Circadian patterns in men acclimatized to intermittent hypoxia

Targeting HIF-1α in sickle cell disease and cancer: unraveling therapeutic opportunities and risks

INTRODUCTION

HIF-1α, a key player in medical science, holds immense significance in therapeutic approaches. This review delves into its complex dynamics, emphasizing the delicate balance required for its modulation. HIF-1α stands as a cornerstone in medical research, its role extending to therapeutic strategies. This review explores the intricate interplay surrounding HIF-1α, highlighting its critical involvement and the necessity for cautious modulation.

AREAS COVERED

In sickle cell disease (SCD), HIF-1α's potential to augment fetal hemoglobin (HbF) production and mitigate symptoms is underscored. Furthermore, its role in cancer is examined, particularly its influence on survival in hypoxic tumor microenvironments, angiogenesis, and metastasis. The discussion extends to the intricate relationship between HIF-1α modulation and cancer risks in SCD patients, emphasizing the importance of balancing therapeutic benefits and potential hazards.

EXPERT OPINION

Managing HIF-1α modulation in SCD patients requires a nuanced approach, considering therapeutic potential alongside associated risks, especially in exacerbating cancer risks. An evolutionary perspective adds depth, highlighting adaptations in populations adapted to low-oxygen environments and aligning cancer cell metabolism with primitive cells. The role of HIF-1α as a therapeutic target is discussed within the context of complex cancer biology and metabolism, acknowledging varied responses across diverse cancers influenced by intricate evolutionary adaptations.

The human blood transcriptome exhibits time-of-day-dependent response to hypoxia: Lessons from the highest city in the world

High altitude exposes humans to hypobaric hypoxia, which induces various physiological and molecular changes. Recent studies point toward interaction between circadian rhythms and the hypoxic response, yet their human relevance is lacking. Here, we examine the effect of different high altitudes in conjunction with time of day on human whole-blood transcriptome upon an expedition to the highest city in the world, La Rinconada, Peru, which is 5,100 m above sea level. We find that high altitude vastly affects the blood transcriptome and, unexpectedly, does not necessarily follow a monotonic response to altitude elevation. Importantly, we observe daily variance in gene expression, especially immune-related genes, which is largely altitude dependent. Moreover, using a digital cytometry approach, we estimate relative changes in abundance of different cell types and find that the response of several immune cell types is time- and altitude dependent. Taken together, our data provide evidence for interaction between the transcriptional response to hypoxia and the time of day in humans.

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Low oxygen availability upon high altitude vastly affects human blood transcriptome

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The transcriptomic changes upon altitude elevation are not necessarily monotonic

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The daily variance in gene expression is dependent on altitude

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The response of several immune cell types is time- and altitude dependent

Circadian patterns of heart rate, respiratory rate and skin temperature in hospitalized COVID-19 patients

Rationale

Vital signs follow circadian patterns in both healthy volunteers and critically ill patients, which seem to be influenced by disease severity in the latter. In this study we explored the existence of circadian patterns in heart rate, respiratory rate and skin temperature of hospitalized COVID-19 patients, and aimed to explore differences in circadian rhythm amplitude during patient deterioration.

Methods

We performed a retrospective study of COVID-19 patients admitted to the general ward of a tertiary hospital between April 2020 and March 2021. Patients were continuously monitored using a wireless sensor and fingertip pulse oximeter. Data was divided into three cohorts: patients who recovered, patients who developed respiratory insufficiency and patients who died. For each cohort, a population mean cosinor model was fitted to detect rhythmicity. To assess changes in amplitude, a mixed-effect cosinor model was fitted.

