Chemoreceptor Afferents Research Articles

Tonic chemoreflex activation does not contribute to elevated muscle sympathetic nerve activity in heart failure.

Sympathetic activation in heart failure may be due to an increase in sympathetic excitatory influences or to a decrease in inhibitory signals to the brain stem. Chemoreflex sensitivity may be increased in patients with heart failure. The present study tested the hypothesis that tonic activation of excitatory chemoreceptor afferents contributes to the elevated sympathetic activity in heart failure. We recorded sympathetic nerve activity to muscle circulation from the peroneal nerve of 12 chronic heart failure patients while the patients were breathing room air and during deactivation of the chemoreceptors while the patients were breathing a 100% O2 gas mixture. All patients except 2 were in class III of the New York Heart Association functional classification. Left ventricular ejection fraction defined by radionuclide ventriculography was 24 +/- 2% (mean +/- SE). We also obtained measurements of resting sympathetic nerve activity in 9 healthy control subjects to document that sympathetic nerve activity was elevated in heart failure subjects. Resting sympathetic nerve activity was 59 +/- 5 bursts/min in heart failure patients versus 36 +/- 4 bursts/min in control subjects (P < .01). In heart failure patients, oxygen administration increased oxygen saturation from 94 +/- 0.9% to 99 +/- 0.3% (P < .0001). This increase in oxygen saturation did not affect resting muscle sympathetic nerve activity (798 +/- 122 U/min while patients breathed room air and 824 +/- 35 U/min during 100% O2 breathing) or blood pressure. Increased efferent sympathetic activity to muscle circulation in patients with heart failure is not explained by tonic activation of excitatory chemoreflex afferents.

S6-2 Aging and the autonomic nervous system

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% O2. However, 100% O2 may decrease cardiac output, thereby offsetting the effects on the chemoreflexes.University hospital.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 O2 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.The application of O2 resulted in an increase of arterial O2 saturation but no significant change in MSNA during resting ventilation. Although voluntary apneas were no longer with O2 (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 O2 (63.5 ± 15.0 vs 59.9 ± 13.9 bursts per minute, respectively; p = 0.02).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 O2 reduces MSNA during apnea may explain the beneficial effects of nocturnal O2 therapy in CHF patients with Cheyne-Stokes respiration.

Dopamine or transmitter release from rat carotid body may not be essential to hypoxic chemoreception.

In anesthetized, paralyzed, and ventilated rats, hypoxia or intracarotid cyanide excited the carotid chemoafferents, whereas intracarotid dopamine and tyramine inhibited the chemoafferent discharges. The inhibition was abolished by chlorpromazine without attenuating the hypoxic excitation. Comparably, the hypoxic excitation was not attenuated by the following: 1) inhibition of nitric oxide synthase with NG-nitro-L-arginine; 2) inhibition of heme oxygenase with zinc protoporphyrin IX; 3) antagonism of ATP receptors with reactive blue 2; 4) antagonism of cholinergic receptors with atropine or trimethaphan; 5) inactivation of adenosine with adenosine deaminase; and 6) blockade of glutamate receptors with kynurenate. Systemic administration of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N'N'-tetraacetic acid, in doses reversibly blocking sympathetic ganglionic transmission, was also without effect. Cyanide microinjection (0.05-0.5 nmol) into the petrosal but not nodose ganglion elicited a rapid dose-dependent elevation of arterial pressure. We conclude that excitation of the chemoreceptor afferents by hypoxia/cyanide cannot be attributed to release of these agents nor to others by Ca(2+)-dependent mechanisms. The results suggest that the afferent nerves themselves might function as oxygen detectors.

