Gene regulation during intermittent hypoxia: evidence for the involvement of reactive oxygen species

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

Reactive oxygen species are the cause of the enhanced cardiorespiratory response induced by intermittent hypoxia in conscious rats

This study was carried out to investigate the role of reactive oxygen species (ROS) in the elevation of cardiorespiratory responses during the development of intermittent hypoxia (IH)-induced hypertension. Rats were exposed to either 30 days of IH [(30s N₂)+(45 s room air (RA)] or RA for 6 h/day. After 5 days of exposure, stable mean arterial pressure, normalized low-frequency power of pulses interval spectrogram (a marker of cardiac sympathetic outflow), and minute ventilation (an index for arterial chemoreflex activation) were significantly increased throughout the observation period in IH-exposed rats, but not in RA-exposed rats. FosB expression in rostral ventrolateral medulla was elevated after IH exposure for 5 days. Intraperitoneal injection of MnTMPyP (a superoxide scavenger) or N-acetylcysteine (an antioxidant) prevented IH-induced elevation of the cardiorespiratory responses and lipid peroxidation of lung tissues. These results suggest that ROS are essential for IH-induced elevation of arterial chemoreflex activation and sympathetic outflow, which may, in turn, contribute to IH-induced hypertension.

OSA: the new cardiovascular disease: part II: Overview of cardiovascular diseases associated with obstructive sleep apnea

Obstructive sleep apnea (OSA), present in 5-15% of adults, is strongly associated with the incidence and poor outcome of hypertension, coronary artery disease, arrhythmia, heart failure, and stroke. Treatment of OSA completely reverses its cardiovascular consequences. In this review, we discuss the clinical evidence for the strong association between OSA and cardiovascular disease and present an argument for approaching OSA as a cardiovascular disease. We particularly focus on the causative relationship between OSA and hypertension, and on the increasingly recognized relationship between OSA and heart failure.

Sleep disorders in systolic heart failure: A prospective study of 100 male patients. The final report

BACKGROUND

Heart failure is a highly prevalent disorder. The main aims of this study were to determine the prevalence, consequences and markers of sleep apnea and the periodic limb movements (PLMS) in heart failure.

METHODS AND RESULTS

This is a prospective study of 100 of 114 consecutive eligible patients with heart failure and LVEF <45%. Forty-nine percent of patients had sleep apnea with an average index of 49 per hour. Thirty-seven percent of patients had CSA and 12% had OSA. Comparing patients with CSA to those without sleep apnea, the markers associated with CSA were poorer functional classification, atrial fibrillation, PaCO2 <36 mm Hg, LVEF <20%, and nocturnal ventricular arrhythmias including >30 PVC's, >1 couplets and >1 episodes of ventricular tachycardia/hour. In contrast, comparing heart failure patients with CSA to OSA, OSA patients were significantly obese (mean body weight 109+/-27 vs 78+/-18 kg) and had habitual snoring (83% vs 38%). Twenty percent of patient with heart failure had PLMS with an average index of 35 per hour. PLMS resulted in a mildly increased number of arousals (3.4+/-2 per hour).

CONCLUSIONS

49% of male patients with systolic heart failure suffer from sleep apnea and 20% have PLMS. CSA occurs in about 37%, and OSA in 12% of patients. Habitual snoring and obesity are the hallmarks of OSA. In contrast, heart failure patients with CSA are commonly thin and mostly do not snore. Hallmarks of CSA are Class III New York Heart, artrial fibrillation, frequent nocturnal ventricular arrhythmias, low arterial PCO2 and LVEF <20%.

Facilitation of dopamine and acetylcholine release by intermittent hypoxia in PC12 cells: involvement of calcium and reactive oxygen species

We have investigated the effects of preconditioning pheochromocytoma (PC12) cells with intermittent hypoxia (IH) on transmitter release during acute hypoxia. Cell cultures were exposed to either alternating cycles of hypoxia (1% O(2) + 5% CO(2); 30 s/cycle) and normoxia (21% O(2) + 5% CO(2); 3 min/cycle) for 15 or 60 cycles or normoxia alone (control) for similar durations. Control and IH cells were challenged with either hyperoxia (basal release) or acute hypoxia (Po(2) of approximately 35 Torr) for 5 min, and the amounts of dopamine (DA) and acetylcholine (ACh) released in the medium were determined by HPLC combined with electrochemical detection. Hypoxia augmented DA (approximately 80%) but not ACh release in naive cells, whereas, in IH-conditioned cells, it further enhanced DA release (ranging from 120 to approximately 145%) and facilitated ACh release (approximately 30%). Hypoxia-evoked augmentation of transmitter release was not seen in cells conditioned with sustained hypoxia. IH-induced increase in DA but not IH-induced ACh release during hypoxia was partially inhibited by cadmium chloride (100 microM), a voltage-gated Ca(2+) channel blocker. By contrast, 2-aminoethoxydiphenylborate (75 microM), a blocker of inositol 1,4,5-trisphosphate (IP(3)) receptors, and N-acetyl-L-cysteine (300 microM), a potent scavenger of reactive oxygen species, either attenuated or abolished IH-evoked augmentation of transmitter release during hypoxia. Together, the above results demonstrate that IH conditioning increases hypoxia-evoked neurotransmitter release from PC12 cells via mechanisms involving mobilization of Ca(2+) from intracellular stores through activation of IP(3) receptors. Our findings also suggest that oxidative stress plays a central role in IH-induced augmentation of transmitter release from PC12 cells during acute hypoxia.

