Comparison of aortic and carotid chemoreceptor responses to hypercapnia and hypoxia

Effects of Prenatal Cannabinoids Exposure upon Placenta and Development of Respiratory Neural Circuits

Cannabis use has risen dangerously during pregnancy in the face of incipient therapeutic use and a growing perception of safety. The main psychoactive compound of the Cannabis sativa plant is the phytocannabinoid delta-9-tetrahydrocannabinol (A-9 THC), and its status as a teratogen is controversial. THC and its endogenous analogues, anandamide (AEA) and 2-AG, exert their actions through specific receptors (eCBr) that activate intracellular signaling pathways. CB1r and CB2r, also called classic cannabinoid receptors, together with their endogenous ligands and the enzymes that synthesize and degrade them, constitute the endocannabinoid system. This system is distributed ubiquitously in various central and peripheral tissues. Although the endocannabinoid system's most studied role is controlling the release of neurotransmitters in the central nervous system, the study of long-term exposure to cannabinoids on fetal development is not well known and is vital for understanding environmental or pathological embryo-fetal or postnatal conditions. Prenatal exposure to cannabinoids in animal models has induced changes in placental and embryo-fetal organs. Particularly, cannabinoids could influence both neural and nonneural tissues and induce embryo-fetal pathological conditions in critical processes such as neural respiratory control. This review aims at the acute and chronic effects of prenatal exposure to cannabinoids on placental function and the embryo-fetal neurodevelopment of the respiratory pattern. The information provided here will serve as a theoretical framework to critically evaluate the teratogen effects of the consumption of cannabis during pregnancy.

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

Carotid bodies contribute to sympathoexcitation induced by acute salt overload

NEW FINDINGS

What is the central question of this study? Does carotid body input contribute to the hyperosmotic responses? What is the main finding and its importance? The response to NaCl overload is sympathorespiratory excitation. Eliminating the carotid body input reduced sympathoexcitation but did not affect the increase in phrenic burst frequency, whereas eliminating the hypothalamus prevented the tachypnoea and sympathoexcitation. We conclude that the carotid body inputs are essential for the full expression of the sympathetic activity during acute NaCl overload, whereas the tachypnoea depends on hypothalamic mechanisms.

ABSTRACT

Acute salt excess activates central osmoreceptors, which trigger an increase in sympathetic and respiratory activity. The carotid bodies also respond to hyperosmolality of the extracellular compartment, but their contribution to the sympathoexcitatory and ventilatory responses to NaCl overload remains unknown. To evaluate their contribution to acute NaCl overload, we recorded thoracic sympathetic (tSNA), phrenic (PNA) and carotid sinus nerve activities in decorticate in situ preparations of male Holtzman rats (60-100 g) while delivering intra-arterial infusions of hyperosmotic NaCl (0.17, 0.3, 0.7, 1.5 and 2.0 mol l^-1 ; 200 μl infusion over 25-30 s, with a 10 min time interval between solutions) or mannitol (0.3, 0.5, 1.0, 2.7 and 3.8 mol l^-1 ) progressively. The cumulative infusions of hyperosmotic NaCl increased the perfusate osmolality to 341 ± 5 mosmol (kg water)^-1 and elicited an immediate increase in PNA and tSNA (n = 6, P < 0.05) in sham-denervated rats. Carotid body removal attenuated sympathoexcitation (n = 5, P < 0.05) but did not affect the tachypnoeic response. A precollicular transection disconnecting the hypothalamus abolished the sympathoexcitatory and tachypnoeic responses to NaCl overload (n = 6, P < 0.05). Equi-osmolar infusions of mannitol did not alter the PNA and tSNA in sham-denervated rats (n = 5). Sodium chloride infusions increased carotid sinus nerve activity (n = 10, P < 0.05), whereas mannitol produced negligible changes (n = 5). The results indicate that carotid bodies are activated by acute NaCl overload, but not by mannitol. We conclude that the carotid bodies contribute to the increased sympathetic activity during acute NaCl overload, whereas the ventilatory response is mainly mediated by hypothalamic mechanisms.

Evolution and physiology of neural oxygen sensing

Major evolutionary trends in animal physiology have been heavily influenced by atmospheric O2 levels. Amongst other important factors, the increase in atmospheric O2 which occurred in the Pre-Cambrian and the development of aerobic respiration beckoned the evolution of animal organ systems that were dedicated to the absorption and transportation of O2, e.g., the respiratory and cardiovascular systems of vertebrates. Global variations of O2 levels in post-Cambrian periods have also been correlated with evolutionary changes in animal physiology, especially cardiorespiratory function. Oxygen transportation systems are, in our view, ultimately controlled by the brain related mechanisms, which senses changes in O2 availability and regulates autonomic and respiratory responses that ensure the survival of the organism in the face of hypoxic challenges. In vertebrates, the major sensorial system for oxygen sensing and responding to hypoxia is the peripheral chemoreflex neuronal pathways, which includes the oxygen chemosensitive glomus cells and several brainstem regions involved in the autonomic regulation of the cardiovascular system and respiratory control. In this review we discuss the concept that regulating O2 homeostasis was one of the primordial roles of the nervous system. We also review the physiology of the peripheral chemoreflex, focusing on the integrative repercussions of chemoreflex activation and the evolutionary importance of this system, which is essential for the survival of complex organisms such as vertebrates. The contribution of hypoxia and peripheral chemoreflex for the development of diseases associated to the cardiovascular and respiratory systems is also discussed in an evolutionary context.

Distribution and innervation of putative arterial chemoreceptors in the bullfrog (Rana catesbeiana)

Peripheral arterial chemoreceptors have been located previously in the carotid labyrinth, the aortic arch, and the pulmocutaneous artery of frogs. In the present study we used cholera toxin B neuronal tract tracing and immunohistochemical markers for cholinergic cells (vesicular acetylcholine transporter [VAChT]), tyrosine hydroxylase (TH), and serotonin (5HT) to identify putative O2-sensing cells in Rana catesbeiana. We found potential O2-sensing cells in all three vascular areas innervated by branches of the vagus nerve, whereas only cells in the carotid labyrinth were innervated by the glossopharyngeal nerve. Cells containing either 5HT or TH were found in all three sites, whereas cells containing both neurotransmitters were found only in the carotid labyrinth. Cell bodies containing VAChT were not found at any site. The morphology and innervation of putative O2-sensing cells were similar to those of glomus cells found in other vertebrates. The presence of 5HT- and TH-immunoreactive cells in the aorta, pulmocutaneous artery, and carotid labyrinth appears to reflect a phylogenetic transition between the major neurotransmitter seen in the putative O2-sensing cells of fish (5HT) and those found in the glomus cells of mammals (acetylcholine, adenosine, and catecholamines).

