Central-peripheral chemoreceptor ventilatory interaction in awake goats

Update on Chemoreception: Influence on Cardiorespiratory Regulation and Pathophysiology

We examine recent findings that have revealed interdependence of function within the chemoreceptor pathway regulating breathing and sympathetic vasomotor activity and the hypersensitization of these reflexes in chronic disease states. Recommendations are made as to how these states of hyperreflexia in chemoreceptors and muscle afferents might be modified in treating sleep apnea, drug-resistant hypertension, chronic heart failure-induced sympathoexcitation, and the exertional dyspnea of chronic obstructive pulmonary disease.

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.

Interdependent feedback regulation of breathing by the carotid bodies and the retrotrapezoid nucleus

The retrotrapezoid nucleus (RTN) regulates breathing in a CO₂ - and state-dependent manner. RTN neurons are glutamatergic and innervate principally the respiratory pattern generator; they regulate multiple aspects of breathing, including active expiration, and maintain breathing automaticity during non-REM sleep. RTN neurons encode arterial PCO2 /pH via cell-autonomous and paracrine mechanisms, and via input from other CO₂ -responsive neurons. In short, RTN neurons are a pivotal structure for breathing automaticity and arterial PCO2 homeostasis. The carotid bodies stimulate the respiratory pattern generator directly and indirectly by activating RTN via a neuronal projection originating within the solitary tract nucleus. The indirect pathway operates under normo- or hypercapnic conditions; under respiratory alkalosis (e.g. hypoxia) RTN neurons are silent and the excitatory input from the carotid bodies is suppressed. Also, silencing RTN neurons optogenetically quickly triggers a compensatory increase in carotid body activity. Thus, in conscious mammals, breathing is subject to a dual and interdependent feedback regulation by chemoreceptors. Depending on the circumstance, the activity of the carotid bodies and that of RTN vary in the same or the opposite directions, producing additive or countervailing effects on breathing. These interactions are mediated either via changes in blood gases or by brainstem neuronal connections, but their ultimate effect is invariably to minimize arterial PCO2 fluctuations. We discuss the potential relevance of this dual chemoreceptor feedback to cardiorespiratory abnormalities present in diseases in which the carotid bodies are hyperactive at rest, e.g. essential hypertension, obstructive sleep apnoea and heart failure.

Integration of Central and Peripheral Respiratory Chemoreflexes

A debate has raged since the discovery of central and peripheral respiratory chemoreceptors as to whether the reflexes they mediate combine in an additive (i.e., no interaction), hypoadditive or hyperadditive manner. Here we critically review pertinent literature related to O2 and CO2 sensing from the perspective of system integration and summarize many of the studies on which these seemingly opposing views are based. Despite the intensity and quality of this debate, we have yet to reach consensus, either within or between species. In reviewing this literature, we are struck by the merits of the approaches and preparations that have been brought to bear on this question. This suggests that either the nature of combination is not important to system responses, contrary to what has long been supposed, or that the nature of the combination is more malleable than previously assumed, changing depending on physiological state and/or respiratory requirement.

Peripheral chemoreceptors determine the respiratory sensitivity of central chemoreceptors to CO2 : role of carotid body CO2

