Short-term potentiation of ventilation after different levels of hypoxia

After-Discharge in the Upper Airway Muscle Genioglossus Following Brief Hypoxia

STUDY OBJECTIVES

Genioglossus after-discharge is thought to protect against pharyngeal collapse by minimising periods of low upper airway muscle activity. How genioglossus after-discharge occurs and which single motor units (SMUs) are responsible for the phenomenon are unknown. The aim of this study was to investigate genioglossal after-discharge.

METHODS

During wakefulness, after-discharge was elicited 8-12 times in healthy individuals with brief isocapnic hypoxia (45-60s of 10%O2 in N2) terminated by a single breath of 100% O2. Genioglossus SMUs were designated as firing solely, or at increased rate, during inspiration (Inspiratory phasic [IP] and inspiratory tonic [IT] respectively); solely, or at increased rate, during expiration (Expiratory phasic [EP] or expiratory tonic [ET] respectively) or firing constantly without respiratory modulation (Tonic). SMUs were quantified at baseline, the end of hypoxia, the hyperoxic breath and the following 8 normoxic breaths.

RESULTS

210 SMU's were identified in 17 participants. Genioglossus muscle activity was elevated above baseline for 7 breaths after hyperoxia (p<0.001), indicating a strong after-discharge effect. After-discharge occurred due to persistent firing of IP and IT units that were recruited during hypoxia, with minimal changes in ET, EP or Tonic SMUs. The firing frequency of units that were already active changed minimally during hypoxia or the afterdischarge period (P>0.05).

CONCLUSION

That genioglossal after-discharge is almost entirely due to persistent firing of previously silent inspiratory SMUs provides insight into the mechanisms responsible for the phenomenon and supports the hypothesis that the inspiratory and expiratory/tonic motor units within the muscle have idiosyncratic functions.

Hyperadditive ventilatory response arising from interaction between the carotid chemoreflex and the muscle mechanoreflex in healthy humans

Physical exercise potentiates the carotid chemoreflex control of ventilation (VE). Hyperadditive neural interactions may partially mediate the potentiation. However, some neural interactions remain incompletely explored. As the potentiation occurs even during low-intensity exercise, we tested the hypothesis that the carotid chemoreflex and the muscle mechanoreflex could interact in a hyperadditive fashion. Fourteen young healthy subjects inhaled randomly, in separate visits, 12% O₂ to stimulate the carotid chemoreflex and 21% O₂ as control. A rebreathing circuit maintained isocapnia. During gases administration, subjects either remained at rest (i.e., normoxic and hypoxic rest) or the muscle mechanoreflex was stimulated via passive knee movement (i.e., normoxic and hypoxic movement). Surface muscle electrical activity did not increase during the passive movement, confirming the absence of active contractions. Hypoxic rest and normoxic movement similarly increased VE [change (mean ± SE) = 1.24 ± 0.72 vs. 0.73 ± 0.43 l/min, respectively; P = 0.46], but hypoxic rest only increased tidal volume (Vt), and normoxic movement only increased breathing frequency (BF). Hypoxic movement induced greater VE and mean inspiratory flow (Vt/Ti) increase than the sum of hypoxic rest and normoxic movement isolated responses (VE change: hypoxic movement = 3.72 ± 0.81 l/min vs. sum = 1.96 ± 0.83 l/min, P = 0.01; Vt/Ti change: hypoxic movement = 0.13 ± 0.03 l/s vs. sum = 0.06 ± 0.03 l/s, P = 0.02). Moreover, hypoxic movement increased both Vt and BF. Collectively, the results indicate that the carotid chemoreflex and the muscle mechanoreflex interacted, mediating a hyperadditive ventilatory response in healthy humans. NEW & NOTEWORTHY The main finding of this study was that concomitant carotid chemoreflex and muscle mechanoreflex stimulation provoked greater ventilation increase than the sum of ventilation increase induced by stimulation of each reflex in isolation, which, consequently, supports that the carotid chemoreflex and the muscle mechanoreflex interacted, mediating a hyperadditive ventilatory response in healthy humans.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

The effect of partial acclimatization to high altitude on loop gain and central sleep apnoea severity

BACKGROUND AND OBJECTIVE

Loop gain is an engineering term that predicts the stability of a feedback control system, such as the control of breathing. Based on earlier studies at lower altitudes, it was hypothesized that acclimatization to high altitude would lead to a reduction in loop gain and thus central sleep apnoea (CSA) severity.