Neural regulation of cardiovascular response to exercise: role of central command and peripheral afferents

During dynamic exercise, mechanisms controlling the cardiovascular apparatus operate to provide adequate oxygen to fulfill metabolic demand of exercising muscles and to guarantee metabolic end-products washout. Moreover, arterial blood pressure is regulated to maintain adequate perfusion of the vital organs without excessive pressure variations. The autonomic nervous system adjustments are characterized by a parasympathetic withdrawal and a sympathetic activation. In this review, we briefly summarize neural reflexes operating during dynamic exercise. The main focus of the present review will be on the central command, the arterial baroreflex and chemoreflex, and the exercise pressure reflex. The regulation and integration of these reflexes operating during dynamic exercise and their possible role in the pathophysiology of some cardiovascular diseases are also discussed.

Breathing and the nervous system

Breathing requires complex interactions of the central and peripheral nervous systems with the respiratory system. It involves cortical (volitional) as well as subcortical (automatic) output. Cortical output is mainly through the corticospinal tract, whereas the brainstem sends signals via the reticulospinal tract. Groups of nuclei in the brainstem (pneumotaxic center, dorsal and ventral respiratory group), situated in the pons and medulla, function as rhythm generators. Some of these nuclei have intrinsic pacemaker activity; however, their output is affected extensively by various chemical (through aortic and carotid bodies), mechanical (stretch reflexes), and neural feedbacks from the peripheral nervous system involving cranial nerves V, VII, IX, X, and XI. Brainstem nuclei also have central chemoreceptors that detect changes in serum carbon dioxide and pH. Various neurologic disorders such as stroke or neurodegenerative diseases (Parkinson's disease, multiple system atrophy) can adversely affect respiration and may even be the first sign of disease onset. Clinicians should have a better understanding of this complex but important physiological process to better appreciate pathologies affecting it. Future research is needed to enhance our understanding of this intricate process.

Autonomic control of the cardiovascular system in the cat during hypoxemia

This study aimed to determine the roles played by the autonomic interoreceptors, the carotid bodies (cbs) and the aortic bodies (abs) in anesthetized, paralyzed, artificially ventilated cats' response to systemic hypoxemia. Four 15min challenges stimulated each of 15 animals: (1) hypoxic hypoxia (10%O₂ in N₂; HH) in the intact (int) cat where both abs and cbs sent neural traffic to the nucleus tractus solitarius (NTS); (2) carbon monoxide hypoxia (30%O₂ in N₂ with the addition of CO; COH) in the intact cat where only the abs sent neural traffic to the NTS; (3) HH in the cat after transection of both aortic depressor nerves, resecting the aortic bodies (HHabr), where only the cbs sent neural traffic to the NTS; (4) COH to the abr cat where neither abs nor cbs sent neural traffic to the NTS. Cardiac output (C.O.), contractility (dP/dt(MAX)), systolic/diastolic pressures, aortic blood pressure, total peripheral resistance, pulmonary arterial pressure, and pulmonary vascular resistance (PVR) were measured. When both cbs and abs were active the maximum increases were observed except for PVR which decreased. Some variables showed the cbs to have a greater effect than the abs. The abs proved to be important during some challenges for maintaining blood pressure. The data support the critically important role for the chemoreceptor-sympathetic nervous system connection during hypoxemia for maintaining viable homeostasis, with some differences between the cbs and the abs.

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.

Inhibitory effects of hyperoxia and methemoglobinemia on H(2)S induced ventilatory stimulation in the rat

The aim of this study was to clarify, using in vitro and in vivo approaches in the rat, the site of mediation of the inhibition of H(2)S induced arterial chemoreceptor stimulation, by hyperoxia and methemoglobinemia. We first determined the ventilatory dose-response curves during intravenous injections of H(2)S. A very high dose of NaHS, i.e. 0.4 μmol (concentration: 800 μM), was needed to stimulate breathing within 1s following i.v. injection. Above this level (and up to 2.4 μmol, with a concentration of 4800 μM), a dose-dependent effect of H(2)S injection was observed. NaHS injection into the thoracic aorta produced the same effect, suggesting that within one circulatory time, H(2)S pulmonary exchange does not dramatically reduce H(2)S concentrations in the arterial blood. The ventilatory response to H(2)S was abolished in the presence of MetHb (12.8%) and was significantly depressed in hyperoxia and, surprisingly, in 10% hypoxia. MetHb per se did not affect the ventilatory response to hypoxia or hyperoxia, but dramatically enhanced the oxidation of H(2)S in vitro, with very fast kinetics. These findings suggest that, the decrease/oxidation of exogenous H(2)S in the blood is the primary effect of MetHb in vivo. In contrast, the in vitro oxidative properties of blood for H(2)S were not affected by the level of [Formula: see text] between 23 and >760 mmHg. This suggests that the inhibition of the ventilatory response to H(2)S by hyperoxia during aortic or venous injection originates within the CB and not in the blood. The implications of these results on the role of endogenous H(2)S in the arterial chemoreflex are discussed.

Effects of chemostimuli on [Ca2+]i responses of rat aortic body type I cells and endogenous local neurons: comparison with carotid body cells

Mammalian aortic bodies (ABs) are putative peripheral arterial chemoreceptors whose function remains controversial, partly because information on their cellular physiology is lacking. In this study, we used ratiometric Ca2+ imaging to investigate for the first time chemosensitivity in short-term cultures of dissociated cells of juvenile rat ABs, located near the junction of the left vagus and recurrent laryngeal nerves. Among the surviving cell population were glomus or type I cell clusters, endogenous local neurons and glia-like cells. A variety of chemostimuli, including hypoxia, isohydric or acidic hypercapnia, and isocapnic acidosis, caused a rise in intracellular [Ca2+] in AB type I cells. The [Ca2+]i responses were indistinguishable from those in carotid body (CB) type I cells grown in parallel cultures from the same animals, and responses to acidic hypercapnia were prevented by the non-specific voltage-gated Ca2+ channel antagonist, 2mM Ni2+. Furthermore, we identified a subpopulation (∼40%) of glia-like cells in AB cultures that resembled CB type II cells based on their approximately equal sensitivity to ATP and UTP, consistent with the expression of purinergic P2Y2 receptors. Finally, we showed that some local neurons, known to be uniquely associated with these AB paraganglia in situ, generated robust [Ca2+]i responses to these chemostimuli. Thus, these AB type I cells and associated putative type II cells resemble those from the well-studied CB. Unlike the CB, however, they also associate with a special group of endogenous neurons which we propose may subserve a sensory function in local cardiovascular reflexes.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Carotid chemoreceptor modulation of blood flow during exercise in healthy humans