We asked if the type of carotid body (CB) chemoreceptor stimulus influenced the ventilatory gain of the central chemoreceptors to CO2 . The effect of CB normoxic hypocapnia, normocapnia and hypercapnia (carotid body PCO2 ≈ 22, 41 and 68 mmHg, respectively) on the ventilatory CO2 sensitivity of central chemoreceptors was studied in seven awake dogs with vascularly-isolated and extracorporeally-perfused CBs. Chemosensitivity with one CB was similar to that in intact dogs. In four CB-denervated dogs, absence of hyper-/hypoventilatory responses to CB perfusion with PCO2 of 19-75 mmHg confirmed separation of the perfused CB circulation from the brain. The group mean central CO2 response slopes were increased 303% for minute ventilation (V̇I)(P ≤ 0.01) and 251% for mean inspiratory flow rate (VT /TI ) (P ≤ 0.05) when the CB was hypercapnic vs. hypocapnic; central CO2 response slopes for tidal volume (VT ), breathing frequency (fb ) and rate of rise of the diaphragm EMG increased in 6 of 7 animals but the group mean changes did not reach statistical significance. Group mean central CO2 response slopes were also increased 237% for V̇I(P ≤ 0.01) and 249% for VT /TI (P ≤ 0.05) when the CB was normocapnic vs. hypocapnic, but no significant differences in any of the central ventilatory response indices were found between CB normocapnia and hypercapnia. These hyperadditive effects of CB hyper-/hypocapnia agree with previous findings using CB hyper-/hypoxia.We propose that hyperaddition is the dominant form of chemoreceptor interaction in quiet wakefulness when the chemosensory control system is intact, response gains physiological, and carotid body chemoreceptors are driven by a wide range of O2 and/or CO2 .

Acute and chronic effects of carotid body denervation on ventilation and chemoreflexes in three rat strains

Brown Norway (BN) rats have a relatively specific deficit in CO2 sensitivity. This deficit could be due to an abnormally weak carotid body contribution to CO2 sensitivity. Accordingly, we tested the hypothesis that CBD would have less of an effect on eupnoeic breathing and CO2 sensitivity in the BN rats compared to other rat strains.We measured ventilation and blood gases at rest (eupnoea) and during hypoxia (FIO2 =0.12) or hypercapnia (FICO2 =0.07) before and up to 23 days after bilateral or Sham CBD in BN, Sprague–Dawley (SD) and Dahl Salt-Sensitive (SS) rats. In all three rat strains, CBD elicited eupnoeic hypoventilation (PaCO2 +8.7–11.0 mmHg) 1–2 days post-CBD (P <0.05), and attenuated ventilatory responses to hypoxia (P <0.05) and venous sodium cyanide (NaCN; P<0.05), while sham CBD had no effect on resting breathing, blood gases or chemoreflexes (P >0.05). In contrast, CBD had no effect on CO2 sensitivity (˙VE/PaCO2) in all strains (P>0.05). Eupnoeic PaCO2 returned to pre-CBD values within 15–23 days post-CBD. Thus, the effects of CBD in rats (1) further support an important role for the carotid bodies in eupnoeic blood gas regulation, (2) suggest that the carotid bodies are not a major determinant of CO2 sensitivity in rats, and (3) may not support the concept of an interaction among the peripheral and central chemoreceptors in rats.

Peripheral chemoreceptors determine the respiratory sensitivity of central chemoreceptors to CO(2)

We assessed the contribution of carotid body chemoreceptors to the ventilatory response to specific CNS hypercapnia in eight unanaesthetized, awake dogs. We denervated one carotid body (CB) and used extracorporeal blood perfusion of the reversibly isolated remaining CB to maintain normal CB blood gases (normoxic, normocapnic perfusate), to inhibit (hyperoxic, hypocapnic perfusate) or to stimulate (hypoxic, normocapnic perfusate) the CB chemoreflex, while the systemic circulation, and therefore the CNS and central chemoreceptors, were exposed consecutively to four progressive levels of systemic arterial hypercapnia via increased fractional inspired CO(2) for 7 min at each level. Neither unilateral CB denervation nor CB perfusion, per se, affected breathing. Relative to CB control conditions (normoxic, normocapnic perfusion), we found that CB chemoreflex inhibition decreased the slope of the ventilatory response to CNS hypercapnia in all dogs to an average of 19% of control values (range 0-38%; n = 6), whereas CB chemoreflex stimulation increased the slope of the ventilatory response to CNS hypercapnia in all dogs to an average of 223% of control values (range 204-235%; n = 4). We conclude that the gain of the CNS CO(2)/H(+) chemoreceptors in dogs is critically dependent on CB afferent activity and that CNS-CB interaction results in hyperadditive ventilatory responses to central hypercapnia.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

An interdependent model of central/peripheral chemoreception: evidence and implications for ventilatory control

In this review we discuss the implications for ventilatory control of newer evidence suggesting that central and peripheral chemoreceptors are not functionally separate but rather that they are dependent upon one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the carotid body chemoreceptors and vice versa i.e., they are interdependent. We examine potential interactions of the interdependent central and carotid body (CB) chemoreceptors with other ventilatory-related inputs such as central hypoxia, lung stretch, and exercise. The limitations of current approaches addressing this question are discussed and future studies are suggested.