A midline area in the nucleus commissuralis of NTS mediates the phrenic nerve responses to carotid chemoreceptor stimulation

The carotid body chemoreceptor afferents have been reported to project to a discrete area located in the nucleus commissuralis of nucleus tractus solitarius [A. Vardhan et al., Am. J. Physiol., 264 (1993) R41–R50]. The afore-mentioned study was done in spontaneously breathing rats and the afferents and efferents located in the chest wall and the respiratory tract of these animals were intact. In order to exclude the role, if any, of these afferents and efferents, in the present experiments respiratory changes were monitored by recording the phrenic nerve activity instead of tracheal airflow. Experiments were carried out in pentobarbital-anesthesized, bilaterally vagotomized, paralyzed and aartificially ventilated rats with a pneumothorax. The carotid body chemoreceptors were stimulated with tracheal administration of nitrogen for 7–10 s. The chemoreceptor stimulation induced an increase in the frequency and amplitude of phrenic nerve bursts. A decrease in the duration of inspiratory ( T I) expiratory ( T E) and total cycles ( T TOT) was observed in the phrenic nerve activity. Inhibition of neuronal cell bodies by microinjections of muscimol (140 pmol/20 nl) into a discrete area in the commissural subnucleus of the nucleus tractus solitarius (coordinates in mm: 0.3 rostral to 0.5 caudal, 0 to 0.5 lateral and 0.3 to 0.5 deep with respect to the calamus scriptorius), attenuated the phrenic nerve responses to the carotid body stimulation. On the other hand, control injections of saline (0.9%) into this site did not alter the phrenic nerve responses to the carotid body stimulation. Microinjections of bicuculline methiodide (a GABA A receptor antagonist) reversed the actions of muscimol. These results further confirm that the carotid body chemoreceptors project to a discrete are in the commissural subnucleus of the nucleus tractus solitarius. This site is tentatively called chemoreceptor projection site (CPS).

Baroreceptor inhibition of the locus coeruleus noradrenergic neurons

The locus coeruleus is involved in the regulation of blood pressure. The present study was undertaken to address the question of how the blood pressure, in turn, changes the activity of the locus coeruleus neurons via the action of baroreceptors. In chloralose- and urethane-anesthetized rats, the central cut end of the aortic depressor nerve, which does not contain chemoreceptor afferents in this species, was stimulated electrically to excite baroreceptor afferents after bilateral vagotomy and sectioning of the carotid sinus nerve. Single train-pulse stimulation of the aortic depressor nerve provoked the inhibition of ongoing activity in 48% of locus coeruleus neurons tested, but 30% of them responded by excitation with subsequent inhibition. However, when the train-pulse stimulation was repeated with a frequency of 5 Hz, which is close to that of the heartbeat in the rat, all neurons were markedly inhibited. Another series of experiments was conducted in vagotomized and carotid sinus nerve-sectioned rats with intact aortic depressor nerves. When blood pressure was elevated by an intravenous injection of a pressor agent, methoxamine or angiotensin II, or by rapid blood loading, ongoing activities of all locus coeruleus neurons tested were suppressed. In contrast, intravenously injected nitroprusside, a depressor agent, increased the activity of locus coeruleus neurons. In rats with all nerves preserved, rapid blood loading markedly inhibited the activity of these neurons. Such inhibition was partially but significantly attenuated by bilateral sectioning of the aortic depressor nerves. It is concluded that activation of aortic baroreceptors provokes inhibitory or excitatory-inhibitory response in locus coeruleus neurons; phasic activation of the baroreceptor afferents with repetitive train-pulse stimulations at a frequency close to that of the heartbeat produces only inhibition, suggesting that, under natural conditions, the locus coeruleus neurons are tonically inhibited by ongoing input from arterial baroreceptors.