Carotid chemoreceptor (CC) inhibition reduces sympathetic nervous outflow in exercising dogs and humans. We sought to determine if CC suppression increases muscle blood flow in humans during exercise and hypoxia. Healthy subjects (N = 13) were evaluated at rest and during constant-work leg extension exercise while exposed to either normoxia or hypoxia (inspired O(2) tension, F(IO(2)), ≈ 0.12, target arterial O(2) saturation = 85%). Subjects breathed hyperoxic gas (F(IO(2)) ≈ 1.0) and/or received intravenous dopamine to inhibit the CC while femoral arterial blood flow data were obtained continuously with pulsed Doppler ultrasound. Exercise increased heart rate, mean arterial pressure, femoral blood flow and conductance compared to rest. Transient hyperoxia had no significant effect on blood flow at rest, but increased femoral blood flow and conductance transiently during exercise without changing blood pressure. Similarly, dopamine had no effect on steady-state blood flow at rest, but increased femoral blood flow and conductance during exercise. The transient vasodilatory response observed by CC inhibition with hyperoxia during exercise could be blocked with simultaneous CC inhibition with dopamine. Despite evidence of dopamine reducing ventilation during hypoxia, no effect on femoral blood flow, conductance or mean arterial pressure was observed either at rest or during exercise with CC inhibition with dopamine while breathing hypoxia. These findings indicate that the carotid chemoreceptor contributes to skeletal muscle blood flow regulation during normoxic exercise in healthy humans, but that the influence of the CC on blood flow regulation in hypoxia is limited.

Carotid chemoreceptor modulation of sympathetic vasoconstrictor outflow during exercise in healthy humans

Recently, we have shown that specific, transient carotid chemoreceptor (CC) inhibition in exercising dogs causes vasodilatation in limb muscle. The purpose of the present investigation was to determine if CC suppression reduces muscle sympathetic nerve activity (MSNA) in exercising humans. Healthy subjects (N = 7) breathed hyperoxic gas (F(IO(2)) approximately 1.0) for 60 s at rest and during rhythmic handgrip exercise (50% maximal voluntary contraction, 20 r.p.m.). Microneurography was used to record MSNA in the peroneal nerve. End-tidal P(CO(2)) was maintained at resting eupnoeic levels throughout and breathing rate was voluntarily fixed. Exercise increased heart rate (67 versus 77 beats min(-1)), mean blood pressure (81 versus 97 mmHg), MSNA burst frequency (28 versus 37 bursts min(-1)) and MSNA total minute activity (5.7 versus 9.3 units), but did not change blood lactate (0.7 versus 0.7 mm). Transient hyperoxia had no significant effect on MSNA at rest. In contrast, during exercise both MSNA burst frequency and total minute activity were significantly reduced with hyperoxia. MSNA burst frequency was reduced within 9-23 s of end-tidal P(O(2)) exceeding 250 mmHg. The average nadir in MSNA burst frequency and total minute activity was -28 +/- 2% and -39 +/- 7%, respectively, below steady state normoxic values. Blood pressure was unchanged with hyperoxia at rest or during exercise. CC stimulation with transient hypoxia increased MSNA with a similar time delay to that obtained with CC inhibition via hyperoxia. Consistent with previous animal work, these data indicate that the CC contributes to exercise-induced increases in sympathetic vasoconstrictor outflow.

Rat carotid body chemosensory discharge and glomus cell HIF-1α expression in vitro: regulation by a common oxygen sensor

Addition of Pco (∼350 Torr) to a normoxic medium (Po2 of ∼130 Torr) was used to investigate the relationship between carotid body (CB) sensory discharge and expression of hypoxia-inducible factor 1α (HIF-1α) in glomus cells. Afferent electrical activity measured for in vitro -perfused rat CB increased rapidly (1–2 s) with addition of high CO (Pco of ∼350 Torr; Po2 of ∼130 Torr), and this increase was fully reversed by white light. At submaximal light intensities, the extent of reversal was much greater for monochromatic light at 430 and 590 nm than for light at 450, 550, and 610 nm. This wavelength dependence is consistent with the action spectrum of the CO compound of mitochondrial cytochrome a3. Interestingly, when isolated glomus cells cultured for 45 min in the presence of high CO (Pco of ∼350 Torr; Po2 of ∼130 Torr) in the dark, the levels of HIF-1α, which turn over slowly (many minutes), increased. This increase was not observed if the cells were illuminated with white light during the incubation. Monochromatic light at 430- and 590-nm light was much more effective than that at 450, 550, and 610 nm in blocking the CO-induced increase in HIF-1α, as was the case for chemoreceptor discharge. Although the changes in HIF-1α take minutes and those for CB neural activity occur in 1–2 s, the similar responses to CO and light suggest that the oxygen sensor is the same (mitochondrial cytochrome a3).

Purines, the carotid body and respiration

The carotid body is essential to detecting levels of oxygen in the blood and initiating the compensatory response. Increasing evidence suggests that the purines ATP and adenosine make a key contribution to this signaling by the carotid body. The glomus cells release ATP in response to hypoxia. This released ATP can stimulate P2X receptors on the carotid body to elevate intracellular Ca(2+) and to produce an excitatory response. This released ATP can be dephosphorylated to adenosine by a series of extracellular enzymes, which in turn can stimulate A(1), A(2A) and A(2B) adenosine receptors. Levels of extracellular adenosine can also be altered by membrane transporters. Endogenous adenosine stimulates these receptors to increase the ventilation rate and may modulate the catecholamine release from the carotid sinus nerve. Prolonged hypoxic challenge can alter the expression of purinergic receptors, suggesting a role in the adaptation. This review discusses evidence for a key role of ATP and adenosine in the hypoxic response of the carotid body, and emphasizes areas of new contributions likely to be important in the future.

