Ageing of the carotid body

Aging and the Carotid Body: A Scoping Review

The carotid body (CB) is a neuroepithelial tissue consisting of O₂-sensitive glomus cells that constantly scan the arterial blood for O₂ and generate a discharge as an inverse function of O₂ content. Aging is a cumulative result of decreased O₂ supply paralleled by a decreased O₂ tissue demand and oxidative damage to cells derived from aerobic metabolism. Here we studied how CB affects the aging process. This is a study of CB ultrastructural morphometry and immunohistochemical expression of proteins underlying CB responsiveness. The study was based on human CBs obtained from cadavers of people who died due to traumatic events in young and old age. The study was supplemented by investigations of CBs obtained from young and old rats subjected to chronic normoxic and hypoxic conditions. We found changes in the old normoxic CBs akin to the effects of chronic hypoxia such as enhanced extracellular matrix, reduced synaptic contacts between glomus cells, fewer glomus cells, secretory vesicles, and mitochondria. These changes were accompanied by enhanced expressions of hypoxia-inducible factor one-alpha (HIF-1α), vascular endothelial growth factor (VEGF), and nitric oxide synthase (NOS2). We conclude that hypoxia and aging share a common background consisting of deficient O₂ tissue supply, mitochondrial dysfunction, and a limited ability to deal with increased cellular oxidative stress. Aging leads to adaptative reductions in CB responsiveness to hypoxia shifting the chemosensory setpoint upward. We submit that the attenuated CB sensitivity at old age may be tantamount to "physiological denervation" leading to a gradual loss of the chemosensing role in the prevention of tissue hypoxia by increasing lung ventilation.

Structural Plasticity of the Carotid Body

The mammalian carotid body (CB) exhibits considerable plasticity of its structure during development and aging and as a consequence of environmental, metabolic and inflammatory stimuli. The structural changes during maturation include an enlargement of the total and vascular volume of the CB. Conversely, aging results in a reduction in the number and volume of glomus cells with progressive cellular degeneration and an apparent increase in the surrounding connective tissue. Age-related structural alterations are similar to those during chronic hypoxia. Long-term hypoxic exposure and sodium nitrate treatment enlarge several-fold the size of the rat CB causing glomus cell hypertrophy and hyperplasia, and evoke changes in its vascular structure, inducing marked vasodilation and neovascularization. In humans, such structural CB adaptation responses to prolonged hypoxia occur during acclimatization to high altitudes. On the other hand, the hyperoxic CB is significantly smaller than those of age-matched normoxic controls. Morphological alterations in the CB in both hypertensive animals and humans are characterized by a slightly enlarged parenchyma without apparent vascular expansion and/or dilation. The CB structural plasticity depends on the existence of a population of multipotent neural crest-derived stem cells, which are activated during hypoxia to proliferate and differentiate into new both neuronal (glomus) and vascular cell types.

Dificultad respiratoria y COVID-19: Un llamado a la investigación

La dificultad respiratoria, también conocida como disnea, es un síntoma frecuente que causa debilidad. Varios reportes han destacado la ausencia de disnea en un subgrupo de pacientes que padecen COVID-19, en la llamada hipoxemia «silenciosa» o «feliz». Los reportes también han mencionado la falta de una relación clara entre la gravedad clínica de la enfermedad y los niveles de disnea referidos por los pacientes. Se ha demostrado en gran medida que entre las complicaciones cerebrales del COVID-19 hay alta prevalencia de encefalopatía aguda, que podría afectar el procesamiento de las señales aferentes o bien la modulación descendente de las señales de disnea. En esta revisión pretendemos destacar los mecanismos implicados en la disnea y resumir la fisiopatología del COVID-19 y sus efectos en la interacción cerebro-pulmón. Posteriormente, presentamos hipótesis sobre la alteración de la percepción de la disnea en pacientes con COVID-19 y sugerimos formas de investigar más a fondo este fenómeno.