Effect of acute and chronic cobalt administration on carotid body chemoreceptors responses

Chronic cobalt exposure leads to release and production of erythropoietin an consequently to polycithemia. Accodingly, cellular elements sensitive to oxygen in the carotid body, would manifest responses during acute and chronic cobalt administration. The carotid body, detects gas changes (P0 2, POo 2/pH) in the arterial blood and regulates ventilation and circulation by the afferent nerve discharge. We hypothesized that cobalt interacts with an oxygen sensitive mechanism in the carotid chemoreception and in erythropoietin producing cells. Twelve cats were anesthetized, paralysed and artificially ventilated; few fiber preparation of carotid sinus nerve were recorded during close intraarterial injection of cobalt. In another protocol, 12 rats received an intraperitoneal dose of CoCl 2 (10 mg/kg) daily for 6 weeks. At the end, the carotid body was fixed in situ by superfusion. Ultrastructural and morphometric studies were made. Acute administration (0.08–2.3 μmol) promptly stimulated the chemoreceptor afferents. Type I cells increased significantly along with erythropoiesis in the chronic cobalt treated rats. The stimulatory effects of cobalt on the carotid body chemoreceptor showed that sensitive mechanisms in the kidney and in the carotid body are similar, and cobalt interacts with the physiological responses of oxygen.

Fetal cardiovascular reflex responses to hypoxaemia

The fetus mounts a coordinated cardiovascular response to an insult of acute hypoxaemia which involves neural and endocrine components. During acute hypoxaemia in late pregnancy there is a transient bradycardia, a gradual increase in arterial blood pressure and an increase in heart rate variability. In addition, there is a redistribution of the combined ventricular output favouring the cerebral, myocardial and adrenal circulations by shunting blood away from the peripheral circulations. A component of the increase in peripheral vascular resistance and the increase in arterial blood pressure during acute hypoxaemia is mediated via increases in plasma concentrations of vasoconstrictor hormones such as vasopressin, angiotensin II and neuropeptide Y. Whilst an increase in plasma ACTH and cortisol is also seen during acute hypoxaemia, their contribution to cardiovascular control in fetal sheep is less clear.Evidence has been presented to suggest that a number of these cardiovascular and endocrine responses to acute hypoxaemia are chemorefiex in nature, mediated principally by carotid chemoreceptor afferents. In addition, this reflex may be modifiable in terms of changes in magnitude and gain: first, by an influence of the intrauterine environment during chronic hypoxaemia and second, through genetic influences, in animals adapted to life at high altitude.

Central integration of chemoreceptor afferent activity.

There is an abundance of literature concerning the possible transduction mechanisms and neurotransmitters that act within the carotid body, the activity of carotid chemoreceptor afferents to various stimulants, and the reflex effects produced by altering chemoreceptor afferent discharge. However, relatively little information is available regarding the mechanisms which intervene between the activity in the afferent nerve fibres and the alterations evoked in motor outflow. In this short review I will summarise what is known regarding the sites of termination of arterial chemoreceptor afferents and the integration of these afferents with other afferent inputs, respiration, and higher centres at the second and higher order neurones. In addition, these will be compared to the pathways underlying the responses evoked by a second set of chemoreceptor afferents, those arising from the pulmonary chemoreceptor afferents variously termed “J-receptors”, “pulmonary C-fibre afferents” or “pulmonary chemoreceptor afferents”. When activated these afferents evoke a marked vagal bradycardia, a depressor response and either rapid shallow breathing or apnoea. These afferents are of interest since the bradycardia they evoke is unaffected by respiratory gating (Daly & Kirkman, 1989; Daly, 1991) whereas the bradycardia evoked by arterial chemoreceptor or baroreceptor afferents is powerfully gated both by the central respiratory cycle and lung inflation (Daly, 1986; Davidson et al., 1976). This may suggest that the central mechanisms underlying the two chemoreceptor reflexes are fundamentally different. Hence, it is worth comparing the pathways responsible for both reflexes. The effects of these two types of afferent on the relevant motor neuronal pools has been discussed elsewhere in this volume (Jones et al., 1994) and will not be discussed in detail here.

Regulation of Oxygen Delivery and Consumption in Anesthetized Rats during Acute Hypoxia.