Results

A total of 429 patients were monitored. Rhythmicity was observed in heartrate for the recovery cohort (p<0.001), respiratory insufficiency cohort (p<0.001 and mortality cohort (p = 0.002). Respiratory rate showed rhythmicity in the recovery cohort (p<0.001), but not in the other cohorts (p = 0.18 and p = 0.51). Skin temperature also showed rhythmicity in the recovery cohort (p<0.001), but not in the other cohorts (p = 0.22 and p = 0.12). For respiratory insufficiency, only the amplitude of heart rate circadian pattern increased slightly the day before (1.2 (99%CI 0.16–2.2, p = 0.002)). In the mortality cohort, the amplitude of heart rate decreased (-1.5 (99%CI -2.6- -0.42, p<0.001)) and respiratory rate amplitude increased (0.72 (99%CI 0.27–1.3, p = 0.002) the days before death.

Conclusion

A circadian rhythm is present in heart rate of COVID-19 patients admitted to the general ward. For respiratory rate and skin temperature, rhythmicity was only found in patients who recover, but not in patients developing respiratory insufficiency or death. We found no consistent changes in circadian rhythm amplitude accompanying patient deterioration.

The Effect of Chronic Intermittent Hypobaric Hypoxia on Sleep Quality and Melatonin Serum Levels in Chilean Miners

High-altitude mining is an important economic resource for Chile. These workers are exposed to chronic intermittent hypobaric hypoxia (CIHH), which reduces their sleep quality and increases the risk of accidents and long-term illnesses. Melatonin, a hormone produced by the pineal gland, is a sleep inducer that regulates the circadian cycle and may be altered in populations subjected to CIHH. This work aimed to assess the relationship between altitude, sleep quality, and plasma melatonin concentrations in miners with CIHH exposure. 288 volunteers were recruited from five altitudes (0, 1,600, 2,500, 3,500, and 4,500 m). All volunteers worked for 7 days at altitude, followed by 7 days of rest at sea level. We performed anthropometric assessments, nocturnal oximetry, sleep quality and sleepiness surveys, and serum melatonin levels upon awakening. Although oxygen saturation progressively decreased and heart rate increased at higher altitudes, subjective perception of sleep quality was not significantly different, and sleepiness increased in all groups compared to population at sea level. Similarly, melatonin levels increased at all assessed altitudes compared to the population at sea level. These data confirm that sleep disturbances associated with CIHH increase morning melatonin levels. Therefore, this hormone and could potentially serve as a biomarker of sleep quality.

Melatonin Relations with Energy Metabolism as Possibly Involved in Fatal Mountain Road Traffic Accidents

Previous results evidenced acute exposure to high altitude (HA) weakening the relation between daily melatonin cycle and the respiratory quotient. This review deals with the threat extreme environments pose on body time order, particularly concerning energy metabolism. Working at HA, at poles, or in space challenge our ancestral inborn body timing system. This conflict may also mark many aspects of our current lifestyle, involving shift work, rapid time zone crossing, and even prolonged office work in closed buildings. Misalignments between external and internal rhythms, in the short term, traduce into risk of mental and physical performance shortfalls, mood changes, quarrels, drug and alcohol abuse, failure to accomplish with the mission and, finally, high rates of fatal accidents. Relations of melatonin with energy metabolism being altered under a condition of hypoxia focused our attention on interactions of the indoleamine with redox state, as well as, with autonomic regulations. Individual tolerance/susceptibility to such interactions may hint at adequately dealing with body timing disorders under extreme conditions.

Gender and the circadian pattern of body temperature in normoxia and hypoxia

Circadian patterns are at the core of many physiological processes, and their disruption can have short- and long-term consequences. This essay focuses on one of the best known patterns, the daily oscillation of body temperature (Tb), and the possibility of its difference between genders. From human and animal studies globally considered, the tentative conclusion is reached that differences in Tb circadian pattern between genders are very small and probably limited to the timing of the rhythm, not to its amplitude. Such similarity between genders, despite the differences in hormonal systems, presumably testifies to the importance that the Tb circadian pattern plays in the economy of the organism and its survival against environmental challenges. The second part of the article presents some previously unpublished experimental data from behaving male and female rats during hypoxia in synchronized conditions. In adult rats hypoxia (10.5% O₂ for three days) caused a profound drop of the Tb daily oscillations; by day 3 they were 55% (♀) and 22% (♂) of the normoxic amplitudes, with a statistically significant gender difference. In pre-puberty rats (26-day old) hypoxia caused a major disruption of the circadian pattern qualitatively similar to the adults but not different between genders. Hence, on the basis of this preliminary set of data, it seems that sex-hormones may be a factor in how the Tb daily pattern responds to hypoxia. The implications of the effects of hypoxia on the circadian patterns, and the possibility that such effects may differ between genders, are matters that could have biological and clinical implications and deserve further investigations.