Mechanisms of action of acetazolamide in the prophylaxis and treatment of acute mountain sickness

Acetazolamide, a potent carbonic anhydrase (CA) inhibitor, is the most commonly used and best-studied agent for the amelioration of acute mountain sickness (AMS). The actual mechanisms by which acetazolamide reduces symptoms of AMS, however, remain unclear. Traditionally, acetazolamide's efficacy has been attributed to inhibition of CA in the kidneys, resulting in bicarbonaturia and metabolic acidosis. The result is offsetting hyperventilation-induced respiratory alkalosis and allowance of chemoreceptors to respond more fully to hypoxic stimuli at altitude. Studies performed on both animals and humans, however, have shown that this explanation is unsatisfactory and that the efficacy of acetazolamide in the context of AMS is likely due to a multitude of effects. This review summarizes the known systemic effects of acetazolamide and incorporates them into a model encompassing several factors that are likely to play a key role in the drug's efficacy. Such factors include not only metabolic acidosis resulting from renal CA inhibition but also improvements in ventilation from tissue respiratory acidosis, improvements in sleep quality from carotid body CA inhibition, and effects of diuresis.

Instability of spontaneous breathing patterns in patients with persistent vegetative state

We investigated the breathing patterns of 27 patients in a persistent vegetative state (PVS) and 15 normal control volunteers. During the baseline period breathing air, 15 patients (the PVS-IB) exhibited irregular breathing (IB), whereas the other 12 (the PVS-OB) displayed oscillatory breathing (OB). Both groups maintained an average value for tidal volume (V(T)), total breath duration (T(TOT)), minute ventilation (V (E)), oxygen saturation (SpO2) similar to the control, but the PVS-OB displayed significantly lower end-tidal CO2 tension (P(ET)CO2) than the control. The V(T), T(TOT), V (E) and P(ET)CO2 of the PVS-OB showed cyclic changes. The coefficients of variation of V(T), T(TOT) and V (I) were: PVS-OB>PVS-IB>control. Inhalation of 100% O2 significantly reduced the respiratory variability and prevented OB of the PVS-OB. We concluded that PVS patients display respiratory instability and that brain damage, hypocapnia, and/or increased loop gain of arterial chemoreceptors may contribute to the pathogenesis of OB, whereas brain damage presumably may be the cause of IB.

Peripheral chemoreceptors in health and disease

Peripheral chemoreceptors (carotid and aortic bodies) detect changes in arterial blood oxygen and initiate reflexes that are important for maintaining homeostasis during hypoxemia. This mini-review summarizes the importance of peripheral chemoreceptor reflexes in various physiological and pathophysiological conditions. Carotid bodies are important for eliciting hypoxic ventilatory stimulation in humans and in experimental animals. In the absence of carotid bodies, compensatory upregulation of aortic bodies as well as other chemoreceptors contributes to the hypoxic ventilatory response. Peripheral chemoreceptors are critical for ventilatory acclimatization at high altitude. They also contribute in part to the exercise-induced hyperventilation, especially with submaximal and heavy exercise. During pregnancy, hypoxic ventilatory sensitivity increases, perhaps due to the actions of estrogen and progesterone on chemoreceptors. Augmented peripheral chemoreceptors have been implicated in early stages of recurrent apneas, congestive heart failure, and certain forms of hypertension. It is likely that chemoreceptors tend to maintain oxygen homeostasis and act as a defense mechanism to prevent the progression of the morbidity associated with these diseases. Experimental models of recurrent apneas, congestive heart failure, and hypertension offer excellent opportunities to unravel the cellular mechanisms associated with altered chemoreceptor function.

CO2/H(+) sensing: peripheral and central chemoreception

H(+) is maintained constant in the internal environment at a given body temperature independent of external environment according to Bernard's principle of "milieu interieur". But CO2 relates to ventilation and H(+) to kidney. Hence, the title of the chapter. In order to do this, sensors for H(+) in the internal environment are needed. The sensor-receptor is CO2/H(+) sensing. The sensor-receptor is coupled to integrate and to maintain the body's chemical environment at equilibrium. This chapter dwells on this theme of constancy of H(+) of the blood and of the other internal environments. [H(+)] is regulated jointly by respiratory and renal systems. The respiratory response to [H(+)] originates from the activities of two groups of chemoreceptors in two separate body fluid compartments: (A) carotid and aortic bodies which sense arterial P(O2) and H(+); and (B) the medullary H(+) receptors on the ventrolateral medulla of the central nervous system (CNS). The arterial chemoreceptors function to maintain arterial P(O2) and H(+) constant, and medullary H(+) receptors to maintain H(+) of the brain fluid constant. Any acute change of H(+) in these compartments is taken care of almost instantly by pulmonary ventilation, and slowly by the kidney. This general theme is considered in Section 1. The general principles involving cellular CO2 reactions mediated by carbonic anhydrase (CA), transport of CO2 and H(+) are described in Section 2. Since the rest of the chapter is dependent on these key mechanisms, they are given in detail, including the role of Jacobs-Stewart Cycle and its interaction with carbonic anhydrase. Also, this section deals briefly with the mechanisms of membrane depolarization of the chemoreceptor cells because this is one mechanism on which the responses depend. The metabolic impact of endogenous CO2 appears in the section with a historical twist, in the context of acclimatization to high altitude (Section 3). Because low P(O2) at high altitude stimulates the peripheral chemoreceptors (PC) increasing ventilation, the endogenous CO2 is blown off, making the internal milieu alkaline. With acclimatization however ventilation increases. This alkalinity is compensated in the course of time by the kidney and the acidity tends to be restored, but the acidification is not great enough to increase ventilation further. The question is what drives ventilation during acclimatization when the central pH is alkaline? The peripheral chemoreceptor came to the rescue. Its sensitivity to P(O2) is increased which continues to drive ventilation further during acclimatization at high altitude even when pH is alkaline. This link of CO2 through the O2 chemoreceptor is described in Section 4 which led to hypoxia-inducible factor (HIF-1). HIF-1 is stabilized during hypoxia, including the carotid body (CB) and brain cells, the seat of CO2 chemoreception. The cells are always hypoxic even at sea level. But how CO2 can affect the HIF-1 in the brain is considered in this section. CO2 sensing in the central chemoreceptors (CC) is given in Section 5. CO(2)/H(+) is sensed by the various structures in the central nervous system but its respiratory and cardiovascular responses are restricted only to some areas. How the membranes are depolarized by CO2 or how it works through Na(+)/Ca(2+) exchange are discussed in this section. It is obvious, however, that CO2 is not maintained constant, decreasing with altitude as alveolar P(O2) decreases and ventilation increases. Rather, it is the [H(+)] that the organism strives to maintain at the expense of CO2. But then again, [H(+)] where? Perhaps it is in the intracellular environment. Gap junctions in the carotid body and in the brain are ubiquitous. What functions they perform have been considered in Section 6. CO2 changes take place in lung alveoli where inspired air mixes with the CO2 from the returning venous blood. It is the interface between the inspired and expired air in the lungs where CO2 change is most dramatic. As a result, various investigators have looked for CO2 receptors in the lung, but none have been found in the mammals. Instead, CO2/H(+) receptors were found in birds and amphibians. However, they are inhibited by increasing CO2/H(+), instead of stimulated. But the afferent impulses transmitted to the brain produced stimulation in the efferents. This reversal of afferent-efferent inputs is a curious situation in nature, and this is considered in Section 7. The NO and CO effects on CO2 sensing are interesting and have been briefly mentioned in Section 8. A model for CO2/H(+) sensing by cells, neurons and bare nerve endings are also considered. These NO effects, models for CO2/H(+) and O2-sensitive cells in the CNS have been considered in the perspectives. Finally, in conclusion, the general theme of constancy of internal environment for CO2/H(+) is reiterated, and for that CO2/H(+) sensors-receptors systems are essential. Since CO2/H(+) sensing as such has not been reviewed before, the recent findings in addition to defining basic CO2/H(+) reactions in the cells have been briefly summarized.