Physiological roles of ventilatory responses to acute hypoxia in the regulation of O2 transport and consumption were evaluated quantitatively in halothane anesthetized and spontaneously breathing rats with or without peripheral chemoreceptor afferents. Ventilation (VE), blood gas values, cardiac output (Q), and whole-body O2 consumption (VO2) were measured at various levels of inspired O2 concentration (FIO2) before and after denervation of carotid chemoreceptor afferents. In the carotid sinus nerve (CSN)-intact rat, the reduction in FIO2 from > 0.27 (hyperoxia) to about 0.10 (mild-moderate hypoxia) elicited an augmentation of VE which was accompanied by a well maintained total amount of O2 delivered to tissue (Q x O2 content of arterial blood, DO2) and VO2. The increase in VE was, however, turned off and the VE value did not differ from that in hyperoxia at FIO2 lower than 0.10 (moderate-severe hypoxia) (hypoxic ventilatory depression, HVD). Significant decreases in Q, DO2, and VO2 were also seen at FIO2 < 0.10. After CSN section, VE, Q, DO2, and VO2 values decreased progressively with the reduction in FIO2 from hyperoxia to mild hypoxia (FIO2 = 0.20-0.15). These findings indicate that the normal DO2 and VO2 are preserved in the presence of carotid chemoreceptor afferents in the FIO2 range > 0.10 and that HVD is accompanied by the decrease in VO2. Summarizing all results, VO2 decreased linearly with the reduction in PaO2 lower than about 60 mmHg or DO2 lower than 4 ml/min/100 g bw (critical PaO2 or DO2 for metabolic suppression), above which VO2 remained unchanged. The decrease in VO2 in lower PaO2 or DO2 levels (hypoxic hypometabolism) ensues perhaps from "adaptive" mechanisms reducing O2 demand when adequate O2 supply to tissue is limited.

Is the ventilatory decline seen in newborns during hypoxaemia centrally mediated?

Breathing pattern in the newborn is often irregular and may be periodic or apnoeic, particularly during REM sleep. Clearly mild levels of hypoxaemia will be regularly experienced by many babies until respiratory control matures. A fall in PaO2 becomes even more significant when the newborn’s respiratory response to acute, mild hypoxaemia is considered. This response is apparently paradoxical, with a brisk initial increase in ventilation (phase 1), being followed shortly afterwards by a decline to, or to below, prehypoxaemic levels (phase 2). Whilst phase 1 is attributed to peripheral chemoreceptor stimulation, the mechanism(s) underlying phase 2 have remained controversial. There is increasing evidence that the brainstem may cause the ventilatory decline in the newborn (Moore et al., 1991) whilst there is maintenance of increased afferent chemoreceptor drive throughout this phase (Blanco et al., 1984). If an inhibitory central mechanism is activated during mild hypoxaemia and acts to reduce ventilation, the central processing of afferent chemoreceptor discharge should be modulated. Therefore, we hypothesised that a carotid chemoreceptor-mediated reflex affecting breathing should be altered during hypoxaemia in young rabbits. We also anticipated that other non chemoreceptor-mediated effects on breathing would not be affected.

Responses of feline caudal hypothalamic cardiorespiratory neurons to hypoxia and hypercapnia.