Contributions of central and peripheral chemoreceptors to the ventilatory response to CO2/H+

The major objective of this review is to evaluate existing information and reach conclusions regarding whether there is interaction between P(CO(2))/H(+) stimulation of carotid (peripheral) and intracranial (central) chemoreceptors. Interaction is defined as a ventilatory response to simultaneous changes in the degree of Pco2/H(+) stimulation of both chemoreceptors that is greater (hyperadditive) or less (hypoadditive) than the sum of the responses when stimulation of each set of chemoreceptors is individually altered. Simple summation of the simultaneous changes in stimuli results in no interaction (i.e., additive interaction). Knowledge of the nature of central/peripheral interaction is crucial for determining the physiological significance of newer models of ventilatory control based on recent neuroanatomic observations of the circuitry of key elements of the ventilatory control system. In this review, we will propose that these two sets of receptors are not functionally separate but rather that they are dependent on one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the peripheral chemoreceptors and possibly other breathing-related reflex afferents as well. The short format of this minireview demands that we be somewhat selective in developing our ideas. We will briefly discuss the limitations of experiments used to study CO(2)/H(+) sensitivity and interaction to date, traditional views of the relative contributions of peripheral and central chemoreceptors to CO(2)/H(+) sensitivity, the evidence for and against different types of interaction, and the effect of tonic carotid chemoreceptor afferent activity on central control mechanisms.

A negative interaction between brainstem and peripheral respiratory chemoreceptors modulates peripheral chemoreflex magnitude

Interaction between central (brainstem) and peripheral (carotid body) respiratory chemosensitivity is vital to protect blood gases against potentially deleterious fluctuations, especially during sleep. Previously, using an in situ arterially perfused, vagotomized, decerebrate preparation in which brainstem and peripheral chemoreceptors are perfused separately (i.e. dual perfused preparation; DPP), we observed that the phrenic response to specific carotid body hypoxia was larger when the brainstem was held at 25 Torr P(CO(2)) compared to 50 Torr P(CO(2)). This suggests a negative (i.e. hypo-additive) interaction between chemoreceptors. The current study was designed to (a) determine whether this observation could be generalized to all carotid body stimuli, and (b) exclude the possibility that the hypo-additive response was the simple consequence of ventilatory saturation at high brainstem P(CO(2)). Specifically, we tested how steady-state brainstem P(CO(2)) modulates peripheral chemoreflex magnitude in response to carotid body P(CO(2)) and P(O(2)) perturbations, both above and below eupnoeic levels. We found that the peripheral chemoreflex was more responsive the lower the brainstem P(CO(2)) regardless of whether the peripheral chemoreceptors received stimuli which increased or decreased activation. These findings demonstrate a negative interaction between brainstem and peripheral chemosensitivity in the rat in the absence of ventilatory saturation. We suggest that a negative interaction in humans may contribute to increased controller gain associated with sleep-related breathing disorders and propose that the assumption of simple addition between chemoreceptor inputs used in current models of the respiratory control system be reconsidered.

