Peripheral chemoreceptor control of ventilation following sustained hypoxia in young and older adult humans

Women at Altitude: Sex-Related Physiological Responses to Exercise in Hypoxia

Sex differences in physiological responses to various stressors, including exercise, have been well documented. However, the specific impact of these differences on exposure to hypoxia, both at rest and during exercise, has remained underexplored. Many studies on the physiological responses to hypoxia have either excluded women or included only a limited number without analyzing sex-related differences. To address this gap, this comprehensive review conducted an extensive literature search to examine changes in physiological functions related to oxygen transport and consumption in hypoxic conditions. The review encompasses various aspects, including ventilatory responses, cardiovascular adjustments, hematological alterations, muscle metabolism shifts, and autonomic function modifications. Furthermore, it delves into the influence of sex hormones, which evolve throughout life, encompassing considerations related to the menstrual cycle and menopause. Among these physiological functions, the ventilatory response to exercise emerges as one of the most sex-sensitive factors that may modify reactions to hypoxia. While no significant sex-based differences were observed in cardiac hemodynamic changes during hypoxia, there is evidence of greater vascular reactivity in women, particularly at rest or when combined with exercise. Consequently, a diffusive mechanism appears to be implicated in sex-related variations in responses to hypoxia. Despite well-established sex disparities in hematological parameters, both acute and chronic hematological responses to hypoxia do not seem to differ significantly between sexes. However, it is important to note that these responses are sensitive to fluctuations in sex hormones, and further investigation is needed to elucidate the impact of the menstrual cycle and menopause on physiological responses to hypoxia.

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Dose-response relationship of intermittent normobaric hypoxia to stimulate erythropoietin in the context of health promotion in young and old people

PURPOSE

Erythropoietin (EPO) has multifactorial positive effects on health and can be increased by intermittent normobaric hypoxia (IH). Recommendations about the intensity and duration of IH to increase EPO exist, but only for young people. Therefore, the aim of the study was to investigate the dose-response relationship regarding the duration of hypoxia until an EPO expression and the amount of EPO expression in old vs. young cohorts.

METHODS

56 young and 67 old people were assigned to two separate investigations with identical study designs (3-h hypoxic exposure) but with different approaches to adjust the intensity of hypoxia: (i) the fraction of inspired oxygen (FiO₂) was 13.5%; (ii) the FiO₂ was individually adjusted to an oxygen saturation of the blood of 80%. Age groups were randomly assigned to a hypoxia or control group (normoxic exposure). EPO was assessed before, during (90 and 180 min), and 30 min after the hypoxia.

RESULTS

EPO increased significantly after 180 min in both cohorts and in both investigations [old: (i) + 16%, p = 0.007 and (ii) + 14%, p < 0.001; young: (i) + 27%, p < 0.001 and (ii) + 45%, p = 0.007]. In investigation (i), EPO expression was significantly higher in young than in old people after 180 min of hypoxic exposure (p = 0.024) and 30 min afterwards (p = 0.001).

CONCLUSION

The results indicate that after a normobaric hypoxia of 180 min, EPO increases significantly in both age cohorts. The amount of EPO expression is significantly higher in young people during the same internal intensity of hypoxia than in old people.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

From the Cover: Prenatal Nicotinic Exposure Attenuates Respiratory Chemoreflexes Associated With Downregulation of Tyrosine Hydroxylase and Neurokinin 1 Receptor in Rat Pup Carotid Body

Maternal cigarette smoke is the major risk of sudden infant death syndrome (SIDS). A depressed ventilatory response to hypoxia (HVR) and hypercapnia (HCVR) is thought to be responsible for the pathogenesis of SIDS and the carotid body is critically involved in these responses. We have recently reported that prenatal nicotinic exposure (PNE) over the full gestation induces depressed HVR in rat pups. Here, we asked whether PNE (1) depressed not only HVR but also HCVR that were dependent on the carotid body, (2) affected some important receptors and neurochemicals expressed in the carotid body, such as tyrosine hydroxylase (TH), neurokinin-1 receptor (NK1R), and α7 nicotinic acetylcholine receptor (α7nAChR), and (3) blunted the ventilatory responses to activation of these receptors. To this end, HVR and HCVR in Ctrl and PNE pups were measured with plethysmography before and after carotid body ablation (Series I), mRNA expression and/or immunoreactivity (IR) of TH, NK1R, and α7nAChR in the carotid body were examined by RT-PCR and immunohistochemistry (Series II), and the ventilatory responses were tested before and after intracarotid injection of substance P (NK1R agonist) and AR-R17779 (α7nAChR agonist) (Series III). Our results showed that PNE (1) significantly depressed both HVR and HCVR and these depressions were abolished by carotid body ablation, (2) reduced the relative population of glomus cells, mRNA NK1R, and α7nAChR and IR of NK1R and TH in the carotid body, and (3) decreased ventilatory responses to intracarotid injection of substance P or AR-R17779. These results suggest that PNE acting via the carotid body could strikingly blunt HVR and HCVR, likely through downregulating TH and NK1R.