Several studies have suggested that the caudal hypothalamus modulates responses to hypercapnia and hypoxia. In addition, this area of the hypothalamus contains neurons that have a sympathoexcitatory discharge. The purpose of the present study was to determine whether the basal discharge of caudal hypothalamic neurons that are stimulated by hypercapnia or hypoxia is related to cardiovascular (sympathetic discharge and/or the cardiac cycle) and/or respiratory activity (phrenic nerve discharge). Hypothalamic single unit activity, phrenic nerve activity, and cervical sympathetic nerve activity were recorded in anesthetized cats. Computer averaging techniques were used to compare temporally the discharge of hypothalamic neurons with cardiovascular and/or respiratory activity. Cardiorespiratory and hypothalamic neuronal responses to ventilation with hypoxic (10% O2/90% N2) and hypercapnic (5% CO2/95% O2) gases were determined in intact and in peripherally-chemodenervated, barodenervated cats. Thirty-two percent of hypothalamic neurons were stimulated by a hypercapnic stimulus in intact cats; of those that were stimulated by hypercapnia, all had a basal discharge related to cardiovascular and/or respiratory activity. Hypoxia significantly increased the discharge rate of 21% of hypothalamic units in intact animals; 90% of those had a cardiovascular and/or respiratory-related rhythm. Only 13% of the neurons were stimulated by both hypoxia and hypercapnia. Similar results were found in barodenervated, peripherally chemodenervated cats. Neurons excited by these stimuli in both the intact and denervated cats were found to be concentrated in the posterior hypothalamic area. The results of this study suggest that a group of caudal hypothalamic neurons contribute to the cardiorespiratory responses to hypoxia and hypercapnia, but via separate subpopulations of neurons. In addition, input from peripheral baroreceptor and chemoreceptor afferents is not required for this modulation.

Hypercapnia and hypoxia which develop during retching participate in the transition from retching to expulsion in dogs

The roles of arterial and central chemoreceptors in the transition from retching to expulsion during vomiting were studied. In spontaneously breathing decerebrate dogs, actual vomiting induced by activation of abdominal vagal afferents always consisted of retching and subsequent expulsion phases. Pulmonary ventilation almost stopped during the retching phase. Arterial blood CO 2 tension gradually increased and reached a maximum near the time of the transition from the retching phase to the expulsion phase. Similarly, when end-tidal CO 2 was maintained higher than 4.6 ± 0.7% in paralyzed, artificially ventilated decerebrate dogs, stimulation of abdominal vagal afferents induced fictive retching and fictive expulsion, which were identified from the characteristic discharge patterns of the motor nerves to the costal and hiatal parts of the diaphragm, the abdominal muscles and the digastric muscle. However, only fictive retching occurred at an end-tidal CO 2 of less than 3.7 ± 0.7%. Although end-tidal CO 2 was at a low level, fictive retching was followed by fictive expulsion when artificial ventilation was interrupted during the fictive retching phase and when sinus nerve afferents were stimulated. Even after sino-aortic denervation, fictive retching and subsequent fictive expulsion could be induced by stimulation of either vagal afferents or the solitary tract and nucleus, but the threshold level of end-tidal CO 2 which enabled the induction of fictive expulsion increased after denervation. These results indicate that the activity of arterial and/or central chemoreceptor afferents must exceed some critical level to induce the transition from the retching phase to the expulsion phase.

Respiratory sensations in subjects who lack a ventilatory response to CO 2

An urge to breathe is perceived during breath hold and hypercapnis (termed ‘air hunger’) and during heavy exercise (often termed ‘shortness of breath’). To better understand the neural mechanisms responsible for these sensations we studied five patients (8–17 years old) with congenital central hypoventilation syndrome (CCHS) who lack ventilatory response to CO 2. CCHS patients reported no respiratory discomfort during CO 2 inhalation or during maximal breath hold which was of much longer duration than age-matched controls. However, all 3 CCHS patients who exercised heavily reported some sensations akin to shortness of breath (they increased breathing nearly as much as controls). Our results are consistent with two possibilities. First, the air hunger of hypercapnia and breath hold is caused by projection to the forebrain of respiratory chemoreceptor afferents which bypass the respiratory centers, while exercise shortness of breath is caused by direct projections of limb afferents or locomotory center activity. Second, air hunger and shortness of breath share the same origin - projection increased brain stem respiratory center motor activity (corollary discharge) to the forebrain.