The Utility of the Swine Model to Assess Biological Rhythms and Their Characteristics during Different Stages of Residence in a Simulated Intensive Care Unit: A Pilot Study

The purpose of this pilot study was to explore the utility of the mammalian swine model under simulated intensive care unit (sICU) conditions and mechanical ventilation (MV) for assessment of the trajectory of circadian rhythms of sedation requirement, core body temperature (CBT), pulmonary mechanics (PM) and gas exchange (GE). Data were collected prospectively with an observational time-series design to describe and compare circadian rhythms of selected study variables in four swine mechanically ventilated for up to seven consecutive days. We derived the circadian (total variance explained by rhythms of τ between 20 and 28 h)/ultradian (total variance explained by rhythms of τ between 1 and <20 h) bandpower ratio to assess the robustness of circadian rhythms, and compare findings between the early (first 3 days) and late (subsequent days) sICU stay. All pigs exhibited statistically significant circadian rhythms (τ between 20 and 28 h) in CBT, respiratory rate and peripheral oxygen saturation, but circadian rhythms were detected less frequently for sedation requirement, spontaneous minute volume, arterial oxygen tension, arterial carbon dioxide tension and arterial pH. Sedation did not appear to mask the circadian rhythms of CBT, PM and GE. Individual subject observations were more informative than group data, and provided preliminary evidence that (a) circadian rhythms of multiple variables are lost or desynchronized in mechanically ventilated subjects, (b) robustness of circadian rhythm varies with subject morbidity and (c) healthier pigs develop more robust circadian rhythm profiles over time in the sICU. Comparison of biological rhythm profiles among sICU subjects with similar severity of illness is needed to determine if the results of this pilot study are reproducible. Identification of consistent patterns may provide insight into subject morbidity and timing of such therapeutic interventions as weaning from MV.

Time of day affects chemoreflex sensitivity and the carbon dioxide reserve during NREM sleep in participants with sleep apnea

Our investigation was designed to determine whether the time of day affects the carbon dioxide reserve and chemoreflex sensitivity during non-rapid eye movement (NREM) sleep. Ten healthy men with obstructive sleep apnea completed a constant routine protocol that consisted of sleep sessions in the evening (10 PM to 1 AM), morning (6 AM to 9 AM), and afternoon (2 PM to 5 PM). Between sleep sessions, the participants were awake. During each sleep session, core body temperature, baseline levels of carbon dioxide (PET(CO2)) and minute ventilation, as well as the PET(CO2) that demarcated the apneic threshold and hypocapnic ventilatory response, were measured. The nadir of core body temperature during sleep occurred in the morning and was accompanied by reductions in minute ventilation and PetCO2 compared with the evening and afternoon (minute ventilation: 5.3 ± 0.3 vs. 6.2 ± 0.2 vs. 6.1 ± 0.2 l/min, P < 0.02; PET(CO2): 39.7 ± 0.4 vs. 41.4 ± 0.6 vs. 40.4 ± 0.6 Torr, P < 0.02). The carbon dioxide reserve was reduced, and the hypocapnic ventilatory response increased in the morning compared with the evening and afternoon (carbon dioxide reserve: 2.1 ± 0.3 vs. 3.6 ± 0.5 vs. 3.5 ± 0.3 Torr, P < 0.002; hypocapnic ventilatory response: 2.3 ± 0.3 vs. 1.6 ± 0.2 vs. 1.8 ± 0.2 l·min(-1)·mmHg(-1), P < 0.001). We conclude that time of day affects chemoreflex properties during sleep, which may contribute to increases in breathing instability in the morning compared with other periods throughout the day/night cycle in individuals with sleep apnea.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Circadian rhythms and sleep-related breathing disorders