METHODS

This study used exposure to very high altitude to induce CSA in healthy subjects to investigate the effect of partial acclimatization on loop gain and CSA severity. Measurements were made on 12 subjects (age 30 ± 10 years, body mass index 22.8 ± 1.9, eight males, four females) at an altitude of 5050 m over a 2-week period upon initial arrival (days 2-4) and following partial acclimatization (days 12-14). Sleep was studied by full polysomnography, and resting arterial blood gases were measured. Loop gain was measured by the 'duty cycle' method (duration of hyperpnoea/cycle length).

RESULTS

Partial acclimatization to high-altitude exposure was associated with both an increase in loop gain (duty cycle fell from 0.60 ± 0.05 to 0.55 ± 0.06 (P = 0.03)) and severity of CSA (apnoea-hypopnoea index increased from 76.8 ± 48.8 to 115.9 ± 20.2 (P = 0.01)), while partial arterial carbon dioxide concentration fell from 29 ± 3 to 26 ± 2 (P = 0.01).

CONCLUSIONS

Contrary to the results at lower altitudes, at high-altitude loop gain and severity of CSA increased.

Episodic hypoxia induces long-term facilitation of upper airway muscle activity in spontaneously breathing anaesthetized rats

We performed these experiments to determine if repeated exposure to episodic hypoxia induces long term facilitation (LTF) in anaesthetized spontaneously breathing rats. A previous study in spontaneously breathing rats was unable to demonstrate evidence of LTF with repeated hypoxia, but this may have been due to the low number of hypoxic episodes used. We hypothesized that with sufficient exposure, episodic hypoxia LTF of genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) activities would be elicited. Experiments were performed in 24 anaesthetized spontaneously breathing rats with intact vagi. Peak and tonic GG, HG and Dia EMG activities were recorded before, during and for 1 h following exposure to eight (n = 8) or three (n = 8) episodes of isocapnic hypoxia ( = 0.1) each of 3 min duration. A third time control series was also performed with exposure to normoxia alone ( = 0.28, n = 8). Short-term potentiation of GG and HG muscle activity developed during the early period after repeated exposure to eight and three hypoxic episodes. LTF, however, occurred only after eight hypoxic episodes. This manifested as an increase in peak GG and Dia inspiratory muscle activity and tonic HG activity. LTF of respiratory breathing frequency was also induced, reflected by a reduction in inspiratory and expiratory time. These findings support our initial hypothesis that LTF in the anaesthetized, spontaneously breathing rat is dependent on the number of exposures to hypoxia and show that the responses to repetitive hypoxia are composed of both short and long-term facilitatory changes.

Effect of stimulus cycle time on acute respiratory responses to intermittent hypercapnic hypoxia in unsedated piglets

To determine whether stimulus frequency affects physiological compensation to an intermittent respiratory stimulus, we studied piglets (n = 43) aged 14.8 +/- 2.4 days. A 24-min total hypercapnic hypoxia (HH) (10% O(2)-6% CO(2)-balance N(2) = HH) was delivered in 24-, 8-, 4-, or 2-min cycles alternating with air. Controls (n = 10) breathed air continuously. Minute ventilation and temperature were not different between the 2-min and 24-min groups, with neither different from controls during recovery. Piglets exposed to 8-min cycles had ventilatory stimulation, whereas those exposed to 4-min cycles had significant depression of ventilation. Despite this, piglets in these intermediate intermittent HH (IHH) groups (8- and 4-min cycles) showed more severe acidosis and attenuated temperature changes (P < 0.001 and P < 0.01 for pH and temperature vs. 24 min, respectively). Cycle time affected the ability of young piglets to tolerate IHH. More severe respiratory acidosis developed when IHH was delivered in intermediate (4 min or 8 min) cycles compared with the same total dose as a single episode or in short (2 min) cycles.