Brainstem PCO2 modulates phrenic responses to specific carotid body hypoxia in an in situ dual perfused rat preparation

Inputs from central (brainstem) and peripheral (carotid body) respiratory chemoreceptors are coordinated to protect blood gases against potentially deleterious fluctuations. However, the mathematics of the steady-state interaction between chemoreceptors has been difficult to ascertain. Further, how this interaction affects time-dependent phenomena (in which chemoresponses depend upon previous experience) is largely unknown. To determine how central P(CO2) modulates the response to peripheral chemostimulation in the rat, we utilized an in situ arterially perfused, vagotomized, decerebrate preparation, in which central and peripheral chemoreceptors were perfused separately (i.e. dual perfused preparation (DPP)). We carried out two sets of experiments: in Experiment 1, we alternated steady-state brainstem P(CO2) between 25 and 50 Torr in each preparation, and applied specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) under both conditions; in Experiment 2, we applied four 5 min bouts (separated by 5 min) of specific carotid body hypoxia (60 Torr P(O2) and 40 Torr P(CO2)) while holding the brainstem at either 30 Torr or 50 Torr P(CO2). We demonstrate that the level of brainstem P(CO2) modulates (a) the magnitude of the phrenic responses to a single step of specific carotid body hypoxia and (b) the magnitude of time-dependent phenomena. We report that the interaction between chemoreceptors is negative (i.e. hypo-additive), whereby a lower brainstem P(CO2) augments phrenic responses resulting from specific carotid body hypoxia. A negative interaction may underlie the pathophysiology of central sleep apnoea in populations that are chronically hypocapnic.

Development of ventilatory response to transient hypercapnia and hypercapnic hypoxia in term infants

Whereas peripheral chemoreceptor oxygen sensitivity increases markedly after birth, previous studies of ventilatory responses to CO(2) in term infants have shown no postnatal development. However, the hypercapnic challenges applied have usually been long-term, which meant that the effect of central chemoreceptors dominated. Oscillatory breathing, apneas, and sighs cause transient Pco(2) changes, probably primarily stimulating peripheral chemoreceptors. We wanted to assess whether the immediate ventilatory responses to step changes in inspired CO(2) and O(2) in term infants undergo postnatal developmental changes. Twenty-six healthy term infants were studied during natural sleep 2 d and 8 wk postnatally. Ventilatory responses to a randomized sequence of 15 s hypercapnia (3% CO(2)), hypoxia (15% O(2)), and hypercapnic hypoxia (3% CO(2) + 15% O(2)) were recorded breath-by-breath using a pneumotachometer. Response rate, stimulus-response time, and response magnitude were analyzed with ANOVA after coherent averaging. Response rate increased with age by 30% (hypercapnia), 318% (hypoxia), and 302% (hypercapnic hypoxia). Response rate during hypercapnic hypoxia exceeded rate during hypercapnia plus rate during hypoxia in wk 8, but not on d 2. Time to half-maximum response decreased by 3.4 s with age for the two hypercapnic stimuli but was unchanged for hypoxia. Response magnitude was unchanged for hypercapnia, but increased for the two hypoxic stimuli. In conclusion, an interaction between the effects of hypercapnia and hypoxia on ventilatory response rate emerged between postnatal d 2 and wk 8 in term infants. Concomitantly, stimulus-response time to hypercapnic stimuli declined markedly. The development of a prompt response to transient hypercapnia may be important for infant respiratory stability.

Multiple sites for central chemoreception: their roles in response sensitivity and in sleep and wakefulness

Central chemoreceptors appear to be widely distributed in the brainstem. Why are there so many central chemoreceptor sites? This review focuses on two hypotheses. (1) The high sensitivity of the respiratory control system as a whole to small changes in systemic P(CO(2)) results from an additive, or greater, effect of the multiple central chemoreceptor sites. Each site provides a fraction of the total response and, importantly, provides tonic excitatory input in eucapnia as well. (2) Individual central chemoreceptor sites vary in effectiveness depending on the arousal or vigilance state of the animal. For example, some sites are more important in wakefulness; others in sleep. Proof for these hypotheses depends critically on obtaining accurate measures of stimulus intensity at each chemoreceptor site in vivo.