Atemnot und COVID-19: Ein Aufruf zu mehr Forschung

Atemnot, auch als Dyspnoe bezeichnet, ist ein häufiges und lähmendes Symptom. In mehreren Berichten wurde die Abwesenheit von Atemnot bei einer Untergruppe von Patienten mit COVID-19 hervorgehoben, die manchmal als «stille» oder «glückliche Hypoxie» bezeichnet wird. Ebenfalls wurde in Berichten erwähnt, dass es an einem klaren Zusammenhang zwischen dem klinischen Schweregrad der Erkrankung und der von den Patienten berichteten Schwere der Atemnot fehlt. Die zerebralen Komplikationen von COVID-19 sind weitgehend nachgewiesen, mit einer hohen Prävalenz akuter Enzephalopathien, die möglicherweise die Verarbeitung afferenter Signale oder die absteigende Modulation von Atemnotsignalen beeinträchtigen könnte. In dieser Übersichtsarbeit möchten wir die an der Atemnot beteiligten Mechanismen hervorheben und die Pathophysiologie von COVID-19 und den bekannten Auswirkungen der Erkrankung auf die Interaktion von Gehirn und Lunge zusammenfassen. Anschließend stellen wir Hypothesen für die Veränderung der Wahrnehmung von Atemnot bei COVID-19-Patienten auf und schlagen Möglichkeiten vor, mit denen dieses Phänomen weiter erforscht werden könnte.

Breathlessness and COVID-19: A Call for Research

Breathlessness, also known as dyspnoea, is a debilitating and frequent symptom. Several reports have highlighted the lack of dyspnoea in a subgroup of patients suffering from COVID-19, sometimes referred to as “silent” or “happy hyp­oxaemia.” Reports have also mentioned the absence of a clear relationship between the clinical severity of the disease and levels of breathlessness reported by patients. The cerebral complications of COVID-19 have been largely demonstrated with a high prevalence of an acute encephalopathy that could possibly affect the processing of afferent signals or top-down modulation of breathlessness signals. In this review, we aim to highlight the mechanisms involved in breathlessness and summarize the pathophysiology of COVID-19 and its known effects on the brain-lung interaction. We then offer hypotheses for the alteration of breathlessness perception in COVID-19 patients and suggest ways of further researching this phenomenon.

Experimental Evidence of A2A-D2 Receptor-Receptor Interactions in the Rat and Human Carotid Body

Adenosine A_2A receptors (A_2AR) and dopamine D₂ receptors (D₂R) are known to be involved in the physiological response to hypoxia, and their expression/activity may be modulated by chronic sustained or intermittent hypoxia. To date, A_2AR and D₂R can form transient physical receptor–receptor interactions (RRIs) giving rise to a dynamic equilibrium able to influence ligand binding and signaling, as demonstrated in different native tissues and transfected mammalian cell systems. Given the presence of A_2AR and D₂R in type I cells, type II cells, and afferent nerve terminals of the carotid body (CB), the aim of this work was to demonstrate here, for the first time, the existence of A_2AR–D₂R heterodimers by in situ proximity ligation assay (PLA). Our data by PLA analysis and tyrosine hydroxylase/S100 colocalization indicated the formation of A_2AR–D₂R heterodimers in type I and II cells of the CB; the presence of A_2AR–D₂R heterodimers also in afferent terminals is also suggested by PLA signal distribution. RRIs could play a role in CB dynamic modifications and plasticity in response to development/aging and environmental stimuli, including chronic intermittent/sustained hypoxia. Exploring other RRIs will allow for a broad comprehension of the regulative mechanisms these interactions preside over, with also possible clinical implications.

Protective effects of SS-31 against SDHB suppression-mitochondrial dysfunction-EndMT axis-modulated CBT sclerosis and progression

Sclerosis variant in carotid body tumor (CBT) is characterized by extensive stromal sclerosis, which results in an uncommon pattern of growth that closely resembles that of an invasive malignant neoplasm. However, the clinical significance and the mechanism remains unclear. In this study, we provide evidence that SS-31 exerts protective effects against SDHB suppression-mitochondrial dysfunction-EndMT axis-modulated CBT sclerosis and progression. In human CBT specimens, sclerosis extent was consistently related to decreased recurrence-, death-, systematic metastasis-, and major adverse event-free survival, decreased SDHB expression, and aggravated EndMT. In human umbilical vein endothelial cells (HUVECs), SDHB KD aggravated hypoxia-induced EndMT, mitochondrial dysfunction and metabolic switch, while SS-31 treatment could significantly attenuate these changes caused by SDHB KD and hypoxia. In patient-derived xenograft (PDX) mice models of CBT, we also observed increased tumor growth speed and extent of EndMT, mitochondrial dysfunction, and metabolic switch in sclerosing carotid body tumor (SCBT) group than in conventional carotid body tumor (CCBT) group. And treating with SS-31 could significantly retard SCBT progression by rescuing the mitochondrial dysfunction-induced EndMT. Altogether, these results show that SDHB suppression-mitochondrial dysfunction-EndMT axis is a critical part of the CBT sclerosis and progression, while mitochondria-targeted drug SS-31 exerts an inhibitive effect on the above-mentioned axis, which opens new strategies to prevent and treat malignancies of CBT.