Periodic breathing and genetics

The episodic waxing and waning of ventilation is a fundamental event in sleep apnea syndromes. Post-hypoxic frequency decline (PHFD) and periodic breathing (PB) are evoked by brief hypoxic exposures in unanaesthetized and unrestrained inbred C57BL/6J mice, but not in A/J mice; and expression of PHFD differs not only among these mice strains but in among rat strains as well. These observations along with the current literature on genetic factors that operate on ventilatory behavior at rest and with chemosensory drive lead to the hypothesis that genetic factors infer some proportion of risk for the ventilatory instability observed in human sleep apnea syndromes.

Periodic breathing in the mouse

The hypothesis was that unstable breathing might be triggered by a brief hypoxia challenge in C57BL/6J (B6) mice, which in contrast to A/J mice are known not to exhibit short-term potentiation; as a consequence, instability of ventilatory behavior could be inherited through genetic mechanisms. Recordings of ventilatory behavior by the plethsmography method were made when unanesthetized B6 or A/J animals were reoxygenated with 100% O(2) or air after exposure to 8% O(2) or 3% CO(2)-10% O(2) gas mixtures. Second, we examined the ventilatory behavior after termination of poikilocapnic hypoxia stimuli in recombinant inbred strains derived from B6 and A/J animals. Periodic breathing (PB) was defined as clustered breathing with either waxing and waning of ventilation or recurrent end-expiratory pauses (apnea) of > or = 2 average breath durations, each pattern being repeated with a cycle number > or = 3. With the abrupt return to room air from 8% O(2), 100% of the 10 B6 mice exhibited PB. Among them, five showed breathing oscillations with apnea, but none of the 10 A/J mice exhibited cyclic oscillations of breathing. When the animals were reoxygenated after 3% CO(2)-10% O(2) challenge, no PB was observed in A/J mice, whereas conditions still induced PB in B6 mice. (During 100% O(2) reoxygenation, all 10 B6 mice had PB with apnea.) Expression of PB occurred in some but not all recombinant mice and was not associated with the pattern of breathing at rest. We conclude that differences in expression of PB between these strains indicate that genetic influences strongly affect the stability of ventilation in the mouse.

Mutant mice deficient in NOS-1 exhibit attenuated long-term facilitation and short-term potentiation in breathing

The objective of the present study is to examine the potential role of nitric oxide (NO) in short-term potentiation (STP) and long-term facilitation (LTF) of breathing. Experiments were performed in wild-type (WT) and mutant mice deficient in nitric oxide synthase-1 (NOS-1), as well as in WT mice administered the NOS-1 inhibitor 7-nitroindazole (7-NI; 50 mg x kg(-1); I.P.). Respiratory responses following either single or recurrent episodes of hypoxia (7% O2, balance N2) were analysed in unanaesthetised animals by body plethysmography along with rate of O2 consumption (VO2)) and CO2 production (VCO2). After a single hypoxic challenge, respiration in WT mice remained elevated for 5 min, suggesting STP in ventilation. Following termination of three consecutive hypoxic challenges, respiration remained elevated during normoxia for as long as 30 min, indicating LTF in breathing under awake conditions. STP and LTF were significantly attenuated or absent in WT mice after 7-NI. A similar attenuation or absence of STP and LTF was also seen in NOS-1 mutant mice. Changes in VO2 and VCO2 were comparable among mice during the post-hypoxic period, suggesting that the absence of STP and LTF was not due to alterations in body metabolism. These results suggest endogenous NO is an important physiological modulator of ventilatory STP and LTF.

S-nitrosothiols signal the ventilatory response to hypoxia

Increased ventilation in response to hypoxia has been appreciated for over a century, but the biochemistry underlying this response remains poorly understood. Here we define a pathway in which increased minute ventilation (&Vdot;E ) is signalled by deoxyhaemoglobin-derived S-nitrosothiols (SNOs). Specifically, we demonstrate that S-nitrosocysteinyl glycine (CGSNO) and S-nitroso-l-cysteine (l-CSNO)-but not S-nitroso-d-cysteine (d-CSNO)-reproduce the ventilatory effects of hypoxia at the level of the nucleus tractus solitarius (NTS). We show that plasma from deoxygenated, but not from oxygenated, blood produces the ventilatory effect of both SNOs and hypoxia. Further, this activity is mediated by S-nitrosoglutathione (GSNO), and GSNO activation by gamma-glutamyl transpeptidase (gamma-GT) is required. The normal response to hypoxia is impaired in a knockout mouse lacking gamma-GT. These observations suggest that S-nitrosothiol biochemistry is of central importance to the regulation of breathing.