Ventilatory responses to hypercapnia and hypoxia following chronic hypercapnia in the rat

This study investigated the effects of an 18 week exposure to 10% CO(2) in air on minute ventilation (V(E)), breathing pattern and the chemoresponiveness of rats to hypoxic and hyperoxic stimuli. We found that V(E) remained elevated over the 18 weeks. Nonetheless, the breathing pattern changed significantly. Tidal volume increased and the durations of inspiration and the total cycle decreased. After the sustained hypercapnia the mean Pa(CO(2)) was 72.0+/-5.1 (S. D.) mmHg. Every 6 weeks the chemoresponiveness of the CO(2)-exposed rats was tested by an acute exposure sequentially to room air, then a 6% O(2), 10% CO(2) and 84% N(2) gas mixture, and finally a 90% O(2) in 10% CO(2) mixture. On either room air or the hyperoxic-hypercapnic mixture V(E) fell to its pre-hypercapnic level. On the hypoxic-hypercapnic mixture V(E) increased significantly. These results demonstrate that the initial stimulating effect of 10% CO(2) on V(E) persisted for the entire 18 weeks without altering hypoxic or hyperoxic ventilatory responses.

Effects of carotid body hypocapnia during ventilatory acclimatization to hypoxia

Hypoxic ventilatory sensitivity is increased during ventilatory acclimatization to hypoxia (VAH) in awake goats, resulting in a time-dependent increase in expired ventilation (VE). The objectives of this study were to determine whether the increased carotid body (CB) hypoxic sensitivity is dependent on the level of CB CO2 and whether the CB CO2 gain is changed during VAH. Studies were carried out in adult goats with CB blood gases controlled by an extracorporeal circuit while systemic (central nervous system) blood gases were regulated independently by the level of inhaled gases. Acute VE responses to CB hypoxia (CB PO2 40 Torr) and CB hypercapnia (CB PCO2 50 and 60 Torr) were measured while systemic normoxia and isocapnia were maintained. CB PO2 was then lowered to 40 Torr for 4 h while the systemic blood gases were kept normoxic and normocapnic. During the 4-h CB hypoxia, VE increased in a time-dependent manner. Thirty minutes after return to normoxia, the ventilatory response to CB hypoxia was significantly increased compared with the initial response. The slope of the CB CO2 response was also elevated after VAH. An additional group of goats (n = 7) was studied with a similar protocol, except that CB PCO2 was lowered throughout the 4-h CB hypoxic exposure to prevent reflex hyperventilation. CB PCO2 was progressively lowered throughout the 4-h CB hypoxic period to maintain VE at the control level. After the 4-h CB hypoxic exposure, the ventilatory response to hypoxia was also significantly elevated. However, the slope of the CB CO2 response was not elevated after the 4-h hypoxic exposure. These results suggest that CB sensitivity to both O2 and CO2 is increased after 4 h of CB hypoxia with systemic isocapnia. The increase in CB hypoxic sensitivity is not dependent on the level of CB CO2 maintained during the 4-h hypoxic period.

The effects of carotid body hypocapnia on ventilation in goats

This study was designed to examine the influence of carotid body (CB) hypocapnia on ventilation by selectively perfusing the CB through an extracorporeal circuit in 19 goats. When PcbCO2 was decreased from normocapnic levels in 14 awake goats (delta PcbCO2 = 10.9 Torr), PaCO2 increased 5.6 Torr (P less than 0.05) and VE decreased 24% (P less than 0.001) (mean values). The ventilatory sensitivity to inspired CO2 was not changed by CB hypocapnia in 5 of these goats, but the response was shifted to the right. During CB hypocapnia, ventilatory instability, including apnea, was observed in 4 of 14 goats; this irregular breathing continued at elevated levels of PaCO2. In 5 anesthetized goats, CB hypocapnia (delta PcbCO2 = 18.0 Torr) decreased VE by 70% in the intact state, but produced no significant ventilatory depression after CB denervation. We conclude that CB hypocapnia depresses ventilation in both awake and anesthetized goats mostly through CB chemoreceptor effects, and suggest that this hypoventilation may predispose to ventilatory instability in some animals.