Comparison of the Effectiveness of High-Intensity Interval Training in Hypoxia and Normoxia in Healthy Male Volunteers: A Pilot Study

Aims

The study investigated the effect of high-intensity interval training in hypoxia and normoxia on serum concentrations of proangiogenic factors, nitric oxide, and inflammatory responses in healthy male volunteers.

Methods

Twelve physically active male subjects completed a high-intensity interval training (HIIT) in normoxia (NorTr) and in normobaric hypoxia (HypTr) (FiO₂ = 15.2%). The effects of HIIT in hypoxia and normoxia on maximal oxygen uptake, hypoxia-inducible factor-1-alpha, vascular endothelial growth factor, nitric oxide, and cytokines were analyzed.

Results

HIIT in hypoxia significantly increases maximal oxygen uptake (p=0.01) levels compared to pretraining levels. Serum hypoxia-inducible factor-1 (p=0.01) and nitric oxide levels (p=0.05), vascular endothelial growth factor (p=0.04), and transforming growth factor-β (p=0.01) levels were increased in response to exercise test after hypoxic training. There was no effect of training conditions for serum baseline angiogenic factors and cytokines (p > 0.05) with higher HIF-1α and NO levels after hypoxic training compared to normoxic training (F = 9.1; p < 0.01 and F = 5.7; p < 0.05, respectively).

Conclusions

High-intensity interval training in hypoxia seems to induce beneficial adaptations to exercise mediated via a significant increase in the serum concentrations of proangiogenic factors and serum nitric oxide levels compared to the same training regimen in normoxia.

Contribution of adenosine and ATP to the carotid body chemosensory activity in ageing

KEY POINTS

Adenosine and ATP are excitatory neurotransmitters involved in the carotid body (CB) response to hypoxia. During ageing the CB exhibits a decline in its functionality, demonstrated by decreased hypoxic responses. In aged rats (20-24 months old) there is a decrease in: basal and hypoxic release of adenosine and ATP from the CB; expression of adenosine and ATP receptors in the petrosal ganglion; carotid sinus nerve (CSN) activity in response to hypoxia; and ventilatory responses to ischaemic hypoxia. There is also an increase in SNAP25, ENT1 and CD73 expression. It is concluded that, although CSN activity and ventilatory responses to hypoxia decrease with age, adjustments in purinergic metabolism in the CB in aged animals are present aiming to maintain the contribution of adenosine and ATP. The possible significance of the findings in the context of ageing and in CB-associated pathologies is considered.

ABSTRACT

During ageing the carotid body (CB) exhibits a decline in its functionality. Here we investigated the effect of ageing on functional CB characteristics as well as the contribution of adenosine and ATP to CB chemosensory activity. Experiments were performed in 3-month-old and 20- to 24-month-old male Wistar rats. Ageing decreased: the number of tyrosine hydroxylase immune-positive cells, but not type II cells or nestin-positive cells in the CB; the expression of P2X₂ and A_2A receptors in the petrosal ganglion; and the basal and hypoxic release of adenosine and ATP from the CB. Ageing increased ecto-nucleotidase (CD73) immune-positive cells and the expression of synaptosome associated protein 25 (SNAP25) and equilibrative nucleoside transporter 1 (ENT1) in the CB. Additionally, ageing did not modify basal carotid sinus nerve (CSN) activity or the activity in response to hypercapnia, but decreased CSN activity in hypoxia. The contribution of adenosine and ATP to stimuli-evoked CSN chemosensory activity in aged animals followed the same pattern of 3-month-old animals. Bilateral common carotid occlusions during 5, 10 and 15 s increased ventilation proportionally to the duration of ischaemia, an effect decreased by ageing. ATP contributed around 50% to ischaemic-ventilatory responses in young and aged rats; the contribution of adenosine was dependent on the intensity of ischaemia, being maximal in ischaemias of 5 s (50%) and much smaller in 15 s ischaemias. Our results demonstrate that both ATP and adenosine contribute to CB chemosensory activity in ageing. Though CB responses to hypoxia, but not to hypercapnia, decrease with age, the relative contribution of both ATP and adenosine for CB activity is maintained.