Guinea Pig as a Model to Study the Carotid Body Mediated Chronic Intermittent Hypoxia Effects

Pulmonary Vascular Responses to Chronic Intermittent Hypoxia in a Guinea Pig Model of Obstructive Sleep Apnea

Experimental evidence suggests that chronic intermittent hypoxia (CIH), a major hallmark of obstructive sleep apnea (OSA), boosts carotid body (CB) responsiveness, thereby causing increased sympathetic activity, arterial and pulmonary hypertension, and cardiovascular disease. An enhanced circulatory chemoreflex, oxidative stress, and NO signaling appear to play important roles in these responses to CIH in rodents. Since the guinea pig has a hypofunctional CB (i.e., it is a natural CB knockout), in this study we used it as a model to investigate the CB dependence of the effects of CIH on pulmonary vascular responses, including those mediated by NO, by comparing them with those previously described in the rat. We have analyzed pulmonary artery pressure (PAP), the hypoxic pulmonary vasoconstriction (HPV) response, endothelial function both in vivo and in vitro, and vascular remodeling (intima-media thickness, collagen fiber content, and vessel lumen area). We demonstrate that 30 days of the exposure of guinea pigs to CIH (FiO2, 5% for 40 s, 30 cycles/h) induces pulmonary artery remodeling but does not alter endothelial function or the contractile response to phenylephrine (PE) in these arteries. In contrast, CIH exposure increased the systemic arterial pressure and enhanced the contractile response to PE while decreasing endothelium-dependent vasorelaxation to carbachol in the aorta without causing its remodeling. We conclude that since all of these effects are independent of CB sensitization, there must be other oxygen sensors, beyond the CB, with the capacity to alter the autonomic control of the heart and vascular function and structure in CIH.

The Effects of Volatile Anesthetics on Renal Sympathetic and Phrenic Nerve Activity during Acute Intermittent Hypoxia in Rats

Coordinated activation of sympathetic and respiratory nervous systems is crucial in responses to noxious stimuli such as intermittent hypoxia. Acute intermittent hypoxia (AIH) is a valuable model for studying obstructive sleep apnea (OSA) pathophysiology, and stimulation of breathing during AIH is known to elicit long-term changes in respiratory and sympathetic functions. The aim of this study was to record the renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA) during the AIH protocol in rats exposed to monoanesthesia with sevoflurane or isoflurane. Adult male Sprague-Dawley rats (n = 24; weight: 280-360 g) were selected and randomly divided into three groups: two experimental groups (sevoflurane group, n = 6; isoflurane group, n = 6) and a control group (urethane group, n = 12). The AIH protocol was identical in all studied groups and consisted in delivering five 3 min-long hypoxic episodes (fraction of inspired oxygen, FiO2 = 0.09), separated by 3 min recovery intervals at FiO2 = 0.5. Volatile anesthetics, isoflurane and sevoflurane, blunted the RSNA response to AIH in comparison to urethane anesthesia. Additionally, the PNA response to acute intermittent hypoxia was preserved, indicating that the respiratory system might be more robust than the sympathetic system response during exposure to acute intermittent hypoxia.

The relationship between hypoxia exposure and circulating cortisol levels in social and solitary African mole-rats: An initial report

Hypoxemia from exposure to intermittent and/or acute environmental hypoxia (lower oxygen concentration) is a severe stressor for many animal species. The response to hypoxia of the hypothalamic-pituitary-adrenal axis (HPA-axis), which culminates in the release of glucocorticoids, has been well-studied in hypoxia-intolerant surface-dwelling mammals. Several group-living (social) subterranean species, including most African mole-rats, are hypoxia-tolerant, likely due to regular exposure to intermittent hypoxia in their underground burrows. Conversely, solitary mole-rat species, lack many adaptive mechanisms, making them less hypoxia-tolerant than the social genera. To date, the release of glucocorticoids in response to hypoxia has not been measured in hypoxia-tolerant mammalian species. Consequently, this study exposed three social African mole-rat species and two solitary mole-rat species to normoxia, or acute hypoxia and then measured their respective plasma glucocorticoid (cortisol) concentrations. Social mole-rats had lower plasma cortisol concentrations under normoxia than the solitary genera. Furthermore, individuals of all three of the social mole-rat species exhibited significantly increased plasma cortisol concentrations after hypoxia, similar to those of hypoxia-intolerant surface-dwelling species. By contrast, individuals of the two solitary species had a reduced plasma cortisol response to acute hypoxia, possibly due to increased plasma cortisol under normoxia. If placed in perspective with other closely related surface-dwelling species, the regular exposure of the social African mole-rats to hypoxia may have reduced the basal levels of the components for the adaptive mechanisms associated with hypoxia exposure, including circulating cortisol levels. Similarly, the influence of body mass on plasma cortisol levels cannot be ignored. This study demonstrates that both hypoxia-tolerant rodents and hypoxia-intolerant terrestrial laboratory-bred rodents may possess similar HPA-axis responses from exposure to hypoxia. Further research is required to confirm the results from this pilot study and to further confirm how the cortisol concentrations may influence responses to hypoxia in African mole-rat.