Recent studies have provided evidence that the human circadian timing system has an influence on respiration and respiratory control. Both sleep and circadian mechanisms combine to mediate the rise in lower airway resistance in nocturnal asthma. In rats, circadian rhythms in minute ventilation are present in both wakefulness and sleep, implying that circadian and sleep mechanisms also combine to influence the control of breathing. The circadian timing system causes a nocturnal increase in the chemoreflex threshold and it is suggested that this may increase the propensity for nocturnal sleep apnea. This hypothesis is supported by a model analysis of human chemoreflex control, and relevant published data are reviewed. The clinical implications of this putative circadian contribution to sleep apnea are potentially very significant, but relevant data are scarce and directions for future research are discussed.

Hypoxia and circadian patterns

In mammals, biological time keeping is organised with a hierarchical and pyramidal structure, under the control of the master clock in the suprachiasmatic hypothalamic nuclei (SCN). Body temperature (Tb) and metabolic rate have robust circadian patterns, and probably represent primary variables controlled closely by the SCN. From the data of studies in animals (mostly rats) and humans, it appears that the most common effect of prolonged hypoxia is to decrease, and in some cases to abolish, the amplitudes of the daily oscillations, irrespective of the state of arousal or activity level. On the other hand, the evidence is that hypoxia causes only minimal and transient perturbation of the period of the rhythm. The fact that hypoxia modifies the circadian oscillations of variables as important as body temperature and metabolism leads to the expectation that the daily rhythms of many other functions are perturbed by hypoxia, according to their link to the primary variables. The data currently available, although sparse and fragmentary, support this view. It is speculated that the alterations of the normal circadian oscillations can contribute to many common symptoms during sustained hypoxia, from sleep fragmentation, to malaise and loss of appetite.

The impact of altered climatic conditions and altitude on circadian physiology

Knowing the output of the "body clock" is fundamental to the science of chronobiology. As the clock resides within the suprachiasmatic nuclei, direct measurement is not feasible and therefore, characteristics of the clock are often inferred from the measurement of marker rhythms, one of which is core temperature. Core temperature is often the marker rhythm of choice due to ease of measurement, particularly in field conditions. However, if the output of the "body clock" is to be inferred from measurement of this variable, it is important to establish whether environmental conditions change or moderate the circadian rhythm of core temperature. Although the majority of circadian patterns do demonstrate independence from such exogenous influences, there does appear to be seasonal variation to their period. Given that humans can easily travel to environments of altered temperature and altitude, there is a need to ascertain the exact effect of such change on the rhythm of core temperature. This review will therefore outline the evidence that the circadian rhythm of core temperature is affected by ambient temperature and by hypoxia. Furthermore, the review will discuss whether these environmental factors act as zeitgebers (affecting the endogenous rhythm) or as masking influences of the inherent rhythm.

Correlations between the circadian patterns of body temperature, metabolism and breathing in rats

