used a mouse model of sleep-disordered breathing that is based on the induction of chronic episodes of intermittent hypoxia (IH), as IH is an important component of clinical OSA. The effect of IH on cardiovascular function was studied in relation to changes in oxidative stress and nitric oxide bioavailability.
Methods: Male mice (n=8-10) were placed in customized cages connected to a system that controls the fraction of oxygen inspired (FiO2) inside the cages. To induce IH, nitrogen gas was flushed inside the cages for 30 seconds bringing FiO2 to 5% followed by immediate flushing with compressed air for the next 30 seconds. Each cycle brings oxyhemoglobin saturation down to 60%, which is similar to values in apneic episodes in patients with severe OSA. Control mice were housed in the same way but were not exposed to nitrogen (IA group). IH cycles were performed during daytime for 12 hours a day (mice are nocturnal animals). After 8 weeks, plasma was collected to measure oxidative stress and inflammatory markers and aortic blood vessels were isolated for studies of endothelial function using a wire myograph.
Results: There was a loss of endothelial dependent relaxation to acetylcholine (ACh) in aortae from IH groups (Emax: IH 67.2 ± 6.5% vs IA 95.1 ± 3.3%; p<0.05). This endothelial dysfunction was related to 1) an increased production of ADMA (IH: 0.63 ± 0.04 vs. IA: 0.46 ± 0.05 μmol/L, p<0.05), an endogenous circulating inhibitor of endothelial nitric oxide synthase (eNOS) and 2) augmented eNOS uncoupling caused by oxidative stress whereby eNOS preferentially produced superoxide over nitric oxide. Exposure to IH also increased levels of plasma markers of oxidative stress: 8-isoprostane (IH: 110 ± 20.2 vs. IA: 59.6 ± 5.6 pg/ml, p<0.05), and inflammation: interleukin 6 (IL-6) (IH: 57.4 ± 2.6 vs. IA: 36.5 ± 3.6 pg/ml, p<0.05).
Conclusion: Episodes of IH, as might occur in patients with OSA, leads to detrimental regulation of endothelial function that is associated with increases in oxidative stress and inflammation, manifesting as reduced bioavailability of nitric oxide. Endothelial dysfunction is known to precede the clinical diagnosis of cardiovascular disease.