Effect of age on long-term facilitation and chemosensitivity during NREM sleep

The reason for increased sleep-disordered breathing with a predominance of central apneas in the elderly is unknown. We speculate that ventilatory control instability may provide a link between aging and the onset of unstable breathing during sleep. We sought to investigate potential underlying mechanisms in healthy, elderly adults during sleep. We hypothesized that there is 1) a decline in respiratory plasticity or long-term facilitation (LTF) of ventilation and/or 2) increased ventilatory chemosensitivity in older adults during non-, this should be hyphenated, non-rapid rapid eye movement (NREM) sleep. Fourteen elderly adults underwent 15, 1-min episodes of isocapnic hypoxia (EH), nadir O2 saturation: 87.0 ± 0.8%. Measurements were obtained during control, hypoxia, and up to 20 min of recovery following the EH protocol, respectively, for minute ventilation (VI), timing, and inspiratory upper-airway resistances (RUA). The results showed the following. 1) Compared with baseline, there was a significant increase in VI (158 ± 11%, P < 0.05) during EH, but this was not accompanied by augmentation of VI during the successive hypoxia trials nor in VI during the recovery period (94.4 ± 3.5%, P = not significant), indicating an absence of LTF. There was no change in inspiratory RUA during the trials. This is in contrast to our previous findings of respiratory plasticity in young adults during sleep. Sham studies did not show a change in any of the measured parameters. 2) We observed increased chemosensitivity with increased isocapnic hypoxic ventilatory response and hyperoxic suppression of VI in older vs. young adults during NREM sleep. Thus increased chemosensitivity, unconstrained by respiratory plasticity, may explain increased periodic breathing and central apneas in elderly adults during NREM sleep.

Clinical predictors and hemodynamic consequences of elevated peripheral chemosensitivity in optimally treated men with chronic systolic heart failure

AIMS

Augmented peripheral chemoreflex response is an important mechanism in the pathophysiology of chronic heart failure (CHF). This study characterizes prevalence and clinical predictors of this phenomenon in optimally managed male CHF patients, and seeks to describe the hemodynamic consequences of chemoreceptor hypersensitivity.

METHODS AND RESULTS

Thirty-four optimally managed CHF patients and 16 control subjects were prospectively studied. Hypoxic ventilatory response (HVR)-a measure of peripheral chemosensitivity-was calculated with the use of short nitrogen gas administrations. Systolic blood pressure (SBP) and heart rate (HR) following transient hypoxic challenges were recorded with a Nexfin monitor. Hemodynamic responses to hypoxia were expressed by the linear slopes between oxygen saturation (%) and SBP (mm Hg) or HR (beats/min). Elevated HVR was present in 15 (44%) of the CHF patients. Patients with elevated HVR exhibited higher levels of N-terminal pro-B-type natriuretic peptide, lower left ventricular ejection fraction, and higher prevalence of atrial fibrillation. CHF patients with elevated HVR had significantly greater SBP and HR responses to hypoxia than CHF patients with normal HVR.

CONCLUSIONS

Despite comprehensive pharmacotherapy, elevated HVR is prevalent in CHF patients, related to severity of the disease and associated with augmented hemodynamic responses to hypoxia. CHF patients with elevated HVR may be prone to unfavorable hemodynamic changes.