It had been demonstrated previously that the circadian patterns of activity and state of arousal are not essential for the manifestation of the daily patterns of pulmonary ventilation (V(E)), tidal volume (V(T)) and breathing frequency (f). In this study we investigated the extent of the linkage between the circadian pattern of breathing and those of body temperature (T(b)) and metabolic rate (oxygen consumption, V(O2), and carbon dioxide production, V(CO2)). Rats were instrumented for measurements of T(b) (by telemetry), and placed in a chamber for continuous 13-day period of measurement of breathing (by a modification of the barometric methodology), and of V(O2) and V(CO2) (by an open flow method). After the first 4 days in control conditions under a 12 h light:12 h dark (L:D) cycle, a perturbation was introduced on day 4, with an L-phase prolongation of 12 h, and on day 9, with an D-phase prolongation of 12 h. During the control days 1-4, all variables had daily oscillations (higher values in D), in phase with each other. During the perturbations (days 4-13), changes in T(b), V(O2) and V(CO2), averaged over the whole period, correlated significantly better with f than with V(T). Day-by-day X-Y loops indicated that V (E), V(T) and f could lead significantly the changes of T(b), V(O2) and V(CO2), and that these relations changed throughout the perturbation period. In addition, f and V(T) did not change necessarily in phase with each other. It is concluded that neither the oscillation in T(b) nor that in metabolism can be considered the direct cause of the daily oscillation of breathing. Presumably, the circadian pattern of breathing reflects the interplay of the daily patterns of many variables, none acting as the primary guide of the breathing daily rhythm.

Physiology of temperature regulation: comparative aspects

Few environmental factors have a larger influence on animal energetics than temperature, a fact that makes thermoregulation a very important process for survival. In general, endothermic species, i.e., mammals and birds, maintain a constant body temperature (Tb) in fluctuating environmental temperatures using autonomic and behavioural mechanisms. Most of the knowledge on thermoregulatory physiology has emerged from studies using mammalian species, particularly rats. However, studies with all vertebrate groups are essential for a more complete understanding of the mechanisms involved in the regulation of Tb. Ectothermic vertebrates-fish, amphibians and reptiles-thermoregulate essentially by behavioural mechanisms. With few exceptions, both endotherms and ectotherms develop fever (a regulated increase in Tb) in response to exogenous pyrogens, and regulated hypothermia (anapyrexia) in response to hypoxia. This review focuses on the mechanisms, particularly neuromediators and regions in the central nervous system, involved in thermoregulation in vertebrates, in conditions of euthermia, fever and anapyrexia.

Chronic hypoxia modulates the function and expression of melatonin receptors in the rat carotid body

Melatonin modulates the carotid chemoreceptor response to chemical stimuli, and chronic hypoxia changes circadian activities and carotid body function. The purpose of this study was to test the hypothesis that chronic hypoxia alters the function and expression of melatonin receptors in the rat carotid body. Effects of melatonin on the carotid responses to hypercapnic acidosis and to hypoxia were determined by spectrofluorometric measurement of cytosolic calcium ([Ca(2+)](i)) in fura-2-loaded type-I (glomus) cells dissociated from carotid bodies obtained from normoxic (Nx) or chronically hypoxic (CH) rats breathing 10% oxygen for 4 wk. In the Nx control, melatonin concentration dependently attenuated the peak [Ca(2+)](i) response to hypercapnic acidosis, whereas it augmented the [Ca(2+)](i) response to cyanide or deoxygenated buffer. Yet, melatonin enhanced the peak [Ca(2+)](i) responses to hypercapnic acidosis or hypoxia in the CH glomus cells. An agonist of melatonin receptors, iodomelatonin also elevated the hypercapnic or hypoxic responses in the CH groups. The melatonin-induced changes in the [Ca(2+)](i) responses were abolished by pretreatment with nonselective mt(1)/MT(2) antagonist, luzindole, and by MT(2) antagonists, 4-phenyl-2-propionamidotetraline or DH97. These findings suggest a functional modulation of melatonin receptors in the glomus cells in chronic hypoxia. To evaluate the level of expression of the melatonin receptors, in situ hybridization study with antisense mt(1) and MT(2) receptor mRNA oligonucleotide probes was performed on the Nx and CH carotid bodies. There were significant increases in the expression of mt(1) and MT(2) receptors in the CH comparing with the Nx group. Taken together, our results suggest an upregulation of the carotid expression of melatonin receptors by chronic hypoxia, which modulates the carotid response to melatonin for the circadian influence on breathing.