Activation of tyrosine hydroxylase by intermittent hypoxia: involvement of serine phosphorylation

Regulation of tyrosine hydroxylase (TH) by intermittent hypoxia (IH) was investigated in rat pheochromocytoma 12 (PC-12) cells by exposing them to alternating cycles of hypoxia (1% O2, 15 s) and normoxia (21% O2, 3 min) for up to 60 cycles; controls were exposed to normoxia for a similar duration. IH exposure increased dopamine content and TH activity by approximately 42 and approximately 56%, respectively. Immunoblot analysis revealed that comparable levels of TH protein were expressed in normoxic and IH cells. Removal of TH-bound catecholamines and in vitro phosphorylation of TH in cell-free extracts by the catalytic subunit of protein kinase A (PKA) increased TH activity in normoxic but not in IH cells, suggesting possible induction of TH phosphorylation and removal of endogenous inhibition of TH by IH. To assess the role of serine phosphorylation in IH-induced TH activation, TH immunoprecipitates and extracts derived from normoxic and IH cells were probed with anti-phosphoserine and anti-phospho-TH (Ser-40) antibody, respectively. Compared with normoxic cells, total serine and Ser-40-specific phosphorylation of TH were increased in IH cells. IH-induced activation of TH and the increase in total serine and Ser-40-specific phosphorylation of TH were inhibited by Ca2+/calmodulin-dependent protein kinase (CaMK) and PKA-specific inhibitors but not by inhibitors of the extracellular signal-regulated protein kinase pathway, suggesting that IH activates TH in PC-12 cells via phosphorylation of serine residues including Ser-40, in part, by CaMK and PKA. Our results also suggest that IH-induced phosphorylation of TH facilitates the removal of endogenous inhibition of TH, leading to increased synthesis of dopamine.

Heart failure and sleep apnea: emphasis on practical therapeutic options

Heart failure is a highly prevalent problem associated with excess morbidity and mortality and economic impact. Because of increased average life span, improved therapy of ischemic coronary artery disease and hypertension, the incidence and prevalence of heart failure will continue to rise into the twenty-first century. Multiple factors may contribute to the progressively declining course of heart failure. One such cause could be the occurrence of repetitive episodes of apnea, hypopnea, and hyperpnea, which frequently occur in patients with heart failure. Episodes of apnea, hypopnea, and hyperpnea cause sleep disruption, arousals, intermittent hypoxemia, hypercapnia, hypocapnia, and changes in intrathoracic pressure. These pathophysiologic consequences of sleep-related breathing disorders have deleterious effects on cardiovascular system, and the effects may be most pronounced in the setting of established heart failure and coronary artery disease. Diagnosis and treatment of sleep-related breathing disorders may improve morbidity and mortality of patients with heart failure [34]. Large-scale, carefully executed therapeutic studies are needed to determine if treatment of sleep-related breathing disorders changes the natural history of left ventricular failure.

Reactive oxygen species in the plasticity of respiratory behavior elicited by chronic intermittent hypoxia

Long-term facilitation (LTF) of breathing elicited by episodic hypoxia (EH) is an extensively studied example of plasticity of respiratory motor behavior. Previous studies employed the paradigm of EH wherein each episode of hypoxia was 5 min. This paradigm is rarely encountered in nature. Brief episodes of hypoxia are encountered frequently with recurrent apneas, wherein hypoxic episodes last a few seconds only. Recent studies suggest that chronic intermittent hypoxia (CIH) represents a form of oxidative stress involving reactive O(2) species. The objectives of the present study were to determine 1) whether acute, repeated, brief EH (15 s) elicit LTF in breathing and 2) whether prior conditioning with CIH modulates acute EH-induced LTF of breathing, and if so whether reactive O(2) species are involved. Experiments were performed on anesthetized, vagotomized, paralyzed, and mechanically ventilated rats, and efferent phrenic nerve activity was monitored as an index of respiratory motor output. In control animals, acute EH (15-s hypoxia; 10 episodes; n = 9) increased minute neural respiration, which persisted during 60 min of the posthypoxic period, suggesting LTF of breathing. EH-induced LTF of respiration was markedly augmented in CIH-conditioned animals (15-s hypoxia, 9 episodes/h, 8 h/day for 10 days; n = 9). By contrast, conditioning with a comparable, cumulative duration of sustained hypoxia (4-h hypoxia; n = 8) did not augment LTF elicited by acute EH. Systemic administration of manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (5 mg. kg(-1). day(-1) for 10 days), a potent scavenger of O(2)(-)*, prevented CIH-induced potentiation of LTF (n = 9). These results demonstrate that 1) acute, brief EH elicits LTF in respiratory motor output; 2) prior conditioning with CIH, but not with comparable, cumulative duration of sustained hypoxia, augments LTF elicited by acute EH; and 3) O(2)(-)* radical scavenger prevents CIH-induced potentiation of LTF of respiration.