Cardiovascular responses to hypoxia and anaemia in the toad Bufo marinus

Amphibians exhibit cardiorespiratory responses to hypoxia and, although several oxygen-sensitive chemoreceptor sites have been identified, the specific oxygen stimulus that triggers these responses remains controversial. This study investigates whether the cardiovascular response to oxygen shortage correlates with decreased oxygen partial pressure of arterial blood (Pa(O(2))) or reduced oxygen concentration ([O(2)]) in toads. Toads, equipped with blood flow probes and an arterial catheter, were exposed to graded hypoxia [fraction of oxygen in the inspired air (FI(O(2)))=0.21, 0.15, 0.10, 0.07 and 0.05] before and after reductions in arterial [O(2)] by isovolemic anaemia that reduced haematocrit by approximately 50%. Toads responded to hypoxia by increasing heart rate (fH) and pulmocutaneous blood flow (Q(pc)) and reducing the net cardiac right-to-left-shunt. When arterial [O(2)] was reduced by anaemia, the toads exhibited a similar cardiovascular response to that observed in hypoxia. While arterial CO(2) partial pressure (Pa(CO(2))) decreased significantly during hypoxia, indicative of increased alveolar ventilation, anaemia did not alter Pa(CO(2))). This suggests that reductions in [O(2)] mediate cardiovascular adjustments, while ventilatory responses are caused by reduced Pa(O(2)).

Nasal oxygen and muscle sympathetic nerve activity in heart failure

AIMS

To evaluate the effects of mild hyperoxia on sympathetic activity during quiet breathing in patients with chronic heart failure (CHF) and, hence, to investigate whether tonic activation of excitatory chemoreceptor afferents contributes to the elevated sympathetic activity in these patients. Sympathetic activation in patients with CHF may result in part from increased chemoreflex sensitivity. Previous studies using microneurography did not demonstrate deactivation of the chemoreceptors while the patients were breathing 100% O(2). However, 100% O(2) may decrease cardiac output, thereby offsetting the effects on the chemoreflexes.

SETTING

University hospital.

PATIENTS AND INTERVENTIONS

Ten patients with moderate-to-severe CHF (mean [+/-SD] age, 53.9 +/- 9.2 years; mean ejection fraction, 21.3 +/- 4.7%) were assigned to breathing 20 min of O(2) as well as room air (3 L/min) applied by nasal prongs. Muscle sympathetic nerve activity (MSNA) was evaluated by microneurography of the peroneal nerve.

RESULTS

The application of O(2) resulted in an increase of arterial O(2) saturation but no significant change in MSNA during resting ventilation. Although voluntary apneas were no longer with O(2) (25.3 +/- 5.8 vs 32.6 +/- 8.6 s, respectively; p = 0.014), MSNA during the last 10 s of voluntary apnea was lower while breathing O(2) (63.5 +/- 15.0 vs 59.9 +/- 13.9 bursts per minute, respectively; p = 0.02).

CONCLUSIONS

The increased MSNA in the patients studied could not be reduced by mild hyperoxia, suggesting that the tonic activation of chemoreflex afferents is unlikely to contribute to the elevated sympathetic activity. That nasal O(2) reduces MSNA during apnea may explain the beneficial effects of nocturnal O(2) therapy in CHF patients with Cheyne-Stokes respiration.

Sleep-Breathing Disorders and Heart Failure

Cheyne-Stokes respiration is known to be associated with severe left heart failure. Because of severe desaturation, sleep fragmentation, arousals, and an increase in sympathetic activity, Cheyne-Stokes respiration may lead to a further impairment of cardiac function and to a worsening of quality of life. Although the pathology of Cheyne-Stokes respiration is not fully understood, enhanced chemoreceptor sensitivity, prolonged circulation time, as well as decreased pulmonary gas stores and increased ventilatory drive may be contributing factors. Therapeutic options include the improvement of cardiac failure; medical treatment, such as using theophylline; continous positive airway pressure ventilation; and low-flow oxygen supply. Because of severe cardiac insufficiency, change of endothoracic pressure may worsen the hemodynamic situation in some patients. Therefore, this form of treatment has to be used carefully. Another possible treatment is a low-flow oxygen supply, which will prevent severe desaturations. This therapeutic approach might be a good alternative to noninvasive ventilation. However, it is controversial whether oxygen supply will improve quality of sleep of the patients, even in long-term treatment.