Scaling the daily oscillations of breathing frequency and skin temperature in mammals

Among mammals, the peak-trough difference (PTD) of the circadian pattern of body temperature (T(b)) drops very little with the increase in body mass (W), despite the large increase in heat capacitance and thermal inertia. We asked whether this might be contributed by systematic differences in the circadian pattern of breathing frequency (f) and skin temperature (Tskin), which are parts of the control mechanisms of heat loss. Measurements had been conducted on animals of eight species, chosen to cover a four-fold range in W, while resting and awake. The oscillation of f preceded that of T(b) in 7 of the 8 species, and its acrophase did not correlate with W. The daily mean and PTD of f scaled with W in a similar manner (respectively, W(-)(23) and W(-)(0.29)), the PTD averaging about 20% of the daily mean. The circadian oscillations of Tskin, measured in specimens of five species at three locations (abdomen, ear and thigh), were in phase with T(b). Neither the PTD nor the acrophase of Tskin changed systematically with W. The differences between T(b) and Tskin (means, peaks and troughs) decreased significantly with W; on average, the T(b)-Tskin difference scaled to W(-)(0.19). In conclusion, the relative amplitudes and the acrophase of Tskin and f did not show systematic inter-species differences. The progressive increase of Tskin with W could be a factor in maintaining the PTD of T(b) within a narrow range among mammals of very different size.

Scaling the amplitudes of the circadian pattern of resting oxygen consumption, body temperature and heart rate in mammals

We questioned whether the amplitudes of the circadian pattern of body temperature (T(b)), oxygen consumption (V (O(2))) and heart rate (HR) changed systematically among species of different body weight (W). Because bodies of large mass have a greater heat capacitance than those of smaller mass, if the relative amplitude (i.e., amplitude/mean value) of metabolic rate was constant, one would expect the T(b) oscillation to decrease with the increase in the species W. We compiled data of T(b), V (O(2)) and HR from a literature survey of over 200 studies that investigated the circadian pattern of these parameters. Monotremata, Marsupials and Chiroptera, were excluded because of their characteristically low metabolic rate and T(b). The peak-trough ratios of V (O(2)) (42 species) and HR (35 species) averaged, respectively, 1.57+/-0.08, and 1.35+/-0.07, and were independent of W. The daily high values of T(b) did not change, while the daily low T(b) values slightly increased, with the species W; hence, the high-low T(b) difference (57 species) decreased with W (3.3 degrees C.W(-0.13)). However, the decrease in T(b) amplitude with W was much less than expected from physical principles, and the high-low T(b) ratio remained significantly above unity even in the largest mammals. Thus, it appears that in mammals, despite the huge differences in physical characteristics, the amplitude of the circadian pattern is a fixed (for V (O(2)) and HR), or almost fixed (for T(b)), fraction of the 24-h mean value. Presumably, the amplitudes of the oscillations are controlled parameters of physiological significance.

Day/night pattern of arterial blood gases in the cow

Both metabolic rate and pulmonary ventilation change throughout 24h with a circadian pattern. Because their changes occur almost in synchrony and by a similar amount, blood gases may remain steady within a narrow range. We tested this possibility in five cows (Bos taurus, Bruna Italiana breed), maintained in a stable at 29 degrees C, under natural light-dark (LD) regime, by measuring arterial blood gases every 3h for 2 days. All cows presented a clear day/night pattern of body temperature (T(b)), with an average peak-trough difference (PTD) of 0.5 degrees C. Breathing rate oscillated significantly in three out of five animals, with a group-mean PTD of 2 breaths per minute, and it was time-advanced with respect to the oscillation of T(b). Significant oscillations in arterial pH, bicarbonate, partial pressure of oxygen and partial pressure of carbon dioxide (Pa(CO2) ) were observed in, respectively, 1 cow out of 5, 1/5, 3/5 and 5/5. Of all these variables, group-mean analysis revealed a significant day/night pattern only for Pa(CO2), and even in this case the average PTD was less than 1 mmHg. We conclude that, in the cow, blood gases remain remarkably stable throughout the 24 h. Hence, the daily oscillations of body temperature, breathing rate, and probably of many other factors affecting metabolic rate and pulmonary ventilation do not preclude an excellent homeostasis of blood gases.