Endothelin concentrations in respiration-related structures of the medulla during the perinatal period of the rat

Endothelin-like immunoreactivity (ET-LI) was quantified in the developing (foetal and postnatal) rat brain stem and cerebellum using radioimmunoassay. The brain stem structures chosen for this study were (a) dorsal medullary region (DMR) including the region of nucleus tractus solitarius where the peripheral chemoreceptor afferents are known to terminate, (b) ventral medullary region (VMR) where the central chemoreceptors are thought to be located and (c) cerebellum (CER), as a control area. Compared to the prenatal period, significantly elevated concentrations of ET-LI were detected in the early postnatal period and thereafter the concentrations decreased: DMR and VMR: in comparison to the prenatal concentrations, a two-fold increase was found on the day of birth which further increased significantly ( P < 0.001) on postnatal day 1 only in the region of DMR; CER: low concentrations of ET-LI were found in the early postnatal period which were not significantly different from the prenatal values. No ET-LI could be detected in any of the three regions in the adult rats. The results are discussed in view of the hypothesis that (1) endothelin appears to play an important role in the perinatal period and (2) it is involved in the chemoreceptor pathway.

Vagal and chemoreceptor influences on abdominal muscle activity in awake lambs during hypoxia.

The ventilatory response to hypoxia is a complex phenomenon involving several control mechanisms. We designed this study to examine the dynamic control of abdominal muscle expiratory electromyogram (EMG) activity during room-air breathing and hypoxia and then to analyze the relative contribution of the chemoreceptors and vagal afferents. We studied 12 11- to 22-day-old awake nonsedated lambs, six intact and six vagotomized. To assess the dynamic influence of peripheral chemoreceptors on abdominal muscle expiratory activity, we performed transient testing of peripheral chemoreceptor function (pure O2 and N2 inhalation, KCN injection). To assess the influence of central chemoreceptor afferents, we compared results obtained during hypocapnic and isocapnic 15-min hypoxic runs (fractional concentration of inspired O2 0.08) in each lamb. We also compared results obtained in intact and vagotomized lambs so that the importance of vagal afferents could be assessed. We consistently observed abdominal muscle expiratory EMG activity in each lamb, whether intact or vagotomized, during baseline room air breathing; further recruitment was observed during hypoxia. We also consistently observed abdominal muscle expiratory recruitment during hypocapnic hypoxia in each lamb, although it was significantly less marked than during isocapnic hypoxia. Our transient testing of peripheral chemoreceptor function showed, furthermore, that peripheral chemoreceptor afferents dynamically modulate abdominal muscle expiratory activity. Thus, during hypoxia in 11- to 22-day-old awake nonsedated lambs, increased afferent information from peripheral chemoreceptors forcefully enhances abdominal muscle expiratory activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Carotid chemosensory response to doxapram in the newborn kitten.

Doxapram stimulates ventilation by acting on peripheral chemoreceptors and medullary respiratory neurons in a dose-dependent fashion (Hirsh and Wang, 1974; Mitchell and Herbert, 1975). In adult cats, doxapram has been found to increase the chemosensory response to hypoxia and hypercapnia (Mitchell and Herbert, 1975; Nishino et al., 1982). Several studies have shown that bilateral section of the carotid sinus nerves abolishes or decreases the ventilatory effect of doxapram in adult animals (Hirsh and Wang, 1974; Mitchell and Herbert, 1975; Folgering et al., 1981; Nishino et al., 1982). Despite a frequent use of this drug in the treatment of neonatal apnea (Bairam and Vert, 1986; Barrington et a1.,1986; Tay-Uyboco et al., 1991), its action on carotid chemoreceptor activity is still unclear. We studied the effects of doxapram on carotid chemosensory responses to hypoxia and hypercapnia in newborn kittens. We found that doxapram stimulates the discharge rate of the carotid chemoreceptor afferents at all levels of FiCO2 and FiCO2 tested. In hypoxia, the effect of doxapram is more variable and this variability is not dependent on age.KeywordsCarotid BodyPancuronium BromideGlomus CellPeripheral ChemoreceptorCarotid Sinus NerveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Maturation of carotid chemoreceptor sensitivity to hypoxia: in vitro studies in the newborn rat.