Defective carotid body function and impaired ventilatory responses to chronic hypoxia in mice partially deficient for hypoxia-inducible factor 1 alpha

To investigate whether the transcriptional activator hypoxia-inducible factor 1 (HIF-1) is required for ventilatory responses to hypoxia, we analyzed mice that were either wild type or heterozygous for a loss-of-function (knockout) allele at the Hif1a locus, which encodes the O(2)-regulated HIF-1 alpha subunit. Although the ventilatory response to acute hypoxia was not impaired in Hif1a(+/-) mice, the response was primarily mediated via vagal afferents, whereas in wild-type mice, carotid body chemoreceptors played a predominant role. When carotid bodies isolated from wild-type mice were exposed to either cyanide or hypoxia, a marked increase in sinus nerve activity was recorded, whereas carotid bodies from Hif1a(+/-) mice responded to cyanide but not to hypoxia. Histologic analysis revealed no abnormalities of carotid body morphology in Hif1a(+/-) mice. Wild-type mice exposed to hypoxia for 3 days manifested an augmented ventilatory response to a subsequent acute hypoxic challenge. In contrast, prior chronic hypoxia resulted in a diminished ventilatory response to acute hypoxia in Hif1a(+/-) mice. Thus partial HIF-1 alpha deficiency has a dramatic effect on carotid body neural activity and ventilatory adaptation to chronic hypoxia.

A mathematical model of CO2 effect on cardiovascular regulation

The effect of changes in arterial CO2 tension on the cardiovascular system is analyzed by means of a mathematical model. The model is an extension of a previous one that already incorporated the main reflex and local mechanisms triggered by O2 changes. The new aspects covered by the model are the O2-CO2 interaction at the peripheral chemoreceptors, the effect of local CO2 changes on peripheral resistances, the direct central neural system (CNS) response to CO2, and the control of central chemoreceptors on ventilation and tidal volume. A statistical comparison between model simulation results and various experimental data has been performed. This comparison suggests that the model is able to simulate the acute cardiovascular response to changes in blood gas content in a variety of conditions (normoxic hypercapnia, hypercapnia during artificial ventilation, hypocapnic hypoxia, and hypercapnic hypoxia). The model ascribes the observed responses to the complex superimposition of many mechanisms simultaneously working (baroreflex, peripheral chemoreflex, CNS response, lung-stretch receptors, local gas tension effect), which may be differently activated depending on the specific stimulus under study. However, although some experiments can be reproduced using a single basal set of parameters, reproduction of other experiments requires a different combination of the mechanism strengths (particularly, a different strength of the local CO2 mechanism on peripheral resistances and of the CNS response to CO2). Starting from these results, some assumptions to explain the striking differences reported in the literature are presented. The model may represent a valid support for the interpretation of physiological data on acute cardiovascular regulation and may favor the synthesis of contradictory results into a single theoretical setting.

Heterogeneity of neuronal nicotinic acetylcholine receptors in 5-HT-containing chemoreceptor cells of the chicken aorta

The effects of nicotinic agonists and antagonists on whole-cell currents and 5-hydroxytryptamine (5-HT) release were studied in order to characterize nicotinic ACh receptors on the 5-HT-containing chemoreceptor cells of the chicken aorta. ACh, nicotine and dimethylphenylpiperazinium (DMPP) evoked concentration-dependent inward currents accompanied by increases in current noise at a holding potential of -70 mV. The peak amplitude of the current response to DMPP was 50% larger than that to either nicotine or ACH: Hexamethonium, alpha - bungarotoxin (alpha - BTX) and methyllycaconitine decreased nicotine-induced inward currents in a concentration-dependent manner. Although hexamethonium (0.1 mM) abolished the current response to nicotine (30 microM), a high concentration (1 microM) of alpha - BTX decreased it only by about 30% of the control response. Methyllycaconitine (0.1 microM) decreased the current response to nicotine to the same extent as did alpha - BTX whilst a high concentration (10 microM) abolished the response. ACh, nicotine and DMPP caused concentration-dependent increases in 5-HT output from the thoracic aorta which effect was blocked by hexamethonium (0.1 mM). Pre-treatment with alpha - BTX (1 microM) for 30 min reduced the output of 5-HT induced by ACh to 70% of the control response. It is suggested that neuronal nicotinic ACh receptors, sensitive and insensitive to alpha - BTX, are present on the chemoreceptor cells of the chicken aorta, the activation of which causes the release of 5-HT.

Heart rate response to transient chemoreceptor stimulation in term infants is modified by exposure to maternal smoking

Modulation of heart rate (HR) during transient hyperoxia, hypoxia, and hypercapnia was studied in 46 healthy term infants on 103 occasions (postnatal d 2 to 82). Twenty-three infants had smoking mothers (median, 11 cigarettes/d). Transient chemoreceptor stimuli (100% O(2), 15% O(2), or 3% CO(2)) were presented repeatedly during quiet sleep. Beat-by-beat HR and breath-by-breath ventilation were recorded continuously. The coherently averaged HR and ventilation responses to each stimulus were calculated for each infant at each age. Outcome variables (HR change from baseline to end of stimulation, maximum HR change, and time to half-maximum) were analyzed by ANOVA. Overall, HR declined during hyperoxia (median change, 4.2 beats/min) and rose during hypoxia (median change, 4.2 beats/min) and hypercapnia (median change, 4.6 beats/min). The percentage change in HR was positively correlated with the percentage change in ventilation (p < 0.001). Increasing number of cigarettes smoked by the mother was correlated with deeper HR declines and smaller HR rises (p = 0.02). For the population as a whole, the HR response lagged 3.8 s behind the ventilatory response during hyperoxia and hypoxia (p < 0.001), whereas during hypercapnia there was no significant lag. The lag in HR response in the smoke-exposed group was 2.5 s greater than that in the control group for all three stimuli (p = 0.001), and the difference increased with the number of cigarettes smoked by the mother (p < 0.01). Both pulmonary reflexes and the type of the chemoreceptor stimulus seemed to influence HR. Maternal smoking affected the magnitude and time-course of the HR response in a dose-dependent manner.

Responses of single-unit carotid body chemoreceptors in adult rats

1. Our goal was to describe the in situ responses in rats of single-unit carotid body chemoreceptors to changes in arterial PO2 and PCO2. We identified single-unit carotid chemoreceptor activity in male, adult Sprague-Dawley rats by their rapid responses to i.v. NaCN (20 microg) and transient (10 s) asphyxia. 2. Single-unit chemoreceptor responses to isocapnic changes in oxygenation within the arterial oxygen pressure range 34-114 mmHg were described by the power function: f(dis) = 74010(Pa,O2)-2.5; (r2 = 0.6), where f(dis) is the discharge frequency (spikes s-1), P(a,O2) is the arterial oxygen partial pressure (mmHg) and r2 is the correlation coefficient. 3. The responses to iso-oxic changes in CO2, assumed to be linear, had a slope of 0.089 spikes s-1 (mmHg Pa,CO2)-1 (r2 = 0.7). 4. We conclude that carotid body chemoreceptors in adult rats have responses to changes in Pa,O2 and Pa,CO2 similar to those of other species.