Ageing and cardiorespiratory response to hypoxia

The risk of severe altitude-induced diseases is related to ventilatory and cardiac responses to hypoxia and is dependent on sex, age and exercise training status. However, it remains unclear how ageing modifies these physiological adaptations to hypoxia. We assessed the physiological responses to hypoxia with ageing through a cross-sectional 20 year study including 4675 subjects (2789 men, 1886 women; 14-85 years old) and a longitudinal study including 30 subjects explored at a mean 10.4 year interval. The influence of sex, training status and menopause was evaluated. The hypoxia-induced desaturation and the ventilatory and cardiac responses to hypoxia at rest and exercise were measured. In men, ventilatory response to hypoxia increased (P < 0.002), while desaturation was less pronounced (P < 0.001) with ageing. Cardiac response to hypoxia was blunted with ageing in both sexes (P < 0.001). Similar results were found in the longitudinal study, with a decrease in cardiac and an increase in ventilatory response to hypoxia with ageing. These adaptive responses were less pronounced or absent in post-menopausal women (P < 0.01). At exercise, desaturation was greater in trained subjects but cardiac and ventilatory responses to hypoxia were preserved by training, especially in elderly people. In conclusion, respiratory response to hypoxia and blood oxygenation improve with ageing in men while cardiac response is blunted with ageing in both sexes. Training aggravates desaturation at exercise in hypoxia, improves the ventilatory response and limits the ageing-induced blunting of cardiac response to hypoxia. Training limits the negative effects of menopause in cardiorespiratory adaptations to hypoxia.

Carotid body function in aged rats: responses to hypoxia, ischemia, dopamine, and adenosine

The carotid body (CB) is the main arterial chemoreceptor with a low threshold to hypoxia. CB activity is augmented by A(2)-adenosine receptors stimulation and attenuated by D(2)-dopamine receptors. The effect of aging on ventilatory responses mediated by the CB to hypoxia, ischemia, and to adenosine and dopamine administration is almost unknown. This study aims to investigate the ventilatory response to ischemia and to adenosine, dopamine, and their antagonists in old rats, as well as the effect of hypoxia on adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in the aged CB. In vivo experiments were performed on young and aged rats anesthetized with pentobarbitone and breathing spontaneously. CB ischemia was induced by bilateral common carotid occlusions. cAMP content was measured in CB incubated with different oxygen concentrations. Hyperoxia caused a decrease in cAMP in the CB at all ages, but no differences were found between normoxia and hypoxia or between young and old animals. The endogenous dopaminergic inhibitory tonus is slightly reduced. However, both the ventilation decrease caused by exogenous dopamine and the increase mediated by A(2A)-adenosine receptors are not impaired in aged animals. The bradycardia induced by adenosine is attenuated in old rats. The CB's peripheral control of ventilation is preserved during aging. Concerns have also arisen regarding the clinical usage of adenosine to revert supraventricular tachycardia and the use of dopamine in critical care situations involving elderly people.

The progressive effects of ageing on chemosensitivity in healthy subjects

The aim of this study was to compare the central inspiratory drive (P(0.1)) response to hypoxia and hypercapnia between different age groups of elderly, nonsmoker, healthy subjects and young healthy controls. A random sample, proportionally stratified by age (65-69, 70-74, 75-79 and 80-84 yrs) from a sample of nonsmoker elderly subjects representative of a general population and 47 healthy subjects aged 20-40 were selected. Arterial blood gas, lung volumes, diffusing capacity, maximal respiratory pressure and oxygen uptake measurements were performed. Breathing pattern and mouth occlusion pressure, as well as P(0.1) responses to hyperoxic progressive hypercapnia and isocapnic progressive hypoxia were evaluated. The elderly subjects had lower P0.1 responses to hypoxia (0.017+/-0.006 vs. 0.031+/-0.008 kPa/%, P<0.001) and hypercapnia (0.042+/-0.018 vs. 0.051+/-0.030 kPa/mmHg, P=0.047) than the young healthy controls. Hypoxic sensitivity gradually decreased as age increased to 70-74 and remained unchanged from 75 years of age onward. CO(2) threshold was lower in the elderly groups than in young healthy controls. Lung volumes, inspiratory muscle strength and baseline metabolic rate were the principal determinants of hypoxic sensitivity. In summary, during old age, a progressive decline in hypoxic sensitivity and a decrease in the CO(2) threshold are experienced. These alterations remain stable from the age of 75 onward.