1. A preparation was developed to record single-fibre chemoreceptor afferent activity from carotid bodies of newborn and adult rats in vitro. The response to severe hypoxia was studied as a function of developmental age in four age groups: 1-2, 4-7, 10-15 days and adult (25-30 days). 2. During superfusion with HEPES-saline at room PO2 and at 26 or 35 degrees C, afferent chemoreceptor activity could be recorded in all age groups. No significant difference was found among groups in baseline discharge frequency at 26 or 35 degrees C. 3. All chemoreceptors responded to anoxia (PO2 congruent to 0 Torr) with a rapid increase in discharge frequency. At 35 degrees C, peak discharge frequency of single chemoreceptor afferents was significantly greater in the adult (15.7 +/- 1.6 Hz, mean +/- S.E.M., n = 18) and 10-15 days (11.2 +/- 4.2, n = 8) compared to rats of 1-2 days (4.3 +/- 0.7, n = 14) and 4-7 days of age (3.9 +/- 1.0, n = 7). 4. At 2 min into the anoxia period, all chemoreceptor activities were reduced from their peak discharge levels. At 35 degrees C, this decrease was significantly greater in the adult compared to the newborn. 5. During the period of decreased activity during anoxia, the chemoreceptor discharge could not be increased by inter-stream injection of acetylcholine (100 micrograms) or dopamine (100 micrograms), although these drugs were effective in increasing discharge rate prior to hypoxia. 6. We conclude that: (1) postnatal maturation of chemoreceptor sensitivity to hypoxia is present in vitro, (2) maturation occurs between the first and second week after birth in the rat, and (3) the decrease in activity during prolonged anoxia is not greater in the newborn compared to the adult. Thus, maturational changes occur in the sensitivity of the glomus cell-nerve ending complex to hypoxia.

Hypoxia decreases intracellular calcium in adult rat carotid body glomus cells.

1. Carotid body chemoreceptors were removed intact from adult rats and subjected to protease and collagenase enzymatic digestion of connective tissue. 2. Recordings from the sinus nerve demonstrated that chemotransduction remains intact for at least 2-3 h after isolation, enzyme exposure, and suspension in N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered saline at room PO2. 3. After mechanical dissociation, the interrelationship between changes in extracellular PO2 and pH and relative changes in intracellular calcium (Ca2+i) were observed in glomus cells with the use of fluo-3 and confocal microscopy. 4. Brief (60-s) decreases in PO2 from 150 mmHg to near 0 mmHg, at nadir, caused a marked reduction in Ca2+i (peak delta F/F0 = -32 +/- 3%, mean +/- SE, n = 43), which rapidly recovered after reoxygenation. The decrease was reproducible from trial to trial and was also observed in HCO3(-)-buffered Ringer solution. 5. Superfusion with Ca(2+)-free HEPES saline with 1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) blocked the hypoxia-induced increase in afferent chemoreceptor activity in vitro. Superfusion of the same solution over isolated cells for 15 min caused a large decrease in Ca2+i (-34 +/- 7%, n = 16). 6. In the presence of Ca(2+)-free HEPES, reoxygenation caused calcium fluorescence to increase. This suggests that the Ca2+ decrease during hypoxia is due, at least partially, to binding to an intracellular site. 7. Extracellular cobalt (1 mM, 15 min) also reversibly blocked the chemoreceptor response to hypoxia, in vitro, and caused a reduction in Ca2+i (delta F/F0 = -37 +/- 8%, n = 11).(ABSTRACT TRUNCATED AT 250 WORDS)

Chemoreceptor A-fibres in the human carotid body contain tyrosine hydroxylase and neurofilament immunoreactivity