Important role of carotid chemoreceptor afferents in control of breathing of adult and neonatal mammals

This review provides a summary and prospective on the importance of carotid/peripheral chemoreceptors to the control of breathing during physiologic conditions. For several days after carotid body denervation (CBD), adult mammals hypoventilate (+10 mmHg increase in Pa(CO(2))) at rest and during exercise and CO(2) sensitivity is attenuated by about 60%. In addition, if the rostral ventrolateral medulla is cooled during NREM sleep after CBD, a sustained apnea is observed. Eventually, days or weeks after CBD, a peripheral ventilatory chemoreflex redevelops and there is a normalization of breathing (rest and exercise) and CO(2) sensitivity. The site (s) of the regained chemosensitivity has not been established. This plasticity/redundancy after CBD appears greater in neonates than in adult mammals. These data suggest the carotid and other peripheral chemoreceptors provide an important excitatory input to medullary respiratory neurons that is essential for breathing when wakeful stimuli and central chemoreceptors are absent.

Nasal oxygen effects on arterial carbon dioxide pressure and heart rate in chronic heart failure

Nasal oxygen applied by nasal prongs reduced tidal volume and increased carbon dioxide partial pressure in patients with chronic heart failure but not in comparable controls, whereas the patients showed a more pronounced decrease in heart rate with oxygen. These findings indicate that nasal oxygen has distinct effects on ventilation and heart rate in chronic heart failure.

Chemoreflex thresholds to CO2 in decerebrate cats

We used a modified rebreathing technique to measure chemoreflex thresholds to CO2 in decerebrate, paralyzed and ventilated cats. Cats were hyperventilated to neural apnea (PaCO2 < 15 mmHg) with one ventilator and then switched to a rebreathing circuit consisting of a balloon inside a bottle connected to a second ventilator. The volume of the circuit was approximately 110 ml. The balloon contained 5% CO2:95% O2 for hyperoxic rebreathing or approximately 5% CO2 with 11 or 6.5% O2 for moderately and severely hypoxic rebreathing. A plateau in CO2 concentration at the onset of rebreathing indicated equilibration of CO2 between the circuit, alveolar gas and venous and arterial blood. After rapid equilibration of CO2 between the cat and the circuit, CO2 increased linearly with time during rebreathing. Under hyperoxic conditions, phrenic activity began to increase at an end-tidal P(CO2) (PET(CO2)) of 35.1 +/- 6.1 (SD) mmHg (n = 8); during hypoxia, phrenic activity began to increase at a significantly lower PET(CO2) of 27.8 +/- 4.8 mmHg (P < 0.01, n = 6). We interpret these values as the central and peripheral chemoreflex thresholds to CO2, respectively. Persistent phrenic activity prevented determination of a threshold during severe hypoxic rebreathing. Our modified method of hyperoxic and hypoxic rebreathing allows detection of the effects of hypoxia on the central and peripheral chemoreflex thresholds and, within a cat, measurements of chemoreflex sensitivities.

Effect of independent changes in mixed-venous PCO2 or PO2 on cardiac output in anesthetized sheep

To determine whether changes mixed-venous PCO2 or PO2 affect cardiac output independent of changes in arterial blood gases, we used extracorporeal gas exchange to increase mixed-venous PCO2 or decrease mixed-venous PO2 in adult sheep. Sheep were anesthetized, mechanically ventilated, and connected to a veno-venous extracorporeal circuit. The circuit included a gas exchanger which was used to increase mixed-venous PCO2 or decrease mixed-venous PO2; the native lungs were ventilated to maintain arterial PCO2 and PO2 at control levels. When mixed-venous PCO2 was increased by 32% above control levels for a period of 60 min, cardiac output increased significantly to 28% above control levels. Cervical vagotomy abolished this response. In contrast, decreasing mixed-venous PO2 by 29% did not increase cardiac output. These results demonstrate that increasing mixed-venous PCO2 can increase cardiac output independent of changes in arterial blood gases and that intact vagus nerves are necessary for this response to occur.

Carbonic anhydrase and control of breathing: different effects of benzolamide and methazolamide in the anaesthetized cat

1. The effect of inhibition of erythrocyte carbonic anhydrase on the ventilatory response to CO2 was studied by administering benzolamide (70 mg kg-1, i.v.), an inhibitor which does not cross the blood-brain barrier, to carotid body denervated cats which were anaesthetized with chloralose-urethane. 2. In the same animals the effect on the ventilatory response to CO2 of subsequent inhibition of central nervous system (CNS) carbonic anhydrase was studied by infusing methazolamide (20 mg kg-1), an inhibitor which rapidly penetrates into brain tissue. 3. The results show that inhibition of erythrocyte carbonic anhydrase by benzolamide leads to a decrease in the slope of the normoxic CO2 response curve, and a decrease of the extrapolated arterial PCO2 at zero ventilation. 4. Inhibition of CNS carbonic anhydrase by methazolamide results in an increase in slope and alpha-intercept of the ventilatory CO2 response curve. 5. Using a mass balance equation for CO2 of a brain compartment, it is argued that inhibition of erythrocyte carbonic anhydrase results in a decrease in slope of the in vivo CO2 dissociation curve, which can explain the effects of benzolamide. 6. The changes in slope and intercept induced by methazolamide are discussed in relation to effects on neurones containing carbonic anhydrase, which may include central chemoreceptors.

Afferent contributions to intermediate area of the cat ventral medullary surface during mild hypoxia

The intermediate area of the cat ventral medullary surface activates to mild hypoxia. Carotid body and vagal afferent contributions to this response were examined by recording activity levels, measured as changes in scattered 660 nm light, from the medullary surface in 7 anesthetized, spontaneously breathing cats following 12% O2 in N2 ventilatory challenge. A miniaturized video camera collected images synchronous with the peak of cardiac R wave at 1/s, from a 3.2 mm diameter area, before, and following bilateral carotid sinus denervation (CSD) and vagotomy. In intact animals, hypoxia increased activity; however, greater increases in activity levels followed CSD, while vagotomy elicited a marked reduction of the response. Thus, carotid body afferents exert inhibitory or disfacilitatory influences on intermediate area neurons, while the vagus appears to play an excitatory role.