Effects of Hypoxia on the Circadian Patterns in Men

We tested the hypothesis that acute hypoxia may alter the circadian pattern of body temperature in adult humans. Six healthy subjects were studied in normoxia, hypoxia (approximately 13% inspired O(2)), and again normoxia, each session lasting >24 h and spaced a few days apart, with a constant routine protocol of sustained wakefulness and minimal activity. Some parameters (e.g., tympanic and abdominal temperatures, heart rate) were recorded continuously; others (e.g., oxygen consumption and pulmonary ventilation) were monitored for approximately 10 min every 2 h. The amplitudes of the circadian oscillation of tympanic, abdominal, and calf skin temperatures were reduced in hypoxia, averaging, respectively, 61%, 80% and 50% of the normoxic amplitude. Oxygen consumption and pulmonary ventilation, which presented a circadian pattern in normoxia, had no longer significant oscillations during hypoxia, whereas the opposite was the case for heart rate and diastolic pressure. Therefore, acute hypoxia can disturb the normal circadian patterns and, specifically, depress those of body temperature. These effects, qualitatively similar to those observed in chronically hypoxic animals and humans, could contribute to sleep disturbances at high altitude.

Do circadian rhythms in respiratory control contribute to sleep-related breathing disorders?

Sleep-related respiratory dysfunction compromises the health and quality of life of millions of people worldwide, underscoring the need for a full understanding of the mechanisms by which the respiratory control system is altered at night. This paper suggests the hypothesis that the circadian timing system may play a role in the pathogenesis of some types of sleep-related breathing disorders. Recent studies have provided evidence that the circadian timing system has an influence on respiration and respiratory control, even in the absence of sleep. These new data are reviewed and potential mechanisms underlying the circadian modulation of breathing are outlined, identifying important gaps in our knowledge. It is proposed that circadian rhythms in respiratory control may increase the propensity for nocturnal respiratory instability and recurrent apnea. Importantly, circadian and sleep mechanisms appear to have additive effects on breathing, suggesting that the circadian timing system can potentially amplify or suppress sleep-related breathing abnormalities, depending upon the characteristics of the circadian output and the time of day at which sleep occurs.

Circadian pattern of ventilation during prolonged hypoxia in conscious rats

In mammals, metabolic rate is well known to present a circadian oscillation. Because metabolism is a major determinant of the magnitude of the hypoxic ventilatory response, we hypothesized that the level of this response would follow a circadian pattern. To this end, we measured pulmonary ventilation (VE), oxygen consumption (V(O(2))), body temperature (Tb) and activity simultaneously and continuously in conscious adult rats by non-invasive methods. Measurements were made in a 12:12-h light (L):dark (D) cycle for 3 days in air, and then for 4 days in 10.5% normobaric hypoxia (HX). In normoxia, all variables oscillated, with the highest values in D. In HX, the circadian Tb and V(O(2)) oscillations were blunted, due to a decrease in their D values. The hypoxic VE response (% increase in VE from the corresponding air value) was greater in L than in D. This L-D difference was proportionate to that of the V(O(2)) response such that the hyperventilatory response (% increase in VE/V(O(2))) was similar throughout the day. The VE/V(O(2)) response was also similar between L and D when it was compared for the same level of activity; in this case, however, there was no L-D difference in the VE or V(O(2)) response. We conclude that the circadian oscillation in the hypoxic VE response was related to the time-of-day changes in the effect of HX on V(O(2)), and that, in the rat, the presence of a circadian clock does not compromise the hyperventilatory response to HX, as this response presents no time-of-day variation.