Previous retrograde tracing studies on rat and guinea-pig showed a projection of sensory tyrosine hydroxylase-immunoreactive neurons to the region of the carotid bifurcation via the carotid sinus nerve. In the present study, focussing on the sensory innervation of the human carotid body, antisera to tyrosine hydroxylase and other catecholamine synthesizing enzymes were applied for an immunohistochemical investigation of carotid bodies obtained at autopsy. In addition, an array of antisera directed to non-enzyme antigens known to be present in viscero-afferent neurons were incorporated in the study. The glomic lobules consisting of glomus cells and sustentacular cells contained a variable number of enzyme-immunoreactive glomus cells. Arteries were supplied by nerve fibres displaying the full phenotype of sympathetic noradrenergic axons, i.e. immunoreactivity to tyrosine hydroxylase, aromatic-L-amino-acid-decarboxylase and dopamine-β-hydroxylase. The glomic lobules, however, were densely innervated by tyrosine hydroxylase-immunoreactive axons lacking immunoreactivity to aromatic-L-amino-acid-decarboxylase and dopamine-β-hydroxylase. These fibres reacted with neurofilament 160kD-antibody but were devoid of immunoreactivity to all neuropeptides tested (calcitonin gene-related peptide, somatostatin, substance P). Ultrastructurally, tyrosine hydroxylase/neuronlament 160kD-immunoreactive axons gave rise to large axonal swellings filled with mitochondria and vesicles, and established extensive contacts to glomus cells. Nerve bundles surrounded by a perineural sheath contained both myelinated (2.0–2.8 μm in diameter) and unmyelinated (0.14–3.0 μm) tyrosine hydroxylase-immunoreactive axons. Most of the unmyelinated immunoreactive axons were running singularly within a Schwann cell-sheath. Judged from the pattern of immunoreactivities as well as their preterminal and terminal ultrastructure, tyrosine hydroxylase-immunoreactive axons innervating glomus cells are of sensory origin. Although final proof by retrograde tracing cannot be presented in man, this conclusion is supported by experimental evidence in laboratory animals. The myelinated immunoreactive axons correspond to chemoreceptor A-fibres whereas the classification of the large unmyelinated immunoreactive axons has yet to be established. The lack of immunoreactivity to the dopamine-synthesizing enzyme, aromatic- l-amino-acid-decarboxylase, in this fibre type does not support the view of dopamine being the primary transmitter of chemoreceptor afferents.

Response of the medullary respiratory network of the cat to hypoxia.

1. The effect of systemic hypoxia was tested in anaesthetized, immobilized, thoracotomized and artificially ventilated cats with peripheral chemoreceptor afferents either intact or cut. Extracellular recordings from different types of medullary respiratory neurones and intracellular recordings from stage 2 expiratory neurones were made to determine the hypoxia-induced changes in neuronal discharge patterns and postsynaptic activity as an index for the disturbances of synaptic interaction within the network. 2. The general effect of systemic hypoxia was an initial augmentation of respiratory activity followed by a secondary depression. In chemoreceptor-denervated animals, secondary depression led to central apnoea. 3. The effects of systemic hypoxia were comparable with those of cerebral ischaemia following occlusion of carotid and vertebral arteries. 4. In chemoreceptor-denervated animals, all types of medullary respiratory neurones ceased spontaneous action potential discharge during hypoxia. 5. Reversal of inhibitory postsynaptic potentials (IPSPs) and/or blockade of IPSPs was seen after 2-3 min of hypoxia. 6. During hypoxia, the membrane potential of stage 2 expiratory neurones showed a slight depolarization to -45 to -55 mV and then remained stable. 7. The neurone input resistance increased initially and then decreased significantly during central apnoea. 8. Rhythmogenesis of respiration was greatly disturbed. This was due to blockade of IPSPs and, in some animals, to more complex disturbances of phase switching from inspiration to expiration. 9. Central apnoea occurred while respiratory neurones were still excitable as shown by stimulus-evoked orthodromic and antidromic action potentials. 10. The results indicate that the medullary respiratory network is directly affected by energy depletion. There is indication for a neurohumoral mechanism which blocks synaptic interaction between respiratory neurones in chemoreceptor-intact animals.