Interactions between the circulatory effects of central hypovolaemia and arterial hypoxia in conscious rabbits

1. Eight conscious rabbits were repeatedly subjected to progressive reduction in central blood volume by gradually inflating a thoracic inferior vena caval-cuff so cardiac index (CI) fell at a constant 8.5% of baseline/min. 2. Caval-cuff inflations were performed after 10 min exposure to 100, 21, 12-14 and 8-10% O2, with and without the addition of 3-4% CO2, in randomized order. 3. The haemodynamic response to progressive reduction in central blood volume was biphasic. In Phase I, systemic vascular conductance index (SVCI) fell linearly, supporting mean arterial pressure (MAP). When CI had fallen to a critical level, Phase II occurred in which SVCI rose abruptly, MAP plummeted and respiratory drive progressively increased. 4. During Phase I, there were independent linear relationships between PaCO2 (but not PaO2) and the rates at which SVCI and MAP changed during the progressive fall of CI. The higher the level of PaCO2, the greater was the rate of fall of SVCI and the less the rate of fall of MAP. 5. There was an inverted U-shaped effect of PaO2 on the level of CI at which Phase II occurred: (a) during hyperoxia (100% O2), Phase II occurred later than during normoxia (21% O2); and (b) across the normoxic and hypoxic gas mixtures (21-8% O2, with and without added CO2), there was an independent linear relationship between PaO2 (but not PaCO2 or PaO2 x PaCO2) and the level of CI at which Phase II occurred. That is, the lower the level of PaO2, the later was the onset of Phase II. This interaction is best explained by an increased level of central sympathetic vasoconstrictor drive during hypoxia.

Carotid chemoreceptor response to natural stimuli in the newborn kitten

The activities of carotid chemoreceptors at three levels of inspired PO2 (55, 145 and 690 Torr) and at two levels of inspired PCO2 (35 and 70 Torr in O2) were studied in 28 anesthetized, mechanically ventilated kittens aged 0-17 days. A biphasic response to hypoxia was found in 46% of them: the chemosensory activity increased to a peak within 30 sec after the initial response to hypoxia and thereafter declined slowly to a stable value. The steady-state single-fiber chemosensory activity at an inspired PO2 of 55 Torr was significantly lower in kittens less than 10 days old (mean +/- SE: 5.8 +/- 0.6 impulses.sec-1) than in the older ones (8.8 +/- 1.3 impulses.sec-1, P less than 0.03). The response curves to arterial PO2 were hyperbolic in both groups but the curve for the younger kittens was displaced to the left of the curve for the older ones. The response to hypercapnia was a progressive increase in chemosensory activity with little evidence of rapid or slow adaptation. The response to hypercapnia was significantly stronger in the older kittens than in the young ones. It is concluded that, in the kitten, the carotid chemoreceptor response to hypoxia may be biphasic. The responses to hypoxia and hypercapnia are already developed but are weak at birth and continue to develop further during the first weeks of postnatal life.

Aortic and carotid body chemoreception in prolonged hyperoxia in the cat

Carotid body chemosensory response to hypoxia is attenuated as a result of prolonged normobaric hyperoxia (NH) in the cat. The effect of NH is likely to be due to high cellular PO2 and O2-related free radicals. Accordingly, the effect would be less if O2 delivery to the chemoreceptor tissue could be compromised. The aortic bodies, which appear to have less of a circulatory O2 delivery, as suggested by their vigorous responses to a slight compromise of O2 flow compared with those of the carotid body, could provide a suitable testing material for the hypothesis. We tested the hypothesis by studying both aortic and carotid body chemoreceptors in the same cats (n = 6) which were exposed to nearly 100% O2 for about 60 h. These chemoreceptor organs were also studied in 6 control cats which were maintained in room air at sea-level. The cats were anesthetized and their carotid and aortic chemosensory fibers were identified by the usual procedure, and their responses to hypoxia and hypercapnia and to bolus injections (i.v.) of cyanide and nicotine were measured. In the NH cats, the carotid but not aortic chemosensory responses to hypoxia and cyanide were attenuated and to hypercapnia (both onset and steady state) augmented. The aortic chemoreceptors were stimulated by hypoxia, hypercapnia, cyanide and nicotine both in the NH and the control cats similarly. The results support the hypothesis that it is presumably a higher tissue blood flow and hence a higher concentration of O2-related free radicals which ultimately led to the specific attenuation of O2 chemoreception in the carotid body.

A dynamic analysis of the ventilatory response to hypoxia in man

1. The dynamics of the ventilatory response to isocapnic hypoxia were studied in seven healthy subjects using four different levels of hypoxia, (inspired oxygen pressures, PI,O2 equal to 110, 100, 80 and 60 mmHg) successively increasing and decreasing stepwise. 2. Five such progressions were performed for each subject, corresponding to five different durations of the steps (t) ranging between 0.33 and 5.00 min. The overall duration of one test (T) was taken as the sum of the seven successive PI,O2 hypoxic steps (t) plus one step t of air breathing. Thus, the values of T ranged between 2.6 and 40.0 min. 3. End-tidal CO2 pressure was maintained constant (+/- 1 mmHg) throughout the test by manipulation of inspired CO2 pressure. 4. We measured, as a function of T, (i) the magnitude of the loops formed by the ventilatory response curves (PA,O2-VE) as measured by their surface area (S), (ii) the magnitude of ventilatory response to each rising hypoxic step, and (iii) the difference between resting VE and VE observed at PA,O2 equal to 50 mmHg (delta V50). On average, we found one maximum in absolute value of S at T = 8 min and one minimum at T = 12 min, along with two maxima of ventilatory response at T values of 8 and 24 min. 5. The same measurements were made on tidal volume response curves (PA,O2-VT) and ventilatory frequency response curves (PA,O2-f): on average we observed two non-significant peaks in the progression with T of VT and S(VT) and two significant peaks in that of delta VT,50 for T = 8 and T = 24 min. No significant peak was observed in the progression with T of f curve parameters. 6. These results are discussed together with the current dynamic model of the ventilatory control system, which includes a central neural controller with no dynamics of its own and a linear response to chemoreceptor inputs. We discuss the physiological meaning of a negative loop area in relation to the previously described depressant effect of hypoxia upon the brain stem. 7. We conclude that the dynamics of the controlling neuronal network are responsible for the observed singularities which result from differential sensitivity properties of the controller. We propose the existence of discrete excitatory states of the controller as a possible explanation of the shape of the steady-state response curve to hypoxia and of the loop variations.