The Microbiome in Obstructive Sleep Apnea

Recent evidence has highlighted important associations between obstructive sleep apnea and the microbiome. Although the intricacies of the pathophysiologic mechanisms are not well understood, available evidence suggests a bidirectional relationship between OSA and microbiota composition. Sleep fragmentation, intermittent hypoxia, and intermittent hypercapnia all play significant roles in altering the microbiome, and initial evidence has shown that alterations of the microbiota affect sleep patterns. Animal model evidence strongly supports the idea that the microbiome mediates disease states associated with OSA including hypertension, atherosclerosis, and obesity. The majority of evidence focuses on changes in the gut microbiome, which may result from OSA as well as contribute to sleep pattern changes, OSA-related CVD, and obesity. Meanwhile, a developing body of work suggests changes in the upper airway microbiome may be associated with OSA and periodontitis-related oral cavity microbiome changes may have significance in OSA-related CVD. Lastly, while evidence is limited, several studies suggest there may be a role for treatment of OSA and OSA-related comorbidities through alteration of the microbiome with probiotics, prebiotics, and microbiota transplantation. These early animal and human studies begin to characterize the interrelationships of the microbiome and OSA and may lead to new avenues for treatment.

Prebiotic administration modulates gut microbiota and faecal short-chain fatty acid concentrations but does not prevent chronic intermittent hypoxia-induced apnoea and hypertension in adult rats

BACKGROUND

Evidence is accruing to suggest that microbiota-gut-brain signalling plays a regulatory role in cardiorespiratory physiology. Chronic intermittent hypoxia (CIH), modelling human sleep apnoea, affects gut microbiota composition and elicits cardiorespiratory morbidity. We investigated if treatment with prebiotics ameliorates cardiorespiratory dysfunction in CIH-exposed rats.

METHODS

Adult male rats were exposed to CIH (96 cycles/day, 6.0% O2 at nadir) for 14 consecutive days with and without prebiotic supplementation (fructo- and galacto-oligosaccharides) beginning two weeks prior to gas exposures.

FINDINGS

CIH increased apnoea index and caused hypertension. CIH exposure had modest effects on the gut microbiota, decreasing the relative abundance of Lactobacilli species, but had no effect on microbial functional characteristics. Faecal short-chain fatty acid (SCFA) concentrations, plasma and brainstem pro-inflammatory cytokine concentrations and brainstem neurochemistry were unaffected by exposure to CIH. Prebiotic administration modulated gut microbiota composition and diversity, altering gut-metabolic (GMMs) and gut-brain (GBMs) modules and increased faecal acetic and propionic acid concentrations, but did not prevent adverse CIH-induced cardiorespiratory phenotypes.

INTERPRETATION

CIH-induced cardiorespiratory dysfunction is not dependant upon changes in microbial functional characteristics and decreased faecal SCFA concentrations. Prebiotic-related modulation of microbial function and resultant increases in faecal SCFAs were not sufficient to prevent CIH-induced apnoea and hypertension in our model. Our results do not exclude the potential for microbiota-gut-brain axis involvement in OSA-related cardiorespiratory morbidity, but they demonstrate that in a relatively mild model of CIH, sufficient to evoke classic cardiorespiratory dysfunction, such changes are not obligatory for the development of morbidity, but may become relevant in the elaboration and maintenance of cardiorespiratory morbidity with progressive disease.

FUNDING

Department of Physiology and APC Microbiome Ireland, University College Cork, Ireland. APC Microbiome Ireland is funded by Science Foundation Ireland, through the Government's National Development Plan.

Autonomic innervation of the carotid body as a determinant of its sensitivity: implications for cardiovascular physiology and pathology

The motivation for this review comes from the emerging complexity of the autonomic innervation of the carotid body (CB) and its putative role in regulating chemoreceptor sensitivity. With the carotid bodies as a potential therapeutic target for numerous cardiorespiratory and metabolic diseases, an understanding of the neural control of its circulation is most relevant. Since nerve fibres track blood vessels and receive autonomic innervation, we initiate our review by describing the origins of arterial feed to the CB and its unique vascular architecture and blood flow. Arterial feed(s) vary amongst species and, unequivocally, the arterial blood supply is relatively high to this organ. The vasculature appears to form separate circuits inside the CB with one having arterial venous anastomoses. Both sympathetic and parasympathetic nerves are present with postganglionic neurons located within the CB or close to it in the form of paraganglia. Their role in arterial vascular resistance control is described as is how CB blood flow relates to carotid sinus afferent activity. We discuss non-vascular targets of autonomic nerves, their possible role in controlling glomus cell activity, and how certain transmitters may relate to function. We propose that the autonomic nerves sub-serving the CB provide a rapid mechanism to tune the gain of peripheral chemoreflex sensitivity based on alterations in blood flow and oxygen delivery, and might provide future therapeutic targets. However, there remain a number of unknowns regarding these mechanisms that require further research that is discussed.