An integrated model of the human ventilatory control system: the response to hypoxia

The mathematical model of the respiratory control system described in a previous companion paper is used to analyse the ventilatory response to hypoxic stimuli. Simulation of long-lasting isocapnic hypoxia at normal alveolar PCO2 (40 mmHg=5.33 kPa) shows the occurrence of a biphasic response, characterized by an initial peak and a subsequent hypoxic ventilatory decline (HVD). The latter is about as great as 2/3 of the initial peak and can be mainly ascribed to prolonged neural hypoxia. If isocapnic hypoxia is performed during hypercapnia (PACO2=48 mmHg =6.4 kPa), the ventilatory response is stronger and HVD is minimal (about 1/10-1/5 of the initial peak). During poikilocapnic hypoxia, ventilation exhibits smaller changes compared with the isocapnic case, with a rapid return toward baseline within a few minutes. Moreover, a significant undershoot occurs at the termination of the hypoxic period. This undershoot may lead to apnea and to a transient destabilization of the control system if the peripheral chemoreflex gain and time delay are twofold greater than basal.

Plasticity of cardiorespiratory neural processing: classification and computational functions

Neural plasticity, or malleability of neuronal structure and function, is an important attribute of the mammalian forebrain and is generally thought to be a kernel of biological intelligence. In this review, we examine some reported manifestations of neural plasticity in the cardiorespiratory system and classify them into four functional categories, integral; differential; memory; and statistical-type plasticity. At the cellular and systems level the myriad forms of cardiorespiratory plasticity display emergent and self-organization properties, use- and disuse-dependent and pairing-specific properties, short-term and long-term potentiation or depression, as well as redundancy in series or parallel structures, convergent pathways or backup and fail-safe surrogate pathways. At the behavioral level, the cardiorespiratory system demonstrates the capability of associative and nonassociative learning, classical and operant conditioning as well as short-term and long-term memory. The remarkable similarity and consistency of the various types of plasticity exhibited at all levels of organization suggest that neural plasticity is integral to cardiorespiratory control and may subserve important physiological functions.

Adaptive and dynamic control of respiratory and motor systems during object manipulation

This investigation was designed to examine the relationship between breathing and prehension movements during object manipulation. Seated subjects (n=12) wore a facemask that was attached to a pneumotachometer which measured airflow. Initially, subjects completed baseline trials that were preceded and followed by an object lift. Subsequently, in response to an auditory signal the subjects reached forward, grasped and lifted an instrumented object that weighed either 150 g or 1000 g while their fingertip forces and movements were measured. The auditory signal was triggered by airflow in response to four experimental conditions (1) expiratory onset (2) inspiratory onset (3) mid-inspiration and (4) mid-expiration. Five trials for each of the four conditions were completed with each weight. The results revealed that inspiratory time was longer under baseline conditions after the subjects lifted the 150 g object as compared to the 1000 g object. In addition, the response latency and reach duration were significantly slower for the 150 g object compared to the 1000 g object during the experimental trials. These temporal measures were significantly correlated to inspiratory time for three of the four experimental conditions but no significant relationship with expiratory time was found. Lastly, lifting of the object occurred during expiration during most experimental conditions. We conclude that an adaptive process is formulated for both the motor and respiratory system in response to changes in motor output and/or sensory inputs associated with object manipulation, that might manifest itself in the pattern of breathing subsequent to removal of these stimuli. Furthermore, we suggest that motor inputs associated with the initiation of object manipulation interact with the control of respiratory timing so that the motor and respiratory systems are coupled. We speculate that this relationship may ensure that some motor tasks are performed during expiration to take advantage of changes in intrathoracic pressure that assist in postural maintenance during completion of the task.