Circadian patterns of breathing

In the rat, a mostly nocturnal animal, activity, body temperature and metabolic rate increase during the dark hours of the day. Since all these variables are known to influence breathing, it is expected that also pulmonary ventilation (VE) will present a circadian pattern. In rats chronically instrumented for measurements of body temperature and activity by telemetry, carbon dioxide production and oxygen consumption (V(O(2))) were measured continuously for several days by an open-circuit method, while VE was monitored by a modification of the barometric technique. Tidal volume, frequency, and VE increased in the dark (D) compared to the light (L) hours, with minor L-D differences in VE/V(O(2)). L-D differences in activity were not responsible for the circadian pattern of VE. Both in hypoxia and in hypercapnia the degree of hyperventilation (percent increase in VE/V(O(2))) was essentially independent of the time of the day, despite the fact that in hypoxia, differently from hypercapnia, the amplitude of the circadian pattern of all variables decreased, activity being the least affected, and body temperature the most. These effects of hypoxia, which occurred before and after sino-aortic denervation and did not compromise the period of the biological clock, may be mediated by the hypothalamic thermoregulatory centers. The data of these experiments and of others reviewed in this article indicate that (1) breathing and its control mechanisms accompany the daily oscillations of numerous physiological variables, and (2) the advantages of a biological clock do not compromise the adequacy of the hyperventilatory responses to chemical challenges.

Plasma melatonin, pinealocyte morphology, and surface receptors/antigen expression on macrophages/microglia in the pineal gland following a high-altitude exposure

The present study examined the effects of high-altitude exposure on the pineal gland, the main source of production of melatonin. It was surmised that hypoxia experienced at high altitude, caused by decreased oxygen tension in the ambient air, might lead to some structural alterations in the pineal gland and, hence, affect its melatonin production. Adult Wistar rats were exposed to an altitude of 8,000 m for 2 hr in an altitude chamber and then sacrificed at various time intervals after the exposure. Normal rats kept at ground level were used as controls. Blood samples were collected at various time intervals for measurement of plasma melatonin level, and the pineal glands from both groups were processed for electron microscopy and immunohistochemistry. The plasma melatonin level showed a steady increase following altitude exposure peaking at 7 days and returned to control levels thereafter. Between 1 and 4 days after altitude exposure, the mitochondrial number and lipid droplets in the pinealocytes appeared to be reduced compared with those in control rats. At 7 days, however, the mitochondrial numbers and lipid droplets were noticeably increased. At the same time interval, the expression of complement type 3 receptors and major histocompatibility class II antigens as detected with the antibodies OX-42 and OX-6, respectively, in macrophages/microglia was up-regulated compared with that in the control rats and those killed at earlier times. This was attributed to the increased serum melatonin after the altitude exposure. By 14 and 21 days, the ultrastructure of pinealocytes and immunoreactivity of macrophages/microglia were comparable with those in the control rats. We conclude from this study that an altitude exposure in rats leads to an increase in melatonin production, which returned to control levels with passage of time.

The circadian pattern of breathing in conscious adult rats

Recently, a circadian oscillation in pulmonary ventilation (VE) was reported in conscious, undisturbed rats [Respir. Physiol. 120 (2000) 179], with a pattern similar to that of body temperature (Tb), oxygen consumption (V(O(2))) and activity. The present study explored the relationship between the daily VE pattern and these rhythms. Adult rats (n=23) were instrumented for measurements of Tb and activity by telemetry, and placed in a chamber for measurement of VE by the barometric method and V(O(2)) by an open-flow method. Simultaneous recordings were made continuously for 3 consecutive days in a 12-h light:12-h dark (L:D) cycle. All variables showed substantial daily oscillations, with significantly higher values in the D phase, and approximately proportionate changes in VE and V(O(2)). Daily changes in tidal volume (VT) relative to inspiratory time (TI), rather than in TI relative to total breath duration, accounted for the oscillation in VE. The VT rhythm was phase-advanced relative to those of V(O(2)) and Tb. L-D differences in VE persisted when comparison between the phases was made for the same level of either very low or very high activity. We conclude that the oscillation in VE does not depend on the daily changes in activity. Rather, the daily pattern of VE is likely shaped by the oscillations of multiple physiological variables, two of which may be Tb and V(O(2)).