Bugs, breathing and blood pressure: microbiota-gut-brain axis signalling in cardiorespiratory control in health and disease

There is clear evidence of physiological effects of the gut microbiota on whole-body function in health and disease. Microbiota-gut-brain axis signalling is recognised as a key player in behavioural disorders such as depression and anxiety. Recent evidence suggests that the gut microbiota affects neurocontrol networks responsible for homeostatic functions that are essential for life. We consider the evidence suggesting the potential for the gut microbiota to shape cardiorespiratory homeostasis. In various animal models of disease, there is an association between cardiorespiratory morbidity and perturbed gut microbiota, with strong evidence in support of a role of the gut microbiota in the control of blood pressure. Interventions that target the gut microbiota or manipulate the gut-brain axis, such as short-chain fatty acid supplementation, prevent hypertension in models of obstructive sleep apnoea. Emerging evidence points to a role for the microbiota-gut-brain axis in the control of breathing and ventilatory responsiveness, relevant to cardiorespiratory disease. There is also evidence for an association between the gut microbiota and disease severity in people with asthma and cystic fibrosis. There are many gaps in the knowledge base and an urgent need to better understand the mechanisms by which gut health and dysbiosis contribute to cardiorespiratory control. Nevertheless, there is a growing consensus that manipulation of the gut microbiota could prove an efficacious adjunctive strategy in the treatment of common cardiorespiratory diseases, which are the leading causes of morbidity and mortality.

Peripheral Dopamine 2-Receptor Antagonist Reverses Hypertension in a Chronic Intermittent Hypoxia Rat Model

The sleep apnea-hypopnea syndrome (SAHS) involves periods of intermittent hypoxia, experimentally reproduced by exposing animal models to oscillatory PO₂ patterns. In both situations, chronic intermittent hypoxia (CIH) exposure produces carotid body (CB) hyperactivation generating an increased input to the brainstem which originates sympathetic hyperactivity, followed by hypertension that is abolished by CB denervation. CB has dopamine (DA) receptors in chemoreceptor cells acting as DA-2 autoreceptors. The aim was to check if blocking DA-2 receptors could decrease the CB hypersensitivity produced by CIH, minimizing CIH-related effects. Domperidone (DOM), a selective peripheral DA-2 receptor antagonist that does not cross the blood-brain barrier, was used to examine its effect on CIH (30 days) exposed rats. Arterial pressure, CB secretory activity and whole-body plethysmography were measured. DOM, acute or chronically administered during the last 15 days of CIH, reversed the hypertension produced by CIH, an analogous effect to that obtained with CB denervation. DOM marginally decreased blood pressure in control animals and did not affect hypoxic ventilatory response in control or CIH animals. No adverse effects were observed. DOM, used as gastrokinetic and antiemetic drug, could be a therapeutic opportunity for hypertension in SAHS patients’ resistant to standard treatments.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

Chronic intermittent hypoxia disrupts cardiorespiratory homeostasis and gut microbiota composition in adult male guinea-pigs

BACKGROUND

Carotid body (peripheral oxygen sensor) sensitisation is pivotal in the development of chronic intermittent hypoxia (CIH)-induced hypertension. We sought to determine if exposure to CIH, modelling human sleep apnoea, adversely affects cardiorespiratory control in guinea-pigs, a species with hypoxia-insensitive carotid bodies. We reasoned that CIH-induced disruption of gut microbiota would evoke cardiorespiratory morbidity.

METHODS

Adult male guinea-pigs were exposed to CIH (6.5% O₂ at nadir, 6 cycles.hour^-1) for 8 h.day^-1 for 12 consecutive days.

FINDINGS

CIH-exposed animals established reduced faecal microbiota species richness, with increased relative abundance of Bacteroidetes and reduced relative abundance of Firmicutes bacteria. Urinary corticosterone and noradrenaline levels were unchanged in CIH-exposed animals, but brainstem noradrenaline concentrations were lower compared with sham. Baseline ventilation was equivalent in CIH-exposed and sham animals; however, respiratory timing variability, sigh frequency and ventilation during hypoxic breathing were all lower in CIH-exposed animals. Baseline arterial blood pressure was unaffected by exposure to CIH, but β-adrenoceptor-dependent tachycardia and blunted bradycardia during phenylephrine-induced pressor responses was evident compared with sham controls.

INTERPRETATION

Increased carotid body chemo-afferent signalling appears obligatory for the development of CIH-induced hypertension and elevated chemoreflex control of breathing commonly reported in mammals, with hypoxia-sensitive carotid bodies. However, we reveal that exposure to modest CIH alters gut microbiota richness and composition, brainstem neurochemistry, and autonomic control of heart rate, independent of carotid body sensitisation, suggesting modulation of breathing and autonomic homeostasis via the microbiota-gut-brainstem axis. The findings have relevance to human sleep-disordered breathing.

FUNDING

The Department of Physiology, and APC Microbiome Ireland, UCC.